spf band 345 cryogenic functional design...

Name Designation Affiliation Signature and Date

Compiled by:

J. Leech SPF Band 345 Lead Engineer

University of Oxford

Approved by:

A.C. Taylor SPF Band

345 Project Manager

University of Oxford

A. Born SPF Band

345 System Engineer

UKATC

I.P. Theron SPF Lead EMSS

Antennas

SPF BAND 345 CRYOGENIC FUNCTIONAL DESIGN DOCUMENT

Document Number..................................................................................317-030000-006 Revision ........................................................................................................................... A Author . J. Leech, R. Watkins, A. Born, M. Jones, L. Liu, A. Hector, A. Aminaei, A. Pollak, D. Banda, T. Ghigna, W. Yang, D. Biao, A. Taylor Date ................................................................................................................ 2019/02/28 Status ......................................................................................................................... Draft

Document No.: Revision: Date:

317-030000-006 A 2019-02-28

Author: J. Leech et. al. of 76

DOCUMENT HISTORY Revision Date of Issue Engineering Change

Number

Comments

A 2019-02-28 - First draft for DDR

DOCUMENT SOFTWARE Package Version Filename

Wordprocessor MsWord Word 317-030000-006_RevA_SPFB345_CryogenicFunctionDesign.docx

Block diagrams

Other

ORGANISATION DETAILS Name SKA Organisation

Registered Address Jodrell Bank Observatory

Lower Withington

Macclesfield

Cheshire

SK11 9DL

United Kingdom

Registered in England & Wales

Company Number: 07881918

Fax. +44 (0)161 306 9600

Website www.skatelescope.org


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TABLE OF CONTENTS

1 Introduction ............................................................................................................10

1.1 Purpose of Document ........................................................................................................... 10

1.2 Scope of Document............................................................................................................... 10

2 Documents ..............................................................................................................10

2.1 Applicable Documents .......................................................................................................... 10

2.2 Reference Documents .......................................................................................................... 10

3 System Description ..................................................................................................12

3.1 Overview ............................................................................................................................... 12

3.2 Functional Architecture ........................................................................................................ 14

3.3 Product Breakdown Structure .............................................................................................. 14

4 Design Description ...................................................................................................15

4.1 Guiding Design Considerations ............................................................................................. 15

4.2 Overall feed package design ................................................................................................. 16

4.3 Vacuum vessel (central hub) and mounting frame .............................................................. 19 4.3.1 Design overview .............................................................................................................................. 19 4.3.2 Required connections and feedthroughs for the central hub ........................................................ 20 4.3.3 Mounting frame .............................................................................................................................. 21 4.3.4 Weather and Sun shield .................................................................................................................. 22 4.3.5 Alignment and datum issues ........................................................................................................... 23 4.3.6 Combined vessel / mounting frame FEA deflection analysis .......................................................... 29 4.3.7 Overall mass budget........................................................................................................................ 34 4.3.8 Future accommodation for Bands 3/4/6 ........................................................................................ 34

4.4 Thermal plumbing design and prototyping .......................................................................... 35 4.4.1 Design Overview.............................................................................................................................. 35 4.4.2 Central cooling hubs........................................................................................................................ 36 4.4.3 Bus-bars and heat shields ............................................................................................................... 39 4.4.4 LNA mounting bracket .................................................................................................................... 40 4.4.5 Wiring / SMA cable routing ............................................................................................................. 41 4.4.6 Proof of concept prototype and rectangular cryostat test results ................................................. 43

4.5 Band 5a/5b Feed/OMT mounting assemblies – the “podules” ........................................... 48

4.6 RF chain mounting ................................................................................................................ 54

4.7 Feed package heat loading budget ....................................................................................... 56

4.8 Vacuum provision and pressure monitoring ........................................................................ 61 4.8.1 Overview of vacuum system ........................................................................................................... 61 4.8.2 Pressure monitoring ........................................................................................................................ 62 4.8.3 Vacuum valves................................................................................................................................. 64 4.8.4 Turbo pump and enclosure ............................................................................................................. 66 4.8.5 Pump-Down Logic ........................................................................................................................... 67 4.8.6 Cryocooler Regeneration ................................................................................................................ 69 4.8.7 Cryosorption and getter pumps ...................................................................................................... 70

4.9 Vacuum integrity .................................................................................................................. 71 4.9.1 Leak testing SMA connectors .......................................................................................................... 73

4.10 Temperature control and monitoring .................................................................................. 74


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5 Maintenance and Logistics .......................................................................................75

6 Power Requirements ...............................................................................................76

7 Safety ......................................................................................................................76

8 Conclusion and further work ....................................................................................76

LIST OF FIGURES

Figure 1: Functional breakdown of SPF Band 345. ................................................................................13

Figure 2: Overview of thermally-regulated components within the cryostat.......................................14

Figure 3: PBS overview showing Cryostat, Vacuum and Thermal Control system elements................15

Figure 4: Front view of the B345 cryostat mounted on the SKA feed indexer together with the feed packages for bands 1 and 2 at the rear. This is the first-light configuration with only the Band 5a and 5b feeds installed. ................................................................................................................16

Figure 5: Top view of the B345 cryostat mounted on the SKA feed indexer showing the clearances between the different feed packages. ........................................................................................16

Figure 6: General arrangement of the B345 cryostat (external view) in its fully-populated configuration, showing the side bolt-on modules for Bands 3 and 4, and Band 6 in the central position. The weather shield is highlighted in blue....................................................................17

Figure 7: General arrangement of the interior of the B345 cryostat showing the distributed cooling system providing 15 K and 85 K cooling to all five feeds together. ............................................18

Figure 8: (Top Left) A view of the B345 feed package with support frame, shown with the Band 6 podule, but without the Bands 3 and 4 side modules. (Top Right) Rear view showing the turbo pump enclosure attached to the side of the cryostat at the Band 4 position with FPC enclosure shown in section to reveal the hermetic feed-through connectors. (Bottom centre) View showing the position of the turbo pump enclosure (below the Band 3 module) and NEG pump (below the Band 4 module) when all of the bands are populated. ............................................19

Figure 9: Views of the vacuum hub with and without blanking plates. The open view shows the ports for bands 5a, 5b and 6 at the front, the two side ports for Bands 3 and 4, the cryocooler’s access port in the base and two small side ports (side and back) for the hermitic connector panels. .20

Figure 10: Section through the vacuum hub showing the thickened corner uprights together with a view of the underside of the lid showing the rib structure. .......................................................21

Figure 11: The mounting frame for the B345 feed package. ................................................................22

Figure 12: Sunshield shown in three-quarter section revealing the foam core and extrusions. ..........22

Figure 13: Sunshield integrated with the cryostat configured for bands 5a and 5b. ............................23

Figure 14: Schematic showing the basic geometry of the SPF345 cryostat on the indexer. The intersection of the horizontal and vertical planes define the optical axis for Band 6 labelled here as the x-axis. ................................................................................................................................24

Figure 15: Schematic showing and defining the two angles β and γ used to set out the angular displacements of the feeds on the cryostat and their relationship to the indexer rotation angle θ. The indexer tilt angle, which has a nominal value of 15 degrees (shown here) is the angle α. .....................................................................................................................................................24


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Figure 16: View of the vacuum enclosure showing the in-plane angles β and the out-of-plane angles γ used to lay out the cryostat (refer to Figure 15). The datum surfaces for mounting the feeds are shown in red (see also Figure 17). ...............................................................................................26

Figure 17: Viewing and alignment geometry showing the position of the cryostat relative to the alignment feature (front dowel hole) on the standard pedestal. ...............................................26

Figure 18: View showing the support frame’s upper and lower alignment dowel holes. ....................27

Figure 19: View showing tooling reference balls and the typical alignment features for each band. .28

Figure 20: View showing the installed feeds for Band 5b before and after the integration of the vacuum enclosure and RF window assembly. ..........................................................................................29

Figure 21: View of the vacuum hub model for the FEA simulations and the distortion of the lid under vacuum forces. The white rectangles are the areas where the model is restrained and correspond to the bolted connections between the support frame and the enclosure. The maximum deflection (at point A) is 0.257 mm............................................................................30

Figure 22: Views of the base and the sides with the lid visibility turned off. The left-hand view is for total deflections in X, Y and Z, while the right-hand view is for deflections in the Y direction. Deflections at indicated points are A = 0.108 mm, B = 0.058 mm, C =.015 mm, D = 0.014 mm. .....................................................................................................................................................30

Figure 23: Views of the model used for the FEA analysis and the deflections experienced by the cryostat when under vacuum and viewing the zenith with the Band 4 feed. Deflections are shown at positions A =0.050 mm, B =0.043 mm, C = 0.040 mm, D = 0.038 mm, E = 0.044 mm. The effects of both gravity and vacuum forces have been considered. This shows that movement of the feed positions is minimal. .................................................................................................31

Figure 24: View showing the deflection of the cryocooler under vacuum loads for zenith viewing with the Band 4 feed assembly. Displacement at point A =0.182 mm. The resulting strain on the thermal plumbing is easily taken up by the flexible mounts between the vacuum hub base and the plumbing. ..............................................................................................................................32

Figure 25: View showing the rotations at points on the cryostat as a consequence of support frame deflections. The gravitational loading was set up for zenith viewing with the Band 4 feed assembly. The rotations are about the origin defined by the geometry in Section 4.3.5 ..........32

Figure 26: View showing the deflections of the support frame under gravitational and vacuum loads with the latter impressed by the distortion of the cryostat’s base plate during evacuation. This orientation has the Band 4 feed pointing at zenith. ...................................................................33

Figure 27: View showing the FEA analysis of the perforated G10 thermal break support for band 5a under maximum gravity loading. The maximum deflection is 0.0035 mm. The results for Band 5b are similar. The axis glyph shows the position of the C-of-G relative to the mechanical-thermal connection between the thermal break and the feed horn. ........................................34

Figure 28: General view showing the distributed cooling system for all five bands. The RF and wiring details have been removed for clarity.........................................................................................36

Figure 29: Close-up view of the copper bus-bars of the thermal plumbing, with the aluminium heat shields removed. .........................................................................................................................37

Figure 30: View showing the 1st stage cooling hub and the mechanical-thermal interfaces between the hub and the bus- bars for the five-arm distributed cooling system. ....................................38

Figure 31: View showing the tubular thermal link between the 6/30 1st stage cooling station and the 85 K bus-bar cooling hub. The bottom of the tube is shown bolted to the adapter flange. ......38

Figure 32: View showing the ambient temperature ends of the cryoharnesses used for testing the five arm distributed cooling demonstration model. ..........................................................................39


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Figure 33: View showing the bus-bars and the radiation shields for the band 5a and 5b only configuration. The LNAs are shown attached to the ends of the 15 K bus-bars and close to the OMTs. Also shown is the carbon cryosorb pump (gold coloured object) fixed to the 15 K stage. .....................................................................................................................................................39

Figure 34: View showing the collected 85 K radiation shields. The feed cylindrical sections bolt to the feed horns. ...................................................................................................................................40

Figure 35: Views showing the Band 5a assembly with its thermal straps and RF connections to the LNAs. ............................................................................................................................................40

Figure 36: The mounting bracket for two LNAs (Left), with heating resistor and temperature sensor (Right). The LNAs are those for the Band 5a feed (indicated by engraved labelling). ................41

Figure 37: View of the band 5a feed assembly showing the noise injection probe on the feed horn. 42

Figure 38: View of an in-line connection assembly using bulkhead SMA connectors. The assembly provides support for the transition from aluminium clad coax to stainless steel-clad coax. .....42

Figure 39: A typical temperature sensor assembly and in-line connector used on the cold plumbing test prototype. .............................................................................................................................43

Figure 40: A 3D CAD model of the prototype thermal plumbing assembly showing the sheet metal aluminium radiation shields covering each arm. The radiation shield over the 85 K cooling hub has been removed to show the routing of the stainless steel semi-rigid SMA cable. ................44

Figure 41: A detailed view of the 1st and 2nd stage cooling hubs with the 85 K radiation shield removed. The model shows the routing of the stainless steel semi-rigid SMA cables from in-line connectors inside the shields to the thermal clamps at the rear of the hub. ............................44

Figure 43: (Left) The thermal plumbing arms (85 K and 15 K), assembled in the rectangular test cryostat, without radiation shields. (Right) Details of the provision for DC wiring in the central hub (six 21-way Micro-D connectors). ........................................................................................45

Figure 44: Detail of 85 K (lower) and 15 K (upper) thermal bus-bar ends. These have been fitted with heater resistors and temperature sensors for experimental testing..........................................45

Figure 45: The prototype thermal plumbing, without radiation shields, installed in the rectangular test cryostat. .......................................................................................................................................46

Figure 46: (Left) The prototype thermal plumbing, with radiation shields fitted to the 785 K bus-bars with the 85 K cooling hub cover off. (Right) Thermal plumbing with all radiation shields and MLI blanket insulation fitted. .............................................................................................................46

Figure 47: The assembled band 5a “podule”. Not shown here is the two-part radiation shield and the thermal straps which connect to the thermal plumbing bus-bars. ............................................49

Figure 48: A cross-section through the band 5a podule. The horn is mounted off the cryostat body via a thin-walled, perforated G10 cylinder (green) and the OMT is mounted off the horn via thin-walled G10 tubes (green). ...........................................................................................................49

Figure 49: An exploded view of the band 5a podule. The first and second items from the right are the cylindrical radiation shield components which link the feed horn to the 85 K bus-bar. The window has a Sheergard SX-12 weather protection membrane. A Zotefoam plug mechanically supports the Mylar vacuum membrane, which is clamped to an O-ring to provide a vacuum seal. .....................................................................................................................................................50

Figure 50: The Band 5a podule with the visibility of the vacuum enclosure switched off. The perforated G10 cylinder which supports and thermally isolates the feed horn is clearly visible. ................50

Figure 51: A 3D CAD model showing how the 5a OMT is mounted at the back of the 5a feed horn by thin-walled G10 tubes. These provide the required thermal isolation between components while maintaining a small gap between them. ...........................................................................51


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Figure 52: A 3D CAD model of the complete Band 5b podule. .............................................................51

Figure 53: The cross-section of the Band 5b podule. ............................................................................52

Figure 54: An exploded view of the Band 5b feed module, window assembly and vacuum enclosure. .....................................................................................................................................................52

Figure 55: Front and rear views of the Band 5b podule, showing the clamping arrangement at the front and part of the OMT at the back with two SMA output connectors for each orthogonal polarization. .................................................................................................................................53

Figure 56: Solid models showing how the 5b OMT is aligned and secured to the feed horn...............53

Figure 57: Three-quarters section through the band 5a window assembly showing the interspace and the Swagelok connection to the desiccator-breather. Also visible is the drop-in Zotefoam plug support. .......................................................................................................................................54

Figure 58: Brownell desiccator-breather, model BLD6927/01-05. .......................................................54

Figure 59: View of a test set-up in the rectangular vacuum chamber. The SMA cabling that routes signals to and from the LNA pair on the right is shown positioned above the 15 K bus-bar and constrained within the volume enclosed by the 85 K aluminium radiation shield.....................55

Figure 60: View of the warm electronics in the cryostat. The assembly shown uses discrete components (filters, amplifiers, attenuators etc.) although a more compact arrangement, with all components integrated onto a single substrate, is also feasible [RD2]. ................................55

Figure 61: Views of the warm electronics modules in various stages of development. (Left) shows the warm RF chain as discrete components, (Centre) RF chain replaced by a single, integrated module. The circuit board (in green) is the local regulated power supply and analogue temperature control PCB. (Right) shows the noise sources and power splitter mounted on a similar temperature-controlled plate..........................................................................................56

Figure 62: A schematic diagram of the overall vacuum system on the dish indexer. ...........................61

Figure 63: BOC Edwards APGX Linear Active Pirani gauge. ...................................................................62

Figure 64: A VAT 264 high vacuum angled solenoid valve. The LED lights indicate open or closed. ....64

Figure 65: An Accu-Glass 113155 pressure relief valve. ........................................................................65

Figure 66: DN10-KF manual vent valve (Part No. 17324). .....................................................................65

Figure 67: Location of the pressure relief valve and up-to-air valve under the cryostat......................66

Figure 68: The pressure relief and up-to-air valves shown attached to side ports welded to the stainless steel cryocooler to cryostat adapter flange. ...............................................................................66

Figure 69: The Turbo Pump enclosure, showing the two Linear Active Pirani Gauges on either side of the VAT solenoid valve before the turbo pump. Two honeycomb shielded air vents with a small DC cooling fan are provided. The fan switches on with turbo pump. ........................................67

Figure 71: Views showing the 3D CAD models of the cryosorption pump. The assembly uses plates with integral aluminium spacers. ................................................................................................70

Figure 72: Views of the HV200 NEG pump on B345 cryostat. The view on the left is for the first light configuration (Band 5a and 5b populated). The NEG pump is attached to the Band 3 blanking plate. The view on the right shows the position of the NEG pump after Band 3 module has been integrated. ...................................................................................................................................71

Figure 76: Leak testing candidate hermetic SMA connectors. ..............................................................73

Figure 77: The Temati CCS (TVO) temperature sensor shown mounted on a copper-on-Kapton heat sink which is itself glued to a small, lapped copper plate. The thermometer mount is screwed to the surface to be sensed using a small amount of interstitial filler (Apiezon N grease). ...........75


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LIST OF TABLES

Table 1: Equipment ports and feedthroughs required for the vacuum hub. ........................................20

Table 2: Correspondence between the indexer rotation angle and the cryostat’s in-plane angle for the different feeds with the Band 6 being the reference or zero position. Positive angles are indicated with the indexer viewed from above and rotated clockwise. .....................................27

Table 3: Mass estimate for the Band 345 first-light configuration. ......................................................34

Table 4: Heat loading and temperatures achieved for the 85 K thermal bus bars. ..............................47

Table 5: Heat loading and temperatures achieved for the 15K thermal bus-bars. The bands 5a and 5b arms are the longest (464mm from coldhead to bar end), the Bands 3 and 4 arms are 383 mm long and the Band 6 arm is the shortest at 363 mm long. ..........................................................47

Table 6: Radiative loading to the 1st stage............................................................................................58

Table 7: Conductive loading to the first stage. ......................................................................................58

Table 8: Radiative loading from the 1st to 2nd stage............................................................................60

Table 9: Conductive loading from the 2nd to 1st stage. .......................................................................60


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LIST OF ABBREVIATIONS

COTS ............................ Commercial off the Shelf

ECP ............................... Engineering Change Proposal

EM ................................. Electromagnetic

FP .................................. Feed Package

FPC ............................... Feed Package Controller

ILS ................................. Integrated Logistic Support

LMC ............................... Local Monitoring and Control

LNA ............................... Low Noise Amplifier

LRU ............................... Line Replaceable Unit

OMT .............................. Ortho Mode Transducer

PEEK……………………PolyEther Ether Ketone

POST ............................ Power-On Self-Test

PID ................................ Piping and Interface Diagram

QRFH ............................ Quad-Ridged Flared Horn

RFI................................. Radio Frequency Interference

SCFM ............................ Square Cubic Feet per Minute

SKA ............................... Square Kilometre Array

SKAO ............................ SKA Project Office

SPF ............................... Single Pixel Feed

SPFC ............................. SPF Controller

SPF1 ............................. Single Pixel Feed Band 1 Feed Package

SPF2 ............................. Single Pixel Feed Band 2 Feed Package

SPF345 ......................... Single Pixel Feed Band 3, 4, 5 Feed Package

SPFC ............................. Single Pixel Feed Controller

SPFHe ........................... Single Pixel Feed Helium Service

SPFVac ......................... Single Pixel Feed Vacuum Service

SRU ............................... Shop Replaceable Unit


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1 Introduction

1.1 Purpose of Document

This document is the design report of the cryogenic function of the SPF B345 cryostat and control assembly, specified in [AD1].

1.2 Scope of Document

This design document provides and justifies performance, interface, physical layout, services, safety, logistic support, special design, construction considerations and reasoning leading to design decisions and requirements which are inputs to the engineering and development of the item.

2 Documents

2.1 Applicable Documents

The following documents are applicable to the extent stated herein. In the event of conflict between the contents of the applicable documents and this document, the applicable documents shall take precedence.

[AD1] A. Born, “SPF Band 345 Development Specification” SKA-TEL-DSH-0000085 Rev 1,

2018‑09‑25

[AD2] G. Smit, “RF and Noise Diode Control Signal Interface between the Single Pixel Feed

Receiver and the Single Pixel Feeds”, SKA-TEL-DSH-0000065, Rev. 2, 2018-06-11.

[AD3] G. Smit, A. Born, “Physical Interface Control Document between the SPF Band 345

Package and the Dish Structure”, SKA-TEL-DSH-0000060, Rev. 1, 2017-03-08 [latest issued

version – update in progress].

[AD4] B. Watkins, “SPF Band 345 Physical Interface Drawing”, 301-000000-020, Rev. 1, 2017-

03-07 [latest issued version – update in progress].

[AD5] G. Smit, “Physical Interface Control Document between the Dish Fibre Network and

the Single Pixel Feeds”, SKA-TEL-DSH-0000066, Rev. 4, 2018-07-03.

[AD6] P.C. van Niekirk, “SPF Controller to SPF Band 345 Data Exchange ICD”, SKA-TEL-DSH-

0000095, Rev. 1, 2018-09-05.

[AD7] A. Krebs, “SPF Band 345 to SPF Vacuum Service Interface Control Document”, SKA-

TEL-DSH-0000100, Rev. 1, 2018-09-05 [latest issued version – update in progress].

[AD8] A. Krebs, “SPF Band 345 to SPF Helium Service Interface Control Document”, SKA-TEL-

DSH-0000103, Rev. 1, 2018-09-05 [latest issued version – update in progress].

[AD9] A. Peens-Hough, et al., “Single Pixel Feed (SPF) Requirements Specification”, SKA-TEL-

DSH-0000012, Rev. 5, 2017-06-01.

2.2 Reference Documents

The following documents are referenced in this document. In the event of conflict between the contents of the referenced documents and this document, this document shall take precedence.


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[RD1] J. Leech et al, “SPF Band 345 Cryogenic Function Design Document”, 317-030000-006, Rev. 1, 2019-02-28.

[RD2] J. Leech et al, “SPF Band 345 Signal Function Design Document”, 317-030000-008, Rev. 1, 2019-02-28.

[RD3] M. Jones et al, “SPF Band 345 Control Function Design Document”, 317-030000-007, Rev. 1, 2019-02-28.

[RD4] A. Born, “SPF Band 345 Product Breakdown Structure”, 317-030000-005 Rev. 1, 2019-02-26.

[RD5] J. Leech et al, “SPF Band 345 Preliminary Design Document”, SKA-TEL-DSH-0000118, Rev. 3, 2018-07-16.

[RD6] “SKA-MID Band 5 Split into Two Bands”, SKA ECP-160022, 317-030000-002, 2016.

[RD7] A. Krebs, “Single Pixel Feed Vacuum Service Design Document”, SKA-TEL-DSH-0000092, Rev. 1, Vacuum Service Design, 2018-10-08.

[RD8] A. Krebs, “Single Pixel Feed Helium Service Design Document”, SKA-TEL-DSH-0000090, Rev. 1, Vacuum Service Design, 2016-11-30.

[RD9] Shu et al., (1986) “Heat flux from 277 to 77 K through a few layers of multilayer insulation”, Cryogenics, vol 26, No. 12. pp.671 – 677


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3 System Description

3.1 Overview

The cryostat function comprises the active major components of the SPF B345 feed packages, which receive incident signals from the dish structure via the SPF B345 horn assemblies and amplify these signals for transmission to the receiver sub-element. The cryostat includes all the components necessary to perform these functions. It excludes power supplies and control-and-monitoring electronics. These components are grouped under the feed package controller (FPC) assembly as discussed in [RD3]. Together the cryostat and FPC assembly form the cryostat & control assembly (see Figure 1).

The SPF B345 feed package is specified to provide the frequency Bands 3, 4, 5a and 5b (Band 5 having been split under a previous ECP [RD6]). An additional Band 6 (covering approximately 15 – 24 GHz) is not yet part of the formal specification, but has been widely discussed and provision is being made for it under reasonable assumptions about its specifications. Only bands 5a and 5b are required at first light, and only the band 5a and 5b systems are being fully detailed at this stage. However, to ensure that it will be possible to upgrade the feed package with Bands 3, 4 and 6 at some point, the mechanical, thermal and electronics infrastructure needed to support them has been designed and forms part of the present design.

The Band 5a and 5b feeds are choked-flange feed horns similar in concept to the Band 2 feed horns, and are cooled to 85 K inside the vacuum chamber. They sit behind a vacuum window/infra-red block and weather protection window. The OMTs are physically separate items, separated from the horn by a thermal break and are cooled to 20 K. RF signals pass from the OMT to LNAs which are mounted on the end of bus-bars which distribute cooling from a central cryocooler. The amplified RF signals pass via semi-rigid cables across the thermal stages to warm RF chain modules that provide additional gain and band-pass filtering, and then exit the cryostat via an array of hermetic SMA connectors. A broad-band calibration noise source is distributed to all of the bands, and is injected in to the OMT via a probe coupler.

A two-stage Gifford-McMahon cryocooler is used to provide cooling for the cryostat with a first-stage cooling power of approximately 30 W at 70 K and a second-stage cooling power of approximately 6 W at 10 K. The second cooling stage is used to cool all the LNAs, the distributed cooling system (the 15 K bus bars) and the OMTs for bands 4, 5a, 5b and 6, with the LNAs cooled to below 20 K. The first cooling stage is used to cool the distributed cooling system (the 85 K bus bars), the radiation shields for all five bands, the OMT for Band 3 and the feed horns for bands 5a, 5b and 6. The second-stage amplification and band-pass filtering of the RF signals, and the calibration noise source and its power splitter, are mounted inside the vacuum enclosure and temperature stabilised at just above ambient temperature (approximately 330 K) – see Figure 2.

The requirements for cryogenic cooling are discussed in [RD5]. The cryocooler operates with high-pressure helium supplied by an external helium compressor. Vacuum maintenance during operation will be accomplished through cryo-pumping and, if necessary, a carbon cryosorb and a non-evaporable getter (NEG) pump. The design currently includes the NEG pump, but if qualification testing shows that it is not necessary it will be removed from the final design.

The low vacuum pressure required for cryocooler start up (typically < 1.0E-2 mbar) is achieved through the use of a small, compact turbomolecular pump (DN63 body size) mounted immediately outside the cryostat. To eliminate risk of radio frequency interference the turbopump, along with the vacuum gauges and the solenoid isolation valve, are housed in a weather-sealed and RFI screened enclosure. The turbomolecular pump backing vacuum is provided by the common vacuum services scroll pump and pumping manifold.

Details of the SPF vacuum and helium services are discussed in [RD7] and [RD8] respectively.


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Figure 1: Functional breakdown of SPF Band 345.


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Figure 2: Overview of thermally-regulated components within the cryostat.

3.2 Functional Architecture

To perform the top-level functions, the cryostat has the following main functions: i. Cryogenic and thermal functions

ii. Environmental functions iii. Mechanical and mounting functions iv. Monitoring functions

This document describes the designs which implement these functions. Figure 1 shows the key functional elements of the B345 feed package.

3.3 Product Breakdown Structure

The physical hardware required to perform each function is shown in Figure 3. The designs are described in Section 4.


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Figure 3: PBS overview showing Cryostat, Vacuum and Thermal Control system elements.

4 Design Description

4.1 Guiding Design Considerations

In addition to performance considerations, the design of the cryostat is strongly driven by requirements based on cost, modularity, ease of assembly and ease of testing. This was discussed in detail at PDR. The following design guidelines form the background for the detail design of all subsystems and components:

1. As far as possible, COTS hardware shall be used. 2. Designs shall be as modular as possible, requiring the minimum matching of components to

function correctly. 3. The design shall maximise hand and tool access during assembly and integration. 4. Close / fine tolerances shall be avoided, unless strictly necessary for the operation of a subsystem

or component. 5. The use of small (<M3) or non-metric fasteners shall be avoided, unless strictly necessary for the

operation of a subsystem or component. 6. Where possible, component material and manufacturing costs shall be minimised by

a. avoiding unnecessarily big raw material sizes,

b. using easily obtainable materials and

c. using standard production techniques and processes.


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4.2 Overall feed package design

Figure 4 and Figure 5 show the B345 cryostat, together with the feed packages for bands 1 and 2, mounted on the SKA feed indexer. The B345 cryostat, like the other two instruments, is mounted on and aligned by a stiff pedestal.

Figure 4: Front view of the B345 cryostat mounted on the SKA feed indexer together with the feed packages

for bands 1 and 2 at the rear. This is the first-light configuration with only the Band 5a and 5b feeds installed.

Figure 5: Top view of the B345 cryostat mounted on the SKA feed indexer showing the clearances between

the different feed packages.


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Figure 6: General arrangement of the B345 cryostat (external view) in its fully-populated configuration,

showing the side bolt-on modules for Bands 3 and 4, and Band 6 in the central position. The weather shield is highlighted in blue.

A solid model cutaway of the overall B345 cryostat is shown in Figure 7. The vacuum vessel consists of three modules: a six-sided central module together with a Band 3 module and a Band 4 module mounted on opposite sides. The central module accommodates the coldhead, the Bands 5a and 5b LNAs, OMTs and horns and ~300 K vacuum-side warm RF electronics. The coldhead is thermally connected to the Band 5a and 5b OMTs and horns by copper “thermal plumbing” cooled to the first and second stage coldhead temperatures. This thermal plumbing allows the OMTs and LNAs to be cooled to the 2nd stage temperature and the feed horns to the 1st stage temperature. There is a narrow thermal break between the waveguide output of each horn and the input waveguide of its corresponding OMT. The Band 5a, 5b and 6 feed horns are mounted inside approximately conical horn “pod / modules” or ``podules'' which bolt onto the front of the central module. Each of these horn podules has an RF transparent vacuum window consisting of a Mylar film with a Zotefoam Plastazote (TM) (hereinafter referred to as Zotefoam) backing mounted on its front face. The Mylar film which is clamped against an O-ring to provides a vacuum seal is protected from the local environment and harsh weather by an outer kevlar-reinforced PTFE membrane (Sheergard SX-12). The interspace between the two membranes is connected to a desiccant-breather to ensure low water vapour levels.


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Figure 7: General arrangement of the interior of the B345 cryostat showing the distributed cooling system

providing 15 K and 85 K cooling to all five feeds together.

As well as providing cooling to the bands 5a, 5b and 6 horns, OMTS and LNAs, the thermal plumbing is also routed into the Band 3 and the Band 4 cryostat modules where it enables cooling of the Band 3 and Band 4 OMTs and LNAs. This is shown in Figure 7 where the side vacuum enclosures have been made transparent for clarity. The horns for Band 3 and Band 4 are mounted externally and at ambient temperature and pressure. The Band 3 and Band 4 modules feature cryostat windows leading into the input waveguide of the ambient temperature horns. These are discussed in more detail in Section 4.3.7 below. Before the Band 3 and Band 4 receivers are commissioned, the Band 3 and Band 4 modules will be replaced with blanking plates which will provide support for the turbo pump enclosure and the NEG pump. When Bands 3 and 4 are installed, these components will be moved to the undersides of the Band 3 and 4 modules (see Figure 8). The thermal plumbing consists of a central copper bar at the coldhead second stage temperature (15 K)1, supported by and above a copper plate held at the temperature of the coldhead’s first-stage (85 K). To the 85 K copper plates are attached sheet aluminium covers. These surround the central copper bar and act as radiation shields. At the rear of the central module are several mounting plates which are temperature controlled to be above a few degrees above ambient temperature. These plates are used for mounting the warm RF components, i.e. the bandpass filters, attenuators and 2nd stage amplifiers. The mounting plates accommodate heaters, temperature sensors and analogue temperature control electronics for the temperature control of the 2nd stage amplifiers. These platforms will accommodate all of the RF components after the LNAs for both polarizations and for all bands. The expected gain of the LNAs is high enough (~39 dB) such that only the LNAs need be cooled below ambient to achieve a good receiver noise temperature.

1 We will refer to the first and second stage temperatures as ‘85 K’ and ‘15 K’ respectively for convenience, but the actual temperatures will vary depending on the heat load and position in the system – the actual LNA temperatures may be lower than 15 K (the coldhead base temperature is 10 K) and outer parts of heat shields may be somewhat warmer than 85 K.


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Mounted on the rear of the central module is the RFI-shielded Feed Package Controller (FPC) enclosure. Amplifier bias wiring, temperature monitoring and control wiring are connected from the FPC enclosure to the vacuum chamber through hermetic feedthroughs. Additional external views of the B345 feed package are shown in Figure 8.

Figure 8: (Top Left) A view of the B345 feed package with support frame, shown with the Band 6 podule, but without the Bands 3 and 4 side modules. (Top Right) Rear view showing the turbo pump enclosure attached to the side of the cryostat at the Band 4 position with FPC enclosure shown in section to reveal the hermetic

feed-through connectors. (Bottom centre) View showing the position of the turbo pump enclosure (below the Band 3 module) and NEG pump (below the Band 4 module) when all of the bands are populated.

4.3 Vacuum vessel (central hub) and mounting frame

4.3.1 Design overview

During initial deployment of the SPF345 feed package the central vacuum hub will support the coldhead, thermal plumbing, warm RF electronics and the bands 5a and 5b feeds, OMTs and vacuum windows. At a later date, Bands 3, 4 and 6 can be optionally accommodated via the addition of side mounting modules (Bands 3 and 4) and a Band 6 vacuum window podule (Band 6). The central hub supports a variable speed Oxford Cryosystems 6/30 coldhead and is connected to the standardized indexer pedestal via its own support frame. It is protected from the environment with a bolt-on sun/weather shield. In this section we will describe the detailed design of the central vacuum vessel, its mounting frame and the sun/weather shield. Alignment and datum procedures are described in Section 4.3.5 and a full FEA analysis of the hub and the frame under vacuum and gravitational forces is presented in Section 4.3.6.


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4.3.2 Required connections and feedthroughs for the central hub

The central hub requires the following connections and feedthroughs as shown in Table 1. The engineered vacuum hub is shown in Figure 9 and Figure 10.

Item Details

Coldhead opening Opening to insert coldhead (with O-ring)

Band 5a podule opening Opening to bolt on the 5a podule

Band 5b podule opening Opening to bolt on the 5b podule

Band 6 podule opening / blanking plate Opening to bolt on the 6 podule (blanking plate at first light)

Band 3 module opening / blanking plate Opening to bolt on the Band 3 side module (blanking plate at first light)

Band 4 module opening / blanking plate Opening to bolt on the Band 4 side module (blanking plate at first light)

Main access lid Lid for the central hub

FPC feed through connector plate Contains two Amphenol connectors shell sizes 19 and 21 to enable connection of FPC electronic box to interior of cryostat.

RF feed through connector plate Contains (up to) 10 hermetic SMA connectors (type TE Connectivity - 1054874_1) for the RF outputs of the H and V channels for each SPF band.

Table 1: Equipment ports and feedthroughs required for the vacuum hub.

Figure 9: Views of the vacuum hub with and without blanking plates. The open view shows the ports for bands 5a, 5b and 6 at the front, the two side ports for Bands 3 and 4, the cryocooler’s access port in the base

and two small side ports (side and back) for the hermitic connector panels.


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Figure 10: Section through the vacuum hub showing the thickened corner uprights together with a view of the underside of the lid showing the rib structure.

The hub will be machined out of a solid block of 5085-O aluminium alloy. This is a cost-effective approach, even though machining on two 3-axis CNC machines will be required to maintain the geometrical and dimensional tolerances associated with the datum planes and axes. Changes to the design from PDR include ‘framing’ out the sides and back with thicker section material and redesigning the lid to minimise the distortions associated with vacuum loads. These vacuum induced distortions and their mitigation are discussed in Section 4.3.6. Lightweighting has gone hand-in-hand with stiffening to yield a mechanically stable and rigid central hub, an essential feature for feed-to-feed and feed-to-telescope alignment. The qualification model vacuum hub and lid will be machined on two 3-axis CNC machines at Oxford, a suitably sized 5-axis machine not being available. This approach will require jigs, fixtures and alignment features on the hub to ensure that the geometrical and dimensional tolerances are maintained between machines. Jigs and fixtures will also be required for post-machining metrology measurements which will be carried out using the University’s moving beam CMM. All external, non-vacuum surfaces will be painted with a high albedo white paint to minimise solar heating. Masks will be required to ensure that vacuum-side surfaces are kept completely free of paint.

4.3.3 Mounting frame

The support frame has been completely redesigned from that presented at PDR. Initial FEA analysis carried out at PDR showed distortion of the vacuum chamber which in turn caused a deflection of the support frame, tilting the front of the vacuum chamber up and away from the pedestal. The redesigned support frame is shown in Figure 11 The requirement to create a stiff light structure has been met by using flat plates and stainless steel electric-resistance welded (ERW) tubing in a welded assembly. The arrangement of struts resists the gravitational loads associated with the different viewing positions, with alignment towards the zenith with either Band 3 or Band 4 being the extreme case. This is borne out by FEA analyses of the assembly comprising the vacuum hub and support frame (Section 4.3.6). The open framework also allows access to the helium gas line connections, the up-to-air valve and the pressure relief valve. After welding, the top and bottom plates are machined (ground) flat and parallel and then fitted with through holes for fasteners, and with reamed holes for dowel pins. A jig will be used to ensure that the dowel holes in the bottom plates (frame-to-pedestal interface) properly align with the dowel holes in the top plates (frame-to-instrument interface). A more detailed discussion of feed-to-telescope alignment is presented in Section 4.3.5.


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Figure 11: The mounting frame for the B345 feed package.

4.3.4 Weather and Sun shield

The weather shield will be constructed from a commercially available high-strength carbon-fibre-skinned foam-cored panel (https://tinyurl.com/yd7hgdhe). This provides a significant mass saving over a sheet aluminium construction. The 11 mm thick panel material has a weight of 3.0 kg/m2, while the equivalent weight for 2 mm thick aluminium sheet is 5.4 kg/m2. Further, being extremely stiff the panel material is self-supporting, obviating the need for an inner support frame. The design for the SPF-345 weather shield is shown in three-quarters section in Figure 12 and integrated with the cryostat in Figure 13. The foam core can be seen in the section view, as can the aluminium extrusions used to protect the exposed foam edges and to attach the side panels. Apart from the removable side panel which uses M5 screws it is a completely bonded structure with a total mass of <12 kg.

Figure 12: Sunshield shown in three-quarter section revealing the foam core and extrusions.

The sunshield is attached to the vacuum hub lid with six M10 bolts, and the leading edges secured with ball-jointed turnbuckle struts. Using the vacuum hub as a primary support structure keeps the arrangement low-profile and avoids shadowing on the sub-reflector. The outer surfaces will be spray painted white to provide a durable reflective finish (high solar reflectance and high thermal emittance). The foam core will also provide a degree of thermal insulation between the inner and outer surfaces - its thermal conductivity is only 0.033 W/m.K (https://tinyurl.com/y8sm7dd6).

https://tinyurl.com/yd7hgdhe

https://tinyurl.com/y8sm7dd6


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Figure 13: Sunshield integrated with the cryostat configured for bands 5a and 5b.

4.3.5 Alignment and datum issues

The B345 system is required to place the phase centres of the various feeds at the focal point of the telescope optics. Also, the feed’s optical axes must be aligned with telescope’s optical axis only by using rotations of the feed indexer about its axis. There is no allowance for adjustment of the individual feeds relative to the vacuum hub; instead, provision is made for accurate positioning and alignment of the feeds, vacuum hub and support frame during manufacture. We assume that the datum surfaces and datum holes provided by the indexer pedestal (top and bottom faces and alignment dowel holes) are aligned and positioned accurately with regards to the telescope optical axis and focal point. It is assumed that the pedestal’s top and bottom faces will be ground flat, with the top face accurately set an angle of 15 degrees relative to the interface plate between the pedestal and the indexer. Below we will first consider the geometry that sets the fixed angles and positions of the feeds in the B345 system, and then describe the features that allow these to be set accurately during manufacture relative to the datum provided by the top of the indexer pedestal.


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Figure 14: Schematic showing the basic geometry of the SPF345 cryostat on the indexer. The intersection of

the horizontal and vertical planes define the optical axis for Band 6 labelled here as the x-axis.

Figure 15: Schematic showing and defining the two angles β and γ used to set out the angular displacements of the feeds on the cryostat and their relationship to the indexer rotation angle θ. The indexer tilt angle,

which has a nominal value of 15 degrees (shown here) is the angle α.

The geometry of the SPF345 cryostat on the indexer is shown schematically in Figure 14 and Figure 15. As the indexer rotates, the focal point and optic axis of the central Band 6 feed traces out a cone (shown in red) relative to the origin of the co-ordinate system. The coordinates of a feed phase centre relative to the apex of the cone (the local origin) are given by: x = (R [cos θ+(1 - cos θ) sin2 α])/cos α y = R sin θ (+ either side of the optical axis, the x-direction) z = -R sin α (1 - cos θ) (always below the ‘reference plane’) The parameters are defined as follows. The parameter R is the base radius of the cone swept out by the optical axis as the indexer rotates; it has a nominal value of 1400 mm. The axis of rotation of the indexer plate is inclined at an angle α to the normal of the ‘reference plane’ which is the horizontal plane through optical axis. The top plate of the standard pedestal is also inclined at the same angle


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relative to the plane of the indexer plate; this angle has a nominal value of 15 degrees. The angle θ is the rotation of the indexer plate about its axis of rotation. It is also the angular displacement monitored and measured by the indexer’s rotary encoder. The phase centres for all bands lie on the circumference of the cone’s base. The optical axes of the cryostat’s feed horns, relative to the ‘reference plane’ and the optical axis for Band 6, are defined by two angles; the in-plane and out-of-plane angles. The in-plane angle β is the rotation about the ‘apparent centre of rotation’ for a rotation θ in the plane of the indexer. This angle is given by following equation: β = arctan[(sin θ cos α)/(cos θ cos2 α+sin2 α)] The out-of-plane angle γ is the rotation about the ‘apparent centre of rotation’ for a rotation θ in the plane of the indexer. This angle is always below the ‘reference plane’ and is given by the following equation: γ = arcsin[sin α cos α (1 – cos θ)] The SPF345 cryostat has been designed so that the optical axis of the Band 6 assembly is in the ‘reference plane’ and aligned to the optical axis of the telescope when the in-plane and out-of-plane angles are zero. Table 2 show the correspondence between the indexer rotation angle and the cryostat’s in-plane angle for the different feeds with the Band 6 being the reference or zero position. The central vacuum hub is used as the component for aligning all five feeds relative to a set of reference features and hard datums machined into the body of the vacuum hub. The primary machining datums for aligning all five feeds with external (to the cryostat) datums, such as dowel holes and surfaces, are as follows:

• The surface defined by the base of the vacuum enclosure.

• The central front face of the vacuum enclosure which is machined orthogonal to the base.

• The axis of the port for the Band 6 position (the optical axis for this feed) positioned 150 mm above the base and in the vertical plane of symmetry of the base. This datum is also defined by the horizontal and vertical planes shown in Figure 14.

• The dowel holes for the alignment dowel pins between the vacuum enclosure and its support frame and for the tooling reference spheres (Figure 19).

Each feed position will be fitted with alignment features and these are illustrated in Figure 19. The overall geometry is little changed from PDR except that the in-plane angle β for bands 5a and 5b has changed from 11.0 degrees to 11.8 degrees. These changes were made to avoid shadowing between bands 5a, 5b and 6 brought about by the final choice of slightly larger RF windows. This change has increased the out-of-plane angle γ slightly. The current geometry and the datum surfaces for mounting the various feeds are shown in Figure 16 and Figure 17.


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Figure 16: View of the vacuum enclosure showing the in-plane angles β and the out-of-plane angles γ used

to lay out the cryostat (refer to Figure 15). The datum surfaces for mounting the feeds are shown in red (see also Figure 17).

The magenta points in Figure 16 are the phase centres that lie on the circumference of the cone’s base, while the yellow points are the vertical projections of the phase centres onto the horizontal ‘reference’ plane. The alignment with the pedestal is shown in Figure 17.

Figure 17: Viewing and alignment geometry showing the position of the cryostat relative to the alignment

feature (front dowel hole) on the standard pedestal.


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Figure 18: View showing the support frame’s upper and lower alignment dowel holes.

Cryostat alignment Cryostat in plane angle β, deg Indexer angle θ, deg

Band 3 28.0000 29.0644

Band 5b 11.8000 12.2224

Band 6 0.0000 0.0000

Band 5a -11.8000 -12.2224

Band 4 -28.0000 -29.0644

Table 2: Correspondence between the indexer rotation angle and the cryostat’s in-plane angle for the

different feeds with the Band 6 being the reference or zero position. Positive angles are indicated with the indexer viewed from above and rotated clockwise.

It is essential that the feed port alignment features be machined accurately, and so during machining the vacuum enclosure will be fitted with additional dowel holes for tooling reference balls (known as “Ickey balls”). These will aid both alignment during machining, particularly if the part has to be moved between CNC machines, and in post-machining metrology. These reference balls are shown in Figure 19 along with an illustration of the typical alignment features for each port. The same pattern of alignment dowel holes for the Ickey balls will be machined in the base of the enclosure with the two sets of holes (top and bottom) aligned to a common set of datums.


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Figure 19: View showing tooling reference balls and the typical alignment features for each band.

The proposed method of integrating and aligning the individual feeds is illustrated in Figure 20. Each feed will be assembled and aligned as a self-consistent sub-system using its own set of datums and alignment features, which ensures that the phase centre of the feed is accurately placed with respect to the alignment face that mates with the vacuum hub. When assembled, the feed position is now accurately placed with respect to the datum features on the base of the vacuum hub. The vacuum enclosures and RF window assemblies that are placed over the installed feed assemblies for bands 5a and 5b also have alignment features so that each window assembly is properly aligned axially with its feed horn. Dowel holes and pins ensure that assemblies can only be installed in one rotational orientation about the centreline. The final step of transferring the indexer datums to the feed positions is via the support frame. During manufacture the top and bottom surfaces of the support frame will be ground parallel and dowel holes machined in the top and bottom attachment lugs to the vacuum hub above and the pedestal below (see Figure 18). The total static alignment error budget is thus the sum of the machining and metrology tolerances of the support frame, vacuum hub and feed assemblies. The accumulated alignment errors are difficult to calculate as each error will have six degrees of freedom and so cannot simply be summed in quadrature, although doing so will give a first order estimate of what the alignment errors might be. If one considers the co-ordinate alignment errors at the pedestal-to-support frame and support frame-to-cryostat interfaces, which are tightly controlled using ground surfaces and locating dowels, then an uncertainty of around +35 um may be assigned to the position of the Band 6 phase centre relative to the pedestal’s datum features. This assumes a perfectly machined cryostat and so to that error must be added the errors associated with machining the vacuum hub out of a single block of aluminium and moving the workpiece between machines. The sequence of machining and the use of dowel pins, tooling reference balls (Ickey balls) and alignment jigs will control the positional errors to better than +70 um although this will have to be borne out by post machining metrology using the University’s moving beam CMM. The alignment of the components of each feed is easily controlled through the use of assembly jigs, alignment spigots and dowel pins with the critical alignment


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component being the large thermal break between the feed horn and the vacuum hub. Using location/clearance fits (H7/h6) between parts, the positional alignment of the phase centre relative to the feed-to-hub interface datums is estimated to be better than +30 um. The positional errors associated with machining the hub will also result in small angular misalignments of each feed relative to its theoretically exact datum axis. The rotation, which is about the feed-to-hub mechanical interface, will result in a small off-axis displacement of the phase centre. An estimate of the angular error for Band 5a can be made by taking the machining uncertainty (+70 um) and dividing it by the port diameter (150 mm) to give an uncertainty of + 0.03 degrees. This small tilt angle will shift the phase centre off the nominal datum axis by around 60 um. A quadrature sum of the above errors gives an estimate for the positional uncertainty of each phase centre relative to the pedestal’s datum of around +50 um. These static errors are comparable to, but do not exceed the dynamic errors introduced by gravity and pressure loading on the feed package (discussed below in Section 4.3.6), but are still well within the allowed budget of +500 um.

Figure 20: View showing the installed feeds for Band 5b before and after the integration of the vacuum enclosure and RF window assembly.

4.3.6 Combined vessel / mounting frame FEA deflection analysis

For the feeds on the cryostat to properly align with the telescope’s optical datums, the assembly comprising the cryostat and its support frame must be tolerant to all applied loads and external forces and not distort in any significant way. The subsystems requiring verification through FEA are the vacuum enclosure, the support frame, and some critical alignment and/or support components associated with individual feeds (in this case Bands 5a and 5b). The vacuum hub and support frame have been designed to reduce the distortions due to gravity loading and external atmospheric pressure to acceptable levels by appropriate use of thickening and stiffening features in the structures, while keeping the total mass to a minimum.


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The primary effect of external pressure on the vacuum hub is to make the lid and the base bow inwards. This distortion is complicated in the case of the base by the presence of the aperture for the coldhead. This gives a tendency for the base of the chamber to roll up and forward towards the Band 6 port, causing the base to fish-tail and foreshorten, with the back going down more than the front. This effect has been mitigated by framing out the back of the chamber with thicker section uprights. Figure 21 and Figure 22 show the effect of external pressure on the vacuum hub without the support frame. Gravitational loading is included in the calculation but is negligible compared to pressure loading. Maximum displacement of the lid is about 0.25 mm but this has little effect on the alignment of any key points (e.g. the feed alignment faces). The faces supporting Bands 5a, 5b and 6 have negligible movement, while the maximum movement close to the Band 3 and 4 attachment positions is around 0.015 mm.

Figure 21: View of the vacuum hub model for the FEA simulations and the distortion of the lid under vacuum

forces. The white rectangles are the areas where the model is restrained and correspond to the bolted connections between the support frame and the enclosure. The maximum deflection (at point A) is

0.257 mm.

Figure 22: Views of the base and the sides with the lid visibility turned off. The left-hand view is for total

deflections in X, Y and Z, while the right-hand view is for deflections in the Y direction. Deflections at indicated points are A = 0.108 mm, B = 0.058 mm, C =.015 mm, D = 0.014 mm.

A

A B

C

D


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The bolted connections between the support frame and the vacuum hub are located around the periphery of the base where the structure is stiffer and experiences smaller deflections when the vessel is evacuated (Figure 23).

Figure 23: Views of the model used for the FEA analysis and the deflections experienced by the cryostat when under vacuum and viewing the zenith with the Band 4 feed. Deflections are shown at positions A =0.050 mm, B =0.043 mm, C = 0.040 mm, D = 0.038 mm, E = 0.044 mm. The effects of both gravity and

vacuum forces have been considered. This shows that movement of the feed positions is minimal.

Figure 23 shows the model used for the FEA simulations. The band 5a and 5b podules were modelled as weighted cylinders, each with the correct mass and centre of gravity. The Band 5b simulated mass is shown transparent with the phase centre as a yellow dot. The turbopump enclosure was modelled in a similar way, as was the FPC enclosure. The cryopump was modelled as a simple shape with the correct mass to better illustrate the deflections of the base. The results show that the vacuum hub moves down uniformly towards the interface between the support frame and the pedestal. The movement is quite small (<0.1 mm), with little or no tilt. Figure 24 shows the deflection of the second stage of the cryocooler, and Figure 25 shows the rotations at the band 5a and 5b positions. As with the X-Y-Z deflections of the cryostat as a whole, the rotation of the cryostat and therefore the angular displacement of the optical axis for a particular feed is very small.

A

B C

D

E


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Figure 24: View showing the deflection of the cryocooler under vacuum loads for zenith viewing with the

Band 4 feed assembly. Displacement at point A =0.182 mm. The resulting strain on the thermal plumbing is easily taken up by the flexible mounts between the vacuum hub base and the plumbing.

Figure 25: View showing the rotations at points on the cryostat as a consequence of support frame

deflections. The gravitational loading was set up for zenith viewing with the Band 4 feed assembly. The rotations are about the origin defined by the geometry in Section 4.3.5

Figure 26 shows the distortions of the support frame under gravitational and vacuum loading. The gravity vector is shown as a red arrow and the simulation was carried out with the cryostat and frame oriented so that the Band 4 feed was aligned with the zenith. The results show that the frame is capable of resisting gravitational loading with very little distortion or movement. The rotations of the

A


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front and rear in-line connection pads mirror the rotation of the cryostat’s base plate as the area around the cryocooler’s access port is rotated up and toward the front of the support frame when the chamber is evacuated. We conclude from the simulations presented in this section that the vacuum enclosure and support frame assembly is stiff enough and stable enough to provide the datum system needed for aligning the separate feeds of the SPF345 cryostat with the telescope’s datums. Evacuating the chamber causes translation down by <0.05 mm with no discernible tilt or rotation. We have also analysed the gravitational deflections of the components of the feed assemblies for bands 5a and 5b that are critical to optical alignment of the feeds with the telescope’s optical axis. In particular, we have analysed the perforated2 G10 tube thermal break assemblies which support and align the components of each feed. Figure 27 shows the thermal break assembly for band 5a, along with the FEA analysis. The mass of each feed assembly is <3 kg. However, it is offset from the horn-to-thermal break mechanical interface and so applies a moment to the assembly. The maximum moment occurs when the g-vector is normal to the feed’s optical axis. Even with the perforations, the deflections at the interface flange with the feed horn are <0.005 mm and so are insignificant.

Figure 26: View showing the deflections of the support frame under gravitational and vacuum loads with the

latter impressed by the distortion of the cryostat’s base plate during evacuation. This orientation has the Band 4 feed pointing at zenith.

2 The perforations reduce the heat flow along the G10 tube by approximately 50% and are needed to maintain a low parasitic heat load on the cooling system.


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Figure 27: View showing the FEA analysis of the perforated G10 thermal break support for band 5a under maximum gravity loading. The maximum deflection is 0.0035 mm. The results for Band 5b are similar. The

axis glyph shows the position of the C-of-G relative to the mechanical-thermal connection between the thermal break and the feed horn.

4.3.7 Overall mass budget

Item Number

Description Mass (kg)

1 Support Frame 19.0

2 Cryocooler assembly 9.2

3 Vacuum hub and lid 66.6

4 NEG pump at Band 3 position 6.3 5 Hermetic SMA assembly 0.5

6 FPC assembly 15.2

7 Hermitic Amphenol assembly 1.1

8 Turbopump enclosure 13.1 9 Warm electronics 4.4

10 Band 6 ( closed off ) 1.7

11 Band 5a 9.9

12 Band 5b 7.9

13 Sunshield 12.8

14 Cold plumbing assembly 11.4

Total 179.2 Table 3: Mass estimate for the Band 345 first-light configuration.

4.3.8 Future accommodation for Bands 3/4/6

The cryostat is designed to allow for expansion to include three additional bands, Band 3, 4 and 6. It is a design requirement that any expansion not compromise the cryostat’s performance as a whole, and/or individual bands in particular. The design presented here meets the objectives for modularity and design flexibility while providing a sound platform for the initial deployment of bands 5a and 5b. The central vacuum enclosure has been designed to provide the datum surfaces, alignment features and all the electrical and cooling services for retrofitting the additional bands. The decision (at delta-PDR) to increase the cooling capacities at both 15 K and 85 K by moving to the larger and more powerful Oxford CryoSystems 6/30 cryocooler provides a greater margin with respect to the additional radiative and conductive heat loads which will arise from Bands 3, 4 and 6, and this presents very little risk. A detailed thermal loading analysis is provided in Section 4.7.


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In order to ensure that the design will accommodate the future bands as well as bands 5a and 5b, it has been necessary to make reasonable assumptions about the design of the feeds for these bands. The design for Band 6 will be a scaled version of band 5b, with the feed components (OMT, horn, shields etc) assembled as a self-consistent and precision aligned sub-subsystem for integration with the vacuum hub, similarly to bands 5a and 5b. Component sizing, such as the vacuum enclosure and window assembly, have been based on a scaled (from Band 5b) feed horn. Similarly, the feed horn for Band 4 will be a scaled version of band 5a, with a similar quad-ridge OMT, and for Band 3 a scaled version of band 2 (with a band 2-style central four-probe OMT). While space has been allowed for fitting in realistically sized components, the detailed design of the side enclosures must await detailed design of the feed components themselves. The transition from a two-band to a full five-band cryostat is slightly complicated by the need to re-site the turbopump enclosure and the NEG pump (if fitted). The proposed re-siting to the undersides of the two side-mounted vacuum enclosures should however be straightforward (refer to Figure 6) with the full five-band configuration staying within the allotted mechanical envelope. The arrangement and placement of the various frequency bands reflects not only their physical size and viewing geometries, but also their tolerance to small spatial and angular misalignments. In particular, the highest frequency feed assembly, Band 6, has been placed in the central position along the principle datum axis of the cryostat where disturbance to cryostat-to-telescope alignment is expected to be lowest. Band 3 and 4, receiving at much lower frequencies are best suited to the side positions and the bolt-on sub-assemblies, as they are more tolerant to misalignments from nominal.

4.4 Thermal plumbing design and prototyping

In this section we describe the “thermal plumbing” system which distributes cooling from the central coldhead to all five feed positions. After describing the detailed design, we describe a “proof-of-concept” 5-arm thermal plumbing prototype (Section 4.4.6) which we designed and constructed before finalising the design presented here. This proof-of-concept prototype was constructed to experimentally demonstrate that such a system was able to provide sufficient cooling to the thermal bar ends at representative heat loads. The experimental tests were very successful, with low thermal gradients measured across the bars. The success of these experimental tests enabled us to confidently proceed in the development of the final design described in Sections 4.4.1 to 4.4.5 below.

4.4.1 Design Overview

The thermal plumbing system distributes cooling from the central coldhead to all five feed positions. It also supports the RF cabling from the cold LNAs to the warm RF electronics modules, and from the calibration noise source to the noise injection point. The overall design of the cold plumbing is illustrated in Figure 28. This design provides room at the back of the cryostat to accommodate the warm RF electronics. It is a simple and compact configuration which allows feed and OMT assemblies on either side of the coldhead to use common cooling paths up to their respective branching points.


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Figure 28: General view showing the distributed cooling system for all five bands. The RF and wiring details

have been removed for clarity.

Copper bus-bars are used to connect the 1st and 2nd cooling stages of the cryocooler to the assemblies that need cooling, and in particular the LNAs and the OMTs (nominally maintained at 15 K) and the feedhorns (maintained at 85K). The 85 K bus-bars are connected to a 1st stage cooling hub and have bolt-on sheet metal covers of 1050-grade aluminium which surround the inner 15 K copper bus-bars and shield them from the surrounding ambient temperature structures. The inner bus-bars are themselves connected to a central cooling hub attached to the 2nd stage of the cryocooler. The radiative loading on the 85 K bus-bars and their heat shields is in turn minimised through the use of multi-layer insulation (MLI) blankets which are fitted after integration of the cooling system.

4.4.2 Central cooling hubs

The bus-bars radiate out from central cooling hubs which are attached to the 1st and 2nd cooling stages of the cryocooler. The cooling hub for the 15 K bus-bars provides the thermal interfaces between the 2nd-stage cooling station and the bus-bars. It is machined from standard electronic-grade copper, lapped flat at the thermal interfaces to minimise thermal contact resistance. It is monitored with a single temperature sensor.


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Figure 29: Close-up view of the copper bus-bars of the thermal plumbing, with the aluminium heat shields

removed.

The 1st stage cooling hub is somewhat different in that it not only provides the thermal interfaces for the 75 K bus-bars, but supports and heat sinks the DC wiring connections (via Micro-D connectors) and the RF connections (via clamps for the stainless steel coaxial cable). The bus-bars are bolted to the top of the cooling hub which is lapped flat. The hub itself is connected to the 1st stage of the cryocooler via a tubular copper extension piece bolted to an adapter flange, the latter being the mechanical-thermal interface between the tube and the 1st stage cooling station. The arrangement for the 6/30 cryocooler is shown in Figure 31. The arrangement introduces four unavoidable thermal contact resistances in the cooling circuit for each bus-bar. These serial resistances will be minimised by using high contact-pressure bolted connections, by lapping the primary mechanical-thermal interfaces (mainly the small interface flange attached to the 1st stage) and by using an interstitial filler (thermal grease). Thermal resistances across metal-to-metal interfaces at cryogenic temperatures can be difficult to model in practice, but the successful test cooling of the proof-of-concept prototype described in Section 4.4.6 shows that the temperature gradients across these interfaces will not prove to be problematic.


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Figure 30: View showing the 1st stage cooling hub and the mechanical-thermal interfaces between the hub

and the bus- bars for the five-arm distributed cooling system.

Figure 31: View showing the tubular thermal link between the 6/30 1st stage cooling station and the 85 K

bus-bar cooling hub. The bottom of the tube is shown bolted to the adapter flange.

The DC wiring circuitry (for temperature sensors and LNA power supplies) are routed via thermally anchored 21-way Micro-D connectors fixed to the 70 K cooling hub. Each of the hub Micro-D connectors is at one end of a woven cryoharness assembly with end connectors potted to the wiring loom. Figure 32 shows the ambient temperature connections for the cryoharnesses used to investigate the performance of the five arm distributed cooling prototype (see Section 4.4.6). The cryoharnesses were supplied by Tekdata Cryoconnect, a specialist group of Tekdata Interconnections Ltd. The thermometer harnesses comprised three 21 x 36 AWG manganin singles woven into a harness using Dupont Nomex with the connector-to-harness connections encapsulated with Stycast 2850/09. The heater and signal harnesses comprised two 21 x 32 AWG copper singles woven and encapsulated as per the manganin harnesses. They were built to ESA ECSS-Q-70-08C standard and inspected to the same ECSS standard.


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Figure 32: View showing the ambient temperature ends of the cryoharnesses used for testing the five arm

distributed cooling demonstration model.

4.4.3 Bus-bars and heat shields

The bus-bars and heat shields for the Band 5a, 5b configuration are shown in Figure 33 .

Figure 33: View showing the bus-bars and the radiation shields for the band 5a and 5b only configuration. The LNAs are shown attached to the ends of the 15 K bus-bars and close to the OMTs. Also shown is the

carbon cryosorb pump (gold coloured object) fixed to the 15 K stage.

The collected aluminium radiation shields are shown in Figure 34 For scale the cylindrical shield on the right is 180 mm long and shields the OMT assembly for Band 5a. The equivalent shield for Band 5b is shaped to go around the turnstile OMT provided by JLRAT. The OMT shields bolt to the feed horns as part of self-consistent, pre-aligned sub-assemblies, and both are cooled via flexible copper straps which link the shields to the distributed cooling system. The thermal straps for the Band 5a assembly are shown in Figure 35, for both the 15 K and the 85 K circuits.


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Figure 34: View showing the collected 85 K radiation shields. The feed cylindrical sections bolt to the feed

horns.

Figure 35: Views showing the Band 5a assembly with its thermal straps and RF connections to the LNAs.

The individual feed assemblies have been designed with assembly in mind and with the requirement to connect each assembly thermally and electrically to the cooling and electrical systems inside the vacuum chamber. Figure 35 shows the thermal straps bridging the gap between the feed components and the cooling bus-bars as well as the RF connections between the OMT and the LNAs. The integration of the feeds will require a staged build with the 85 K cold plumbing going in first. This ensures that there is access to the screws fixing the 1st stage thermal straps to the underside of the bus-bars. Each feed will be assembled with its MLI blankets before being fitted to the vacuum enclosure. The 85 K bus-bar and cooling-hub MLI blankets will be fitted to the underside of these assemblies before installation and then closed at the top after fitting the aluminium radiation shields. The MLI blankets will be spaced off the radiation shields adequately to minimise thermal conduction through the blankets using small off-the-shelf plastics spacers which were used successfully in the proof-of-concept prototype (Section 4.4.6).

4.4.4 LNA mounting bracket

The mounting bracket for the two LNAs required for each distinct polarization for a particular band is illustrated in Figure 36.


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Figure 36: The mounting bracket for two LNAs (Left), with heating resistor and temperature sensor (Right).

The LNAs are those for the Band 5a feed (indicated by engraved labelling).

The design aims to minimise the volume of the mechanical envelope while providing an isothermal support for the amplifiers, a degree of thermal decoupling from the source of cooling and close thermal coupling to the common heater and temperature sensor. Thermal decoupling is affected by two rectangular PEEK plastic shims at the tie-down screw positions. The assembly for a particular feed is mounted to the 15 K bus-bar at a point close to the OMT so that the coax cable runs between the OMT and the LNAs are kept short and the RF losses low. The LNAs are temperature controlled to better than + 50 mK about a set point using the heater (a power resistor) and a temperature sensor mounted as shown in Figure 36. The temperature control is provided by a PID loop provided by the FPGA in the FPC, with the LNA bracket maintained at a few tenths of a Kelvin above the temperature of the 15 K bus-bar. In addition to the PEEK shims, and to minimise conduction, each LNA support assembly is secured to its 15 K bus-bar with PEEK screws. The shims and screws introduce a small thermal resistance so that the LNAs can be held at the slightly elevated temperature with the provision of a few mW of heating from the resistor.

4.4.5 Wiring / SMA cable routing

Managing and controlling both DC and RF wiring will be important if the build sequence is to go smoothly. Unlike in the more open configurations typical of single pixel feed cryostats, the semi-rigid coaxial cables and DC wiring have to follow well-defined paths through close-fitting radiation shields and so the coaxial cables will be accurately pre-formed before fitting. This is particularly important for the stiffer stainless steel clad semi-rigid coaxial cable. This is difficult to bend in-situ and so must be shaped to follow the routing and the requirements of installation such as compliance when torquing up the SMA connectors. The requirement for pre-bent semi-rigid coaxial cables should be straightforward to fulfil as many cable suppliers offer pre-forming as a service. The coax cable routings for bands 5a and 5b are divided up into the same routing segments: from the OMT to the LNA assembly, from the LNA assembly to the 85 K cooling hub (thermal clamps), and from the cooling hub to the warm electronics. There are also the coaxial cable connections to the noise injection probes fitted to the feed horns. These are routed to the warm electronics via clamps on the 85 K cooling hub for heat sinking. To facilitate assembly, the noise connection cables will require in-line connectors near the LNA assemblies and inside the 85 K shielding. They will also require physical support off the 85 K bus-bar as they are quite long. The noise connection coax route for band 5a inside the OMT shield is shown in Figure 37.


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Figure 37: View of the band 5a feed assembly showing the noise injection probe on the feed horn.

Figure 38 shows an in-line connection using a PEEK support and SMA bulkhead connectors. The connectors are side mounted to facilitate integration. The assembly marks the transition from aluminium clad coaxial cable to stainless steel-clad coaxial cable, and was used on the proof-of-concept prototype (Section 4.4.6).

Figure 38: View of an in-line connection assembly using bulkhead SMA connectors. The assembly provides

support for the transition from aluminium clad coax to stainless steel-clad coax.

Each feed assembly has two temperature sensors; one on the feed horn and one on the OMT. The temperature sensor wiring harnesses will require in-line connectors between the sensors and the Micro-D connector on the 85 K cooling hub. A typical in-line connection for the temperature sensor is shown in Figure 39; it was successfully used on the proof-of-concept five-arm cold plumbing prototype (described in Section 4.4.6). The 2-wire sensor shown in the photograph is wired out as a 4-wire resistance measurement at the connector. The arrangement for bands 5a and 5b will be slightly different in that the 4-wire connection will be made at the solder pads on the sensor’s copper-on-Kapton heat sink.


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Figure 39: A typical temperature sensor assembly and in-line connector used on the cold plumbing test

prototype.

Low conductivity G10 supports inside the 85 K radiation shielding will be used to support and constrain all DC wiring routed to the Micro-D connectors from the sensors, heaters and LNAs.

4.4.6 Proof of concept prototype and rectangular cryostat test results

In order to experimentally verify that our conceptual design of the thermal plumbing presented at PDR would be able to provide suitable cooling performance, we constructed a “proof-of-concept” prototype of the thermal plumbing in our rectangular test cryostat [RD5]. The intention was to construct a prototype with five complete arms of thermal plumbing (for Bands 3, 4, 5a, 5b and 6), each of realistic length connected to the central coldhead via a central cooling hub. The prototype included sheet metal aluminium radiation shielding, MLI blankets as well as provision for DC wiring and SMA cabling for the RF signals. One should note that this prototype was designed and built before we re-baselined the coldhead from an Oxford Cryosystems 2/9 to the more powerful Oxford Cryosystems 6/30. We note that the successful experimental results presented here were made with a prototype using the less powerful Oxford Cryosystems 2/9 coldhead, so we can be even more confident of successful performance with the more powerful 6/30 coldhead which we will use in our CDR qualification model prototype. A 3D CAD model of the prototype thermal plumbing is shown in Figure 40.


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Figure 40: A 3D CAD model of the prototype thermal plumbing assembly showing the sheet metal

aluminium radiation shields covering each arm. The radiation shield over the 85 K cooling hub has been removed to show the routing of the stainless steel semi-rigid SMA cable.

Figure 41: A detailed view of the 1st and 2nd stage cooling hubs with the 85 K radiation shield removed. The model shows the routing of the stainless steel semi-rigid SMA cables from in-line connectors inside the

shields to the thermal clamps at the rear of the hub.

At the end of each of the five thermal plumbing arms we installed heating resistors and temperature sensors so that we could inject a predetermined heating power along the 15 K and 85 K sections of each arm. We could then measure the resultant temperature gradient between the ends of the arms and central hub for the likely heat flows, thus measuring the complete thermal conduction of the thermal plumbing structure, including the difficult-to-calculate thermal contact resistances across metal-to-metal interfaces. To ensure that the measured temperature differences were accurate, 15 temperature sensors were cross-calibrated on simple copper brackets mounted to the coldhead. The LNA noise temperature performance begins to degrade above 20K, so the ultimate aim of the experimental tests was to show that we can confidently achieve less than this at the thermal plumbing


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bar ends. The temperature of the 1st stage bar ends, which cool the feed horns, are less critical, but we would like to keep the temperatures below 120 K to ensure the horns are cold enough to give good cryopumping.

Figure 42: (Left) The thermal plumbing arms (85 K and 15 K), assembled in the rectangular test cryostat, without radiation shields. (Right) Details of the provision for DC wiring in the central hub (six 21-way Micro-

D connectors).

Figure 43: Detail of 85 K (lower) and 15 K (upper) thermal bus-bar ends. These have been fitted with heater resistors and temperature sensors for experimental testing.


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Figure 44: The prototype thermal plumbing, without radiation shields, installed in the rectangular test cryostat.

Figure 45: (Left) The prototype thermal plumbing, with radiation shields fitted to the 785 K bus-bars with the 85 K cooling hub cover off. (Right) Thermal plumbing with all radiation shields and MLI blanket insulation

fitted.

The IR loading calculations presented in Tables 17 and 18 of the PDR document [RD5] show that we can expect, in the worst case, 0.55 W of IR loading on the Band 5a feed horn. This calculation assumes


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that the interior of the Zotefoam window undergoes no radiative cooling. In practice, the vacuum facing side of the Zotefoam window will cool, and if this is accounted for, a lower figure of 0.29 W is found. Nevertheless, for test purposes we chose to apply a worst-case loading of 0.55 W (using the heater resistor) to the end of the 85 K thermal plumbing arm to simulate the heat input from the IR loading of the front of the 5a feed horn. The results of the experimental tests are summarised in Table 4 below.

Heat applied (W) Temp. of Band 3 bar end (K)

Temp. of central hub (K)

Temperature difference (K)

0.552 83.8 K 81.12 K 2.68 K

Table 4: Heat loading and temperatures achieved for the 85 K thermal bus bars.

Given the published thermal conductivity of the grade C101 (UK spec) copper stock material used for the 85 K bus-bar (not electronic grade) we expect a temperature gradient of around 1.5 K to arise from the finite conduction from the bus-bars. In addition we expect a further ~1 K to arise due to finite thermal conduction across the copper-to-copper interfaces [RD5], giving a total expected gradient of 2.5 K. Our measured temperature difference of 2.68 K is within good agreement and shows that we have achieved a good thermal conduction, i.e. low contact resistances, across the metal interfaces. In our final qualification prototype, there will be a few more metal-to-metal thermal contact resistances between the thermal plumbing bar ends and the horns, but our results show that each of these should give no more than about 1K each of additional thermal gradient. We are therefore confident that the horn temperature should equilibrate at no more than a few Kelvin above the central 1st stage hub temperature, even with a pessimistic IR thermal loading from the window. Finally, we note that the exact equilibrium temperature of the horn is not particularly critical to the overall receiver noise temperature since the ohmic losses of the feed horn are expected to be quite low [RD2]. However, we intend to keep the horns lower than 120K, because above this temperature the horns’ surfaces become much less efficient at cryopumping residual gasses in the vacuum chamber. For the 15 K arms, we applied ~0.6 W which is the very worst case 2nd stage loading (i.e. based on the conductive load expected from the Band 3 OMT (see Section 4.7)). Note in practice the other channels’ cold plumbing arms (5a, 5b etc.,) will experience a much lower heat load than this (i.e. ~0.08 W of heat dissipation for 2 LNAs, with a negligible contribution from DC wiring and SMA coaxial cabling). The experimental results obtained are shown in Table 5.

Bus Bar Band

T (hub), (K)

T (bar end), (K)

Temp. difference (K)

Heat applied (W)

Total Conductivity (W/K)

Band 4 14.37 14.8 0.43 0.5885 1.369

Band 5a 14.43 14.98 0.55 0.625 1.136

Band 6 14.38 14.74 0.36 0.583 1.619

Band 5b 14.45 14.93 0.48 0.588 1.225

Band 3 14.45 14.9 0.45 0.594 1.320

Table 5: Heat loading and temperatures achieved for the 15K thermal bus-bars. The bands 5a and 5b arms are the longest (464mm from coldhead to bar end), the Bands 3 and 4 arms are 383 mm long and the Band 6

arm is the shortest at 363 mm long.


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Very low temperature differences were observed (all were around 0.4 K), so we would expect our LNA in the Band 3 case to be only 0.4K warmer than the hub. Thus, as long as the central hub cools to below 19.6 K we can be confident that all of the LNAs will be below our target of 20 K. Our heat load estimates in Section 4.7 show that we expect our 2nd stage temperature to be as low at 10K if we use a 6/30 coldhead. In conclusion, these experimental tests demonstrate we have a sizeable engineering margin for the goal of keeping the LNA temperatures below 20K.

4.5 Band 5a/5b Feed/OMT mounting assemblies – the “podules”

The feed horns and OMTs, which are mounted behind their respective Zotefoam/Mylar vacuum windows, are maintained at two different temperatures: ~15 K for the OMTs and ~85 K for the feed horns. The mounting structures for these components, together with the vacuum windows, are collectively referred to as “pod modules” or shortened to “podules” within this document. The podules for each band bolt onto the central vacuum hub, and are connected to the thermal plumbing to deliver cooling to the horns and OMTs. The Band 5a podule is shown in Figure 46. The Kevlar-reinforced PTFE outer weather protection membrane (Sheergard SX-12) can be seen. A Swagelok fitting for a tube connection to the desiccant breather is located under the cryostat and inside the weather shield. The desiccant is required to keep the small airspace between the weather membrane and the inner Mylar vacuum membrane dry. The assembly as shown is comprised of two distinct sub-assemblies: the conically-shaped vacuum enclosure and window assembly, and the feed assembly comprising the horn, the OMT, the radiation shields, the thermal breaks and the flexible conductive links. The assembly comprising the window components and vacuum enclosure is not a precision aligned part and is simply fitted over the feed assembly after it has been installed on the vacuum hub. The cylindrical radiation shield, which is visible towards the bottom left of the figure, shields the OMT from radiative heat loads and provides the conductive link between the 85 K bus-bar and the feed horn. The quad-ridge OMT and copper cooling bracket is seen protruding from the bottom left. This copper cooling bracket is connected to the 15 K bus-bar via a flexible copper strap (not shown in the figure).


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Figure 46: The assembled band 5a “podule”. Not shown here is the two-part radiation shield and the

thermal straps which connect to the thermal plumbing bus-bars.

Figure 47: A cross-section through the band 5a podule. The horn is mounted off the cryostat body via a thin-

walled, perforated G10 cylinder (green) and the OMT is mounted off the horn via thin-walled G10 tubes (green).

A cross-section of the Band 5a podule is shown in Figure 47. Working from the outside toward the inside we see a roughly conical outer shell which bolts onto the central hub and supports vacuum window assembly. Next we see a green, perforated G10 cylindrical sleeve which bolts to the cryostat’s outer wall (at 313 K) and supports the feed horn (at 85 K). Connected to this feed horn are thin-walled G10 tubes which support the OMT (at 15 K) from the feed horn. The tubes also accurately maintain a small waveguide gap (to act as a thermal break) between the waveguide of the horn and the


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waveguide of the OMT. The horn is connected to the 85 K thermal plumbing via a cylindrical radiation shield and the OMT is connected to the 15 K plumbing via a copper thermal strap (not shown). Figure 48 shows an exploded view of the podule with all the components displaced along the optical axis. Other views of the 5a OMT are shown in Figure 49 and Figure 50.

Figure 48: An exploded view of the band 5a podule. The first and second items from the right are the

cylindrical radiation shield components which link the feed horn to the 85 K bus-bar. The window has a Sheergard SX-12 weather protection membrane. A Zotefoam plug mechanically supports the Mylar vacuum

membrane, which is clamped to an O-ring to provide a vacuum seal.

Figure 49: The Band 5a podule with the visibility of the vacuum enclosure switched off. The perforated G10

cylinder which supports and thermally isolates the feed horn is clearly visible.


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Figure 50: A 3D CAD model showing how the 5a OMT is mounted at the back of the 5a feed horn by thin-

walled G10 tubes. These provide the required thermal isolation between components while maintaining a small gap between them.

Figure 51: A 3D CAD model of the complete Band 5b podule.

The Band 5b podule is constructed similarly to the Band 5a podule, with some differences due to the different shape of the OMT, which is a turnstile design rather than the quad-ridge design used in Band 5a. The complete assembly is shown in Figure 51. In a similar way to the Band 5a podule, a Sheergard outer weather-protection membrane can be seen on the upper right, with a Swagelok pipe fitting for connection to the desiccant breather. As with the Band 5a assembly, the desiccant breather is needed to keep the interspace between the closely spaced weather and vacuum membranes dry. The turnstile OMT is completely enclosed by a closely shaped radiation shield, which can be seen in part on the lower left of the figure.


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Figure 52: The cross-section of the Band 5b podule.

A cross-section of the Band 5b podule is shown in Figure 52. The mounting concept for the 5b horn and OMT is similar to that for the 5a horn and OMT but with modifications made to support the more complicated shape of the 5b OMT. The outer vacuum shell supports the circular vacuum window, but bolts to the cryostat body at a rectangular aperture, which accommodates the shape of the 5b turnstile OMT more closely. Again, we can see a green G10 cylindrical perforated sleeve which bolts to the cryostat outer (at 313 K) and supports the feed horn (at 85 K). Connected to this feed horn are thin-walled G10 tubes which support the OMT (at 15 K) from the feed horn and maintain a small (~0.5 mm) waveguide gap which provides the thermal break. An 85 K radiation shield, which directs cooling to the feed horn is visible, and the OMT itself is cooled by connecting a flexible copper strap (not shown in this view) to the 15 K thermal plumbing through a rectangular aperture in this radiation shield, visible at the bottom right of the figure.

Figure 53: An exploded view of the Band 5b feed module, window assembly and vacuum enclosure.

An exploded view of the Band 5b podule is shown in Figure 53. The turnstile OMT is the item visible to the right of the perforated G10 thermal sleeve (green). It is enclosed in a shaped 85 K radiation shield, which also directs cooling to the 5b feed horn. Because of the shape of the turnstile OMT, the


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parts have to be assembled in a particular order and cleats must be used to clamp and secure certain assemblies which are loaded over the feed horn.

Figure 54: Front and rear views of the Band 5b podule, showing the clamping arrangement at the front and

part of the OMT at the back with two SMA output connectors for each orthogonal polarization.

Figure 55: Solid models showing how the 5b OMT is aligned and secured to the feed horn.

The Mylar RF windows for bands 5a and 5b will be protected from UV exposure and weather by an outer membrane of Sheergard SX-12 from Saint-Gobain. The membrane has a 3-layer construction with 2 barrier films designed for use in RF applications. It uses high strength aramid fibres (Kevlar) and a PTFE matrix to give a highly flexible and tear resistant weave, which is fully protected by its chemically impermeable outer layers. The composite exhibits a very low dielectric constant and loss tangent as well as excellent hydrophobic properties (https://tinyurl.com/ydcbpfh4). A clamp and O-ring ensure an air-tight seal between the membrane and the window assembly. The interspace between the vacuum window membrane (Mylar) and the weather window membrane (Sheergard) is shown in Figure 56. The small enclosed volume is kept dry via use of a desiccator and the percentage relative humidity of the air above the Mylar membrane is kept well below the local dew point.

https://tinyurl.com/ydcbpfh4


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Figure 56: Three-quarters section through the band 5a window assembly showing the interspace and the Swagelok connection to the desiccator-breather. Also visible is the drop-in Zotefoam plug support.

A suitable desiccator-breather is available from Brownell Ltd., model BLD6927-01-05. The interspace for each window (the volume between the two membranes) will be connected to the breather by PTFE tubing using Swagelok pipe fittings. In principle it should be possible to extend the desiccator-breather’s network to include the horns for Bands 3 and 4 if radomes are used for weather protection. The desiccator-breather is shown below in Figure 57.

Figure 57: Brownell desiccator-breather, model BLD6927/01-05.

4.6 RF chain mounting

As described in Section 4.4.4 above, the LNA pair for each band is supported and grouped by a compact, temperature-controlled bracket which is itself mounted close to the band’s OMT and cooled by that band’s 15 K bus-bar. The stainless steel RF signal cabling from the LNAs is then routed along and above the 15 K bus-bar and supported, where needed, with low thermal conductivity clips. In addition to the two SMA cables for each polarization, there is a third SMA cable which routes the signal from the noise diode module on the warm electronics to the noise injection probe at the throat of the feed horn. Figure 58 shows the arrangement used for feeding in and taking out RF signals to/from an LNA pair mounted to a 15 K bus bar inside the rectangular test chamber. The arrangement for the qualification cryostat will not be as cramped, as the shields will be wider and taller.


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Figure 58: View of a test set-up in the rectangular vacuum chamber. The SMA cabling that routes signals to and from the LNA pair on the right is shown positioned above the 15 K bus-bar and constrained within the

volume enclosed by the 85 K aluminium radiation shield.

The SMA cables will be heat sunk to the 85 K cooling hub using the same clamping arrangement as described in Section 4.4.5. The cables from the 85 K cooling hub to the warm electronics will be pre-formed so that they can be easily fitted. Similarly, the SMA cables from the warm electronics.

Figure 59: View of the warm electronics in the cryostat. The assembly shown uses discrete components (filters, amplifiers, attenuators etc.) although a more compact arrangement, with all components integrated

onto a single substrate, is also feasible [RD2].


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The accommodation of the warm electronics in the cryostat is shown in Figure 59. The arrangement shown is for all five bands and uses discrete components such as amplifiers, attenuators and filters for each of the bands. The components are mounted on temperature-controlled plates with local temperature control and regulated power for the amplifiers provided by a dedicated analogue circuit board. A thermal link to the vacuum hub is used to maintain temperature control of the plates such that they are held at several degrees above the maximum anticipated ambient temperature (40 C). A separate plate will be used to support the two noise sources and power splitter. The spatial relationship between the warm electronics, the 85 K cooling hub and the hermetic SMA vacuum feedthrough panel is clearly illustrated. The design and RF performance of these components, together with the details of the temperature control and experimental verification is described in [RD2]. Cabling will have to be carefully controlled and sequenced to ensure ease of installation. There is scope to reduce both the mechanical envelope and the part count for each panel by replacing the discrete components with an integrated design. Such a design is shown for a single band in Figure 60 (left hand assembly). The discrete amplifiers, attenuators and filters shown in the central assembly have been replaced by a single unit powered by the local circuit board. The right-hand assembly shows the noise sources and power splitter on a separate panel. Although the baseline design is to use discrete components, both options will be prototyped to ascertain the most suitable (and practical) solution.

Figure 60: Views of the warm electronics modules in various stages of development. (Left) shows the warm RF chain as discrete components, (Centre) RF chain replaced by a single, integrated module. The circuit

board (in green) is the local regulated power supply and analogue temperature control PCB. (Right) shows the noise sources and power splitter mounted on a similar temperature-controlled plate.

4.7 Feed package heat loading budget

In the PDR document [RD5], we used preliminary estimates of component surface areas to estimate the radiative loading and combined these with estimates of conductive and dissipative loading from wiring and LNAs. We established that, with the use of MLI covered radiation shielding, we would have a total load of 5.2 W at the coldhead’s first stage (85 K) and 0.87 W at the second stage (15 K). This was within the cooling capacities of the then baseline Oxford cryosystems 2/9 coldhead, i.e. less than 9 W at 85 K and 2 W at 15 K. At delta-PDR, we decided to re-baseline the coldhead to a more powerful


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Oxford Cryosystems 6/30 coldhead, which offers several advantages (see Appendix B in [RD5]). These include longer lifetime, reduced cooldown times and a wider engineering margin between the likely heat loading and the available cooling capacity (6W @15 K and 30 W @ 85 K). Now that we have a detailed design we can present an updated and more realistic thermal loading budget. Radiative loading to the 1st stage

We estimate the radiative loading onto the 1st coldhead stage as follows.

1) From our CAD models we sum up the total surface area of outward facing 85 K metal surfaces which will be exposed to 313 K radiation from the cryostat outer walls. This will include the copper thermal plumbing bars, the aluminium radiation shielding and the side surfaces of the feed horns etc.

2) The radiative loading is calculated assuming plane-parallel transfer from the machined

aluminium outer cryostat walls to the metal surfaces in question. We assumed an emissivity of 0.09 for machined or sheet aluminium surfaces and 0.05 for copper surfaces.

3) We then reduce this radiative loading by a factor of 8.6, which is typical for the radiative shielding achieved with three 10-layer blankets of aluminised Mylar multilayer insulation [RD9].

Another significant source of parasitic heat load on the first stage is that arising from the radiative loading from the 313 K Zotefoam window plug (emissivity = 0.9) to the apertures of the 5a and 5b feedhorns. This loading cannot be reduced by MLI since it would block the RF signal. We propose reducing this loading by gold plating the interior of the feedhorns, thus reducing the emissivity by around one third to 0.03. The final source of radiative loading will arise from the Band 4 vacuum window (which is situated in the horns’ RF waveguide, the Band 4 feed horn being located outside of the main vacuum vessel). This is modelled as a Zotefoam window (emissivity = 0.9) facing an 85 K cooled Zitex woven polyethylene radiation shield. The contributions to the radiative heating are also summarised in Table 6 below.

Material at 85K

Material at 313K

Components MLI reduction factor

Total Area (m2)

Heat flow (W)

MLI covered Al. em 0.09

Machined Al. em. 0.09

Rad shields, horns, 5a, 5b.

0.116 0.473 1.40

MLI covered Cu. em. 0.05


Thermal plumbing

bars, 5a, 5b.

0.116 0.125 0.26

Gold plated Al. (0.03)

Zotefoam em. 0.9

5a, 5b horn apertures

None

0.058 0.95

MLI covered Al. emm 0.09


Rad shields, horns, 3,4,6

0.116 0.461 1.36

MLI covered Cu. em. 0.05


Thermal plumbing bars, 3,4,6

0.116 0.0597 0.12

Gold plated Zotefoam 6 horn None 0.00551 0.089


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Al. (0.03) em. 0.9 aperture

Zotefoam em. 0.9

Zotefoam em. 0.9

Band 4 window to Band 4 rad

shield

None

0.00395 1.75

Total 5.94

Table 6: Radiative loading to the 1st stage.

Conductive loading to the first stage The combined conductive loading through the LNA power supply wiring, stainless steel coax and temperature sensor wires are summarised in Table 7 below. The table also includes the conduction along the G10 support for the feed horns as well as the estimated total conductive/radiative heat loading from the Band 3 OMT (based on EMSS measured results from the SKA band 2 OMT).

Item Comments Total No.

Size/material Heat Flow (W)

LNA PSU wires 3 per LNA, 5 chans. 30 32 awg, Cu. 7.82E-3

LNA heater wires 2 per LNA, 5 chans 20 32 awg, Cu. 5.21e-3

Temp. sensor wires, (Both 15 K and 85 K

sensor wires are heat sunk to 85 K)

Monitoring LNA, OMT, horn and cold hub temps

52

36 awg, Manganin

5.44E-3

Stainless steel RF coax 3 per channel, H, V and noise injection

30

See [RD2] 0.174

G10 5a feed horn support

1 G10 fibreglass polymer

0.374

G10 5b feed horn support

1 G10 fibreglass polymer

0.695

Band 3 window assembly (EMSS

estimate)

1 Various 1.8

Total 3.06

Table 7: Conductive loading to the first stage.

Dissipative loading on the first stage Based on the current consumption of the LNAs and their temperature control headers for all 5 bands, the heating via power dissipation on the first stage is 0.022 W.


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Combined heat loading to the first stage The total heat loading is the sum of the radiative, conductive and dissipative loading as is found to be 5.94 + 3.06 + 0.022 = 9.0 W. This is less than ⅓ of the available first stage cooling power for the baseline Coolstar 6/30 coldhead, providing a substantial engineering margin. Radiative loading from the 2nd to 1st stage Radiative loading from the second to first stage arises from the following surfaces

1) Metal-to-metal radiation from 85 K to 15 K.

2) 15 K Band 5a OMT entrance to 313 K Zotefoam window.

3) 15 K Band 5b OMT entrance to 313 K Zotefoam window.

4) 15 K Band 6 OMT entrance to 313 K Zotefoam window.

5) 15 K Band 4 OMT entrance to 85 K waveguide Zitex radiation shield.

Warm material, emissivity

temperature

Cold material, emissivity,

temperature

Comment Total Area (m2)

Heat flow (W)

Machined Al. em. 0.09, 85K

Machined Al. em. 0.09 15 K

85 K Radiation shield to 15 K LNAs and OMTS B5a,B5b

0.0806 0.011


Cu. em. 0.05., 15K

85 K Radiation shield to 15 K top 15 K thermal bus

bars B5a, B5b

0.109 0.0107



85 K Radiation shield to 15 K LNAs and OMTS B5, B6

0.07 0.010


Cu. em. 0.05., 15K 85 K Radiation shield to 15 K

thermal bus bars B3,B4,B6

0.063 0.006

Zotefoam em. 0.9 313K


15K B5a OMT entrance to B5a to

313 K Zotefoam window

0.00138 0.067



15K B5b OMT entrance to B5b to

313 K Zotefoam window

0.00039 0.019

Zitex em. 0.9 313K


15K B4 OMT entrance to 85 K Zitex radiation shield across

0.0040 0.001


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waveguide



15K B6 OMT entrance to B6 to 313 K Zotefoam

window

0.00017 0.008

Total 0.134

Table 8: Radiative loading from the 1st to 2nd stage.

Conductive loading from the 1st to 2nd stage The combined conductive loading through the LNA power supply wiring, stainless steel coax and temperature sensor wires is summarised in Table 9. The table also includes the conduction along the G10 support for the 15 K OMTs as well as the estimated total conductive/radiative heat loading from the Band 3 OMT (based on EMSS measured results from the SKA band 2 OMT).

Item Comments Total No.

Size/material Heat Flow (W)

LNA PSU wires 3 per LNA, 5 chans. 30 32 awg, Cu. 4.31E-4

LNA heater wires 2 per LNA, 5 chans 20 32 awg, Cu. 6.46E-4

Temp. sensor wires, (Both 15 K and 85 K

sensor wires are heat sunk to 85 K)

Monitoring LNA, OMT, horn and cold hub temps

44

36 awg, Manganin

3.8E-4

Stainless steel RF coax 3 per channel, H, V and noise injection

30

See [RD2] 4.8E-3

G10 5a OMT support 1 G10 fibreglass polymer

0.055

G10 5b OMT support 1 G10 fibreglass polymer

0.027

Band 3 OMT assembly (EMSS estimate)

1 Various 0.6

Total 0.688

Table 9: Conductive loading from the 2nd to 1st stage.


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Dissipative loading on the 2nd stage Based on the current consumption of the LNAs and their temperature control heaters for all 5 bands, the heating via power dissipation in the wiring on the first stage is 0.022 W. We also have a total of 10 LNAs which dissipate ~0.04 W each. The total dissipation is therefore 0.422 W. Combined heat loading to the second stage The total heat loading is the sum of the radiative, conductive and dissipative loading and is found to be 0.134 + 0.688 + 0.422 = 1.24 W. This is approximately 1/5 of the available second stage cooling power for the baseline Coolstar 6/30 coldhead, providing a substantial engineering margin. Conclusion In conclusion, we expect “clean metal” (i.e. in the absence of cryocondenstes) thermal loadings of 9W (first stage) and 1.24 W (second stage), well within the capacity of a Coolstar 6/30 coldhead. By examining the Oxford Cryosystems load curves, we find that this loading corresponds to an expected 1st stage temperature of 27 K and an expected 2nd stage temperature of 10 K when driven at full speed. We expect build-up of crycondensates (chiefly ice from water vapour) will act to gradually increase the emissivity of the metal surfaces and thus increase the radiative loading over time. We intend to mitigate this problem, if necessary, with the use of charcoal cryosorb and a Non-Evaporable Getter (NEG) pump (see Sections below) to achieve a long (multi-year) cryogenic hold time before cryocooler regeneration is necessary.

4.8 Vacuum provision and pressure monitoring

4.8.1 Overview of vacuum system

Figure 61: A schematic diagram of the overall vacuum system on the dish indexer.


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A schematic diagram of the overall vacuum system on the dish indexer is shown in Figure 61. The Band 2 cryostat uses a scroll pump to achieve its required vacuum level (10E-2 mbar), whereas we intend to use the scroll pump effectively as a roughing pump for a turbo pump which is attached directly to the side of the B345 cryostat. This combination will be able to achieve a vacuum of 10E-4 mbar before the coldhead is started. A detailed justification for the use of the turbo pump, with an experimental demonstration can be found in Appendix C of [RD5]. We found experimentally that a turbo pump is definitely necessary for cooling with a Coolstar 2/9 coldhead on our rectangular test cryostat. Since we have now re-baselined the coldhead to a more powerful Coolstar 6/30 coldhead, it is possible that we can achieve cooling with a lower starting vacuum and thus we may not need to employ a turbo pump after all. We will investigate this experimentally once we have constructed the final qualification cryostat described in this document.

4.8.2 Pressure monitoring

The vacuum pressure is measured at two points in the system: the vacuum pressure inside the cryostat and the vacuum pressure within the indexer’s vacuum manifold (separated from the main vessel by a solenoid vacuum valve and turbo pump). Both of these vacuum levels will be measured using Edwards Linear Active Pirani Gauges (APGX) mounted within the RFI shielded turbo pump enclosure. These gauges offer vacuum monitoring for pressures from 1333 mbar down to 3.0E-4 mbar. The DN16-KF version is shown in Figure 62. The gauge is activated by a 15 to 36 V DC power supply and outputs a 2 to 9 V DC linear analogue signal related to pressure. An 8-way electrical connector socket is used to connect the gauge to its power supply and pressure monitoring electronics (in this case an analogue-to-digital convertor channel within the FPC enclosure). A stainless-steel mesh filter is fitted to the end of the gauge tube to protect the gauge filament from contamination and from the effects of turbulence in the vacuum system when it is pumped down or vented to atmospheric pressure. The APGX also contains two patented temperature sensing devices which compensate the output for the effects of changes in ambient temperature.

Figure 62: BOC Edwards APGX Linear Active Pirani gauge.

One potential disadvantage of using this type of gauge is that its lowest measurable pressure is 3.0E-4 mbar so, while it can measure to pressures where the coldhead may be safely turned on (while warm) it would be unable to measure to the ultimate vacuum level when the cryostat is cold (which is likely to be <1.0E-7 mbar). The limited pressure range offered by the APGX somewhat restricts its use as a diagnostic tool. For example, it could not be used below 3.0E-4 mbar to determine the cause of an unwanted temperature rise, such as a vacuum leak or excessive cryodeposits or a malfunction


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in the cryocooler. However, given the fact that there is no requirement of feed package maintenance on the telescope, the first steps for diagnosing the problem would in fact be the same, in either of these two fault scenarios i.e.

1. Remove entire cryostat from telescope (replacing with a functioning spare).

2. Take the faulty cryostat to the centralised repair lab.

3. Attach to a vacuum pump with a wide range (inverse magnetron) type gauge and test whether

it can be pumped down to below 1.0E-4 mbar vacuum at room temperature.

4. Perform a helium leak test on the warm cryostat under vacuum.

5. If tests (3) and (4) are successful, a vacuum leak is unlikely, so one could turn the coldhead on to attempt to cool to the cryostat. If the cryostat then did not cool, this would indicate a problem with the coldhead.

In other words, in both cases, one could quickly test whether there were problems with vacuum leaks in the repair lab before attempting to cool the coldhead. For this reason, knowing that the root cause of a warmup problem is a vacuum related issue before the cryostat is taken off the telescope does not offer any advantages in the likely real-world maintenance procedure. Thus the fact that the Linear Active Pirani cannot measure pressures below 3.0E-4 mbar is not as great a practical disadvantage as one might assume. Before settling on the Edwards Linear Active Pirani gauge to monitor vacuum levels, we considered the use of an Edwards Wide Range Gauge (WRG), which can measure pressures down to 1.0E-9 mbar. These feature a combined inverted magnetron gauge (for low vacuum) and pirani gauge (for high vacuum) in the same unit, with digital control electronics to switch between the two measurement methods at an appropriate pressure level. We have used these gauges routinely in our laboratory based Edwards turbo pump equipped pumping stations. While they can measure down to the likely working vacuum pressures when the cryostats are fully cold, they have several disadvantages compared to Linear Active Pirani gauges, namely:

1. Potentially higher levels of RFI emissions. In addition to integrated digital control electronics and a Pirani gauge, the WRGs feature a high voltage inverted magnetron which may generate additional unwanted RFI and require extra shielding. We considered the possibility of installing the WRG inside the cryostat body (i.e. with the whole unit surrounded in vacuum) to mitigate RFI emissions, but we were advised by Edwards that this would most likely lead to problems with high voltage arcing across the inverted magnetron supply lines.

2. Physical size. Since the WRGs cannot be operated in vacuum, the only other RFI-tight enclosure available to house them is the turbo pump enclosure. Available space is already somewhat limited within this enclosure since it already needs to contain the turbo pump and somewhat bulky solenoid operated vacuum valve.

3. Cost. The WRG unit cost is £900 vs. £267 for an active Linear Pirani gauge.

For these reasons, combined with the fact that the limited readout range of the Linear Active Pirani gauge does not, in fact, present significant disadvantages for fault finding and maintenance, we decided to select Linear Active Pirani Gauges. These will be used for monitoring the vacuum pressure on either side of the main vacuum valve i.e. inside the cryostat and in the vacuum services vacuum manifold.


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4.8.3 Vacuum valves

The proposed main vacuum valve is a standard DN16 ISO-KF VAT 264 angled solenoid valve. (http://tinyurl.com/y4a46dk8) . This is a mains (240 V) operated vacuum valve, with a body and seat leak rate each below 1.0E-9 mbar.l/s (Figure 63). The valve (part number 26424-KE61) has a conductance of 5 l/s (molecular flow). The valve is attached to the backing port of the turbo pump and its relatively low molecular flow conductance is unimportant as roughing out happens in the turbulent and laminar flow regimes and before the turbo pump reaches operational speeds. We know this to be true experimentally as the pumping system was simulated experimentally using our large rectangular test cryostat, a DN63 turbopump, a manual DN16 isolation valve and a 3 m length of flexible DN25 backing line to a scroll pump. This test is described in detail in the delta-PDR document [RD5]. It was shown to take approximately 30 minutes to evacuate the chamber to a point where the turbopump could be switched on and approximately 60 minutes to evacuate the chamber to ~5.0E-5 mbar. The valve has a leak rate of 1.0E-9 mbar.l/s and is fitted with OPEN/CLOSED limit switches and a single acting closing spring. Solenoid valves of this size use either 115 Vac or 240 Vac solenoids.

Figure 63: A VAT 264 high vacuum angled solenoid valve. The LED lights indicate open or closed.

A pressure relief valve is needed in addition to the isolation valve, since significant over-pressures can develop when a cryostat which has been cold for many months is warmed up. These over-pressures are the result of the rapid evaporation of any cryodeposits which may have built up when the cryostat was cold. These over-pressures can be quite large (>2 bar) and would stress the Mylar membranes on the vacuum windows, with the band 5a window most affected with its 340 mm diameter membrane. The proposed pressure relief valve is manufactured by Accu-Glass (part number 113155, (https://tinyurl.com/y79lcswk) and is routinely used on cryostats for precisely this purpose. The DN16-kF valve is shown in Figure 64. The actuation pressure is adjustable from 0.5 psi to 2 psi (35 mbar to 138 mbar).

http://tinyurl.com/y4a46dk8


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Figure 64: An Accu-Glass 113155 pressure relief valve.

It is proposed to fit a manual vent valve to the main vacuum enclosure so that the cryostat can be vented in a controlled way for maintenance and without the need to power up the solenoid valve. A DN10-KF vent valve of suitable design is available from by Oerlikon Leybold (part number 17324) and is shown in Figure 65.

Figure 65: DN10-KF manual vent valve (Part No. 17324).

The pressure relief valve and up-to-air valve are located under the cryostat and within the support frame. This is shown in Figure 66.


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Figure 66: Location of the pressure relief valve and up-to-air valve under the cryostat.

Figure 67: The pressure relief and up-to-air valves shown attached to side ports welded to the stainless steel

cryocooler to cryostat adapter flange.

4.8.4 Turbo pump and enclosure

As described in [RD5], we will use a turbo pump connected directly to the side of the cryostat in order to obtain a vacuum below 1.0E-4 mBar before turning on the coldhead and begin cooling of our system. We selected an Edwards nEXT 85 turbo pump and tested the RFI emissions at the SKA RFI test facility in South Africa [RD5]. The conclusion was that we needed an enclosure to provide additional shielding for the turbo pump at the level of at least 85 dB. The design of this turbo pump enclosure is show in Figure 68.


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Figure 68: The Turbo Pump enclosure, showing the two Linear Active Pirani Gauges on either side of the VAT solenoid valve before the turbo pump. Two honeycomb shielded air vents with a small DC cooling fan are

provided. The fan switches on with turbo pump.

The enclosure contains the turbo pump, the solenoid actuated vacuum valve (VAT 264 HV angled valve) and the two Edwards Linear Active Pirani gauges. One of the Pirani gauges measures the vacuum level in the cryostat (through a hole in the interface flange) and the other measures the vacuum level in the vacuum service manifold on the roughing side of the turbo pump. RFI shielded electrical inputs are DC (24 V for the turbo and 240 V for the valve) or slowly changing analogue signals (analogue vacuum level (2 to 9 V) from the Pirani gauges). Peak electrical power draw of the turbo pump can be as high as 80 W, so provision for fan assisted cooling air flow is provided via long honeycomb structure inlets and outlets (to provide RFI isolation). The turbo pump enclosure is connected to the cryostat outer via a high quality RFI tight gasket of a similar type to that successfully used on the Band 2 FPC.

4.8.5 Pump-Down Logic

The proposed pump-down procedures are outlined in [RD5]. It is reproduced here for convenience. As far as operations are concerned, there are two distinct scenarios when the B345 turbo pump would need to be operated:


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1. When all of the single pixel feed packages (across the entire array) are warm and at, or close to, atmospheric pressure. This scenario might occur after a lengthy site, (1 day+), site-wide power cut.

2. When just the B345 cryostat is warm and at atmospheric pressure on one antenna. This might

occur e.g. when the B345 packages are being attached individually to each antenna during commissioning, or after the replacement of a faulty B345 feed package with a spare from the central maintenance facility.

In case (1) the following pump down / cooldown procedure would be observed.

1. The ScrollVac scroll pump would be switched on. 2. Immediately afterwards the solenoid valves to the Band 2 would be opened, the Band 2

cryostat evacuated to a pressure of 1.0E-2 mbar. 3. The Band 2 coldhead would then switched on, beginning the cooldown of the band 2 cryostat.

The band 2 cryostat will be cold within 2 hrs. Once cold (or at least to the temperature where it will cryopump), the Band 2 valve can be closed.

4. The ScrollVac scroll pump would then be switched off. The relief valve in the vacuum manifold would be opened, which brings the vacuum manifold and hoses up to atmospheric pressure.

5. The ScrollVac scroll pump would be switched on and the valve to the B345 cryostat would then be opened, and the B345 cryostat and vacuum manifold would be pumped until a base pressure of <5.0E-1 mbar was reached.

6. The valve to the B345 cryostat would remain open and the turbo pump would be switched on. In this configuration the scroll pump is now acting as the backing pump for the turbo pump.

7. The turbo would run until a base pressure of below 1.0E-4 mbar is obtained in the B345 cryostat.

8. The B345 cryostat coldhead would be started, beginning the cooldown of the B345 cryostat which should take around 16 hours to cool completely with a 6/30 coldhead. The turbo pump would continue to run until the 1st stage temperature reaches around 80 K where cryopumping will occur (estimated to be around 6 hrs).

9. At this point the valve to the B345 cryostat (and turbo pump) would be closed and the power to the turbo pump would shut off. The turbo pump will spin down under vacuum over a period of around 20 minutes. The scroll pump can be switched off at this point.

10. The relief valve in the vacuum manifold would be opened, which brings the vacuum manifold and hoses up to atmospheric pressure.

11. Within 1 day all cryostats would be cold and under vacuum.

Note that observing can begin across the entire array using Band 1 and Band 2 during the time it will take the B345 cryostat to cool. In case (2), the following pump down cooldown procedure would be observed:

1. The ScrollVac scroll pump would be switched on. 2. Immediately afterwards the solenoid valves to the B345 cryostat would be opened. The

solenoid valve for the Band 2 receiver would remain closed, as this receiver is already cold and under vacuum.

3. The scroll pump would run until the B345 cryostat and vacuum manifold reach a base pressure of 1.0E-1 mbar.

4. The valve to the B345 cryostat would remain open and the turbo pump would be switched on. In this configuration the scroll pump is now acting as the backing pump for the turbo pump.

5. The turbo would be run until a base pressure of 1.0E-4 mbar is obtained in the B345 cryostat.


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6. The B345 cryostat coldhead would be started, beginning the cooldown of the B345 cryostat which should take around 1 day to cool completely with a 6/30 coldhead. The turbo pump would continue to run until the 1st stage temperature reaches around 80 K where cryopumping will occur (estimated to be around 6 hrs).

7. At this point the valve to the B345 cryostat (and turbo pump) would be closed and the power to the turbo pump would shut off. The turbo pump will spin down under vacuum over a period of around 20 minutes. The scroll pump can be switched off at this point.

8. The relief valve in the vacuum manifold would be opened, which brings the vacuum manifold and hoses up to atmospheric pressure.

9. After around 16 hours the B345 cryostat would be cold and under vacuum. The Band 2 cryostat valve would have remained closed, with the receiver cold and under vacuum (and potentially observing) during this time.

If we are just pumping down and cooling one B345 cryostat on one antenna, observations can proceed using the rest of the array. Note that the vacuum of the B345 cryostat will be monitored via a vacuum gauge located inside the turbo pump enclosure itself, to mitigate RFI risk.

4.8.6 Cryocooler Regeneration

The gradual build-up of cryodeposits (chiefly ice from water vapour) will act to increase the emissivity of cold components within the cryostat and increase radiative loading. As the required cooling capacity increases, the temperature of the cold components will gradually rise. We intend to mitigate this problem, if necessary, with the use of charcoal cryosorb and a Non-Evaporable Getter (NEG) pump (as described in Sections 4.8.7.1 and 4.8.7.2). The temperature rise due to build-up of cryodeposits will become unacceptable for the continued operation of the feed package when the LNA temperatures rise above 20K, degrading the on-sky sensitivity. At this point the cryostat would need to be bought up to room temperature to clear the surfaces of cryodeposits. This is a remote maintenance procedure known as cryocooler regeneration. The procedure would be as follows:

1. Turn off the NEG pump. 2. Turn off the cryocooler motor. 3. Allow the cryostat to warm up to ambient. 4. Initiate a complete new pump down using the indexer scroll pump. 5. Restart the turbo pump. 6. Regenerate the NEG pump. 7. Turn on the cryocooler motor. 8. Allow to system to cool as normal.

How often this procedure needs to be instigated is difficult to predict without further experimental work, since it depends on how quickly cryodeposits will form. This will depend on the overall cryostat leak rate and the effectiveness of the charcoal cryosorb and NEG pump at removing water vapour from the vacuum. Even with significant cryodeposit formation the effect this will have on the base temperatures will depend on the excess cooling capacity available to the coldhead, which will itself degrade slowly over time due to wear. With our representative rectangular test cryostat, we found that the gradual heating effect due to cryodeposits was quite slow, even with the less powerful 2/9 coldhead, no carbon cryosorb and no NEG pump. Based on this experiment alone, we would therefore not expect to have to perform a cryocooler regeneration more than once every a year, potentially less or never if we use a cryosorb and a NEG pump.


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4.8.7 Cryosorption and getter pumps

After pumping down the cryostat and cooling to base temperature, the net gas load due to real and virtual leaks will vary strongly with time. Initially the gas load will be the high, as trapped gas inside the cryostat begins to be released and materials such as MLI out-gas at their highest rates. After a few weeks, the gas loads will stabilise, and will be dominated by permeation through the O-rings and window materials. As long as cryopumping ensures that the vacuum level remains low enough such that the mean free path is below the characteristic size of the cryostat (<1e-4 mBar), there will be no significant increase in direct gas conduction due to these gas loads. However, the water vapour component of these gas loads will freeze onto cold surfaces, increasing their IR emissivity and thus act to gradually increase the radiative loading of the cryostat. For this reason, it is sensible to reduce the effect of these permeating gas loads as much as economically possible, to maintain long time cycles between cryocooler regeneration. In this section, we describe the use of charcoal cryosorbs, which are most effective at reducing the gas load immediately after pump down, and Non-Evaporable Getter (NEG) pumps which are most effective at reducing equilibrium gas loads due to continuous permeation through O-ring and window materials.

4.8.7.1 Cryosorption using charcoal sorbs

A charcoal trap installed on the second stage of a cryocooler will absorb those gases which are not cryocondensed on first-stage cooled surfaces. A concept design for a 15 K cryosorption pump was presented at PDR. A full design has now been developed and is shown in Figure 69. It is a modular, custom design which can be accessed easily by removing the 85 K cooling hub’s radiation shield (see Figure 21).

Figure 69: Views showing the 3D CAD models of the cryosorption pump. The assembly uses plates with

integral aluminium spacers.

The use of carbon pellets rather than granules allows for a more open structure (to residual gases) and is typically used in commercial cryopumps. The design in Figure 69 provides 1.5 g of charcoal per fin. The pellets are attached to the fins with an adhesive that stays fairly flexible during cool-down.


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4.8.7.2 Non-evaporable getter pump

A getter pump, if used, will augment and not replace the cryocondensation and cryosorption pumping offered by the cryocooler and its charcoal sorb. The power requirements for operating and regenerating the pump have already been factored into the baseline power budget for the full SPF345 configuration. The power requirements published at PDR document are reproduced below for convenience. The non-evaporable getter (NEG) pump proposed at PDR, the SAES CapaciTorr HV200, has been purchased, but not yet tested on the rectangular test chamber with the five-arm cold plumbing test model. This test will take place shortly and will coincide with testing the LNA assemblies with their prototype warm electronics in the chamber. The pump, which has a very compact design, provides high pumping performance in the high vacuum

range of pressures (<1.0E-7 mbar) for all getterable gases like H2, N2, H2O, CO/CO2 and O2. The

published sorption capacities for the HV200 correspond to the continuous sorption of an air leakage rate of approximately 5.0E-6 mbar.l/s for 1 year. The getter cartridge, which can be regenerated 20 times, is easily replaced and so the pump has a long service life. Depending on whether the cryostat is configured for first-light or with its full-complement of feedpackages, the pump can be accommodated in one of two places as shown in Figure 70.

Figure 70: Views of the HV200 NEG pump on B345 cryostat. The view on the left is for the first light configuration (Band 5a and 5b populated). The NEG pump is attached to the Band 3 blanking plate. The view

on the right shows the position of the NEG pump after Band 3 module has been integrated.

The power requirements for operating and regenerating the getter pump are summarised as follows.

● A power of 8.6 W will be needed by the pump to maintain its ~200 C operating temperature.

● During regeneration under vacuum, a power of 54 W will be needed to heat the NEG pump’s

active core to 500 C.

4.9 Vacuum integrity

The vacuum system comprising the vacuum enclosures for all five bands, the O-ring seals, the Mylar membrane windows, the foam window supports and the electrical-vacuum feedthroughs has not changed materially since PDR and so only differences will be described and considered in this document. The core of the cryostat is a kite-shaped vacuum hub to which is bolted a number of smaller enclosures, each providing the vacuum insulation required by a particular feed. All volumes are


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interconnected and sealed using standard O-ring technology. As discussed at PDR, the many internal surface areas and O-ring seals result in both short-term and long-term gas loads. These gas loads have to be minimised and managed if the cryostat is to perform well through its different stages of operation. The gas load calculations presented at PDR remain the baseline estimates. The concerns at PDR were the high initial gas loads and the ultimate pressure achieved inside the cryostat when evacuated by the scroll pump through low conductance pipework (the indexer manifold). Simulations carried out by Edwards High Vacuum showed that the pressures achieved within a reasonable time would not be low enough to start the cryocooler. Further simulations and measurements demonstrated the need for a small turbopump which is now part of the baseline configuration. The experimental testing of the proposed turbo pump is described in Section 4.11.2 of the delta-PDR document [RD5].

Another issue discussed at PDR was the hermeticity of the electrical-vacuum feedthroughs and the overall leak rate presented by the connectors. Using the supplier’s published leak rate, an ensemble of 10 hermetic bulkhead SMA connectors would have set the baseline leak rate at 1.0E-6 mbar.l/s or higher. Experimental measurements which have since been carried out on an ensemble of 5 connectors showed that the overall leak rate was significantly lower than published, so removing the hermeticity of the connectors as a cause for concern. The measurements and results are discussed in detail in Section 4.9. The long term effects of gas permeation through O-ring seals and Mylar membranes can be effectively mitigated by using cryocondensation, cryosorption and a non-evaporable getter (NEG) pump. The NEG has been added to the baseline design for the first light configuration (Bands 5a and 5b only), but will only be used if found necessary for long hold times. Very little structurally has changed since PDR, but there are a few notable changes in size which are detailed below.

a) The vacuum hub remains essentially the same although it has been strengthened to be structurally stable under vacuum loads. This is important as the hub provides the datum system for aligning all five feeds. The total volume and surface area have grown slightly with the change in lid design from flat to rib-supported.

b) The window support foam has been changed from Plastazote HD 110 to the less dense HD30, and the foam support ring changed accordingly to support the lower strength material. The volume of foam exposed to the vacuum is slightly larger and reflects the larger window assemblies for Bands 5a and 5b.

c) The volumes of the vacuum enclosures for Bands 5a and 5b have increased to accommodate the new window assemblies. The O-ring seals for the Mylar membranes are also larger in diameter and so have longer cord lengths.

These small volume increases add to the pump-down time to cryocooler switch on, but not significantly and so are not of concern. Laboratory experiments carried out on test window assemblies using both HD110 (presented at PDR) and HD30 indicate that they behave in a very similar way when exposed to vacuum with an initial high gas load followed by a slow release of nitrogen. The trapped nitrogen gas in the cellular structure of the foam will add to the pump-down time to cool down, but the turbopump should handle the gas loads comfortably and so this not considered an issue.


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4.9.1 Leak testing SMA connectors

One of the concerns at PDR was the leak rate of readily available, low-cost, commercial hermetic SMA bulkhead connectors from suppliers such as TE Connectivity. It was understood that suppliers tended to be cautious when quoting leak rates, but even a moderately good leak rate of 1.0E-7 mbar.l/s translates into a total leak rate of 1.0E-6 mbar.l/s when one considers the ten hermetic SMAs needed to services Bands 3, 4, 5a, 5b and 6. A fully welded assembly from CeramaTec with a total leak rate <1.0E-9 mbar is expensive and so it was decided to measure experimentally the leak rate of an ensemble of five lower-cost hermetic SMA bulkhead connectors that have been regularly used on other cryostats. The electrical feedthrough chosen for evaluation was a TE Connectivity F-F SMA hermetic connector, Part No. 1054874-1, details of which can be found here https://tinyurl.com/ybppj368. The test equipment and test configurations used for leak testing both the individual connectors and the ensemble of five connectors are shown in the photographs below.

Figure 71: Leak testing candidate hermetic SMA connectors.

The total leakage rate for each connector is comprised of two parts: a ‘real’ and a virtual’ part. The ‘real’ part is simply a pathway or hole through the body seal around the central pin while the ‘virtual’ part is comprised of body seal porosity and O-ring permeability. Seal porosity and O-ring permeability are manifest by a slow rise in the leak rate while a sudden and sharp rise would indicate a hole. The leak detector used was a turbo-pumped Pfeiffer ASM 142 with a useable sensitivity of 1.0E-11 mbar.l/s. Conflat vacuum components and copper gasket seals were used to ensure that the only O-rings contributing to the ‘virtual’ leak rate signals were those of the hermetic bulkhead connectors themselves. The body seal integrity for each connector was determined using stoppered rubber tubes. These were filled sequentially with helium gas using a hypodermic needle connected to the helium supply. The gross or total leak rate comprising both ‘real’ and ‘virtual’ leaks was determined using a helium-filled metallised bag placed over the connector flange and sealed to the test volume using

https://tinyurl.com/ybppj368




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Kapton tape. The bag was again filled with helium gas using a hypodermic needle and then refilled periodically over an interval of one hour to arrive at a steady state leak rate for all five connectors. The results were very good with individual body seals showing leak rates of <5.0E-10 mbar, while the steady state gross or total leak rate for the ensemble of five connectors was no worse than 5.0E-8 mbar, suggesting a total leak rate of around <1.0E-7 mbar.l/s for an ensemble of 10 connectors. The low leakage rates for helium also suggest that molecular flow applies to the gas flowing through the leakage paths and so to first approximation the equivalent leakage rates for air can may be estimated by multiplying the leakage rates for helium by the square root of the ratio of masses or [4/29]0.5 = 0.37 (https://tinyurl.com/ydeccyuf). Thus, the air leakage rate for an ensemble of ten connectors of the type specified is estimated to be <5.0E-8 mbar.l/s, which is acceptable. All hermetic connectors will be leak tested on goods inward and prior to installation to screen out faulty assemblies.

4.10 Temperature control and monitoring

The cryogenic temperatures inside the cryostat will be monitored at the following locations:

1. The 85 K stage of the central thermal plumbing hub.

2. The 15 K stage of the central thermal plumbing hub.

3. The temperatures of the LNAs, at their mounting bracket at the ends of the 15 K thermal bus

bar.

4. The band 5a and 5b OMTs (around 15 K).

5. The band 5a and 5b feed horns (around 85 K).

This gives 8 cryogenic temperature sensors for 5a and 5b deployment. In addition, the Band 3, 4 and 6 OMTs and LNA packages would be monitored giving an additional 6 temperature sensors. We would also monitor the cryogenically cooled Band 6 feed horn. This leads to a total of 15 cryogenic temperature sensors. In addition, the ambient temperature of the cryostat body will be monitored by an ambient temperature sensor (AD590) (https://www.digikey.co.uk/product-detail/en/analog-devices-inc/AD590KF/AD590KF-ND/611801) mounted in the centre of the base of the central vacuum hub. The cryogenic temperature sensors will consist of Carbon Ceramic Sensors (CCS) temperature sensors supplied by Temati (https://www.temati-uk.com/). These are sometimes referred to as “TVO” sensors (from their original Russian acronym), and have been widely used for temperature monitoring at CERN. They are low cost (relative to e.g. Lakeshore diode-based sensors), offer a fast response time (1 ms at 4.2 K) and have a good long-term stability (1 mK/year). We mount these sensors on a small copper mounting plate Figure 72. The sensors are read out via 4-wire monitoring using a circuit similar to that used for the Band 2 temperature sensors [RD3].

https://tinyurl.com/ydeccyuf

https://www.digikey.co.uk/product-detail/en/analog-devices-inc/AD590KF/AD590KF-ND/611801

https://www.digikey.co.uk/product-detail/en/analog-devices-inc/AD590KF/AD590KF-ND/611801


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Figure 72: The Temati CCS (TVO) temperature sensor shown mounted on a copper-on-Kapton heat sink which is itself glued to a small, lapped copper plate. The thermometer mount is screwed to the surface to be

sensed using a small amount of interstitial filler (Apiezon N grease).

In addition to this temperature monitoring, there will be active temperature control of the following components (see [RD2]and [RD3] for further details):

1. 5a and 5b Low noise amplifiers. This will be controlled via a heater resistor attached to the

LNA mounting brackets. The control loop will be implemented by the FPGA in the FPC.

2. Warm RF electronics mounting brackets. These will be temperature controlled by a self-

contained analogue temperature control circuit mounted on the mounting brackets itself.

These control circuits incorporate temperature sensors which will also be read out by the FPC

electronics.

3. Mounting bracket for the calibration noise source (noise diode). This will be temperature

controlled in the same way as the warm RF electronics mounting brackets.

In order to provide sufficient overall cooling power, while minimising electrical power consumption, we also plan to control the cold head speed and differential helium pressure within the cold head compressor. In practice, this will involve adjusting the speed of the cold head (via the FPGA in the FPC) so that the temperature of the 15 K central thermal plumbing hub is maintained within a suitable set point temperature range, against a background of ambient temperature changes of the cryostat outer. We will carefully tune the time constants and the PI parameters of the LNA control loop and the coldhead speed control loop to avoid any unwanted cross talk and interactions.

5 Maintenance and Logistics

The Oxford Cryosystems 6/30 coldhead has a recommended service interval of 15000 hours with the servicing requiring the removal of the coldhead and the replacement of seals and rotary valve. The cryocooler manufacturer states that the need for service is generally indicated by a degradation of ultimate temperatures, and can generally be postponed until this loss becomes unacceptable, without any risk of damage to the cryocooler. It has been the experience at other radio astronomy observatories that warming up and transporting the cryostat increases the general risk of component failure. We therefore envisage that no scheduled maintenance will be done and the cryocoolers will be operated until they can no longer maintain temperature.


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While Oxford cryosystems recommend a service interval of 15000 hours (1.7 years of continuous operation), the likely lifetime between necessary maintenance is likely to be considerably longer for two reasons. Firstly, we can afford to run the cryostat beyond the recommended interval until we can no longer maintain an acceptable operating temperature. Secondly, these service intervals refer to operation with the coldhead speed at 60 rpm. Given the fact that we will use a variable speed compressor, we should be able run at a lower steady state speed (~40 rpm) given the fact that our cooling power requirement will be considerably lower the maximum cooling capacity (6W/30W) of the coldhead. Wear on the seals is proportional to coldhead speed, so we expect a mean time between coldhead seal replacement of much more than 1.7 * (60 /40) = 2.4 years.

If a feed package develops a fault, or can no longer maintain temperature due to coldhead wear, the complete feed package and mounting frame will be removed from the indexer via a crane and replaced with a working spare. The faulty feed package will be transported to a central repair facility for coldhead replacement or fault diagnosis and repair. The feed package will incorporate suitable handles and lifting points, integrated into the cryostat body lid, for ease of handling during integration, testing, installation and maintenance, as specified in [AD1].

6 Power Requirements

All power to the cryostat and its components is supplied by the power supply unit located in the Feed Package Controller (FPC) enclosure. This is discussed in [RD3].

7 Safety

Unsupervised or unwarranted opening of an operating cryostat, or one that has not been properly shut down, carry a risk of equipment damage and injury. Appropriate warning labels and markings as specified in [AD1] will be attached to the cryostat.

The SPF Band 345 cryostat is a vacuum vessel. According to the South African Occupational Health and Safety Act of 1993, Pressure equipment Regulations [RD7], vacuum vessels are not regulated in South Africa. Care was taken when designing the cryostat and safety factors were implemented in design to ensure safe operation.

8 Conclusion and further work

We have described the detailed design of the SPF Band 345 cryostat assembly. Key components, such as the critical thermal plumbing have already been successfully prototyped and tested. Following DDR we will begin construction of the final qualification model cryostat at the Oxford Astrophysics workshops in Oxford. There will then follow an assembly and testing stage, incorporating the full RF chains for Band 5a and 5b. The level of mitigation against cryodeposit build-up will be investigated by cryosorb and NEG pump tests in our existing rectangular test cryostat and also in the final qualification model cryostat.