CERN BE/Group

BE-BI

EDMS Document No.

Date: 14 AUGUST 2017

CERN

CH-1211 Geneva 23 Switzerland

Meeting Minutes

Beam Gas Curtain Design Review at Cockcroft

Institute and Liverpool University

Abstract: The aim of the design review was to examine the status

of the Beam Gas Curtain instrument in view of the tests made and the experimental program foreseen on the existing Cockcroft set-

up, revise the status of the 2nd system to be assembled and operated this year at Cockcroft and the requirements for the

installation to measure the circulating proton beam in a hollow

electron lens.

The presentations can be seen at https://indico.cern.ch/event/646235/

Location: Liverpool University

Date: 27.06.2017

Present:

CERN: Marton Ady, Elena Barrios-Diaz, Tom Dodington, Stefano Mazzoni, Adriana Rossi, Gerhard Schneider, Raymond Veness

Cockcroft Institute: Carsten Welsch, Hao Zhang

GSI: Serban Udrea

Prepared by :

G. Schneider

R. Veness

Checked by :

Approved by :

Page 2 of 13

Contents

1. INTRODUCTION AND WELCOME TO THE REVIEW ......................................... 3

1.1 ACTIVITY OF THE PHYSICS DEPARTMENT OF THE LIVERPOOL UNIVERSITY ............ 3

1.2 CERN PERSONAL ON THE PROJECT ................................................................... 3

2. BRIEF HISTORY OF GAS JET APPLICATIONS AND COCKCROFT COLLABORATION ................................................................................................ 3

3. BGC AS PROFILE DIAGNOSTICS FOR ELECTRON LENSES IN HL-LHC (HALO DEPLETION AND BEAM-BEAM LONG RANGE COMPENSATION ............................... 4

4. STATUS OF QUESTIONS AND ACTIONS FROM THE MEETING OF 10/2016 ..... 5

5. RESUME OF THE EXPERIMENTAL PROGRAM UP TO DATE .............................. 6

6. COCKCROFT PLANS FOR THE NEXT 12 MONTHS ........................................... 6

7. GAS JET SIMULATIONS................................................................................ 8

8. FLUORESCENCE PROFILE MONITOR CAMERA SYSTEM .................................. 8

9. DESIGN OF THE BEAM GAS CURTAIN NO. 2................................................ 10

10. PRODUCTION AND COMMISSIONING OF THE 2ND BGC ........................... 10

11. MANUFACTURING AND ALIGNMENT OF THE SKIMMER AND NOZZLE ASSEMBLY ........................................................................................................ 11

12. INSTALLATION OF AN INSTRUMENT ON A CERN MACHINE ...................... 11

13. DISCUSSION .......................................................................................... 11

13.1 WRAP-UP ................................................................................................. 11

13.2 QUESTIONS .............................................................................................. 12

13.3 ACTIONS .................................................................................................. 12

13.4 RESEARCH TOPICS .................................................................................... 13

Page 3 of 13

1. INTRODUCTION AND WELCOME TO THE REVIEW

1.1 ACTIVITY OF THE PHYSICS DEPARTMENT OF THE UNIVERSITY OF LIVERPOOL

C. Welsch

C. Welsch presented the staff and students structure of the Physics Department.

At present, are • +50 academic staff • 40 research staff

• +100 PhD students • +300 undergraduate students (all years) (~6 per academic)

• Projected intake ~110 students

The range of the core activities of the physics department range from condensed

matter, nuclear, particle to accelerator physics.

1.2 CERN PERSONNEL ON THE PROJECT

R. Veness

R. Veness presented the persons of the CERN side of the project. At present, a total of 7 persons are working on the project in the fields of vacuum, physics,

optics and mechanics.

2. BRIEF HISTORY OF GAS JET APPLICATIONS AND

COCKCROFT COLLABORATION

C. Welsch

C. Welsch showed that the initial applications for gas jets came from very low

energy regime to study the difference between matter and anti-matter. The objective was to have a ‘clean’ process with few reaction channels. The interaction

time is in the fs range, as this is the ‘orbit’ time.

Anti-protons were studied using a supersonic low temperature He gas jet. The vacuum chamber background pressure was in the order of 10-12 mbar.

The advantage of a gas jet is the ability to adjust the gas jet properties to image any beam. The project started in 2008. The activities are in the frame of the

QUASAR Group research.

The history of which lead to the present BGC system started with simulations which were made unto 2010, including skimmer geometries. The set-up was made in

2012 by Massimiliano. Vasilis was working on the BGC from 2010 to 2016. He did the set-up and published the first results.

The project start date was 1/4/16, but STFC money came 3 months later. 1.56 MCHF equipment from STFC for the whole UK project (not just the BGC). The project is still not fully signed and approved.

Page 4 of 13

Hao started 1/4/17 for 3 years, payed 50% by CERN, 50% Cockcroft.

Carsten showed the financial data and the planning a copy of the key planning figures.

The BGC Set-up No. 3 is targeted for the installation in LHC machine in June 2019, which

is compatible with LS2.

3. BGC AS PROFILE DIAGNOSTICS FOR ELECTRON LENSES IN

HL-LHC (HALO DEPLETION AND BEAM-BEAM LONG RANGE COMPENSATION)

A. Rossi

The hollow e-lens allows for depletion of the beam halo and reduces load on collimators. A review in 10/2016 demonstrated the effect and benefits. The hollow

electron lens was recommended for HL LHC:

A 5 A electron beam is needed to produce desired halo depletion and 10 kV to

transport through the structure. The main solenoid has a magnetic field inbetween 3 and 5 T over about 3m length.

The proton beam is about ~1.2mm at 4 sigma. The measurement for the proton beam needs 30 µm accuracy (0.1 σ of proton beam), the time resolution is 1 ns.

The electron beam size is between d4σ≈ 6.93 mm and d6σ≈ 10.4 mm. The electron

beam resolution range is 48-50 um.

BPMs are used in addition to the gas jet monitors.

The technical design is limited in transverse space. The e-gun and the e-collector are in the vertical plane. Some space is reserved for the BGC.

The e-beam is sensitive to geometrical imperfections in the vacuum chamber wall

due to the large beam size. It can also therefore induce instabilities in the proton beam.

RHIC uses back-scattered electrons for overlap monitor.

In an ideal world one would have a full 2D image of both the proton and electron

beams. Ray commented that we will have one imaging system so will have to optimise for the overall systems.

Serban comments that presently the resolution of the optical system is at best ~50 um.

Hollow e-lens timeline:

Page 5 of 13

October 2017 Conceptual review

Test stand at CERN in 2018 of the solenoid (resistive) to measure existing

5 A electron gun

LS2 (end 2018) Technical Design Report

LS3 (end 2023) Installation during LS3

One could imagine to install the gas-jet monitor on this test stand, using an e-gun

with 1x10-4 duty cycle.

An alternative application of the BGC could be the BBLR.

The purpose of the BGC in the BBLR is to measure the distance between the electron beam and the proton beam:

Proton beam (±2σ) ~ 2.8 mm with a required 50 µm resolution (0.1 σ of

proton beam)

Electron beam smallest ~ 2mm with a required 50µm resolution

Low impedance for p+ beam is mandatory

A 20 A at 35 kV e-beam over 3m length would be the upper limit. A possible integration for local compensation is in IR 1 and 5 of the LHC, but space is limited.

BBLR timeline:

2017 first demonstration test with wire collimator

2018 new demonstration test

Adriana pointed out that for both applications, space and impedance

considerations are necessary to be taken into account.

4. STATUS OF QUESTIONS AND ACTIONS FROM THE MEETING OF 10/2016

R. Veness

Ray presented the action and questions.

Many of these things were done and answered in the meantime.

The remaining ones are:

We need to measure the actual lower pressure in the nozzle chamber with a

Penning gauge (Gerhard). <Still needs to be done>

Can we obtain an absolute value for the gas molecular density - will need to scale

the simulation from Roberto and compare with the vacuum gauge data (Roberto/Hao) <status?>

Page 6 of 13

Possible research topics:

Continue with the high pressure gas dynamics study from Paolo - need new resources (FCC fellow?). No new CFD analysis since summer 2016. <Need a

solution here.>

Lacking data for cross sections of gas interactions at LHC energies - extrapolation

over more than an order of magnitude. Do we need data or simulations at higher energies? <Still need investigation, Data available with LHCb or BGV?>

Movement of the gas in the space charge field. Is this studied by GSI? <Status?>

5. RESUME OF THE EXPERIMENTAL PROGRAM UP TO DATE

H. Zhang

Hao describes the main goals achieved:

Proof-of-principle result of BIF mode

Gas curtain distribution study

Faraday cup installation

Lab move and resume of the experimental program

All BIF measurements were done with the “big” skimmer of 7.2×1.8 mm2.

The estimation of number of protons equation is 0.08xdT, if one needs 100

photons, then this would give some 1000 seconds which compares well with experiment.

An integration time of 8000 s was used, but only 1000 needed to get an image.

This is for the low intensity electron gun with ~7 uA with 2.6 A filament current at 3.5 keV.

The optics will get better light collection by about a factor of 5. A gun with higher intensity will further increase this factor by about 10 to 20.

After the lab move, problems with e-beam steering occurred. This has happened

during the filament change.

Adriana raised the question if we can reduce the thickness of the 3rd skimmer to

improve resolution? Gerhard replied yes, already with the smaller 3rd skimmer which is available with 4x0.4 mm. We have not found the limits yet.

6. COCKCROFT PLANS FOR THE NEXT 12 MONTHS

H. Zhang

The e-gun could be up-graded from 100 to 300 uA for a few 100 pounds. It was agreed to do that.

Page 7 of 13

The first tests are foreseen with the new gun with large skimmer, than put in a

0.83 mm skimmer to improve resolution.

Serban has brought an amplifier for the Faraday Cup, which should improve precision for the time of his stay at Cockcroft.

Concerning the skimmer production, the question was raised if one could use just a flat plate instead of a complex shaped- skimmer. No argument against the slit

instead of the skimmer was found in the meeting. Could this be studied in more detail by Marton?

The first tests with the 2nd BGC system are foreseen in November 2017. A new PhD can help with the assembly of the new system.

The Fresnel zone plate was studied by Bergen University. They looked at the optical aspects of the plate. The zone plates were produced by a company in

Switzerland.

The proposed timeline for 2017 is:

Until 30 June: repeat the BIF experiment (this was skipped following the

time constraints) and install the new optics system.

1 July-20 July: install the new E-gun and have BIF image with large

skimmer (lower resolution), need to reduce the integration time a lot.

21 July-25 July: another density measurement using large skimmer

26 July -10 August: change to small 3rd skimmer and repeat the BIF

experiment for higher resolution image and get a estimation of

integration time.

11 August- 30 August: Cross section study using both Nitrogen and

Neon.

1 Sep- 10 Sep: Build the frames, support and rails for new setup

11 Sep- 20 Sep: Assemble the vacuum chamber

21 Sep- 30 Sep: Assemble pumps and sensors

1 Oct -20 Oct: Assemble nozzles and skimmers and alignment system

21 Oct – 30 Oct: Install the optics system and E-gun

1 Nov – 10 Nov: Pumping and baking

November: BIF experiment with new setup

December 2017-January 2018: Old setup, Jet density study, External

field benchmark experiment, Zone plate test on old setup.

January-March 2018: Test new nozzles and skimmers, have Jet density

scan and benchmark simulation if available.

March-May 2018: Give best nozzle size and skimmer size from

experimental operation and simulation comparison for HLLHC

parameter.

May to June 2018: Begin to design 3rd gas jet monitor

Page 8 of 13

7. GAS JET SIMULATIONS

M. Ady

Marton explains Monte-Carlo method. The difference between pressure,

impingement rate and density is shown.

A MolFlow simulation with 7000 polygons was used.

It is not very sensitive to flow direction on the first skimmer. The geometry was divided into 3 structures to speed-up the simulation with a ‘link’ between the structures. Only 65% were sent through first time. A model was made for the

transport to represent ratio of direct to scattered passage.

A first comparison was made with the experimental set-up. A problem is seen on

skimmer 3, but otherwise the predictions are good. It is suspected that skimmer 3 is leaking into the interaction chamber.

The difference between background and gas-jet-on was studied. This seems to

validate the general model.

The new BGC was analysed.

It is estimated from mean free path that flow is still not molecular at the exit of first skimmer. Various simulations for gas divergence coming out of first skimmer were made.

The 2nd and 3rd skimmer volumes should be separated with a cone that is ~99% leak tight to remove the gas scattered on the 2nd skimmer. Else, the background

density versus the gas jet density is too similar in the interaction chamber.

Ray questions how critical is the input from the high pressure end is. Marton replied that it makes a significant difference. Perhaps one could use the pressure

ratios in the different regions. However, velocity is not a direct issue.

Next work will be simulation for a 3rd skimmer with just a plate rather than a cone.

Marton asked where exactly the start of the molecular flow is and how collimated it is.

Discussion: Is a new calculation on the high-pressure side needed?

8. FLUORESCENCE PROFILE MONITOR CAMERA SYSTEM

S. Udrea

Serban explained the different wavelength of fluorescence depending on the gas

type and the live times. Candidate gases are neon and nitrogen.

Typically, the emission of N2 is strong but results from ionized molecules and the

life time of the upper level is approx. 60 ns.

Ne has a broader emission spectrum with strong lines at relatively long wavelengths, which is not in a good range for the photocathode of the image intensifier, but these lines are emitted by neutrals and the life time of the upper

level is about 20 ns. It is not clear if some of the fluorescence lines observed in the UV are due to impurities or Neon ions. Data is limited, especially for Ne.

Page 9 of 13

One of the (smaller) N2 peaks can only be seen with electrons. However, the cross

section would be far too small at 10 keV. We still need to extrapolate over more than one order.

Ne cross sections are approx. 2 orders of magnitude lower than N2.

The cross sections for protons at LHC energies can only be estimated by extrapolation over several orders of magnitude and, in case of Ne, by means of

the principle of equal velocities.

At LHC conditions and Ne as working gas, the photon rate is only ~1.9

photons/sec.

The optics requirements and equipment were shown.

The peak (~3/4 of the max with the gas jet) for BIF signal and also with gas jet

off, is due to the high background in the interaction chamber. This would be much lower for the LHC case.

Presently the total depth of field (DOF) is up to 4.5 mm with reasonable blur. This could be relaxed if an appropriate set-up geometry is used, e.g. a camera looking perpendicular to the curtain plane or the application of the Scheimpflug principle.

Serban proposed an alternative option to the 3D movable gauge to measure the

density of the gas jet. The main idea is to use a so called open MCP (OMCP), i.e. one without photocathode, since we do not want to have a conversion to photons at the input

surface.

In front of the MCP and mechanically connected to it a set of 2-3 grids should be placed to ensure for acceleration of positive ions and retention of secondary electrons. For the duration of the measurement this assembly should be positioned

such that the gas jet atoms or molecules impinge on the open surface of the OMCP and generate secondary electrons which get multiplied in the channels and after

colliding against a phosphor screen give rise to a light signal. This signal can be acquired with a usual camera through a vacuum window.

The gas jet atoms/molecules have a velocity which is less than 1 km/s and as such a kinetic energy which is far too low to allow for generation of secondary electrons.

Thus, the idea is to partly ionize the gas jet and accelerate the ions in the space between the first grid and the OMCP surface. The ionisation can be achieved either with an electron beam with a large enough transversal size or by quickly scanning

with a thin one through the gas jet.

Advantages of this method: - fast and direct measurement of the transversal profile of the gas jet, no need to

scan

- by using a well characterized electron beam with an energy for which the ionization cross-section is known and a calibrated OMCP one can also determine

the flux of gas jet atoms; if the molecular velocity is also known, the density can be calculated

Page 10 of 13

- expected to have a relatively small size Disadvantages:

- there are many open questions and development work is needed

- not cheap

Concerning the circumstances, it was agreed for this week’s experimental

programme that the cross-check with the initial parameters will get dropped and go ahead with the new optics from Serban at Cockcroft.

Stefano raised the question why an iris is put before the lens, which reduces light by factor of 4. Serban replied that this is done to reduce geometric aberrations and increase DOF. The question is whether this is good to do or not. Serban will

test this idea.

Stefano questioned the choice of the photocathode, since there are lots of different

types with different wavelength sensitivities. Serban replied that these have much higher dark counts.

9. DESIGN OF THE BEAM GAS CURTAIN NO. 2

E. Barrios-Diaz

Elena presented the various sections of the BGC No. 2. The 3D models exists, the flange to flange length of the interaction chamber is 337 mm. The remaining work is the support for the optical system and the details of the integration of a cone

between the 2nd and the 3rd skimmer.

The 2nd BGC is a simplified version of the first prototype, with easy access to the

skimmers and a modified aligned concept.

10. PRODUCTION AND COMMISSIONING OF THE 2ND BGC

H. Zhang

Hao presented in detail the cost of the various parts of the 2nd beam gas curtain.

The grand total cost is estimated to be about 110 kGBP. The biggest part are the vacuum pumps with 45 kGBP, followed by the vacuum chambers.

The installation schedule was shown:

1 Sep- 10 Sep: Build the frames, support and rails for new setup

11 Sep- 20 Sep: Assemble the vacuum chamber

21 Sep- 30 Sep: Assemble pumps and sensors

1 Oct -20 Oct: Assemble nozzles, skimmers and alignment system.

21 Oct – 30 Oct: Install the optics system and E-gun

Page 11 of 13

1 Nov – 10 Nov: Pumping and baking

November: BIF experiment with new setup

Discussion at the end of the presentation: The type of gauges, namely penning

versus Bayard-Alpert should be thought about. The Penning gauge has the advantage of not emitting light, while the Bayard-Alpert gauge is less sensitive to

a magnetic field. Both criteria should be checked.

11. MANUFACTURING AND ALIGNMENT OF THE SKIMMER AND

NOZZLE ASSEMBLY

T. Dodington

Tom presented the requirements of the alignment of the nozzle to the first and second skimmer. Both the nozzle holder and the second skimmer will be aligned

relative to the first skimmer. An optical system in a laboratory environment is considered for the alignment. Diffraction in the nozzle might be an issue.

The initial idea is to have all parts pinned together. It is not clear if the required concentricity tolerance of below 50 um can be achieved. The backup solution is to remove the pins and align the 3 parts using a micrometrical table.

Until now, no company was found capable to drill a cylindrical hole of 30 um in a wall thickness of 300 um. The proposed company would make a 2 degree angle.

Alternative companies are welcome.

There remains some work to do, and the alignment can be tried out when the parts arrive.

There is an option with the BI optical lab using a laser.

12. INSTALLATION OF AN INSTRUMENT ON A CERN MACHINE

R. Veness

The collaboration includes the design and production of a BGC by Cockcroft

Institute for installation in the LHC. Design issues for LHC must be considered: RF, vacuum, integration, tunnel access and impedance for both electrons and protons.

The proposed strategy is to go in phases.

The installation is foreseen in LS2 with a delivery in June 2019 to CERN.

13. DISCUSSION

13.1 WRAP-UP

The pros and cons of Ne versus N2 were discussed. From a gas jet point of view,

the difference should be minor since the molecular weight is similar (28 for N2 versus 20 for Ne). Stefano commented that from an optics point of view, there

Page 12 of 13

should also not be a big difference between the 2 types of gasses. This should be

tested at Cockcroft in order to be sure.

Serban will contribute to the knowledge transfer of the optical system to CERN.

The sensitivity to operate in a stray magnetic field should be checked.

13.2 QUESTIONS

Ray asked what are the requirements concerning the precision and resolution for the BGC function as an overlap monitor. Could a specification be made to answer

this question?

Ray asked if a BGC monitor was needed before and after the solenoid magnet.

Adriana answered that this would be good, but need to consider the effect on the electron beam. To be discussed.

Could we use similar monitors in other areas of the machine, e.g. replace the BGI

in the LHC and ELENA?

13.3 ACTIONS

Can we look at 3rd skimmers with a lower width to improve the resolution and

reduce the gas load? Action: Cockcroft to check the fluorescence measurements with the 4x0.4 mm 3rd skimmer. Make a simulation for a flat plate rather than a cone. Can we increase the gas jet angle and reduce the expansion section length?

(Action: Marton)

After the measurement and the simulation, decide if a thinner slit or thinner 3rd

cone should be produced.

Look for a more powerful electron gun (Action: Adriana)

Buy an upgrade to the existing gun (Action: Hao)

Need to agree the priorities for the new student that starts in October also taking into account the other people in the team. (Action: Carsten will make a proposal)

Serban consider the modifications to the optics and possible other photocathodes from Stefano (Action: Serban)

Experimental programme for the new BGC (Action: Hao, Edward)

Updating the costings and cost estimates. (Action: Hao)

Costing for an installation in the LHC for a design review. (Action: To be decided)

Simulation for the illumination by a LED of the alignment target. (Action: Serban)

Assure availability of a retractable gauge and retractable mirror on new system.

(Action: Hao, Edward)

Selection of the gauges. (Action: Hao, Gerhard)

Space reservation for the BGC demonstrator, to be installed in LS2. (Action:

Adriana)

Change vacuum sectorisation (Action: Gerhard and Adriana with VSC)

DIR, DIC, DIF requests (2 racks, cables for vacuum pumps, gauges, camera system) (Action: Gerhard)

Page 13 of 13

Integrate present BGC system in tunnel environment in order to see boundaries

(Action: Tom) Prepare for CERN internal committees (TTC maybe September/October 2017,

LMC?)(Action: Ray)

Write ECR (after TCC, before LMC) (Action: Gerhard)

Pump design: Cryopump at the dump? Which capacity is needed? (Action: Adriana, Gerhard)

Check radiation hardness requirements (Action: To be discussed)

13.4 RESEARCH TOPICS

Cross-sections for fluorescence, possibly get data from the BGV or from LHCb?

Need someone to do the CFD analysis.

Beam-gas interactions and dynamics.

Next meeting late October 2017