PH I LI PS TE C H N I C'A L. REVIEWVOLUME 27, 1966, No. 12

Masers for a radio astronomy interferometerF. W. Smith, P. L. Booth- and E. L. Hentley

621.375.9:522.617.3

The maser, with its very low equivalent noise temperature, has found a number of appli-cations, particularly in radio astronomy. An interesting feature of the maser systemdescribed here is the superconducting magnet; the current in this ,magnet is adjusted bya superconducting dynamo following the principle described in thisjournal some time ago.

Introduetion

In applications where a receiving aerial is directedtoward cold space, so that the background noise islow, full use may be made of the low-noise propertiesof solid-state maser amplifiers. One such application isthe detection and measurement of the discrete sourcesof radio noise which are present throughout the uni-verse. In recent years the study of these sources has ledto the discovery of a previously unknown type ofstellar body, the "quasi-stellar object" or "quasar",and the investigation of these bodies is proving ofgreat importance to the astronomer. Some of thestellar sources of radio noise have been positivelyidentified with visible objects such as distant galaxies.Such identification requires precise measurement ofthe position and the angular diameter of the radiosource.One method which is particularly applicable to this

. purpose is the use of a radio interferometer. The noisefrom the source is received in two aerials and the corre-lation is measured between the noise outputs of thetwo aerials [11. In principle correlation will be appreci-able. only if the difference in the pathlengths from thesource to the two aerials is around one wavelength orsmaller. A high precision is obtained by spacing theaerials many wavelengths apart: correlation will then bemeasured only if the source is in a very small angle a-round the normal to the base line. (The system may beaimed in a direction different from the normal by intro-ducing the appropriate signal delay in one of the bran-

F. W. Smith, B.Sc., P. L. Booth, B.Sc., and E. L. Hentley,A.M.I.E.R.E., are with Mullard Research Laboratories, Redhill,Surrey, England.

ches.) The diameter of the source is obtained by mea-suring the correlation as a functionof the distance be-tween the aerials [21. '.

To be able to measure weak sources a low noise tem-perature of the receiver is required, and it is desir-able to use a maser, which can have an equivalent noisetemperature of only a few "K, as the first stage. Thesensitivity of the system may be expressed in terms ofa minimum detectable source temperature LIT givenby:

LIT = Ts/ V2BT , (1)

where TB is the receiver system noisetemperature (main-ly determined by the first stage), E the bandwidth ofthe receiver and T the post-detector integration time.At given values ofTs and E, long integration times areoften required to obtain the desired sensitivity ..To measure accurately the correlation which may

exist between the two signals received, any distortion ofthe signals during amplification must be avoided; or,more precisely, the two signals should not be distorteddifferently. It is therefore a requirement that the masergain and phase characteristics match, and, in view ofthe possibility of long integration times, that they arehighly stable.The radio interferometer at Defford, near Malvern,

England, has two 25 ill diameter parabolic aerials

[1) R. R. E. Journal, No. 50, Oct. 1963.[2) This principle and its application to "optical" stars have been

treated in: R. Hanbury Brown and A. Browne, The stellarinterferometer at Narrabri, Australia, Philips tech. Rev. 27,14H59, 1966 (No. 6).

314 PHILlPS TECHNICAL REVIEW VOLUME 27

(fig.l). One of these can be moved along a substantiallyEast-West railway track, the other along a track at6T to the first. This arrangement allows the length andorientation of the base line to be varied.

In this article a short description is given of themasers that have been built for this system. These aretravelling wave masers, operating at 3.025 Gels (ap-proximately 10 cm). In many respects they resemble thetravelling wave maser developed in this laboratory foruse in satellite communications, which has been de-scribed extensively in a previous article [3] in this jour-

microwave losses of the input and thereby the noisetemperature.

As the interferometer system requires a considerabledistance between tbe aerials to obtain a high angularresolution, it is desirable to have remote tuning of thecentre frequency of operation of the masers, to enabletheir gain-frequency characteristics to be made identi-cal. This is achieved in the present system by the use ineach maser of a superconducting dynamo controlling themagnetic field of the superconducting magnet, whichin turn determines the centre frequency.

Fig. I. The radio interferometer at Defford, near Malvern, England. Noise from a radiosource is received at each of the two parabolic aerials (diameter 25 m). The noise signal is am-plified in each channel and the receiver noise can be strongly reduced by using a maser as thefirst amplifier stage. Finally the correlation between the signals is measured. To improve thenoise performance and to facilitate the fitting of the masers the aerials are being modified to aCassegrain system. The aerials can be moved along the railway tracks in two different di-rections to vary the base line in length and orientation.

nal. Two possibilities for improved performance whichwere discussed in the earlier article have been realizedin the present masers. The first is the use of persistentcurrent superconducting electromagnets which ensure avery stable magnetic field at the maser crystal, and thusa high stability of phase and gain. The second is theuse in the helium dewar vessel of waveguide for theinput lead instead of a coaxial line: this reduces the

The travelling wave maser

For an extensive description of the maser principleand of considerations for the design of a travelling wavemaser the reader is referred to the article [3] mentioned

[3] J. C. Walling and F. W. Smith, Solid state masers and theiruse in satellite cornrnunication systems, Philips tech. Rev.25,289-310, 1963/64.

1,966,No. 12 MASERS FOR RADIO ASTRONOMY

above, We will confine ourselves to a short descriptionof the present masers.Let us recapitulate in a few words the main elements

of the travelling wave maser. The microwave signalto be amplified travels from input to output along themaser crystal, a paramagnetic crystal, which deliverspower to the wave. The crystal is mounted in a slowwave structure to intensify the interaction of the crystalwith the wave. The maser crystal is activated by amicrowave pump signal usually at an appreciably higherfrequency than the signal to be amplified. The crystalis placed in a magnetic field tuning it to the frequenciesof signal and pump. Finally a low temperature is essen-

the group velocity is approximately I/80'times the freespace velocity of light. One side of the structure isloaded with maser material. The other side containsyttrium iron garnet (YIG) discs held in place by adielectric material (with a dielectric constant of 9,whichapproximately matches that of the maser material).By this arrangement the device is made non-reciprocal:under operating conditions forward waves interact.substantially only with the maser material and areamplified whereas backward waves interact only withthe YIG discs and are attenuated (cf. ref. [31). The combstructure is attached directly at one end to the wave-guide transmitting the pump power .

.....m-I II I

w

y-.~',,'

Fig. 2. Cut-away view and cross-section of the 3.025 Gel» travelling wave maser. The dimen-sions are in mm. Electromagnetic waves are propagated slowly (group velocity 1180 timesthe free space velocity of light) along the comb structure C; forward waves are amplified bythe maser crystal R (ruby), backward waves are attenuated by the YIG discs Y held in placeby the dielectric slab A. The maser crystal is tuned by a magnetic field H and activated by apump signal (26.4 Gc/s) transmitted by the waveguide W. The orientation of the optic axis cis indicated.

tial to ensure appreciable gain and low noise: themaser is usually operated in a bath of liquid helium.In a travelling wave maser, the "electronic gain" G

(the gain produced by the maser material not countingstructure losses) is given (in dB) by:

fLF 1G=27.3--,

. Vg Qmf being the frequency, L the length of the slow wavestructure, Vg the group velocity, F the fraction of thetotal magnetic energy stored in the maser material, andQm the magnetic quality factor of the maser material.The slow wave structure of the present masers is of

the comb type. The arrangement and the dimensionsare shown infig. 2. The length ofthe structure is 12 cm,

The maser material is a ruby single crystal (0.04%chromium by weight), about 12 cm long, with the op-tic axis at 60° to the longitudinal direction (i.e. the di-rection of growth). In the structure the ruby is mountedwith the optic axis perpendicular to the pins of the comb;these are parallel to the static magnetic field.

Fig. 3 shows the paramagnetic energy levels of theruby in a magnetic field perpendicular to the optic axis;the pumping scheme is indicated.Both at the input and at the output the comb struc-

ture is matched to a rigid low loss (helical membrane)coaxial line. Final adjustment of the passband of theslow wave structure is made by means of slight altera-tions to the dimensions of the dielectric loading.The intrinsic loss of the structure is 12 dB; when

(2)

315

316 PHILlPS TECHNICAL REVIEW

Fig. 3. Pararnagnetic energy levels of the maser crystal (ruby)shown against H when H is perpendicular to the optic axis. 25GclsThe pump transition 1-4 (26.4 Gc/s) and the signal transition1-2 (3.025 Gc/s) in a field of 2800 Oe are indicated.

operated at 1.5 "K in a uniform magnetic field the elec-tronie gain is 48 dB, giving a net gain of 36 dB. Underthese conditions the 3 dB bandwidth is 14 Mc/s, Thebandwidth may be increased at the expense of gain byusing a non-uniform (staggered) static field, so thatdifferent parts of the crystal experience different mag-netic fields and thus have different centre resonantfrequencies. A greater bandwidth thus obtained is ad-vantageous (cf. eq. 1) if the lower gain is still sufficientto render the noise contributions from later stages negli-gible. The overall noise temperature is not affected aslong as the pump levels remain saturated in all sectionsof the crystal and the maser noise temperature is itselfnot increased. The maser noise temperature is not af-fected provided all sections of the crystal contributesignificant net gain over the increased bandwidth rai

The superconducting magnet and the superconductingdynamo

The magnetic field which tunes the maser crystal to

°0~----_L----~2~---+~3~~--~4L_----~5~k~O~e-_H

20

15 -

-5

-10

-15

-20

VOLUME 27

4

3

2

the signal and pump frequen-cies is provided by a super-conducting magnet, builtclosely around the slow wavestructure, the magnet andthe slow wave structure beingimmersed in the helium bathtogether as one unit (jig. 4).

The magnet [4J consists ofa yoke of mild steel with twosuperconducting coils; across section IS shown III

jig. 5. The gap volume is3.5 X 3.0 x 22 cm. A slab oflead-bismuth alloy(50Pb50Bi)is placed on either side of thepole pieces and the coils toact as a superconductingdiamagnetic screen and re-duce the leakage from thegap [5]. Fig. 5a and b showthe effect of these shields. Theuniformity as measured overthe ruby crystal volume isbetter than 1 part in 103

(cf. jig. 6).The coils are made of cop-

per-covered niobium-zirco-nium wire (75Nb25Zr) 0.25

Fig. 4. The unit containing the slow wave structure and the magnet. The magnet J can be ro-tated over a few degrees by the gears which can be seen at the top of the maser on theright. 2 lead from external magnet current supply. 3 input waveguide. 4 output coaxial line.

1966, No. 12 MASERS FOR RADIO ASTRONOMY 317

Fig. 6. The distribution of the magnetic field in the magnet gap. In the dark grey area thefield homogeneity is better than I : 104, in the light grey area better than : lOa On the lefta cross-section of the magnet (as in fig. 50) is shown. R is the ruby crystal.

5

Q

R

Q

mm in diameter, and nylon-insulated. Each coil contains600 turns. They are wound on coil formers of a tita-nium alloy with the same thermal expansion coef-ficient as the wire.

The coils make up a superconducting closed circuittogether with a superconducting dynamo which is usedto obtain a fine variation of the magnet current. This dy-namo is described below. In principle it could be usedto build up the whole magnetic field; this, however, isnot a practical procedure in this case as it wouldtake some 9 hours. The arrangement by which thecurrent in the superconducting magnet is establishedinitially is shown in .fig. 7. An external power supplyis connected to the coils, in parallel with the supercon-ducting dynamo. The superconduction in the dynamobranch is disturbed by a thermal switch, and the currentdelivered by the power supply is taken completely bythe magnet coils. After the current has been set to thecorrect value the thermal switch is de-energized; super-conduction is restored in the dynamo branch and a per-

[4] E. L. Hentley, Mullard Research Laboratories Report, Sept.1964.

[5] P. P. Cioffi, J. appl. Phys. 33, 875, 1962.[6] J. Volger and P. S. Admiraal, Physics Letters 2, 257-259,

1962. See also J. Volger, Philips tech. Rev. 25, 16-19,1963/64, and J. van Suchtelen, J. Volger and D. van Houwe-lingen, Cryogenics 5,256-266, 1965 (No. 5).

Fig. 5. a) Cross-section of the superconducting magnet. Y yokeand P pole pieces of mild steel. C superconducting coils of nio-bium-zirconium wire. 5 superconducting shields of lead-bis-muth. The lines of magnetic field are shown dashed.b) To show the effect of the shields 5, the field pattern that wouldbe obtained without the shields is shown.

sistent current flows in the superconducting circuit.The external supply can then be disconnected.

The thermal switch consists of a coil of niobium-zir-conium wire interwound with a heater coilof constan-tan wire; a current of 50 mA in the latter will cause nor-mal conduction in the niobium-zirconium coil.

The superconducting dynamo, originally developedat Philips Research Laboratories, Eindhoven [6], is

E

30V lOA

-A

Fig. 7. Magnet current supply diagram. The persistent current inthe superconducting circuit (within the contour B) is establishedby: a) activating the "thermal switch" TTl (i.e. disturbing thesuperconduction in the coil Tl by a small current through Theating T and Tl); b) connecting the external supply E andsetting the current; c) de-energizing the thermal switch TTl. Assoon as Tl is superconducting again, a persistent current flowsin Band E can be disconnected. SM superconducting coils.M, D superconducting dynamo. The parts drawn above line Aare outside the cryostat.

318 PHILIPS TECHNICAL REVIEW VOLUME 27

11Mi)-

~N •I \ I: 1 I

I

D

\,s

Fig. 8. Diagram of the superconducting dynamo. The permanentmagnet M induces a normal, non-superconducting region N inthe superconducting disc D and some flux from M penetratesthrough N. When M rotates around the centre of D so that Npasses repeatedly through the terminals of the superconductingcircuit S, flux is pumped into S and the persistent current ischanged.

shown diagrammatically III jig.8. The dynamo (or"flux pump" as it is often called) consists of a super-conducting disc - connected to the superconductingcircuit at its centre and at its circumference - andsmall permanent magnets rotating close to the disc.These magnets induce normal, non-superconducting,regions in the disc and part of their flux penetratesthrough these regions. Each time a normal region passesthe "terminals" of the circuit, flux is pumped into thecircuit, causing a change in the persistent current. Thecurrent may be increased or decreased by rotating themagnets either clockwise or anticlockwise with respectto the disc. The flux pump is mounted at the base ofthe maser magnet (jig. 9). The disc is lead, 25 fLmthick,mounted on a Terylene support disc. Two "Ticorial"

permanent magnets provide a field of I 000 Oe at thedisc; they are mounted in an aluminium block whichcan be rotated at 500 r.p.m. by a motor on the cryostattop plate. This corresponds to a field variation of5 Oe/min which provides adequate controlover themaser centre frequency.

The maser crystal resonant freq uencies are dependentnot only on the magnitude of the magnetic field butalso on its direction with respect to the crystal axes.To enable the field to be orientated accurately duringoperation of the maser the magnet is mounted so thatit may be rotated a few degrees around the longitudi-nal direction of the crystal; it can be aligned by a geardrive adjusted from the cryostat head.

In a superconducting circuit such as the above the making ofsuperconducting joints requires great care. To join two niobium-zirconium wires, they are cleaned carefully, twisted togethertightly, wrapped in a piece of 0.08 mm copper sheet and mountedin a stainless steel clamp. Non-superconducting connections (theleads of the external supply) can be soft-soldered to the coppersheet. The connection between the lead disc of the flux pump andthe niobium-zirconium wire of the coils is made using niobiumsheet with a layer of niobium-tin as an intermediate conductor.The layer is obtained by heating the sheet in tin surrounded byan inert atmosphere. The niobium-zirconium wires are spot-welded to the niobium sheet; a lead-bismuth wire is solderedboth to the niobium-tin layer and to the lead disc.

The cryostat

The double dewar vessel of stainless steel is showninfig. 10, and the maser assembly, as it is mounted inthe dewar vessel, in fig. 11. Details are indicated in the

Fig. 9. The superconducting dynamo. I lead disc. 2lead-bismuth wires connecting the dynamoto the magnet coils. 3 shaft, 4 gear for rotating the "Ticonal " magnets in aluminium block 5.

1966, No. 12 MASERS FOR RADIO ASTRONOMY

captions, and the figures do not require much furthercomment. A few points, however, may be noted.

At a certain height a thick copper block is attachedto the neck of the helium vessel. This block is kept at

o

o

Fig. 10. Sectional drawing of the cryostat. Internal diameter11.5 cm, maximum diameter 28 cm, length 95 cm. Clliquid nitro-gen container. C2 liquid helium container. VI, V2 vacuum spaceslinked by the holes L in the copper block H2. The "tie-point"H2 is kept at 77 "K by the copper sheet HI reaching into theliquid nitrogen. S radiation shield. The top of S, connected to thenitrogen exhaust tubes Tl, is cooled by nitrogen vapour. T2 ni-trogen filling tube. The helium capacity is 17 litres, the nitrogencapacity 21 litres.

•

319

77 "K by a length of copper sheet reaching into theliquid nitrogen. At this level the maser assembly con-tains a copper radiation shield, with spring fingers forthermal contact to the inner dewar wall, keeping allparts at this level roughly at 77 "K.

As mentioned in the introduction, the signal input

Fig. 11. The maser assembly. 1 maser magnet surrounding theslow wave structure. 2 input waveguide. 3 output coaxial line.4 pump power waveguide. A horizontal radiation shield is keptat 77 OK by spring fingers 5 contacting the 77 "K tie-point of thecryostat (H2 in fig. 10). The copper baffles 6 obstruct the heliumvapour, leaving only a meandering path through irregular holes,thus providing a good heat exchange between the vapour and theassembly of leads. The lead 7 from the external magnet currentsupply which has to carry 6.8 A is of copper; it is kept at 77 "Kat the radiation shield 5. Platinum wire resistance thermometersare used as helium level indicators (8) [7J. 9 guide tube for fillingwith liquid helium. la gear box connecting to the motor fordriving the superconducting dynamo.

[7] E. L. Hentley, Mullard Research Laboratories TechnicalNote, Jan. 1966 .

320

lead is a waveguide, to re-duce the input loss, andthereby keep the noise tem-perature at a low value.It is made of stainless steelto prevent an unacceptableheat influx to the liquidhelium. The section fromthe 77 "K tie-point level up-wards is copper plated onthe inside. A waveguide-coaxial transition (standing-wave ratio 1.1) connectsthe waveguide to the inputcoaxial line of the maser.The output lead to the topof the cryostat is a thin-walled stainless steel co-axial line. The pump poweris fed to the structurethrough a thin-walled cop-per-nickel waveguide. Bothwaveguides and the coaxialline are vacuum sealed atthe top of the cryostat.

The cryostat is mountedin a cradle fixed to the

PHILIPS TECHNICAL REVIEW VOLUME27

Fig. 12. The top of the cryostat. Jinput waveguide. 2 coaxial output. 3 pump waveguide. 4motor for driving the superconducting dynamo. 5 magnetic field orientation adjustment. 6helium filling tube. 7 helium exhaust. 8 electrically heated "bunsen valve" preventing waterfrom entering the nitrogen dewar. 9 nitrogen safety valve. 10 nitrogen filling tube.

aerial. When charged withliquid nitrogen and heliumit can be operated at anglesup to ±45° from the vertical position. The extremepositions correspond to the aerial being directed to thezenith and to the horizon.

The top of the cryostat is shown infig.12 and the twocomplete masers infig. 13.

Performance

A few data concerning the masers are shown in thefollowing table:signal centre freq uencypump frequencypump powermagnetic fieldmagnetic field current

3.025 Gcl« (9.92 cm)26.4 Gcl« (1.15 cm)

120mW2800 Oe6.8 A

operating temperature 1.5 "KThe following performance figures for the two masers

have been obtained, using a staggered magnetic fieldacting on the maser crystal:

Maser A Maser B

Net gain3 dB bandwidthNoise temperature

26.2 dB24.5 Mc/s

4.5 ± 3.0 oK

28.8 dB23.0 Mc/s

6.0 ± 3.0 oK

The noise temperature has been obtained by a weilknown method [81. Matched loads at room tempera-ture and at 77 "K are connected to the input, and thedifference in noise output is recorded.

The gain stability of the maser is affected mainlyby variations in the temperature of the helium bathand by variations in the power and the frequency ofthe microwave pump. Gain changes of the maser willalso affect the phase. Another important cause ofchange of phase is the variation of the effective dielec-tric constant in the input waveguide and the outputcoaxialline as the helium level drops in these leads.

The phase stability of a single maser was measuredin a phase bridge. In a period of 19 hours a steadydrift in phase of 19° was observed, 11° of this driftcould be accounted for by the change in the heliumlevel. Some of the residual phase drift appears to bedue to a slight reduction of the helium bath tempera-ture as the amount of the liquid in the dewar decreases.This reduction in temperature contributes to the phasechange through the resulting increase in electronicgam.

The effect of variations in the pump is slight: a5 Mc/s change in pump frequency introduced a phase

•

1966, No. 12 MASERS FOR RADIO ASTRONOMY 321

Fig. 13. The two complete masers.

change of 1°, and a I dB reduction in pump powerresulted in a phase shift of 1.20

A second experiment was carried out with one maserin each arm of the phase bridge. The two cryostatswere filled with liquid helium to the same level and acommon vacuum pump was used to reduce the pres-sure above the liquid. The differential phase drift be-tween the two masers, which is the parameter of im-portance in the interferometer application, amountedto only 5° over a period of 18 hours.

In continuous operation one helium filling of themasers will last about 44 hours. A significant fractionof the evaporation of the helium is due to the micro-wave pump power. The duration of a helium filling cantherefore be increased by switching the pump powerinto a matched load when no measurements are beingtaken. In the radio interferometer application this is auseful procedure as there are long periods in whichthe maser is not being used.

A further increase of 30 % in operational time can beachieved by completely filling the cryostat with liquid

[8] A. L. McWhorter, J. W. Meyer and P. D. Strum, Noise tem-perature measurement on a solid state maser, Phys. Rev.IOS,1642-1644, 1957.

helium under reduced pressure. For this purpose spe-cial sealing valves have been constructed which makeit possible to connect the cryostat to the storage dewarwhilst maintaining a pressure of a few torr in bothvessels.

The work described in this article was carried outon behalf of the British Ministry of Defence, NavyDepartment.

Summary. A description of two travelling wave masers developedby Mullard Research Laboratories for the radio astronomy in-terferometer at Defford, England. This interferometer, which isused for determination of the position and angular diameter ofsources of radio noise, does so by measuring the correlation be-tween the signals received by two large parabolic aerials. In orderto obtain a very high receiver sensitivity an extremely low noisetemperature is required, and a maser is therefore used for thefirst stage of each receiver. The masers must be very stable andhave closely similar characteristics to enable the correlation be-tween the two received signals to be measured accurately. Highstability was obtained by making use ofa superconducting magnetto give the magnetic field which tunes the ruby maser crystal tothe signal and pump frequencies. The magnet and maser are ar-ranged in a single unit in a helium cryostat. The magnetic field ofeach of the masers is adjusted by a supercond ucting dynamo, andthis enables the centre frequencies of the two masers to be in-dividually set to the same value by remote control. The masersoperate at 3.025 Gels at an operating temperature of 1.5 "K, ina field of 2800 Oe. With a single maser, a phase shift of 19°was measured in a 19 hour run; the differential phase shift meas-ured between the two masers was only 5° in an 18 hour run.

322 PHILIPS TECHNICAL REVIEW VOLUME27

The laboratory of an X-ray equipment factory

The photograph shows part of the electrotechnical laboratoryof the C. H. F. Müller GmbH X-ray equipment factory in Ham-burg. Work is carried out here on the development of low-voltagecircuits for X-ray generators; the equipment meets the widelydiverse requirements of modern X-ray technology.

Each row of laboratory benches is provided with variablestabilized voltage sources, which deliver a voltage at a frequencyof 50 els or 60 els, as required, so that the appropriate voltage isalso available for investigating export equipment. Each benchhas a separate rack for holding various X-ray generator compo-nents, which are available as units of standard dimensions.

Equipment such as voltage sources, stabilizers, time switchesand counters can thus be set out tidily above the table. The unitsused are principally types in current production, or units derivedfrom these types. When this photograph was taken a new fast-starting drive was being developed for a tube with a rapidly ro-tating anode ("Super-Rotalix").

The flexibility achieved with this lay-out is indispensable inorder to keep pace with present-day progress in X-ray technology.Typical of this progress is the increasing employment of electro-nic components where, only a few years ago, almost exclusiveuse was still being made of conventional power components.