AN AUTOMATED SYSTEM FOR EXPERIMENTS
AT LOW AND VERY LOW TEMPERATURES
AN AUTOMATED SYSTEM
FOR EXPERIMENTS
AT LOW AND VERY LOW
TEMPERATURES \
Application to dynamic
specific heat measurements
PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus,
prof. drs. P.A. Schenck,
in het openbaar te verdedigen
ten overstaan van een commissie door het
College van Dekanen daartoe aangewezen,
op donderdag 22 september 1988
te 14.00 uur
door
Wim Adriaan Bosch,
geboren te 's-Gravenhage,
natuurkundig ingenieur.
1988
Dit proefschrift is goedgekeurd door
de promotor: prof. dr. H. Postma.
STELLINGEN
STELLINGEN
behorende bij het proefschrift van V.A. Bosch
1) Niinikoski schrijft verstoringen die optreden tijdens het uitlezen van veerstandsthermometers toe aan kosmische straling. Gezien de ervaringen opgedaan tijdens het hier beschreven onderzoek, zouden meet technische problemen ook een belangrijke oorzaak kunnen zijn. T.0.Niinikoski in: "Cosmic-Ray Disturbances in Thermometry and Refrigeration", Liquid and Solid Helium, Proceedings of EPS Topical Conference, Haifa 1974, Halsted Press (1975), edited by C.G.Kuper, S.G.Lipson and H.Revzen, p.145;
dit proefschrift, p.38.
2) De meetresultaten van Tang et al. van de soortelijke varmte van indium voor T<5mK in een veld van respectievelijk 30mT en 60mT lijken te onnauwkeurig om hun bepaling van het teken van de qua-drupoolkoppeling van de indiumkernen te rechtvaardigen. Y.-H. Tang, E.D.Adams, K.Uhlig, and D.N.Bittner, J. Low Temp. Phys. 60_, 351 (1985).
3) Bij de bepaling van de hyperfijnbijdrage tot de soortelijke warmte van Er door Hill et al. wordt verwezen naar de afleiding van de effectieve spin-hamiltoniaan door Sano et al.. Deze afleiding vordt echter door hen niet gegeven. Tevens wordt geen acht gesla-gen op de gemiddelde fout van 0.3X in de door Sano et al. gemeten resonantiefrequenties. Daarom kan de door Hill et al. aangegeven bovengrens van de onnauwkeurigheid van de hyperfijnbijdrage ook niet bij O . U liggen.
N.Sano, H.Teraoka, K.Shimizu and J.Itoh, J. Phys. Soc. Jap. 32, 571 (1972);
R.W.Hill, J.Cosier and D.A.Hukin, J. Phys. F 14, 1267 (1984). 4) De voorwaarden voor het kunnen toepassen van
singuliere-waarde-decompositie bij twee-dimensionale signaalanalyse zijn minder stringent dan door Kung et al. worden gesuggereerd.
S.Y.Kung, K.S.Arun and D.V.Bhaskar Rao, J. Opt. Soc. Am. 73_, 1799 (1983).
5) Greywall toonde recentelijk aan, dat de eerder binnen enige pro-centen nauwkeurig veronderstelde temperatuurschaal tussen 1 en 10 mK, ondersteund door tvee overgangstemperaturen van 3He, mogelijk tussen de 10% tot 25X afwijkt. De tijdspanne tussen eerdere publi-caties van Alvesalo et al. en Paulson et al. enerzijds en die van Greywall anderzijds toont aan welke experimentele problemen men in het betreffende temperatuurgebied moet overwinnen.
T.A.Alvesalo, T.Haavasajo, H.T.Manninen and A.T.Soinne, Phys. Rev. Lett. 44_, 1076 (1980);
D.N.Paulson, M.Krusius, J.C.Wheatley, R.S.Safrata, M.Kolac, T.Tethal, K.Svec and J.Matas, J. Low Temp. Phys. 34_, 63 (1979);
D.S.Greywall, Can. J. of Phys. 65, 1328 (1987).
6) Om zinloze speculaties te voorkomen moet bij gepresenteerde meet-data een toename van de soortelijke warmte bij afnemende tempera-tuur rond de laagst haalbare temperaturen van de gebruikte meet-opstelling met argwaan worden geïnterpreteerd.
B.C.Passenheim and D.C.McCollum, J. Chem. Phys. 51, 320 (1969); M.Schörmann and G.Adang, Nederlands Tijdschrift voor Natuurkunde B53, 111 (1987);
Fischer et al. in Proc. Trends in Physics EPS-7, Helsinki 1987, to be published;
H.C.Hamaker, H.B.MacKay, M.S.Torikachvili, L.D.Uoolf and M.B.Maple, J. Low Temp. Phys. 44_, 553 (1981);
dit proefschrift, Fig. 5.5, blz. 95.
7) De berekening van twee-electron-correlaties in relativistische atoomstruktuurberekeningen m.b.v. paar-functies is inferieur aan de multi-configuratie Dirac-Fock (MCDF) methodiek gebaseerd op het variatieprincipe, omdat bij deze laatse methode de z.g. Brown-Ravenhall "disease" vermeden wordt.
G.E.Brown, Physica Scripta 36_, 71 (1987); J.Sucher, Physica Scripta 36_, 271 (1987);
H.M.Quiney, I.P.Grant and S.Wilson, Physica Scripta 36_, 460 (1987).
8) — H e t is in het belang van de wetenschap zelf geen voorbarig of be-vooroordeeld standpunt in te nemen bij onderzoek dat uit het oog-punt van de gevestigde wetenschap twijfels opwekt of betiteld wordt b.v. als "pseudo-science".
G.C.Dijkhuis, Nederlands Tijdschrift voor Natuurkunde B51, 125 (1985);
C.H.Braams and J.J.Lodder, Nederlands Tijdschrift voor Natuurkunde B51, 151 (1985);
E.Davenas et al., Nature 333, 816 (1988);
J.Maddox, J.R.Randi and W.W.Stewart, Nature 334_, 287 (1988); J.Benveniste, Nature 334, 291 (1988).
9) Het aantal feilloos werkende electronische hoogte-indicatoren voor vloeibare stikstof is teleurstellend veel minder groot dan het aantal hierover gepubliceerde schakelingen.
10) In navolging met het buitenland lijkt een ontkoppeling van het Te-letekst aanbod voor de drie Nederlandse televisieprogramma's op zijn plaats.
11) De infraroodreflectie van optisch doorzichtig Hylar folie maakt interessante huishoudelijke toepassingen mogelijk.
12) Gezien het toenemend aantal vrouwelijke zendgemachtigden zouden ontvangers zodanig moeten worden ingericht dat bij het afstemmen op enkelzijbandgemoduleerde telefoniesignalen voor zowel mannen-als vrouwenstemmen dezelfde verstaanbaarheid wordt verkregen. 13) Het verdient aanbeveling om de bruikbaarheid te onderzoeken bij
zeer lage temperaturen van het kontaktloos meten van hoogfrequente electrische signalen via electro-optische laser-sample-technieken, vooral daar vaar anders golfpijpen in het kryogene systeem zouden moeten worden aangebracht.
G.A.Hourou, "Electro-optic sampling: device embodiments and possibilities", Characterization of Very High Speed Semiconductor Devices and Integrated Circuits, Ravi Jain, Editor, SPIE 795, 300 (1987).
page,line: 12, 19, 23, 23, 25, 26, 26, 26, 27, 29, 30, 30, 31, 33, 35, 37, 39, 42, 48, 48, 49, 50, 50, 59, 60, 72, 73, 73, 73, 77, 80, 82, 82, 85, 87, 90, 92, 94, 94. 101 3 bottom 24 bottom 34 10 23 bottom 4 23 14 bottom 6 22 4 bottom 27 18 3 8 3 23 23 2 5 30 14 24 bottom Table 4.1 Fig.4.8 108,20 Table 4.3 1 2 13 9 8 11 ,14 List of errata ' should rea oSMn(Fe) Table 3.5 B 0.5«T Table 3.5 RRR 5 S e c 2.3.2. X 2200s. ../20 0.28 (T/mK) JT, «3.8mK decrease (Sec. 3 ) . T 1.2s Sec 3.4 continuous cryostat and and the and its offer C43 C73 catagories Pb_Rh,_ ,Ru, ,07 2 (2—x) (x) Ru02 attemps 4K AVo 4Ta dT/Öt. . . / d e n . . date C193 xl03 brass boIds about 54mm2 330K/U at 0.066K. Kalsruhe 4Ö-4T in the ..=Q.RJ!(T) = .. C =CA1.T2+... s Itho id: S"Mn<Fs) Table 3.2 B=0.5AIT Table 3.2 RRR«5 Sec. 2.3.1. T«2200s. ../20S0.28 T/mK /JTfi3.8mK decreases (Chp. 3 ) . fsl.29 Sec. 3.4 continuous 1y cryostat, and and in the and of its offers C73 C43 categories Pb0Rh,0 ,Ru, , 07 2 (2—x; ( x) ( Ru02 attempts 2K JTfa dla 4T/Öt. ../dol.. data C183 xl02 brass bolts about 100mm2 330K/U. Karlsruhe ffl-AT in the memory ..=Ö.RJB(T)=.. P _ A1.A2 (Tl-r2)2 s (A2.tl+Al.T2)Ri<Al+A2) Itoh
Aan allen, die het tot stand komen van
dit proefschrift hebben mogelijk gemaakt.
CONTENTS CHAPTER 1
INTRODUCTION AND SURVEY 9
References chapter 1 13
CHAPTER 2
CRYOGENIC COOLING SYSTEM, MEASURING AND CONTROLLING EQUIPMENT
2.1. The cryogenic system 15 2.1.1. Experimental region 18
2.1.2. Magnetic shielding of the experimental region 19
2.1.3. Cooling power at various levels 24
2.1.4. Demagnetization stage 25 2.2. Measuring and controlling equipment 27
2.2.1. Instrumentation in the Faraday cage 30 2.2.2. Interconnections Faraday cage — cryostat . . 35
2.2.3. Interference reductions 37 2.2.4. Peripheral instrumentation 43 2.3. Automation of the experiments 45
2.3.1. Interfacing computer and experiments . . . . 46
References chapter 2 . . . 47
CHAPTER 3
THERMOMETRY 49
3.1. Resistance thermometry 49 3.1.1. Temperature dependence of the resistance . . 51
3.1.2. One—point calibration AB220 resistors . . . . 55 3.1.3. Heat—treating sliced Matsushita resistors . . 57 3.1.4. Resistance thermometry in magnetic fields . . 58 3.2. Cerium—magnesium—nitrate susceptibility thermometry 59
3.3. Nuclear orientation thermometry 60 3.4. Comparison NBS-CTS-1, EPT-76 and Delft thermometers 63
References chapter 3 68
CHAPTER 4
THERMAL CONDUCTIVITY EXPERIMENTS 71 4.1. Macor machinable glass ceramic 71
4.2. Staybrite stainless steel 74 4.3. Superconducting heat switch, Cu/A,6/Cu welded joints 76
4.4. Using the Uliedemann—Franz—Lorentz relation 85
4.5. Titanium hydride 86 References chapter 4 87
CHAPTER 5
EXPERIMENTS CONCERNING SOME MAGNETIC SUPERCONDUCTORS
5.1. Introduction 89 5.2. Experimental methods 89
5.2.1. Electrical resistance and susceptibility . . 89
5.2.2. Heat capacity 89 5.3. Tm. .Y,. xRh.B. (x=0.7,x=l) 96 (x) (1—x; 4 4 5.4. ErRh^B^ 97 5.5. (RE)Rh, ,Sn, , (RE=Sm,Tm) 98 (y) (z) 5.6. Er, .Y,. .Rh. .Sn- , ( x=0 .4, x=0 .8, x=l) 98 lx) (1 — x ) 1.1 3.6 5.7. Er 98 5.8. Conclusions 99 References chapter 5 100 PAPER 1
Behaviour of thick film resistors (Philips type RC-01) as
low temperature thermometers in magnetic fields up to 5 T 103
PAPER 2
The magnetic phase transition of the magnetic superconductor
TmRh4B4 109
PAPER 3
The specific heat of Er. .Y .. )Rh, , Sn~ , for x=0.4, 0.8
and 1 ! . Ill
SAMENVATTING 113 APPENDIX A
BLOCK DIAGRAMS OF THE DEVELOPED MEASURING INSTRUMENTS . . 117 APPENDIX B
LIST OF ABBREVIATIONS AND ACRONYMS 123
CHAPTER 1
INTRODUCTION AND SURVEY
For many aspects in solid state physics it is important to carry out fundamental research at low and very low temperatures. The removal of thermal vibrational energy can reveal phenomena related to weak interactions in condensed matter like magnetic ordening or superconductivity. Because of the difficult and rather time consuming character of most measurements at very low temperatures, an automated experimental set—up can be of great hel p.
This thesis gives a description of the construction and performance of an automated and, to a large extent, computer controlled cryogenic system for experiments at temperatures between a few millikelvins and about 4K. The developed system consists of three main parts, (A), (B) and (C) which are indicated in figure 1.1 at the following pages.
Part (A) is a 3He—circul ating dilution refrigerator system,
capable of cooling fairly large and complicated experiments in a
magnetically shielded region of about 2dm3 (Section 2.1.1) down
to about 15mK in a continuous mode and down to about 4mK after adiabatic demagnetization of a hyperfine enhanced nuclear magnet PrNic-. Introductional knowledge about the topics mentioned can be found in Refs. £1,2,33. The precooling of the cryogenic part of the system from room temperature down to T14.2K, as well as the refilling of cryogenic fluids to the pumping and cryostat baffles is performed automatically, enabling long-term unattended experiments, which require reproducible conditions and accurate determination of parameters. Part (B) is a Faraday cage containing the sensitive electronic measuring equipment connected to experiments inside the refrigerator (Sec. 2.2.). The influence of interference sources such as digital electronics or power supplies was examined and various measures were taken to minimize spurious power dissipation in the experiments (Sec. 2.2.3). Part (C) consists of two microcomputers, a minicomputer and other equipment for automatic control of the experiments, for data acquisition, storage and
The research was characterized by rather limited resources, which implied that most of the instrumentation (the dilution refrigerator and part of the measuring equipment) had to be designed, developed and constructed during this research in the
institute: a large amount of time was spent in trouble shooting, inevitable during the period of development and construction. Experimental skills and techniques were continually updated. The experimental results described in this thesis reflect the actual state of the technical developments at the time they were obtained.
After the initial building period first of all attention was paid to get acquainted with precision low temperature thermometry (Chapter 3 ) . Preparation techniques were developed and performances were compared of several kinds of resistance thermometers, based on Al 1 en—Bradley 220fl, on Speer 100Q, 220Q and 470Q, on Matsushita 68fJ and 82°. carbon-composition resistors, and Philips RTC—01 chip—resistors; the latter type was introduced and studied in detail (Paper 1) while developing
instrumentation for heat capacity experiments. Functional relations between the temperature and the resistances were determined in order to interpolate the calibration data and to
enable automated experiments involving thermometry and conversion of measuring data. A comparison between 6° C o ( C o ) and
*5M n ( F e ) y—radiation thermometry revealed some aspects of the
influence of the experimental circumstances upon the temperature reading of these primary thermometers. The EPT—76 temperature scale C4D is reproducible in the institute uith a germanium resistor thermometer calibrated at the Kamerlingh Onnes 1ou temperature Laboratory (KOL, The Netherlands) and secondly it is also incorporated in the transition temperatures of a SRM-767 device H5II (NBS, National Bureau of Standards, U S A . ) . The EPT—76 scale was extrapolated from its lowest temperature of 500mK downwards by measuring the susceptibility of cerium magnesium nitrate (CMN) C7D. This extrapolation was compared with the NBS—CTS—1 scale, which is available in the institute as the fixed temperatures of a SRM—768 device L63.
The anisotropy of }—radiation from oriented 6° C o nuclei in a hep
single^crystal Co needle C8D was used for obtaining supporting points in the temperature region below about 40mK. A second germanium resistor thermometer (calibrated between 50mK and 13K by Lake Shore Cryotronics, USA.) supported the calibration of secondary thermometers. The two lowest transition temperatures as stated by the NBS of the SRM—768 device were compared with the temperature reading of the e oC o ( C o ) thermometer.
In order to support the construction and development of the refrigerator system, several thermal conductivity experiments were performed, mainly at temperatures between about 50mK and 500mK (Ch. 4 ) . The thermal conductivity was determined for Macor machinable glass ceramic, for stainless steel (type Staybrite), for copper/aluminum/copper welded joints, made by high energy impact (explosive) welding, and for several gold or silver plated copper/copper press—contact joints. For metallic samples a comparison was made between the residual electrical resistance at 4.2K and the coefficient of the thermal conductivity at low temperatures. Also the thermal conductivity of a compressed TihU sample was measured between 30mK and 500mK. TiH_ might be useful as an alternative material for adiabatic demagnetization cooling C9D.
investigations in Delft on physical properties of rhodium— borides and rhodium—stannides, uhich are alloys with mixed or alternately magnetic and superconducting properties C103. Up till now only for some of these alloys data were available for Ti400mK. The a.c. magnetic susceptibility and electrical
resistance of TmRh.. qSn» „, SmRh, oSn» ~, Ef"n A^ri ^R^1 -Sn, ,,
and Tm, xY,, ,Rh.B« (x=0.?,x=l) samples were measured as a (x) (1—x) 4 4
function of temperature in some magnetic fields. The specific
heat of T m(x)Y(l-x)R h4B4 <x=0.7,x=l), ErRh^B^ and
Er, ■)Y(1_ %Rh1 <Sn_ . ( x=0 . 4 , x=0 .8 , x=l) and Er samples was
determined between about 25mK and IK by means of a thermal relaxation method C U D . This dynamic method enables small— sample spec ific—heat experiments at very low temperatures, which is of interest when only limited amounts of material are available. The dynamic specific heat experiments are a fair example of the possibilities of the automation of the measuring system.
References chapter 1
C13 O.V.Lounasmaa, J. Phys. E12, 668 (1979).
C23 K.Andres and E.Bucher, J. Low Temp. Phys. 9, 267 (1972). C3I1 H.R.Folle, M.Kubota, C.Buchal, R.M.Muller, and F.Pobell,
Z. Phys. B41, 223 (1981). C4D Metrologia 15, 65 (1979).
C5D J.F.Schooley, R.J.Soulen,Jr. , and G.A.Evans,Jr., NBS Special Publication 260-44 (1972).
C63 R.J.Soulen.Jr. and R.B.Dove, NBS Special Publication 260-62 (1979).
C73 R.P.Hudson, H.Marshak, R.J.Soul en,Jr., and D.B.Utton, J. Low. Temp. Phys. 20, 1 (1975).
C8D H.Marshak, J. Res. Natl. Bur. Stand. 88, 175 (1983).
C9D U.Heeringa, R.Aures, R.Maschuw and F.K.Schmidt, Cryogenics 25, 369 (1985).
C10I1 L.N.Bulaevskii, A.I.Buzdin, M.L.Kulic and S.V.Panjukov, Adv. in Phys. 34, 175 (1985).
C113 R.Bachmann, F.J.DiSalvo, Jr., T.H.Geballe, R.L.Greene, R.E.Howard, C.N.King, H.C.Kirsch, K.N.Lee, R.E.Schwall, H.-U.Thomas, and R.B.Zubeck, Rev. Sci. Instrum. 43, 205, (1972).
CHAPTER 2
CRYOGENIC COOLING SYSTEM, MEASURING AND CONTROLLING EQUIPMENT
2.1. The cryogenic system
In this section the cryogenic cooling system will be briefly reviewed. Basically the system consists of a cryostat (Fig. 2.1) and a gas-handling/circulation station. The cryostat contains
internal vacuum cans, a 3He-^c ircul at ing dilution refrigerator
and a demagnetization stage. The pumping lines lead to the gas handling/circulation station with rotary and diffusion pumps,
pressure and flow indicators and a 3He/4He^gas storage. The
cryogenic cooling system was designed, manufactured and tested in the institute.
The cryostat is suspended from a concrete block, located on the laboratory floor. An earlier installed gas—spring suspension of the block was not used in order to obtain a better vibrational isolation between pumping lines and cryostat and to secure the position of the cryostat with respect to a f^detector (see also Fig. 1.1 and Sec. 3.3).
A liquid nitrogen vessel is located in the upper part of the cryostat in the outer vacuum chamber (OVC). It cools an aluminium radiation shield in the lower part of the OVC, and by means of thermal links one of the horizontal radiation shields
in the upper section of the main bath with liquid 4H e . This
horizontal shield cools the various incoming tubes. The left-hand section of Fig. 2.1 shows the inner vacuum chamber (IVC) in more detail. The IVC is situated in the main bath at 4K. It contains the IK—pot, the dilution unit and demagnetization
stage. The IK-pot (volume ca. 1dm3) has two filling lines with
inlet valves, which can be controlled from the top of the cryostat. One of these lines has a flow impedance for automatic
refill, enabling continuous operation C1D. The 3He evaporation
chamber (still) is located beneath the IK-pot. The incoming 3He
gas finds its main f 1 ow—impedance for condensation below the
still. Next the 3He liquid flows through a continuous
heat-exchanger, consisting of a doubly—spiralized CuNi capillary inside a stainless steel tube, followed by three step heat—
IVC pumping line wiring feedthrough
3H e pumping line 1K pumping line top flange IVC SQUID IVC 1K pot supporting tube, 3x He condensor still, 0.7K 1K shield diluted fluid capillary heat exchanger 0.1K plate 3 discrete heat exchangers still radiation shield mixing chamber, T ^ i 4 m K sintered silver superconducting heat switch Cryoperm shield mix. chamber rad. shield experimental area 3 x 2 copper supporting rods thermometer 6 small compensation coils
main compensation coil
superconducting magnet: 5.7 T at 68A demagnetization stage: 75gr. PrNis 1K pumping line 3H e pumping line IVC pumping line
4H e evaporation line OVC (outer vacuum chamber) radiation shields, Sx thermal links OVC pumping line
liquid N2( m a x . 24I) radiation baffle, 3x 2 radiation traps Dquld4He (max. 50I)
needle valve 1K pot tiffing circuit (2x) inlet capillary filling capillary 1 K P 0 t 77K shield IVC > dilution unit
superconducting heat switch
> experimental area — m u - m e t a l shield
> demagnetization stage
INNER VACUUMCHAMBER (IVC) WITH REFRIGERATOR UNIT
CRYOSTAT
Fig.2.1» Cryostat with dilution refrigerator and demagnetization stage.
Fig. 2.2. Lower part of the cryogenic system and profile of the field at 68A of the main demagnetization magnet with compensation coils at the experimental region» note: s.s.=sta in 1 ess steel » Cu=copper, Al=aluminium.
mu-metal shield 8.8. OVC Al radiation shield s.s. inner cryostat can -Cryoperm shield s.s. IVC Cu 1K shield Cu thermal l i n k s -to mix. chamber Cu supporting rods -magnet experimental region ' NO experiment -connected to PLT 2 via . Cu thermal link Cu still shield Cu mix. chamber shield welded joint compensation coils -3 rods 25g PrNi5 -- Cu mix. chamber - heat switch — Cu thermal links PLT 1 & PLT 2(6x) - Cu frame 2T magnet thermal link — experiment in high field region
(mm) 600 5S0 500 4S0 350 200 150 100 50 0 -50 -100 < ( ' / S "«-/ \ / —— -20 0 20 —»B(10"4T) . -/ 0 1 2 3 4 5 6 I.68A — B(T)
exchangers with sintered silver powder to enlarge the heat exchanging surface. The copper mixing chamber is in thermal contact to the 3H e also via sintered silvei—powder. A copper
cage consisting of two plates and interconnecting bars is in thermal contact with the mixing chamber via an aluminium heat switch; it forms the experimental region (Sec. 2 . 1 . 1 ) . A concentric system of two metal shields with high permeability reduces the magnitude of external magnetic fields inside the region to a value less than IJAT (Sec. 2.1.2). The refrigerator is able to cool the experiments in the region continuously down to about 15mK (Sec. 2.1.3) with the experimental cage connected to a PrNic- stage (Sec. 2 . 1 . 4 ) . About 4mK can be reached by adiabatic demagnetization of the PrNi,- stage using a 5.5T superconducting magnet.
Various thermometers, heaters and a small 2T magnet for magnetizing samples are present in the IVC. This implies that numerous current and signal wires, some of them specially filtered or shielded (Sec. 2.2.3), run from the cryostat top plate to various points in the IVC. They are thermally anchored at various points to reduce heat input via thermal conduction. The pressures in the circulation and OVC vacuum system are guarded by Pirani controllers (type 101, Edwards, U K . ) . During automated and unattended runs the system automatically switches between the diffusion pumps and rotary pumps depending upon the pressure situation. All liquid nitrogen baffles of the circulation and OVC diffusion pumps and of the liquid 4H e
main-bath are automatically refilled.
2.1.1. Experimental region
Fig. 2.2 shows the lower part of the cryostat system in detail. The experimental region is situated in the IVC between the upper copper plate, PLT1, which is thermally and mechanically connected to the mixing chamber via the superconducting heat switch, and the lower plate, PLT2. The plates are interconnected with 6 copper bars via pressed contact joints. Both plates are equipped with threaded holes enabling thermal connections to various heaters, thermometers and experiments, which are described in the next chapters. Where possible, the copper
surfaces uere gold plated. Most of the signal leads from the top flange of the IVC to the devices in the experimental region are superconducting wires, 0.2mm in diameter with a cupro— nickel matrix! some of the wires are twisted in pairs and run inside cupro—nickel tubings for shielding.
Some experiments were carried out in magnetic fields, like the reported specific heat measurements on magnetic superconductors. A small split-pair magnet system was developed to produce at a
current of 5A a field of 2T, which is homogeneous to within 1°4
in a volume of about 30x30x30mm3. This magnet was suspended from
and also thermally connected to the mixing chamber instead of to the PLT1 or PLT2 plates, mainly to reduce parasitic heat flows after adiabatic demagnetization. The magnet housing is manufactured from a single piece of oxygen—free high-conductivity copper (OFHC). The wires of the split—pair coils (superconducting mul tif i 1 ament wire with copper matrix, <t)0.12mm) were fixed by impregnation with a varnish (type GE 7031, General Electric, USA.). They had to be electrically joined close to the magnet housing. This required a contact resistance
<10-1°fl, because at a current of 5A the ohmic heating had to be
much less than the cooling power of the mixing chamber at the lowest temperatures, which is about O.ljuU. The joint was realized by resistance spot—welding of bare superconducting' filaments inside a Nb covet—foil . The magnet current is stabilized to suppress interferences to the experiments (heat generation or electrical disturbances in the measuring wires). A resistive shunt of lmfl is placed over the magnet terminals at the 4K flange and a polycarbonate capacitor of ljuF leads high frequency currents to ground (see also Fig. 2.15).
2.1.2. Magnetic shielding of the experimental region
The experimental region contains two temperature—reference devices supplied by the National Bureau of Standards (NBS, USA.); see also Sec. 3.4 and Refs. C2,3D. These devices are sets of mutual inductances, containing several metals as standard reference materials (SRM's) with different superconducting transition temperatures (T ' s ) , to which specific values (T , )
c nbs have been assigned at the NBS (Table 3.5). In order to reproduce
these temperatures to within O.lmK a residual field of less than
about IJJT is required. A system of two concentric metal
shields with high permeability was chosen to reduce ambient magnetic fields, including low frequency components (e.g. 50Hz interference), see Figs. 2.1 and 2.2. A drawback of using these materials is, that after a demagnetization cooling, a magnetic
field of several tens of a til can be trapped and thermometers
sensitive to magnetic fields cannot be used. The inner shield must operate at TOOOK in order to make its diameter as smal 1 as possible to obtain sufficient field reduction. Cryoperm—10 (Vacuumschmelze GMBH, FRG., C4D) was chosen as shielding material, because, after machining and a heat treatment a
permeability JJ>105 can be obtained in a field B<5juT at 4K; to
reach this permeability an outer shield at room temperature must
be present to reduce e.g. the local magnetic field of 50JJT. It
should be noted that it is not sufficient to use only one mu— metal shield at 4.2K; its permeability is not higher than iron at these temperatures, resulting in a very low shielding factor. The dimensions of the vacuum cans of the cryostat determine the diameters of the concentric shields, so only the length of the shields can be optimised. The assumption that an external field B of 50MT parallel to the cryostat axis must be reduced to B.<0.1/JT, dictates a longitudinal shielding factor of order 500.
In Refs. C5,6,7D formulas can be found to calculate shielding factors of cylindrical shells. The transversal shielding factor S. of a infinite cylinder is given by!
St=BQ/Bi=Md/D (1.1)
with B : magnetic field outside the shield; B.t the field
o l
inside; y: permeability of the shield (JJ>>1)J d: thickness of
the shield and D: diameter of the cylinder (D>>d).
In the case of a mu-metal outer shield: S.292.3, with ji-25000 C5D, for d=lmm and D=271mm. The total longitudinal shielding S of an open cylinder can be written as!
S=(l/S6 +l/SB)-i (1.2)
with S„ = 4NS.,.+1 (1.3) 2 t
„ . c expC2.26(l/D)3 ,. ..
with i'. length of the cylinder; S.: longitudinal shielding of the cylinder jacket; N: demagnetization factor of an ellipsoid with dimensional ratio J6/D C6D (Fig. 2.3a depicts N versus JB/D) and S
.Bo' longitudinal s h i e l d i n g d u e t o t h e open e n d s of t h e c y l i n d e r . F i g . 2.3b g i v e s S . for S = 9 2 . 3 , F i g . 2.3c S . and F i g . 2.3d t h e total s h i e l d i n g S v e r s u s 2/Q. x 1 0 ^ 30 20 10 ' I ' I -\ - \ - \ \ v -, I -, I I I I I I
a>^
-^^^-^_^^ 125 100 03 75 50 25 n £/D £/D CO 4U 30 20 10n
■ i ' / - / i I i i ' i . ' . d ) . ^•v -I , I , 1 2 3 4 5 f/DFig. 2.3. Shielding parameters in the center of the mu-metal
cylinder as a function of its length to diameter ratio £/D} (a)
demagnetization factor N, (b) longitudinal shielding S.8 cylinder jacket, (c) longitudinal shielding Süo due to openings and (d) total longitudinal shielding factor S.
Due to constructional restrictions, the length of the mu-metal
value to obtain the maximum S factor) and B.S50/32.5=1.5yT. The inner Cryoperm—10 shield has a permeability J J > 1 05 in a field of 1.5JJT C4H; that is, S >650 is obtainable for D=154mm and d=lmm. Uith the material available a cylinder with jB=392mm was constructed, resulting in Sw58. A lower limit of the total shielding of the concentric system is the product of the individual shielding factors, resulting in Bi<<3xl0_2jjT.
It is important that the shields are properly degaussed after mounting. A 50Hz field of about 9mT is used, generated by a coil, which is slowly moved up and down along the cryostat, with both shields at room temperature. No noticable improvement was found, like higher T 's of the SRM devices or smaller
c
hysteresis loops at the transitions, when the inner Cryoperm—10 shield was degaussed at 4K instead (this latter procedure was suggested by Col well of the N B S ) .
Care was taken in selecting the materials near the SRM devices. E.g., stainless steel AISI 304 can cause 0.5mT at a distance of lcm after the material has been machined, because of ferromagnetic impurities. Stainless steel threads and rings were avoided when possible and brass ones were used instead. During electric welding of type AISI 304, normally used for the construction of cryogenic vacuum cans or supports, carbides are formed, which speed up the tendency for transformation from a paramagnetic austenitic phase to a ferromagnetic b.c.c. martensitic phase C8D, especially when the material is frequently cycled between room temperature and temperatures below 77K. A welded AISI 304 steel cylinder (<))10cm) appeared to trap a field of about 3JJT near the welding area; in its centre a value of 0.5juT was found. Therefore the more expensive stainless steel type 316 has been used for the IVC vacuum can, because in this type the formation of ferromagnetic impurities is limited. At room temperature some field measurements were performed with a flux-xiate magnetometer to get a rough indication of the quality of the shielding. The residual field B. inside the outer mu—metal cylinder was S3.5JAT, and S*10, S »40, all in fair agreement with the expected values. B . < 5 X 1 0 ~2M T , when the
Cryoperm—10 shield was put inside the outer cylinder. At 4.2K an even lower B. value is expected for this shield in its center.
However, the magnet system prevents the actual mounting position of the Cryoperm—10 shield to be symmetrical around point C, Fig. 2.2, which is the mounting level of the SRM devices. This results in a degradation of the local performance. No direct field measurements have been undertaken at low temperatures. Instead, the shift of the temperature at which the superconducting to normal transition of the beryl 1ium SRM occurs, was examined, when direct currents I . were superimposed
dc
on the primary excitation current of the SRM device. In this way residual field components parallel to the coil axis of the devices could be measured.
Fig.2.4. Rel magnetization during temper Tc = Tnbs arrows indica of the a.c additional di primary coil JJA, C : 6 0 JUA. ative change of of the Be SRM ature scans near = 23.35mK; the ite the direction
recorded traces; rect current in
a:0 juA, b:30
A maximum transition temperature is achieved, when I , «30jiA (see Fig. 2.4, trace b ) . Taking the primary coil constant B/I . =0.0158T/A C3H into account, a longitudinal field component
□c
B 0.5JJT must have been compensated in this way. The apparent
shift in T , JT « 2 1 J J K , corresponds to B = 0 . 1 5 M T , because |dT/dB | = c c
is smaller than 10~3. Thus the shift is negligible compared to the accuracy of the temperature as stated by the NBS for this transition point. The NBS advises to reduce ambient fields to values much smaller than ljiT, but estimates that their
calibrations were carried out in fields betueen about 1 to 2JJT
C3H.
2.1.3. Cooling power at various levels
Fig. 2.5 shows temperature T. measured at a copper plate mounted against the bottom of the stainless steel IK—pot (area
A=113xl0-4m2, thickness d=10~2m) versus electrical heater power
Q, simulating the load of the 3He condensor.
2.2r
D Fig. 2.5. Temperature
P |- n | «He fluid (Tf4) and
bottom plate (Tb) of the IK pot versus heater power Q.
40 50 Q(mW)
Vapour—pressure measurements showed that the temperature of the wall of the pot, half—way between top and bottom, corresponded
to the 4He-fluid temperature T,... The temperature difference
T, —Tn, and its dependence upon Q can be thought to be the
result of a thermal resistance R=(d/A)/(aT+bT2), with
experimental values a»l. 3xl0~2U/K2m and ba*l .9xlO"2UI/K3m. Suppose R=R +R. ,
ss k with R the resistance of the stainless steel bottom ss
and steel
R, the Kapitza boundary resistance between the stainless
and *He, and Rk<10~2/AT3 (K/U), C9H, then:
R/R, ->d*102*T2/(a+bT)U0 (T>1.2K) and thus R *R
k ss R is about 5 ss
times higher than expected from the results of Haasbroek C10D. During normal circulation and continuous refilling T.c/i»1.2K, which means CKlOmUj it is sufficiently low to guarantee proper
condensation of the 3He in the condensor attached to the IK pot.
locations in the dilution unit versus temperature. The curves differ, because of the different thermal resistances between the various points. A theoretical curve of the maximum obtainable
cooling power Q=84nT2 C U D at the nominal 3He-circul ation rate
of about 250 jumol/s, is included. The discrete heat exchangers operate at temperatures T>30mK, which is not favourable because of the relatively high thermal resistance between the sintered silver powder inside (0.07jum particle diameter, Vacuum
Metallurgical Co. Ltd., Japan) and the 3He fluid, see e.g. Refs.
£12,13,143. To lower the minimum mixing-chamber temperature and to reduce thermal instabilities i) a coal pump was mounted in the IVC to absorb residual exchange gas and ii) a mechanical supporting structure for the discrete heat exchangers was
introduced; finally stable temperatures were reached lower than about 15mK.
Fig. 2.6. Temperatures versus the heater power Q, with the experimental region and de magnetization stage mounted:
3He—fluid in mixing chamber
(Tf3), bottom plate mixing chamber (Tb), PLT1 <Tp); the dashed 1ine represents the
optimum cool ing power Q=84nT2
at a 3He-circulation rate n of 250 jjmol/s.
The stage for adiabatic demagnetization cool ing consists of 3 rods, 25g each, of the hyperfine enhanced nuclear magnet
PrNi,-(Ames Laboratory, Iowa, USA.), C15,16D. Electroforming of copper was used C17D to cover the irregular shaped, brittle and poorly thermally conducting C183 PrNic rods. The thermal conductivity of the electroformed copper appeared to be comparable to that of unannealed copper strips and sheets (RRR 5 ) . Copper wires, welded to PLT2, were connected to the copper layer, again by electroformingJ for each rod 6 wires, 01mm, were used.
200 20 10 -' -' 1 Tf3 i I I I I , I I 7 / ' - " V 8 4 A ' / h=250/umol/s i i I — i - . i _ • 10" 10° 10' QinW 10z 2.1.4. Demagnetization stage
The magnet system for adiabatic demagnetization uas made from NbTi superconducting multifi1ament wire (MCA, U S A . ) , wound onto a stainless steel former and afterwards vacuum impregnated. Fig. 2.2 shows the profile of the field components parallel to the magnet axis. The maximum field B is about 5.7T at 68AJ the longitudinal field in the experimental region without the magnetic shields is reduced to a value smaller than 50mT by a set of compensation coils.
The magnetic cooling process is controlled by a CBM3032 microcomputer (Commodore, U S A . ) , see Sec. 2.3.2. After magnetization to B=5.5T about 15 hours are required to cool the demagnetization stage to T*20mK. 20 E 10 5 "
- i\
B = 5 . 5 T \ I "\ I \ I v_ I I I I I I I B =0Ti
i i • i / / .' 50 100 150 t(min.) 200 250 Fig. 2.7. Temperatures of PLT2 indicated by a soCo(Cp) thermometer as a function of time during and after demagnetization cooling starting at t=0.Fig. 2.7 gives the temperature of PLT2 measured with a 6° C o ( C o )
nuclear orientation (NO) thermometer (see Fig. 2.2 and Sec. 3.3) after demagnetization to a final field B.p=0T, when the field is ramped down approximately exponentially in time with t 2200s.
About 100 minutes after the lowest measured temperature Tf«3.8mK
is reached, the temperature increases rapidly. Similar results
w e r e found using other t's or linear demagnetization.
Demagnetizations starting from 20mK and BS4.5T resulted in T.p>5mK. A constant Tf»6mK during some 4 hours was possible, when
B . was gradually ramped down in small steps. Inspection of the entropy versus temperature diagram of PrNi,- (measured by the Jülich group C163) in Fig. 2.8 shows that the demagnetization process is very irreversible.
B. =65xlO_3T C16D the internal field due to interactions in PrNic-, a final To»0.2mK is expected at B=0T. The discrepancy is
likely to be caused by: i) a maximum heat leak of about 50nU) to the PrNig stage, which also explains a 4T,-»3.8mK between PrNi,-and the thermometer; ii) an irreversible demagnetization process, caused by eddy current heating due to field components at the level of PLT2 and by instabilities in the field at B<1T due to wire movement inside the magnet, probably as a result of an incomplete impregnation. One should realize, that these demagnetization coolings were carried out with the full experimental cage in position.
Fig. 2.8. Entropy PrNi5
versus temperature (af ter C16D); for each curve the magnetic field strengh B is indicated.
0.1 1.0 10 20 50 100 Temperature (mK)
2.2. Measuring and controlling equipment
A major part of this research was concerned with the development of the measuring and controlling instruments. Though this was a rather time consuming task, it gave a good insight into the problems which arise when measuring physical quantities, especially at low temperatures. Furthermore, the latest developments in electronic components were implemented as much as possible; the designs and constructions were adapted to the special requirements of 1ow—temperature measurements.
Measuring leads introduce heat from the environment to the low-temperature experiments by thermal conduction and by dissipation of electro—magnetic energy in electrically conductive parts. The thermal resistances of materials and boundaries are relatively high at low temperatures; e.g. at
lOOmK a heat flow of IOJJU through a commercial copper wire of
(p2mm will give rise to a thermal gradient of 3mK/cm. The influence of thermal conduction can be limited by the proper
(O CO f~ h e a t - c a p a c i t y experiment ""N 2T coil & heat switch coil various heaters various CRTs 200mA c u r r e n t s o u r c e s T - c o n t r o l s t i l l j 1 I 1 250mA | 5A current sources HP3468A wer | 7 » plies I ; po supplies GRTs & precool sensors CRTs & TFRs TFRs heatsink resistance bridge resistance bridge VS3 X - t r e c o r d e r s DVMs manual sensor s e l e c t i o n TFRs sample plates heater sample plates heater heatsink CMN. SRM d e v i c e s RESEBO RESEBOI
h
"-» PCB V S * h) VS4y>
scsUjn
> DICAM TECO — » C M N - P S D | SQUID O 30 O </>2
o SQUID c o n t r o l * * s i g n a l s e l e c t i o n , t r a n s f o r m a t i o n , r » c o m p e n s a t i o n * SRM-PSDJ-
-1
ADA COBY PIA x - y recorder flux counte3
scope}
J 2 3D D O Ik O m analogue - » signals -2V 2VFig. 2.9. Block diagram instrumentation Faraday cage; <==>: digital control signals; < — > : analogue signal 5 the acronyms and abbreviations are explained in the text and in appendix B.
choice of materials (e.g. superconducting leads) and thermal
anchoring. Uith the very large background level of
electromagnetic fields due to external sources (radio/tv transmitters, electrical discharges, digital equipment, pumping systems, electrical power regulators etc.) several tens of
microwatts will penetrate the cryogenic system, unless proper
shielding, filtering and earthing techniques are applied. Partly this interference is indirectly introduced by means of electromagnetic radiation and partly directly via the mains of the electronics. Low—frequency magnetic fields due to magnets or mains transformers in the vicinity of the cryostat can also cause problems. The interferences may prevent the dilution refrigerator from reaching its lowest temperatures, or hinder resistance thermometry below about lOOmK. At these temperatures a power dissipation in the resistance thermometer
greater than 10~14U can take it significantly out of thermal
equilibrium. Such an excitation is e.g. easily exceeded by a digital multimeter operating in the vicinity of the measuring wires leading to the sensor. The sensitivity of the sensors decrease due to the saturation effects, the indicated resistance value is noisy and the reproducibi1ity of calibrations will depend e.g. on the arrangement of the measuring leads or equipment. A general description of electromagnetical interference problems and solutions can be found in Ref, C19D.
A solution to the problem of heat introduced by electrical leads and by electromagnetic interference would be the use of glass fibers, combined with GaAs low-power digital electronics, which can be used at low temperatures for analogue <""* digital conversions. However, complex GaAs IC's are not yet generally available. Of course at all times d.c. power leads to the cryo—electronic circuitry or magnet systems are necessary. For the present a sufficient solution was obtained by putting the measuring electronics in a Faraday cage (Sec. 2.2.1), connected to the cryostat via a shielding link (Sec. 2.2.2), and by taking
special precautions for interference reduction (Sec. 2.2.3). A computer located outside the Faraday cage controls the equipment
inside via a special optical interface (Sec. 2.2.1 and 2.3.1).
2.2.1. Instrumentation in the Faraday cage
Fig. 2.9 shows a diagram of the instruments in the Faraday cage and their interconnections as used for the computet—controlled heat capacity experiments (Sec. 5 . 2 ) . The instruments are hooked up to several items inside the IVC: the 2T magnet, the heat switch magnet^coi1s, the heaters and thermometers, which control or monitor the cooling of the refrigerator (GRT's, CRT's! germa nium and carbon resistor thermometers), the heaters and thermo meters of the heat capacity experiment (TFR's: thick—Film resistor thermometers) and the CMN, SRM thermometric devices
(Sec. 3 ) .
The next overview emphasizes the functioning of these instruments with some typical modifications or features necessary for low temperature research. The indication "TUD" signifies that the instrument concerned was developed in the institute during this research.
Conductance Bridge PCB (Biomagnetics Inc./S.H.E.. USA)
T w o — or fout—wire measurements of the conductance of resistance thermometers are performed with PCB. Interference problems were minimized by various modifications to the bridge, which enabled i) to locate its power supply externally, ii) to switch off its scanning LED-rdisplay and iii) to have an output voltage available, proportional to the measured conductivity: this voltage can be displayed e.g. by an external digital voltmeter
(DVM) with low power LC—display in a shielded box. PCB is used e.g. for the registration of the temperature decays of samples during the relaxation—bype heat capacity experiments.
Resistance bridge VS4 (Instruments for Technology. Finland)
The purpose of VS4 is a.c. 2—wire measurements of resistance thermometers. Like PCB, it has been facilitated with an external read—out. The internal time constant T and the system output noise are a function of the excitation voltage of the resistance measurement. When the VS4 is used in a temperature control —1oop, 3 0 0 M V is the optimum excitation mode, with "f 1.2s. Additional
loop stability is achieved with a low-pass filter (f (—3dB)= c
50Hz) added to the recorder voltage output.
Resistance bridge VS3 (Instruments for Technology, Finland) VS3 is used for a.c. 4-uire measurement of calibrated germanium resistor thermometers. The temperature reading error due to the instrumentational accuracy is described in Sec 3.4. Its d.c. recorder output cannot be used for external readings or temperature control due to spurious signals, instability and a settling time of about 10s.
Phase sensitive detector CMN-PSD (TUP)
Fig. Ala, Appendix A, gives the block diagram of a phase sensitive detector, CMN-PSD, for measuring mutual inductances. The instrument is used for CMN thermometry and for a.c. susceptibility and resistance measurements of samples of magnetic superconductors (Sec.5.2). A CMN thermometer contains a sample of cerium magnesium nitrate (CMN), located inside a mutual—inductance coil system. Basically the sample susceptibility x varies as x»C/T, uith C the Curie constant of CMN. Ulhen a low frequency current I of constant amplitude is
prim
passed through the primary coil, the voltage V over the sec
secondary coil varies proportionally to the inverse of the temperature. V is in principle 90° out of phase with I . .
sec prim It is often referred to as the "0° signal-component'. Also
another signal is present due to dissipation effects in phase with I : the "90°-T:omponent". Both signals show an extra
prim
shift compared to I due the impedances of other components prim
in the measuring network.
In principle, CMN-PSD is an analogue two—phase automatically compensating lock—in detector at 38Hz. The 0 ° — and 9 0 ° — components are compensated at the differential input amplifier. The d.c. output signals are proportional to the effective value of the compensated components. Automatic compensation takes place within 0.1s. A reference resistor (Fig. Alb) enables an accurate and reproducible phase adjustment.
Phase sensitive detector SRM-PSD (TUP)
Fig. A2a gives the block diagram of the detection circuitry for the superconducting «■-» normal transitions of the metals in the SRM devices. The principle of the instrument is a single—phase
400Hz lock-in detector with the ability to compensate dynamically the input signal for its major part. A small step on a large back-ground signal can easily be detected. Compared to standard units as described by the NBS C2,3,20D the signal-to-noise ratio of this instrument is about 20 times better, while it is considerably lower in price. The optional d.c. field compensator (Fig. A2b) superimposes a direct current upon the measuring current to compensate d.c. magnetic fields at the devices.
Computer interface PIA/ADA/COBY (TUP)
Fig. 2.10 shows the sét-up which enables automatic measurements with the dilution refrigerator (Sec. 2.3.1).
6 5 A MAGNET POWER SUPPLY i/0 TO VARIOUS I N S T R U M . ANALOGUE &■ DIGITAL SIGNALS HP 1 0 0 0 MINI COMPUTER CBM3032 MICRO COMPUTER , , ' V CBM BUS COMPUTER
0 0 D DO
INTERFACE 1 DECODERS & FAST OPTIC COUPLERS . i f~ ADA COBY CONVERTORS 18 BIT D/A 18 BIT D/A 12 BIT A/D 12 BIT A/D 16 BIT D/A FILTER UNITS 16 BIT A/DJ1
SPECIAL LOW-PASS FILTERo o o o <r $
i . SLOW OPTIC COUPLERS llIT
I I 0 I I 5
PIA CARDS COMPUTER INTERFACE 2 FARADAY CAGEFig. 2.10. Block diagram of the CBM3032 computer interfaced to
the equipment inside and outside the Faraday cage; <==>: digital control signals! < — > : analogue signal.
An optic coupler unit enables the tranfer of control and data words between a CBM3032 microcomputer (Commodore, USA.) and peripheral interface adaptor (PIA) cards (BEM-PIA-1A, Brutech Electronics, The Netherlands) located in a completely shielded box (computer interface 2, CI2) in the Faraday cage. The
(filtered) 1/0 ports of some PIA cards in CI2 communicate with a unit of A/D and D/A convertors (ADA unit), which controls or reads various analogue voltages of the measuring electronics, see Fig. 2.9. Other PIA cards provide, in combination with a set
of special 1ow-pass filters, the "control bytes" (COBY). These bytes control the settings of the DICAM, RESEBO, TECO and SCS units, which will be described next.
Digitally controlled acquisition module DICAM (TUP)
See Fig. A3; DICAM enables to measure, under control of the computer, several d.c. voltages (e.g. the output voltage of resistance bridges or the voltages developed over electrical heaters) with the one 16—bit A/D convertor available in ADA. The computer selects via COBY one out of 8 input signals and its amplification. Changing from one setting to another will take about 0.5s. The relays used for the selection circuitry are of the bistable type: only when a switching change is needed, a short low—energy setting pulse is required. This gives a low interference level, and sufficient thermal stability of the relay contacts to measure micro-volts.
Resistor selection box RESEBO (TUP)
RESEBO, see Fig. A4, enables to select manually or via COBY, one out of six resistance thermometers for 2—wire measurements. The unit makes use of shielded bistable reed relays in a compartment, shielded and filtered against digital switching noise (left part of Fig. A 4 ) . The low powei—dissipation of these relays enables a continuous operating battery power supply, free of (capacitive) loops to other equipment or earth. Adding the optional invertor enables the control of a second RESEBO via the same nibble.
Temperature controller TECO (TUP)
Fig. A5a gives an outline of the circuitry necessary for temperature control of a heat sink, which forms the base of most experiments described in this thesis. The central part is the temperature control unit (TECO). The difference between a resistance bridge output voltage (V, . , ) and a reference
b r i d g e
v o l t a g e (V ,.) i s a m p l i f i e d p r o p o r t i o n a l l y ( P ) ( m e a s u r a b l e a t ret"
t h e e r r o r m o n i t o r o u t p u t ) , i n t e g r a t e d ( I ) and d i f f e r e n t i a t e d ( 0 ) . The PIP components a r e added t o a b i a s v o l t a g e (V, . )}
bias the resulting voltage controls the heater current-source. The controller is optimally balanced if the mean error voltage is zero and if V, . determines the major part of the heater
bias
saturation effects. A change of V ~ results in an alteration of
ret
the heat sink temperature. The bias voltage unit (Fig. A5b) provides an adjustable fixed output voltage; the reference voltage (Fig. A5c) can be ramped up and down at various rates or can be held at a fixed value. Fig. A5d shows the PID-unit. At each temperature the P,I and D components have to be tuned for optimum thermal stability and step response. Generally the thermal stability obtainable is better than 0.1 of the display resolution of the controlling resistance bridge. During computer controlled experiments V c and V, . are generated via ADA.
ret bias Special current sources SCS (TUP)
Voltage controlled current sources (linearity error <0.01%) supply the electrical power to the heaters of the heat capacity experiment. The computer controls via COBY the range and output selections and via ADA the current magnitude.
Still temperature controller (TUP)
The principle of this temperature controller of the still is basically the same as TECO. It is especially of great use during the startup phase of the dilution refrigerator and it helps to achieve long term stabilization of the mixing chamber temperature.
Heater current sources (TUP)
A unit with various current sources supplies electrical power to various heaters in the IVC, e.g. at the still.
Magnet current sources/sweep units (TUP)
High power current sources with internal sweep drive were developed for the 2T magnet (5A) and heat—switch coils (250mA). Optic fiber transceivers (TUP)
A set of optic fiber transceivers provides transmission of analogue signals between the Faraday cage and the peripheral
instrumentation with full galvanic isolation. The optic transmitter converts an analogue input voltage V(t) to a block signal with a frequency proportional to V ( t ) , and transmits it
in the form of digital light pulses via an optic fiber to the optic receiver and frequency—to—voltage—converter. The latter reconstructs the original analogue voltage V ( t ) .
2.2.2. Interconnections Faraday cage — cryostat
Measurements were performed to obtain relevant parameters and ideas about the configuration of the earthing of the Faraday cage and cryostat and the shielding of their interconnecting signal leads. Table 2.1 gives the 50Hz pick—up differentially measured over a lOOkfi resistor in a metal box connected to a
lock-in amplifier in the Faraday cage while varying the
earthing connections A...D (see Fig. 2.11).
METAL BOX
ELECTRICAL INSULATION
FARADAY CAGE
Fig. 2.11. Set—up to investigate the influence of various
earthing conditions of Faraday cage and cryostat upon resistance thermometry; see also the text.
This set—up simulates a resistance thermometer connected to a measuring bridge. The minimum voltage V J;£=0.3JLIV (Table 2.1) is caused by the pick—up of spurious signals of the part of the wiring near the lock—in detector and is independent of whether the test box is located outside or inside the Faraday cage. The best suppression of 50Hz pick—up is obtained, when the cryostat is electrically isolated from parasitic earthings like the pumping system, the helium—gas evaporation system or cooling-water supplies, and additionally is connected to the Faraday cage, which in its turn is equipped with a proper, "clean" earthing connection.
The cables between the Faraday cage and the cryostat run inside a tubing system that connects the cage and the cryostat electrically and provides a continuous shielding for the
measuring signals. In order to investigate this shielding, three test boxes of rectangular cross section made of copper and iron were constructed, see Fig. 2.12.
Fig. 2.12. Test box and a r t i ficial 50Hz noise generator (line transformer T) to compare the electromagnetic shielding of tubings with rectangular cross sections and thickness d.
Table 2.2. The 50Hz shielding S of
tubings with rectangular cross sections; see also Fig.2.
shielding type none Cu (d=1.5) Fel(d=1.5) Fe2(d=4.0) none Cu (d=1.5) Fel(d=1.5) probe type 1 1 1 1 2 2 2 Veff (JJV) 47 40 17 9 4000 1.3 1.4 12} d in mm. Vbias (juV) <-0.2 SO.2 <-0.2 *0.2 310 »1 «1 S 1 1.2 2.8 5.3 1 «10* *10« Table 2.1. 50Hz volta
ges measured with set up of Fig.2.11. connection DAC DA DACB DCB ACB AC C no connection Veff <iiV> SO.3 <0.3 12 35 8 1.2 0.2*106 0.2*106
The 50Hz voltage V f f developed over probes inside the boxes was
measured differentially with a lock—in detector. Probe 1, consisting of loop ABC, is mainly sensitive for magnetic couplings, probe 2 (part AB only) for electrical components of a field generated by a small mains transformer T. Table 2.2 gives the results; the bias voltage V. . is measured, when T is not
bias active.
Iron, 1.5mm thick, was chosen as a constructional material for the tubing system. The main parts of the Faraday cage and of the tubing system are constructed of metals of low permeability with a 50Hz shielding factor of about 3. Further, the copper mesh shielding of most coaxial cables is only effective against capacitive coupling, but has little effect upon low frequency
inductive coupling. That is why the actual location and orientation of equipment outside the Faraday cage producing magnetic stray fields was thoroughly considered to ensure
proper resistance thermometry at very low temperatures.
2.2.3. Interference reductions
Fig. 2.13 depicts the situation of the mains-power connections to the Faraday cage (Siemens, FRG.) and the peripheral instrumentation.
Fig. 2.13. Mains power
connections.
r
R O -10A 0 O -shunt,2kW' r,
—*
■ih - / h isolation transformer TRX2500 Claude LyonsÏ
15/*FT-H T-H
low-pass filter SiFi/C Siemens low-pass filters Siemens 15^F / 1 O -f f-FARADAY CAGE 10A_L '
low-pass filter SiFi/C Siemens ALPES 50 Merlin Qerin i resonant isolation transformer DLC 1052 Oeltec / / > I I ' ' remote cor trolI
1
power fail detection
65A magnet power supply CBM computer disk drive interface system printer
The cage is provided with a mains 1ow—pass filter to block high frequencies from entering the cage; in this filter capacitors to earth (some juF's) suppress the common mode interference. An isolation transformer (TRX2500, Claude Lyons, UK.) minimises, because of its very low capacitive coupling (O.OlpF) between primary and secondary windings, the currents of frequencies f>50Hz, which, as a result of the filter capacitors, run in the
the measuring leads. For the same reason computer signals enter the cage via an interface with optic couplers (Fig. 2.10).
A 1 oui—pass filter (SIFI/C, Siemens, FRG.) was added to the original cage filter to improve its attenuation for f<100kHz. The attenuation for f>100kHz is better than 150dB, as was detected with a mains—receiver. Remaining interferences are generated in the cage itself; e.g. neon-gas discharge lights or power supplies increase the noise level with 30dB at f>10kHz. Fig. 2.14 gives the output of two resistance bridges, VSda and VS4b, of equal sensitivity and connected to thermometers at the same spot. This is a typical example of possible disturbances in sensitive measurements concerning random variations rendering measurements impossible.
Fig. 2.14. Output of two VS4 resistance bridges a,b as a function of time during variations of the mains
impedance; see also the text.
OUTPUT VS4b
i mum » , ^ » — * » i i i P I I I I I i w i n l
It turned out to be caused by a small change of the mains impedance in the laboratory during evenings. The combination of the isolation transformer, the 1ow—pass filters (resulting in an strongly imaginary mains impedance) and the power supplies in the cage (drawing pulse—shaped currents) results in an oscillatory behaviour of the mains voltage superimposed on its 5GHz sine—wave, which can lead to occasional erroneous triggering of the 25Hz excitation circuitry of the resistance bridges. A resistive shunt (25P.) over the mains lines and additional capacitors (15jiF) lower the amplitude and frequency of this parasitic ringing. An additional 50Hz band—pass filter at the input of the VS4 excitation circuitries was used to immunize them for these mains distortions.
Gerin, France) in Fig. 2.13 ensures a proper continued operation of the computer system and magnet power—supply in case of short interruptions or surges of the mains. The resonant isolation transformer (DLC1052, Del tec, USA.) stabilizes the mains voltage and forms an energy buffer between the UPS and the power supply. Uhen this transformer is not used, peaks of 20A occur upon the 2A effective sinusoidal UPS output current uhen the magnet is fully energized. The resulting electromagnetic radiation leads to unstable and incorrect resistance thermometry.
Fig. 2.15 shows schematically the filtering, shielding and earthings in the Faraday cage and the cryostat. Troublesome high frequency interferences generated inside the Faraday cage were located and elimimated. They would cause systematic errors in the temperature reading, when calibrated resistors are measured with an alternative arrangement of equipment, even if the excitation voltage is the same: switching off the LED-tdisplay of a VS4 bridge apparently increases the resistance of a Speer 100Q CRT at 25mK by about 20% in case of unfiltered wiring. During the measurements it was avoided to switch high (mains) voltages and to discharge static voltages; this may generate high frequency electro-magnetic pulses after which, at the lowest temperatures, it may take the resistance thermometers about 15 minutes to return to their equilibrium values. Interferences can be detected by means of a simple portable radio, which acts as an easy to use, sensitive and selective frequency analyser. Its sound—signals are a direct indication of the interference source and its location. The interconnecting resistances between all parts of the metallic housing and the shielding of the
input and output connectors of the measuring equipment were kept as small as possible. The instruments were interconnected with heavy copper strips to the common earthing—point of the cage. Because of microphonic interference (cable capacitance varation as the outer shield moves with respect to the inner conductor), the most sensitive measuring wires were carefully positioned and tied down at as large a distance as possible from power transformers. It should be noted that a minimum 50Hz pick—up can be achieved at locations where two or more magnetic fields compensate each other: if one of the
o 0.1/1 F — ± x— / ; jlnF =jnnF ~~I / ; X3S3 0 i /)
h
« ( T ^ 4x100pF x Ft
8x330pF tubing ; 1mH minichoke - I 2 X 1 0 0 P F T - T , . ,. : 0.2mH ferrite bead 4x1.8nF 1Fig. 2.15. The shielding and filtering in the Faraday cage and
the cryostat; (a)...(n) depict the configurations of the various filters applied, see also the text.
instruments is switched off after checking the pick—up, the interference level might increase again without noticing. BNC coaxial cable was used for most of the signal interconnection between the instruments, with both sides of the cable shielding connected to earth in most cases. Only the inner conductors of the RG174 coaxial cables between instrumentation (il,i2..i4; Fig. 2.15) in the Faraday cage and the connector box of the cryostat carry signals; the shielding is connected to earth level near the instruments only, to avoid parasitic currents induced by high frequency interferences generated inside or outside the shielded room.
Fig. 2.15 also shows details of some of the filters (a)..(n) used. Table 2.3 illustrates their influence. It gives the relative indicated resistance r(A) due to deliberately induced r.f.-heating for a Matsushita 100ft CRT with leads filtered near the resistor (filter type (n)) and a similar one but unfiltered, both at 50mK. "A" is the signal amplitude of a r.f.— generator, which produces a current of frequency "f" in a wire, wrapped around the leads to the sensor. Clearly the filtering dramatically decreased the influence of the interference upon the result of the measurement.
Table 2.3. Relative indicated resistance
r(A)=R(A)/R(0) due to a spurious signal with amplitude A and frequency f; T=50mK.
f (MHz) 1 1 10 A (V) 0.1 1.0 0.455 r(A) unfi1tered 0.9 0.15 0.05 r(A) fi1tered 1.0 0.8 0.94 See Fig. 2.15:
(a) reduces high frequency currents in earth loops of the shieldings of the BNC interconnecting cables.
(b) is used in the input section of a DMM (type 3468A, Hewlet Packard, USA.), which is entirely put inside a metal cover. When such digital multimeters (e.g. type 1200, Keithley, USA.) would be operated unscreened in the Faraday cage, the dilution
unit can not reach temperatures lower than about 30mK.
(c). The shield parts of the BNC input and output connectors of e.g. the DICAM and TECO units are connected to "0" of the power supply and isolated from chassis earth.
(d) was constructed for the 1ou-frequency digital interconnection between COBY (Sec. 2.2.1) in the computer interface (i5) and other instruments (i4). Minichokes were used, which give an extra lOOdB attenuation compared to ferrite beads of components of the 1MHz computer c1ock—pul se. These components are fed to earth via the InF feed—through capacitors. Employing beads instead will result e.g. in r.f. radiation from the shielding of the interconnecting cable.
(e) stops the generation of high frequency components due to the switching of P-N junctions in the rectifier section of power supplies. To minimize 50Hz pick-up external power supplies
(psl..ps2, ±15V, 5V) were used for most instruments, located as far from the measuring wires as possible. Some of them are equipped with a type of toroidal transformers, which offer a low capacitive coupling between primary and secondary windings.
(f),(g). The attenuation of the wiring from the Faraday cage to the CRT's/TFR's/heaters in the IVC was measured at room temperature as a function of frequency.
Fig. 2.16. Attenuation A
as a function of fre quency f at room tem perature for the signal wires between the in struments in the Faraday cage and the devices in the cryostat: (Dtsymme— trically and (2): asym metrically measured! (3), (4): some filters included, see also the text.
0.5 1 2 3 4 5 6 7 f(MHz)
See Fig. 2.16J curve (1): symmetrically measured between two unfiltered signal wires; curve (2): asymmetrically measured between one unfiltered signal wire and earth! curve (3): filter (f) included (suitable for 25Hz resistance thermometry,
