DOUBLE UNSATURATED HeII BATHS FOR NESTING SUPERCONDUCTING SOLENOIDS PRODUCING FIELDS GREATER THAN 14 T

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DOUBLE UNSATURATED HeII BATHS FOR NESTING SUPERCONDUCTING SOLENOIDS

PRODUCING FIELDS GREATER THAN 14 T

G. Faure-Brac, A. Greenberg, B. Hébral, J. Vallier

To cite this version:

G. Faure-Brac, A. Greenberg, B. Hébral, J. Vallier. DOUBLE UNSATURATED HeII BATHS FOR NESTING SUPERCONDUCTING SOLENOIDS PRODUCING FIELDS GREATER THAN 14 T.

Journal de Physique Colloques, 1984, 45 (C1), pp.C1-75-C1-78. �10.1051/jphyscol:1984116�. �jpa-

00223633�

DOUBLE UNSATURATED Hell BATHS FOR NESTING SUPERCONDUCTING SOLENOIDS PRODUCING FIELDS GREATER THAN 14 T

G. Faure-Brac, A.S. Greenberg, B. Hebral and J.C. Vallier*

Centre de Recheroh.es sur lea Ires Basses Temperatures, C.N.R.S., B.P. 166 X, 38042 Grenoble Cedex, France

^Service National des Champs Intenses, C.N.R.S., B.P. 166 X, 38042 Grenoble Cedex, France

Résumé - Nous avons construit un ensemble cryogénique pour des solénoides supraconducteurs concentriques. Ils sont placés dans des bains indépendants d'hélium superfluide non saturé et produisent actuellement 14 T dans un diamètre de 0 = 120 mm. Avec des aimants supplémentaires, un champ de 20 T dans 0 = 48 mm sera disponible. Cet ensemble est destiné à effectuer des expériences à très basse température (T < 10 m K ) .

Abstract - We have constructed a cryogenic system for nesting superconduc- ting solenoids. They are placed in two independant unsaturated Hell baths and are now producing up to 14 T in a 0 = 120 mm bore. Additional concentric superconducting solenoids will permit a 20 T field in 0 = 43 mm. This sys- tem is designed to provide stable fields for experiments in a very low temperature environment (T < 10 m K ) .

New developments for magnetic fields higher than 10 T have appeared in recent years. On the technical side, testing and developping new superconducting materials

(A15 compounds, Chevrel phases) and magnets with high critical fields is best per- formed in coils with large experimental volumes. In the research domain, in parti- cular for milliKelvin physics, such magnetic fields are useful for experiments as polarized hydrogen and helium, quantized Hall effect, magnetic phase diagrams, etc.

Superconducting coils are well suited to these purposes. In particular, the vibration problem induced by the cooling refrigerant in typical resistive coils is absent thus removing one of the main limitations for the lowest achievable tempe- rature. The use of unsaturated superfluid He baths HI may even increase the per- formances of the milliKelvin apparatus.

We have constructed a cryogenic system for superconducting coils producing 14 T in a bore of 120 mm and ultimately 20 T in 48 mm. The system is schematically des- cribed on fig. 1. It consists of two independent unsaturated Hell baths in which are placed the superconducting coils.

The outer cryostat (0 -v. 1 m) holds a 10 T Nb-Ti coil.The two coil windings are in series : the outer coil is a stack of pancakes, the inner coil (bore 0.32 m) a clas- sical solenoid. The total coil weighs 900 kg and stores 6 MJ at 10 T. Above 1000 A current sharing effects appear and at 1150 A the total 10 W dissipation exceeds the present cooling capacity of the superfluid bath. More details on the cryostat and the magnet are given in ref. 2.

The space reserved for experiments and additional superconducting coils is con- tained in a reentrant dewar (0i = 0.28 m) which is equipped with its own unsaturated Hell bath. This dewar has three walls, the inner one being easy to extract providing quick access to the experiment. Typical cooling times are 12 hours from 300 K to 100 K (measured with Pt thermometers), 4 hours to fill the dewar with liquid He, 2 hours to reach 1.8 K in the inner unsaturated Hell bath.

The lower Hell bath is separated from the upper 4.2 K He bath by an epoxy plate 3.5 cm thick. Two safety release valves in the plate guard against possible over- pressure in this bath in case of quenching of the magnets.

The dissipated heat in the Hell bath is transferred to a pumped liquid He bath contained in a Cu exchanger(500 cm

of surface area).The exchanger is continuously filled/3/froni the upper 4.2K bath through a variable-impedance consisting of needle valve whose aperture is in a feedback loop thus maintaining a constant level of He

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984116

Cl-76 JOURNAL DE PHYSIQUE

in the exchanger. This level i s monitored by ..

the resistance of a Nb-Ti wire $l = 35 p with the emerged section driven normal by a 50 mA current.

A Nb3Sn c o i l , manufactured by IGC /4/, i s fixed t o the experimental vacuum can.

The coil ( L - 2 H) i s formed of 56 pan- cakes with a 0.12 m bore. The outer diameter i s 0.2 m and the height 0.39 m.

The vapor cooled current leads consist of 850 copper wires (0 = 0.15 mm) in an epoxy tube (gi = 8.2 mm) passing through the top of the cryostat and are connected a t 4.2 K to Nb-Ti wires. These l a t t e r wires go throu@two leak-tight feed throughs i n the epoxy plate. A t 305 A the typical voltage across the current leads i s 60 mV with a flow r a t e of 500 l/h of gaseous He. The resistance between the coil and ground i s a t l e a s t 130 MS?.

Cryogenics and magnetic t e s t s have been performed t o measure various temperatures, heat leaks, total and available cooling capacities, pumping r a t e s , etc. The temperatures were recorded by 100 0 Speer r e s i s t o r s placed throughout the apparatus e.g. on the Cu exchanger, and a t the bottom of the cryostat. The pressures inside the Cu exchanger and on the pumping l i n e a t the top of the cryostat are also measured giving the temperature of the liquid He inside the heat exchanger and the drop of pressure along the pumping line. A 100 Q-25 W heater i s placed a t the bottom of the cryostat and i s used t o

determine the cooling capacity of the I l m

Fig. 1 - Schematics of the apparatus : 1. Verepoxy plates; 2 . Heat exchanger of the inner dewar; 3. Heat exchanger of the outer dewar; 4. Inner extracta- ble wall; 5. Heater; 6. Vacuum jacket;

7. Safety release valves; 8. 3 ~ e bulb;

9. Nb3Sn c o i l ; 10. NbTi c o i l . system. In order t o c a l i b r a t e the r e s i s t o r s a 3 ~ e vapor-pressured bulb i s immersed i n the superfluid bath. The f i e l d measurements are obtained through the voltage induced in a 12000 turns coil placed i n the experimental can. This v01 tage i s in turn applied t o an integrator magnetometer with low d r i f t .

The cryogenics t e s t s are performed in two ways :

a ) a t constant He level i n the Cu exchanger, the temperature i s measured f o r various applied heat powers ;

b) a t constant temperature, the applied heat power i s measured versus the He level in the Cu exchanger.

Curve A or1 f i g . 2 features the temperature of the unsaturated He11 bath as a function of the total power determined from the flow of pumped heljum, Q which i s the sum of the heat losses, Qo, and the electrical power applied, Q,. The yinimum temperature (1.57 K ) i s reached f o r zero applied power and corresponds t o

of 0.9 W. Close t o TA, T=2.155 K (T~=2.163 1< a t 1 atm) a power of 7 W i s extracted.

Curve B refers to the temperature inside the Cu exchanger. The drop of

temperature between the inside and che outsidesof the exchanger can be related to the Kapitza resistance :

2SAT/Q = 10 cm2 K/IJ.

This i s in good agreement,within our temperature uncertainty,with the value For the

a t 300 i n the pur~iping li n e . The difference between curve B and C corresponds then t o the drop o f pressure i n the pumping l i n e i n s i d e t h e c r y o s t a t . This i s mainly due t o the impedance Z o f the f i l l i n g exchanger /3/ constructed o f 50 Cu g r i d s w i t h a 0.35 mm h o l e s i z e : Z = A P / ~ = 1.6

104 Pa

mole-l, located i n the pumping tube o f the e changer. The pump has a 250 m /h f l o w r a t e . 4

The powers Q, Q, and 4 are p l o t t e d i n f i g . 3 versus t h e temperature o f the unsaturated H e I I bath. The exchanger i s h a l f f i l l e d i n t h i s run and the 10 T

Fig. 2 - T o t a l extracted power versus : A. t h e unsaturated H e I I bath temperature.

B. t h e temperature i n s i d e the Cu heat exchanger.

C. the f i c t i v e T* temperature defined i n the EYE-

c o i l was a t 100 K (holding temperature when n o t i n use). The minimum temperature i s s l i g h t l y higher than i n f i g . 2 f o r two reasons : f i r s t the heat exchange surface area i s halved and second, a d d i t i o n a l losses

100 mW) occur due t o the r a d i a t i o n from the outer dewar a t 100 K. The system has a usable power o f 4 W a t 2.05 K and a maximun o f 5 W a t TA. The heat losses are e s s e n t i a l l y constant

1 W ) up t o 2.05K and s l i g h t l y increase t o 2.2 W

a t TA. This i s mainly c o r r e l a t e d --

w i t h the l a r g e thermal g r a d i e n t occuring i n t h e upper bath. The

HeIIIHeI p o i n t i n the upper bath

decreases as t h e applied power increases and crosses the t o p o f t h e epoxy p l a t e a t T = 2.05 K.

A d i f f e r e n t increase of t h e heat

l o s s a t h i g h applied power i s

a l s o observed when t h e exchanger i s f i l l e d . This l o s s i s r e l a t e d

t o the r e d u c t i o n o f t h e f r e e

helium surface t o t h e cross s e c t i o n o f the pumping tube

(P = 15 mm) instead o f the f u l l

'

U

Fig. 3 - Total extracted power Q, cooling capacity Q, , heat losses Q as a f u n c t i o n o f t h e He11 unsaturated bath ?emperature .

cross s e r t i o n o f the exchanger. For l a r g e powers the f r e e surface i s n o t w e l l defined and convection e f f e c t s thermally connect the upper and lower baths through t h e i n s i d e o f the tube. It must be noted than a good f i t t i n g o f the epoxy p l a t e i n i t s seal i s e s s e n t i a l t o minimize the heat losses i n H e I I through the small chan- n e l s between t h e two baths. I n an e a r l y run poor f i t t i n g increased the heat losses by a f a c t o r o f two corresponding t o a continuous channel o f 15 p around the 0.28 m diameter epoxy p1 ate.

The c r i t i c a l c u r r e n t o f the Nb3Sn c o i l i s given on t a b l e I f o r various external

f i e l d s (produced by the Nb-Ti c o i l operating also i n unsaturated He11 f o r 8 and

9.6 T).

Cl-78 JOURNAL DE PHYSIQUE

The increased operating efficiency in super- f l u i d He i s clearly demonstrated. The r e s i s t i v e behavior of the magnet has been observed in zero external f i e l d and i s well described by an equivalent resistance of 3.4

10-6 Q. This resistance i s probably located a t a contact between the pancakes. During the quench in

zero external f i e l d a t 320 A the temperature .V

²⁵⁰

of the lower bath warmed up t o 4.2 K and in _9.6

₂₂₅ less than three hours the temperature dropped

back t o 1.9 K. W e thus performed steady s t a t e Table I - Critical current ( A ) of t e s t s a t 300 A where the heat losses of the the NbjSn coil f o r various external c o i l a r e 0 . 3 1 W . T h e o r i g i n o f t h e q u e n c h i s f i e l d s .

not y e t clearly understood. Quench detectors

are used t o anticipate anomalous behavior of the system. For example in case of quenching of the ,lb-Ti c o i l , t o avoid a sudden large increase of current in the Nb3Sn coil i t s power supply i s also disconnected and the current in the coil i s discharged into a r e s i s t o r whose value i s chosen in order t o have : L/X = 0.5 S .

The maximum f i e l d achieved so f a r i s 14.0 T obtained with 9.6 T produced by the Nb-Ti c o i l . However the t e s t with 250 A in 8 T corresponds t o larger electromagnetic forces on the Nb3Sn conductors.

More intense f i e l d s will be obtained by adding extra superconducting solenoids.

In the present empty space between the wall of the inner dewar and the NbgSn coil we plan to place another NbgSn c o i l . Magnetic f i e l d s larger than 17 T should then be produced in the same 0 = 0.12 m space. The possibility to obtain even larger magnetic f i e l d s by f i l l i n g part of the existing bore with a coil using new materials (V3Ga, Chevrel phases) i s being studied. A VgGa prototype coil 0 = 22 mm has already been tested /6/ t o determine the characteristics of a coil with a bore of 48 mm which will give a t o t a l f i e l d of 20 T when inserted i n the previous coils.

As an example of the application of t h i s system, a powerful dilution refrigerator designed t o perform experiments on spin polarized hydrogen i s now assembled and i s compatible with the c o i l ( s ) configuration(s). I t i s designed to have a cooling capa- c i t y of 1 mW a t 50 mK and will reach temperatures lower than 10 mK. Special care has been given t o avoid vibrations. The magnet cryostat i s on a massive compacted sand and concrete mass giving a mechanical resonance frequency of about 1 Hz. Stainless steel flexible lines a r e inserted a t crucial points f o r vibration isolation from the many mechanical pumps involved in t h i s apparatus.

Such a system allying low temperatures and the advantages of large superconducting f i e l d s will offer a powerful new tool f o r research in candensed n a t t e r currently performed i n Grenobl e.

G. Bon-Mardion i s greatly acknowledged f o r his advise during the developments of the unsaturated He11 bath. Thanks a r e due t o G. Arnaud f o r the operation of the 10 T system and t o H. Jurek f o r his careful assembly work of the inner cryostat.

References

/l/ BON MARDION G., CLAUDET G . , VALLIER J.C., Proceedings of ICEC 6 (1976) 159.

/2/ AUGUERES J.L., AYMAR R., BON MARDION G., CLAUDET G . , FAURE-BRAC G., PLANCOULAINE J . and SENET L., Cryogenics 20 (1980) 529.

ARNAUU G., BON MARDION G., F m E - B R A C G., VALLIER J .C., t o be published.

/3/ BON MARDION G., CLAUDET G., Cryogenics 19 (1979) 552.

/4/ Intermagnetic General Corporation, GUILLK'RLAND, N.Y. 12084, USA.

/5/ See f o r example CONTE R.R., Elements de CryogGnie, Masson e t Co. Edit., Paris (1970) 236.

/6/ MARTY J . , VALLIER J . C . , Proceedings of MT7, IEEE on Magnetics, Mag-17 (1981)

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