A REFRIGERATOR WITH A DOUBLE NUCLEAR STAGE FOR RESEARCH ON SUPERFLUID 3He

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Submitted on 1 Jan 1978

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A REFRIGERATOR WITH A DOUBLE NUCLEAR

STAGE FOR RESEARCH ON SUPERFLUID 3He

A. Ahonen, W. Gully, O. Lounasmaa, M. Veuro

To cite this version:

JOURNAL DE PHYSIQUE Cofloque C6, suppl6ment au

no

8, Tome 39, aoiit 1978, page

C6-

1 1 3

A REFRIGERATOR WITH A DOUBLE NUCLEAR STAGE FOR RESEARCH ON SUPERFLUID 3 ~ e

A.I. Ahonen t, W.J. Gully tt, O.V. Lounasmaa and M.C. Veuro

Low Temperature Laboratory, HeZsinki University o f Technology, SF

-

02150 Espoo 15, Finland

Rdsum6.- Nous dscrivons un rdfrigdrateur qui utilise un double Btage de dgsaimantation nucldaire en cuivre et une cellule

P

3 ~ e en argent. L'appareil a refroidi de 1I3He superfluide 1 0,38 mK et il reste en dessous de 0,5 mK pendant 30 heures.

Abstract.- A refrigerator employing a double copper nuclear stage and a silver 3 ~ e cell is described. The apparatus has cooled superfluid 3 ~ e to 0.38 mK and the 3 ~ e sample stays below 0.5 mK for 30 hours.

In designing and building this apparatus, which will be described in detail elsewhere /l/, we benefited from the long experience our laboratory had with an older nuclear refrigerator /2/. Certain innovations were incorporated into the new cryostat the most important ones being a double nuclear sta- ge and a silver 3 ~ e cell. Our main aim was to reach as low 3 ~ e temperatures as possible by reducing the heat leak directly to the 3 ~ e specimen.

The two nuclear stages, shown in figure 1,

were each made of 10moles of insulated copper wires of 0.20 mm diameter. The resistance ratio is 220.

T I NHEAT SWITCH

R

SILVER SUPPORT FRAME

LEAD HEAT SWITCH

INNER NUCLEAR STAGE

OUTER NUCLEAR STAGE

The outer nuclear stage is annular in shape and it is directly attached, by means of a pressure con- tact, to the silver support frame. The inner stage is cylindrical. The nuclear stages were thermally connected to each other at their top ends by a lead heat switch, operated by the fringe field of the main magnet. All leads and tubes were carefully thermally anchored to the outer nuclear stage. The silver 3 ~ e cell was connected, by means of a pres- sure contact, to the silver cell foot which, in turn, was welded to the top of the inner nuclear stage.

Figure 2 is a drawing of the sample chamber. For thermal contact with liquid 3 ~ e the cell con- tains IOg of silver sinter made of 0.07 um diameter powder.

CARBON BLACK COIL

sv3"

THERM0METER

P t THERMOMETER COlL B U L K ' H ~ COlL

&

(1

1

lomm

'HEATER

Fig. 2 : A detailed drawing of the 3 ~ e cell. Becau- se the thermal conductivity of silver sinter is poor many narrow grooves have been machined to the bottom of the cell. The "tower arrqngement", often Fig. 1 : The nuclear stages and the 3 ~ e cell- used in cryostats for research on He, is rather

t

Present address : Department of Physics, Cornell - convenient : the same cell can be employed for ma- University, Ithaca, New York 14853 USA ny types of experiments.

tt

Present address : Department of Physics, Ohio State University, Columbus, Ohio 43210 USA

Mailing address : O.V. Lounasmaa, see above The total surface area against 3 ~ e is 13 m2 as de-

termined by gas absorption techniques. The cell has a Stycast extension tower for NMR experiments. The volume for 3 ~ e in the cell is about 3 cm3, one third of which is in the tower. The temperatures are measured by the pulsed NMR technique with pla-

tinum powder employed as the thermometric substan- ce. The powder is in contact with liquide 3 ~ e only.

The reason for using silver, instead of cop- per, for the 3 ~ e cell and for the supports through which heat must flow, is to avoid a large thermal reservoir outside the nuclear stage. A high para- sitic heat capacity would lengthen the time cons- tant of the apparatus. With silver this problem is eliminated : the nuclear magnetic moment of this metal is small and its heat capacity per unit volu- me is 200 times lower than that of copper.

The idea of employing two nuclear stages was borrowed from the old demagnetization cryostats which often used a main salt and a guard pill. In this way the heat leak is rather effectively pre- vented from reaching the inner parts of the appara- tus.

The cryostat works as follows. The dilution refrigerator first cools the nuclear stages, in an rms applied field of 5.3 T, to 18 mK in 50 hours. Demagnetization to 40 mT is then performed in about

3 hours. With 2.8 T in the main solenoid, corres- ponding to an 80 mT fringe field, the lead heat switch becomes superconducting whereby thermal con- tact between the two nuclear stages is broken.

Until 0.5 mK the time constant of the appara- tus is short : the 3 ~ e specimen is lagging behind the nuclear stage only a few minutes. Below 0.5 mK

however, cooling proceeds slowly and the lowest 3 ~ e temperature, 0.38 mK, is reached after 10hours. This behaviour shows that somewhere thel~is in the apparatus a heat reservoir which is in poor thermal contact with the rest of the system. Stycast is per- haps the culprit. The apparatus stays below 0.5 mK for 30 hours and below 1 mK for 50 hours. There is thus no hurry at all with the experiments.

The heat leak to the 3 ~ e specimen and to the inner nuclear stage was found to be 1.2 nW, of which

0.5

nW entered directly to the 3 ~ e specimen. We thus note that not only is the total heat leak small but also that only 20 % of it goes directly into 3 ~ e . This,is the main reason for the success of our cry- ostat in cooling superfluid 3 ~ e .

At 0.40 mK the temperature difference between

3 ~ e and the conduction electrons in the silver sin-

ter is 50 pK. There is a further temperature drop of 150 VK between the conduction electrons of the cell body and those of the nuclear stage. This se- cond temperature drop is caused by the relatively high thermal resistance of the silver cell foot. On the other hand, the difference between the conduc- tion electron and the nuclear spin temperatures in the nuclear stage itself is small so that the cop- per nuclei in this apparatus, in a magnetic field of 40 mT, are at about 0.20 mK.

References

/ l / Veuro,M.C., Ph. D. Thesis, Acta.Polytechnica Scandinavica (to be published in 1978) /2/ Ahonen,A.I., Berglund,P.M., Haikala,M.T.,