Physica B 1658~166 (1990) 47-48 North-Holland

DEVELOPMENT OF A COMPACT DILUTION REFRIGERATOR WITH A CRYOGENIC ‘He J-T CIRCULATION SYSTEM FOR RAPID COOLING DOWN AND CONTINUOUS OPERATION*

Takeshi Igarashi, Masahito Sawano”, Yoshitomo Karaki, and Minoru Kubota, Institute for Solid State Physics, University of Tokyo, Roppongi 7-22-l,Minato-ku Tokyo 106

Yu Hiresaki, Suzuki Shokan Co.,Ltd, Koji-machi 3-1, Chiyoda-ku Tokyo 102 Kiyoshi Akiyama, Rigaku Denki Co.,Ltd. Matsubara-cho 3-9-12, Akishima, Tokyo 196 Toyoichiro Shigi Dept. Appl. Phys. Okayama Univ. of Science Ridai-cho l-l, Okayama 700

JAPAN

A compact dilution refrigerator without pumped &He bath has been developed to reach as low as 32 mK continuously with a simple counterflow continuous heat exchanger alone below the still. This refrigerator utilizes heat exchange with evaporated 3He from the still and J-T expansion of the incoming 3He in order to condense it. Although similar machines are reported by a few authors, we have investigated other possible paths in the ‘He T-H diagram by use of a specially designed film burner heat exchanger. Another dilution cryostat with a cryogenic ?He circulation system is being constructed. Application of such a system for a millikelvin refrigerator without liquid He bath, but with a 4 K refrigerator is also studied.

1. Introduction A dilution refrigerator is a well established

tool to cool samples to millikelvin temperatures and it can be used continuously by circulating 3He. Ordinarily a pumping system at room temperature is used for 3He and also for &He, the latter is evaporated from a 1K pot to liquify the circulating =He. A few authors have already proposed3-s’ dilution refrigerators without the 1K pot, but a condensation stage utilizing 3He gas cooling capacity from the still. and the J-T expansion of the incomming 3He, which is precooled to 4.2K before entering this stage.

We have studied various paths in the temperature - enthalpy diagramm oE 3He in a dilution refrigerator (DR) without 1K pot, using a home refrigerator compressor in the 'He gas handling system.l’ With this room temperature system, we obtain 32 mK in the mixing chamber with simple construction 3He gas heat exchangers including one at a specially designed f i lmburner . For farther simplication of the system and easier operation,a compact dilution refrigerator with a cryogenic 3He J-T circulation system is being developed, which is applicable either for a system confined in liquid He bath or for a system precooled with a 4K small refrigerator, a Cifford-McMahon cycle plus J-T refrigerator, for example.

2. 3He H-T(P) Diagram and Dilution Refrigerator The low temperature thermal properties of

-7He has been studied in the pioneering work” by Wiedemann and his coworkers in Muenchen and the first proposals for dilution refrigerator

* Work supported through Grant in Aid for Developmental Scientific Research, Ministry of Education, Science and Culture, Japan. * Permanent address: Rigaku Denki Co.,Ltd.

applications were made by J.Kraus” and by A. de Waele,et.al,” independently. Much more recently K.Uhlig has reported” a very successful dilution machine. Fig.1 shows the enthalpy - temperature diagram from ref.21 on which we added present study’s paths and one by K.Uhlig. If the still is kept at 0.6K, the dilute solution in the still has up to 7.7 J/g cooling power per 3He, which can be compensated by a heater power in an ordinary dilution refrigerator. The power can be applied at a film burner whereas in the present study we place a heat exchanger. This film burner is ordinarily kept at about 1 K and offers a good heat exchange position in addition to a 3He gas heat exchanger in the still pumping line.

- x4 k-

Fig. 1 Enthalpy-Temperature diagram of 3He from ref.2. condensation paths As?, B-S and C-S are our experiments and U-S is from Uhlig’s . The importance of the filmburner heat exchanger (H.E.2) is seen. 7.7 J/g3He is the maximum possible cooling power of the still exchanger.

0921-4526/90/$03.50 @ 1990 - El sevier Science Publishers B.V. (North-Holland)

48 T. Igarashi, M. Sawano, Y. Karaki, M. Kubota, Y. Hiresaki, K. Akiyama, T. Shigi

3. Experiments to study 3He condensation Process order of 7 to 10 FW and T* dependent cooling with room temperature gas handling system (GHS) power at &-lOOumol/s.

Our set up uses a DR vacuum can immersed in a He bath. There is a test section above the still where we have studied heat exchange and pressure and temperature drop relations for more than 10 dif Eerent arrangements and recorded the corresponding paths in the T-H(P) diagram. (Fig.11 The inlet gas passed through the test section is turned to a mixture of liquid and gas and led to the still heat exchanger where all of 3He is expected to become liquid. This point is S in Fig.1. The liquid ‘He is then led to a counterflow heat exchanger and finally to the mixing chamber in just the same manner as in ordinary dilution refrigerators. All of our heat exchangers and flow impedances are made out of CuNi capilaries after simplicity and reliability considerations.

As to the operation after completion of room temperature setting up it could be precooled with ‘He exchange gas to liquid Nitrogen temperature and during the initial liquid He transfer all the mixture gas is precooled in the cryostat to 4.2K. The circulation can be started after 30min pumping of the vacuum can and the initial condensation of mixture gas could be performed quickly with the use of a bypass to the No.1 impedance by a l/10 impedance. The lowest temperature could be reached within 2.5 hours after the liquid He transfer start.

Fig.3 Test adsorption pump. A miniature adsorption pump and a heater controlls the operation.

The higher inlet gas pressure at T>3..5K is preferable in the view point of maximum heat exchange possible with the evaporated ‘He from the still as see in Fig.1. However when one trys a combination of adiabatic J-T expansion with heat exchange, the high inlet gas pressure needs certain caution, because the path may pass through out side of the ‘He inversion curve, That is, by adiabatic expansion the ‘temperature may go up and heat exchange may not be performed properly. This problem can be avoided by the multiple “J-T and heat exchanger” combinations

4. Cryogenic ‘He circulation system Adsorption pumps have been used for cryogenic

refirigerators quite extensively.7’B’ Yet 3He circulation system with the regenerated gas at elevated pressure above 0.5 kg/cm’ has not been reported. Although it may bring obv i ous advantages, the cyclic adsorption and regeneration process poses a cryoengi neer i ng problem. We are studying to achieve short time constant of the system and small heat load to the He bath using a model charcoal pump depicted in fig.3. We have 10 gram of activated charcoal. Each grain of 3mm diameter is made thermal contact to the copper inner chamber wall by means of glueing to copper mesh and then rolling of the Cu mesh to fit tightly to the inner diameter of the chamber. While adsorption is performed reasonablly with moderate heating of 25-40mW, for lOOpmol/s circulation, study for regeneration with least possible He consumption is underway.

as are tried by us (B,C see Fig.2). Our heat exchanger at the film burner is

expected to function at the lowest temperature namely below 2K before entering to the still exchanger and the actual importance is seen in the results. At the film burner extra heating capability is facilitated by an electric heater. The film burner functions in reasonable manner when the still temperature is below 0.7K. The ratio ‘He / “He was measured for the circulating gas to check the film burner function. We obtain 98% ‘He with 0=0.5mW.

With a still shield installed the cooling power of our refrigerator was measured as a

function of temperature. The result could be analyzed as a sum of a constant heat leak of the

Heat Exchanger 1

Impedance I

Heat Exchanger 2 (Film burner)

Impedance 2

Fig.2 Experimental set UP in the test section for the case C.

5. Summary A ‘He condensation stage with J-T and ‘He gas

heat exchanger functions quite well. Conbination with cryogenic ‘He circulation system seems to be promissing yet further study is needed to minimize the heat load to the bath, A 4K refrigerator offers another way of running this dilution refrigerator as being tested by us.

REFERENCE (1) Preliminary results are given in M.Sawano et.al. Proc.3.Japan-Sino Joint Seminer on Small Refrigerators and Related Topics. Okayama (1989) 27-30. Details will appear in Proc.ICEC13(1990)

(2) J.Kraus, K.Uhlig, W.Wiedemann Cryogenics _l& (1974) 29

(3) J.Kraus. Cryogenics 17 (1977) 173 (4) A.de Waele, et-al. Cryogenics u (1977)175 (5) K.Uhlig. Cryogenics 27 (1987) 454 (6) W.P.Kirk, E.D.Adams, Cryogenics & (1974) 147 (7) V.A.Mikheev, et.al. Proc. ICEC .lQ (1984) 263 (8) R.Salmelin,et.al. J.Low Temp.Phys.B(l989) 83