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An Autonomous Dilution Microcryostat-Insert for Nano Physics

An Autonomous Dilution Microcryostat-Insert for Nano Physics. V. S. Edelman, Institute for Physical Problems RAS, Moscow, Russia.

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An Autonomous Dilution Microcryostat-Insert for Nano Physics

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  1. An Autonomous Dilution Microcryostat-Insert for Nano Physics V. S. Edelman, Institute for Physical Problems RAS, Moscow, Russia The cryostat with base temperature ~0.04 K is designed to cool ultimately sensitive photo detectors or samples in experiments that do not require a high refrigerating capacity.It can be used to study electronic properties of various nano objects: quantum dots and wires; multilayer structures; Q-bits, etc., and for educational purposes.

  2. The operation of the Dilution Refrigerator (G. Frossati, J. Low. Temp. Phys., 1992, vol. 87, nos. 3/4, p. 595.) Cooling power of a dilution refrigerator: Q is the heat fed to the mixer, T and Tc are the temperatures of the mixer and incoming concentrated 3He at the inlet to the mixer, and dn/dt is the 3He circulation rate. Typically for commercially available dilution refrigerators dn/dt = 10-4=10-3 mol/s. At T=0.1K this corresponds to dQ/dt >100W – much higher than is needed for most experiments in nano physics in which overheating of a sample can easily arisen. This excess obviously requires additional expenses in course of experiment (for electric power, liquid helium, etc.) and leads to a complicated design and large dimensions.

  3. Dilution refrigerator with condensation pumping (V.S. Edelman, Cryogenics, v.12, p.385 (1972)) The mixer temperature versus power fed to the still This very simple DR operates at power fed to the still of about 0.25 mW, or at circulation rate of about 0.01mmol/s. Presumably, this limitation is due to some processes in heat exchanger. Later it was shown that dilution refrigerator with condensation pumping can operate at circulation rate of about 1 micro mol/s (V.S. Edelman, Physica B 329-333 (2003) 1574).

  4. Top view on open refrigerator.

  5. Examples of time dependences of the mixer, 3He bath, and still temperatures and the power supplied to the still. Time dependences of the mixer and sample-holder temperatures at different levels of power supplied to the holder.

  6. PARAMETERS OF THE CRYOSTAT Minimum registered temperature of the sample holder, 0.04 K. Time of maintenance a temperature <0.1 K, 10– 13 h. Stable operation at a circulation rate of ~1.5–15.0 µmol/s. At a circulation rate of 1.5 µmol/s and a heat power of 0.5 µW supplied to the sample holder, its temperature is no higher than 0.1 K. Time of mixer cooling from 1.0 to 0.1 K, ~30–40 min (depending on circulation rate). Minimum temperatures of the 3He bath, 0.34 K, and the 4He bath during 3He regeneration, 0.9 K. Temperature of the screen of 100 K, 80–100 K. Time of preliminary cooling to nitrogen temperature in liquid nitrogen, the night duration. Time of cooling to a level of 4–5 K, the magnetic switch being switched on, 4–6 h. Helium loss by preliminary cooling, ~5 l. Time of operation with one portable cryostat with helium, 6 days. The amounts of gases for filling the cryostat: 0.22 mol 4He, 0.11 mol 3He, and 0.05 mol of a mixture of 40% 4He + 60% 4He. Pressures of gases in the warm filled apparatus: 50 atm (4He), 25 atm (3He), and 50 atm (mixture). Masses of the insert and the Dewar vessel filled with liquid helium and containing the insert, 7.5 and at most 35.0 kg, respectively. Principally cryostat can operate with a pulse tube refrigerator instead of a Dewar with liquid helium.

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