Electric cooling from room temperature down to 200 mK M.Tarasov, L.Kuzmin, and I.Agulo, Chalmers University of Technology, S41296, Göteborg, Sweden V.Mikheev, P. Noonan, and A. Adams Oxford Instruments Superconductivity, Old Station way, Eynsham Witney OX29 4TL, UK Title

Outline Building blocks:  Pulse tube rifrigerator  He3 sorption cooler  SIN electronic refrigerator  Experimental results  Estimated margins, other experiments

General view of the equipment Cryostat D300, H850

View of the He3 sorb View of the internal part of the cryostat Oxford Instruments  with He3 sorption cooler, removed outer vacuum can (OVC), and removed radiation shields

Pulse tube cooler & compressor Sumitomo 

Cryocooler SRP-052  Two stage pulse tube cryorefrigerator  Cold head unit RP-052A1  First stage 20 W at 45 K, second stage 0.5W at 4K  Compressor unit CSW-71D, water cooled 7 l/min,  W450, L500, H687 mm, 120 kg,  Electrical requirement 3 phase 9kVA  Operation pressure 25 bar, steady-state 17 bar

Top of the cryostat To reduce interference and noise from grounding we placed our room- temperature battery feed electronics at the top of the cryostat close to the connectors.

Electron cooler chip layout 4 junction structure for cooling/heating at the top and botton Log-periodic antenna for THz range Double-dipole antenna for 600 GHz Double-dipole antenna for 300 GHz

SPM view of SIN electron cooler

SIN cooler with Au trap

SINIS cooler

Electron cooler with trap

IV curves of SIN thermometer A 7 k  SINIS thermometer IV curve at T ph =290 mK without cooling (X-es), and under electron cooling (circles)

Cooling curves Electron cooling starting from phonon temperatures in the range of mK

Ideal SIN tunnel junction IV curve The IV curve of SIN junction have simple analythic form The electron temperature under absorbed power Zero-bias resistance with leakage current

Calculated ZBR Resistance ratio calculated for bias voltages 0, 200, 300  V, thermometer normal resistance 10 k  and leakage resistance 35 M 

Dynamic resistance of SIN Dependencies of sensor resistance measured in dilution refrigerator at 20 mK & 250 mK and co ler bias 0, 150, 400  V

Optimal cooling Electron temperature estimated from dynamic resistance at 300  V (boxes)

Calculation of cooling power Cooling power Effective electron temperature

Heat balance Curve T250 (circles) corresponds to the electron-phonon power transfer P ep =(T ph 5 -T e 5 )  v at 250 mK, other curves from the left to the right present cooling and heating power balance P v =P cool -V 2 /R s -P bg at bias 392, 384, 372, 356, 340  V

Discussion  Obtained cooling down to 190 mK has a reserve of improvement down to 100 mK as in He4 liquid precooled He3 sorption cooler  We are still suffering from electric noise coming from high power supply line and its grounding directly to the cryostat via high pressure supply from connector  Acustic vibrations also affect operation of electron cooler and cold electron bolometer  Operation wth a pulse tube refrigerator at 3.5 K instead of 2.8 significantly prolongs the precooling period and available lowest temperature is 290 mK instead of 275 mK.

Conclusion We have demonstrated the first cryogen-free electric cooling from room temperature down to electron temperature below 200 mK. The key idea behind this device is to develop a millikelvin range cryocooler as simple in operation as a conventional kitchen refrigerator. It does not need filling with any cryogen liquid and you need just to switch it on in the evening to have electron temperature of the sample below 200 mK in the morning. The first building block of the device is a double-stage pulse tube cryocooler Sumitomo  that provides cooling down to ~3 K. The second building block is He3 sorption cooler of original design by Oxford Instruments  that brings for moderate thermal load a basic temperature of about 280 mK. The third building block is a Superconductor-Insulator-Normal metal (SIN) electron cooler. This type of cooler is analogous to the Peltier effect and in general can provide electron cooling by up to 200 mK. In our very first tests of the whole system we already achieved cooling down to 191 mK.

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