Non-Monotonic Thermal Relaxation in the Nanopore Heat Exchanger for Dilution Refrigerator

Thermal resistance and the thermal relaxation time are important physical properties in order to cool down samples at low temperatures. In the case of a dilution refrigerator, the relation between the thermal relaxation time of working fluid in a heat exchanger and the circulation rate should be taken care of. Recently, we reported on the nanopore heat exchanger with dramatically reduced thermal resistance. Experimental results suggested that the thermal relaxation time in the sinter pad is very short (less than 1 s), while the exact value was not determined. In order to verify its relation to the circulation rate, we performed numerical simulations of the thermal relaxation time. The expected monotonic increase with decrease of temperature has been confirmed down to 50 mK. However, at lower temperatures unexpected decrease is found. This unusual behavior is considered due to relatively small thermal resistance of liquid helium compared with the thermal boundary resistance.