Development of Active Gas-Gap Heat Switch for Double-Stage Adiabatic Demagnetization Refrigerators

We designed and fabricated an active gas-gap heat switch (AGGHS), which ON/OFF the heat conduction between the 1st stage (0.05-2 K) and the 2nd stage (1-4 K) of a double-stage adiabatic demagnetization refrigerator (DADR). Our design geometrically separates two components which dominates the ON or OFF performance, and achieved heat conductivity of 6 mW/K (ON) or 4 mu W/K (OFF) at 2 K. The ON/OFF is controlled by a heater attached to the charcoal box to adsorb/deadsorb He-4 gas inside. We introduced the AGGHS to the DADR and successfully cooled the detector stage down to 60 mK, working properly more than a year.