A He-3 gas heat switch for use in a cyclic magnetic refrigerator

We describe a heat switch which we have constructed to work in the range fr om 0.5K to 1.8K as a component of a cyclic demagnetization refrigerator. We measure a thermal conductivity of about 50 mu W/K at 1.5K when the switch is in the 'off' state, and in excess of 10 mW/K at 0.5K in the 'on' state. The switching time into the conducting state is a matter of several seconds , while the transition to the insulating state achieves a factor of 100 red uction of conductivity within about 1 minute, followed by a slower reductio n by another factor of 5 over an hour time scale. This paper describes some details of the construction, discusses the limiting factors in the thermal conductivity in the on and off states, and the switching times between the se states. The requirements of the intended application are discussed.