Hot gas temperature controller for a cryostat insert having high stability

A very low-noise, variable temperature insert for a standard liquid-helium cryostat has been developed. The design provides a 10-100 K operating range, with temperature stability better than 20 mK in the range 25-100 K, improving to 2 mK below 25 K. This temperature stability figure is obtained by balancing the cooling power due to the thermal coupling of the sample to liquid helium, with the heating power of a stream of room-temperature He gas flowing through a heat exchanger. The heating power is continuously regulated within a feedback loop by throttling the gas flow with a proportional valve, using the error signal from the temperature controller. The system provides good temperature stability without the need for low-resistance leads in the vicinity of the sample holder as those needed to feed an electrical heater. This is relevant to experiments where electromagnetic interference must be minimized. (C) 1997 American Institute of Physics.