Design and test of a liquid helium cryostat with automatic level control for cooling a superconductive single-flux-quantum circuit chip

A small-scale liquid helium cryogenic system has been designed for cooling the superconductive single-flux quantum circuits. With a special double inner cylinders structure and a PID control system, automatic control of the liquid helium level could be achieved. The cooling capacity recovered structure was added to the cold shield of the system in order to reduce the heat leakage of the system. Besides, A special composite magnetic shielding structure was designed to ensure the circuits working in an extremely low magnetic environment. Experiments were conducted to test the thermal performance and the magnetic shielding efficiency of the liquid helium cryogenic system. The results show that the storage time of the liquid helium is about 2 days with about 110 coaxial cables and the magnetic flux density at the chip location is below 5 nT at the temperature of 4.2 K.