A thermally insulated vessel contains a thermal insulation barrier defining an upper compartment above the barrier and a lower compartment below the barrier. The compartments are interconnected by a passage to allow pressure equalization. High temperature superconductor is mounted within the lower compartment for immersion in the liquid cryogen. A cryogenic refrigerator has a cold head thermally coupled to the high temperature superconductor for maintaining the high temperature superconductor below a superconductive transition temperature. A temperature controller maintains a temperature of the liquid cryogen in the upper compartment at a temperature of at least a boiling point of the liquid cryogen at atmospheric pressure when the lower compartment and at least a portion of the upper compartment are filled with the liquid cryogen.