Helium boil-off experiments for a superconducting magnet cryostat with pulse tube cryocooler

Recent helium re-condensing technology has been widely applied in the field of superconducting magnet. The pulse tube (PT) cryocoolers have achieved great application for its advantage of no moving parts in the cold end, high reliability, low vibration and noise. In our study, a two-stage PT cryocooler is employed to provide the cooling power for a nuclear magnetic resonance (NMR) superconducting magnet cryostat. In our tests, it has been shown that the superconducting magnet cryostat can realize a zero liquid helium boil-off during static operation. To obtain the liquid helium boil-off losses when the cryocooler is stopped, the cryocooler is shut down for seven hours continuously. The liquid helium evaporation rate test results show that the volume of liquid helium decreases from 63.6% to 56.3%, and the liquid helium evaporation rate is 0.83 L/h approximately. Based on the test results, the liquid helium will wholly evaporate in several days for similar superconducting magnet system even when the liquid helium cryostat is refilled.