Impact of Trapped Helium Gas Bubble in Liquid Helium on the Cooling in High Magnetic Field

Liquid helium is used as the coolant for high field superconducting magnets or samples tested in the high magnetic field at the National High Magnetic Field Laboratory. When the magnetic field is ramping up, the boil-off helium gas generated by heat losses in the coil and from external environment of the cryostat can be trapped in the liquid helium bath in a region where Bz · dBz/dz is less than -2100 T2 due to the weakly diamagnetic property of helium. It results to the surfaces of the magnet or samples being covered partially or even fully with the accumulated helium gas instead of liquid helium. The very low thermal conductivity of helium gas may cause an inefficient cooling for the magnets or samples. In the beginning stage of YBCO coils development, the trapped gas bubble caused a temperature increase in the region of trapped gas bubble and prevented the test coils ramped up to a higher current. This raised an issue for the cooling of the 32-T magnet, which is to be cooled in a liquid helium bath, because the trapped gas bubble may cause inefficient cooling of the magnet in this gas bubble region. Relevant tests were performed, and the trapped gas bubble was observed in recent tests of the 32-T magnet prototype coils. The impact of the gas bubble on the cooling of the 32-T YBCO prototype coils was tested, and the test results are presented.