Lessons learned from the cool down of a superconducting magnet using a thermal-siphon cooling-loop

The two Michigan State University (MSU) cyclotron gas-stopper magnet superconducting-coils were designed to be cooled down and to be kept cold using three pulse-tube coolers per coil cryostat. These coolers are designed to produce from 1.3 to 1.7 W per cooler when the cooler first-stage is at 40 K. The cyclotron gas stopper coils can be separated while cold, but unpowered. The two coil cryostats were cooled down separately in 2014, and room temperature helium gas was liquefied into the coil cryostats. The magnet temperature at the end of the cool-down was 4.55 K for one coil and 4.25 K for the other with and added 1.6 W of heat. The coil-down time for the coils was three and a half times longer than expected. The time to liquefy the helium was also much longer. The reasons for the disparity between the calculated cool-down time and measured cool-down time are discussed in the paper.