Design and fabrication of the cryostat for the floating coil of the Levitated Dipole Experiment (LDX)

The Levitated Dipole Experiment (LDX) is a new, innovative magnetic confine ment fusion experiment being designed and installed in collaboration with C olumbia University at the Massachusetts Institute of Technology (MIT). The primary objective of the experiment is to investigate the possibility of st eady-state, high-beta plasma confinement with near classical transport. The main component of the experiment is a levitated cryostat with a 5.7 T Nb3S n superconducting magnet, housed in an Inconel high pressure helium vessel. The pressure vessel is surrounded by a large thermal mass radiation shield and an outer vacuum shell, all of which are magnetically levitated inside a much larger vacuum chamber. The cryostat, now under construction, is desc ribed in this paper. The cryostat keeps the magnet temperature between 5 an d 10 K during 8 hours of levitated operation. A low heat leak support syste m for the helium vessel and the shield is designed to withstand impact forc es of 10 g in case of a levitating failure. The helium vessel is filled to 125 atm at room temperature with 1.4 kg of helium. The helium vessel and th e shield are equipped with a tube heat exchanger for initial nitrogen magne t cooling and daily helium re-cool from a high of 20-25 K back down to 5K. This cooling system uses hermetically sealed, retractable cryogenic transfe r lines when the cryostat is resting in the bottom charging station of the LDX vacuum chamber. The magnet is charged and discharged inductively by an outer charging magnet.