Performance of the Conduction-Cooled LDX Levitation Coil

Abstract

The Levitated Dipole Experiment (LDX) was developed to study plasma confinement in a dipole magnetic field. Plasma is confined in the magnetic field of a 680‐kg Nb3Sn Floating Coil (F‐coil) that is electromagnetically supported at the center of a 5‐m diameter by 3‐m tall vacuum chamber. The Levitation Coil (L‐coil) is a 2800‐turn, double pancake winding that supports the weight of the F‐coil and controls its vertical position within the vacuum chamber. The use of high‐temperature superconductor (HTS) Bi‐2223 for the L‐coil minimizes the electrical and cooling power needed for levitation. The L‐coil winding pack and support plate are suspended within the L‐coil cryostat and cooled by conduction to a single‐stage cryocooler rated for 25‐W heat load at approximately 20 K. The coil current leads consist of conduction‐cooled copper running from room temperature to 80 K and a pair of commercially‐available, 150‐A HTS leads. An automatically filled liquid‐nitrogen reservoir provides cooling for the coil’s radiation shield and for the leads’ 80‐K heat stations. This paper discusses the L‐coil system design and its observed cryogenic performance.