Effect of Surface Oxidation on Stability of LHD Conductor Immersed in Pressurized<tex>$rm He$</tex>II

Abstract

Stability tests were performed on two small test coils wound with the Large Helical Device (LHD) conductors with different surface conditions. One used the conductor with a chemically oxidized copper surface, which is the same as that used for the LHD helical coils. Another used the same structural conductor with a polished copper surface. The stability tests were performed for the magnetic flux densities from 3 T to 7 T and at the bulk liquid helium temperatures from 2.0 K to 4.2 K at atmospheric pressure. Stability limit current for the LHD test coil was defined as a minimum dynamic propagation current, beyond which a short normal zone initiated by a thermal disturbance dynamically propagated to both sides along the conductor. There was a slight difference in the stability limit currents for the two test coils in case of He I cooling. However, the stability limit current for the polished coil was significantly improved as compared with that for the oxidized one in case of He II cooling. The Kapitza conductance on an oxidized copper surface is known to be quite smaller than that on a polished copper surface. One of the main factors that govern the stability of the LHD conductor cooled by He II is the Kapitza conductance