PBX-M passive coil system reliability improvements

The major cause of lost run time during the start-up year of the Princeton Beta Experiment-Modified (PBX-M) was electrical insulator failures in the passive coil system. The passive coil system forms a close-fitting conducting stabilizing shell around the highly indented bean-shaped plasma. The system consists of five pairs of single-turn coils formed from stainless-steel-clad, explosion-bonded aluminum plates; each pair is connected in a saddle-coil arrangement for stabilizing n=1 kink modes. Because the passive coil system was retrofitted into the former PBX/PDX vacuum vessel with access only through man-access ports, the plates were divided into 70 segments which were bolted together to form the coils. In all, 723 electrical insulators are used. Failures experienced were of three major types: (1) severe mechanical damage due to I×B forces from currents flowing unexpectedly in the coil supports; (2) sudden momentary rises in the vacuum-vessel background pressure due to outgassing of failed polyimide insulators; (3) arcing over great distances along the toroidal field lines. The nature of the failures, the results of voltage measurements which were used to help guide the design changes, and the design changes made to improve reliability are described