Dielectric breakdown and tokamak start-up with FBX spherical torus

Summary form only given. One of the main engineering breakthroughs in tokamak devices can be obtained by decreasing the excess magnetic flux consumption in the initial plasma production phase. Without this bottle-neck, the tokamak designers would be relieved from the yoke of ´the millisecond trial´: (1) Transformer coils should have a tolerance of 10/sup 2/ times the normal operation voltage. (2) Sophisticated vacuum vessels with high resistivity. Both of them are required only for the first few milliseconds, less than 10/sup -4/ of total plasma operation duration. The FBX is a spherical torus. However, in the initial phase, the dielectric breakdown is initiated in the local average minimum B zone and a plasma with an aspect ratio around five is produced. Just as the other tight aspect ratio system, this excessive flux consumption is critical. For the steady operation, it carries six rod-coils to provide a poloidal 400 kA plasma current and a 0.5 Tesla toroidal field in the magnetic axis. Toroidal multipole field coils are equipped to produce 100 kA toroidal plasma current and the 300 eV plasma is sustained in a poloidal diverter formation. The modification of plasma operation in this initial current rise phase is discussed with theory.