Experimental studies of compact toroids

The Berkeley Compact Toroid Experiment (BCTX) studies a spheromak-type magnetically confined fusion configuration. In BCTX the compact toroid plasma is formed in a 70 cm diameter flux conserver by injection from a magnetized Marshall gun. The BCTX experiment has an RF heating system which can produce up to twenty megawatts of RF power at 450 MHz for 100 microseconds. Experiment and computer modeling predictions show good agreement for the formation of a spheromak. Equilibrium magnetic field measurements show that the plasma tends to equilibrate to a low-beta Taylor state for a broad range of PF coil currents. The appearance of toroidal fields at the plasma edge support the existence of a Z-pinch on the geometric axis in some shots. RF power can be used to lengthen the gun current pulse and potentially can sustain a Z-pinch discharge in some cases. The production of higher electron temperatures in RF-heated BCTX discharges is critically dependent on lower electron densities. The experimentally observed lack of correlation between core electron temperatures and the magnetic decay time is a result which is inconsistent with the presence of parallel heat conduction throughout the plasma volume. A model of the system which couples a highly conducting core plasma with significant MHD activity to a relatively cold and lossy edge plasma may describe the observed performance of BCTX. (Summary modified)