Transient loss from a theta pinch with an initial trapped reverse magnetic field

Results of an experimental study of the transient phenomena occurring with the loss of plasma from a 25 cm long theta pinch discharge which trapped an initial reverse magnetic field are presented. The plasma was doubly-ionized helium with ion density Nsub(i)= 2 x 1016 cm-3 and temperature Tsub(i) = 15 eV at midcoil, and with Nsub(i) = 0.5 x 1016 cm-3 and Tsub(i) = 6 eV at a position 2.5 cm outside the theta coil. Local, high temporal resolution (0.1 μs) diagnostics included: diamagnetic loops, plasma luminosity, magnetic field, electron density, electron temperature, and impact pressure of the flowing plasma. Rotation was observed to produce an m = 2 instability after annihilation of the trapped reverse magnetic field. Plasma flow from the coil did not begin until after collapse of the plasma column. By combining impact pressure measurements with local density data, the velocity of the end-loss plasma is obtained. Axially directed electromagnetic force interactions are found to have had a minimal effect on the plasma loss; the primary driving mechanism for plasma loss is identified as the axial pressure gradient arising from axial gradients in both density and temperature. In the end region of the theta pinch, the expanding plasma is preceeded by an outward propagating front which forms; it is found to ionize and transfer momentum to the ambient background gas. (author)