ION ENERGY MEASUREMENTS IN THE SPHEX SPHEROMAK

A neutral particle analyser (NPA) and high-resolution spectrometer have been used to make ion energy and rotation measurements during the driven and decaying phases of SPHEX, the UMIST spheromak. During the driven phase NPA spectra reveal the presence of a small (less than or equal to 0.5%) population of ions, indistinguishable from a Maxwellian population, at a temperature of 300 eV. Measurements made in decaying plasmas indicate that the hot ions are closely associated with the current drive process and, in particular, with a region of high potential in the plasma known as the 'central column'. At the onset of the decay phase we find the central column density, gun current and hot-ion population all decay at a similar rate (<< 50 mu s). The surrounding toroidal plasma, however, has a separate and much slower decay rate (similar to 250 mu s) which is better correlated with the properties (rotation and temperature) of the bulk of the ion distribution.