Spectroscopic studies of plasma collapse in the advanced toroidal facility

Wall conditioning and chromium gettering have reduced Zeff in the Advanced Toroidal Facility (ATF) to the range of 1.5-2.2. As a result, it is possible to achieve essentially steady state conditions using electron cyclotron heating alone, without the uncontrolled growth of the electron and impurity densities observed in the very earliest discharges, which always collapsed to low temperature afterglows. Plasma collapses are, however, still observed during neutral beam injection. It is known that large islands have been present in ATF during its initial periods of operation, but it cannot be determined whether they influence any aspect of the collapse. The impurity behaviour and the radiation losses in this perturbed magnetic configuration are discussed. (author). Letter-to-the-editor. 19 refs, 8 figs, 1 tab