Results from HBTX1C with close and distant resistive shells

Summary Form only given, as follows. Results are presented on the HBTX reversed field pinch operating with thin (resistive) shells. In experiments with a shell having a time constant tau /sub W/=0.5 ms, the pulse length was limited by thin shell modes, and there was an increase in the plasma resistance and a degradation in the confinement time. Theoretical studies show the growth of m=0 and m=1 resistive thin shell modes, resonant outside the field-reversal surface, and ideal nonresonant thin shell modes originating both inside and outside this surface. The main groups of unstable modes predicted by linear theory are observed experimentally and either rotate slowly or not at all in contrast to the thick shell. With a secondary shell, tau /sub w/=5.5 ms, positioned outside the windings, the pulse duration is doubled and the electron temperature increased. The growth rate of low-n modes is reduced in accord with theory, but that of high-n modes is reduced by more than the predicted amount due to nonlinear processes. The horizontal position of the plasma with the secondary shell has been controlled to +or-1 mm by feedback. A preliminary experiment to control thin shell modes by feedback has given promising results.<>