Plasma confinement in HBTX with a distant resistive shell

When HBTX was operated with a resistive shell (τw=0.5 ms) located at 1.15 times the minor radius, pulse lengths were limited to 3-5 ms. This limit was not imposed by the available voltseconds but was associated with the growth of thin shell modes. Using arrays of saddle coils placed far from the plasma outside the windings system (at 1.6 times the minor radius), radial magnetic fields up to 50 Gauss were detected from these modes. Analysis of these modes showed a strong correlation between the growth of the (m,n)=(1,2) external kink mode and the plasma duration. These studies prompted the idea that a secondary shell placed at the position of these coils might have a beneficial effect of suppressing the radial field and the growth of the (m,n)=(1,2) mode in particular. (author) 6 refs., 3 figs., 1 tab