Modified magnetic confinement configurations based on understanding helicity dissipation and pressure driven modes in the spheromak

Recent results from the spheromak work at Los Alamos include: In decaying spheromaks in the mesh flux conserver, the rate of current decay dI/dt depends only on the density n/sub e/ (not on I or T/sub e/ as might be expected classically). The particular dependence of dI/dt on n/sub e/ suggests that most of the helicity is dissipated in the field error regions at the edge where electron-neutral hydrogen collisions dominate the electrical resistance. A new solid-wall, titanium-gettered flux conserver has been commissioned, resulting in less field errors than with previous mesh flux conserver. A factor of four decrease in dI/dt has been observed. In this new flux conserver, clear evidence of a pressure-driven instability has been obtained. To our knowledge, this is the first time a pressure driven interchange mode has been directly observed in a toroidal geometry. Peak β values observed before the onset of the mode are of the same order as those predicted at the corresponding Mercier limits (β/sub c/ ∼ 1%). As a result, flux conserver shapes with higher β/sub c/ are now being considered by us. The β/sub c/ calculations have been made using Taylor-like (minimum energy) magnetic profiles. By changing the flux conserver shape, the q profile of the minimum-energy state can be varied continuously from RFP-like to tokamak-like. The highest β/sub c/ (∼10%) are found at these two extremes of the range of magnetic configurations. 3 refs