FORMATION OF A FLUX CORE SPHEROMAK

An alternate design for compact tori, specifically of the spheromak type, is studied. In this design, the "flux core spheromak" [Nucl. Fusion 29, 219 (1989)], the externally imposed bias field links the confinement region of closed flux surfaces. The advantages of this configuration are: (i) it enjoys greater stability to magnetohydrodynamic (MHD) modes, particularly the tilt and shift; (ii) it has a poloidal divertor, and an amount of poloidal flux separating the closed flux surface region from the walls; and (iii) it might be sustained by helicity injection. Results are presented showing the dependence of the geometry on the distribution of bias flux on the conducting walls and showing the optimization of the 2-D formation scheme to minimize the contact of the plasma with coils, electrodes, and walls. This last topic involves taking advantage of current sheet formation and subsequent tearing, as in formation of the MS spheromak [Phys. Fluids 28, 3154 (1985)]. The parameters which can be varied to produce this favorable formation scheme via tearing, rather than a formation that proceeds off the reversal coils, are explored. In addition, it is found that there is strong viscous heating of the ions in this early reconnection phase.