DIRECT ION HEATING EFFECT ON IGNITION CONDITIONS IN SPHEROMAKS

As a member of the compact toroidal class of magnetic fusion devices, the spheromak [Nucl. Fusion 19, 489 (1979)] offers substantial advanta ge as a fusion reactor concept over larger, more complicated, and more costly re-entrant devices like the tokamak. The compact and simply do sed geometry affording high energy density, the inherent diverted natu re of the magnetic topology, the force free ((j) over right arrow x (B ) over right arrow = 0 contains del x (B) over right arrow B = lambda (B) over right arrow) nature of the spheromak equilibrium minimizing e xternal coil requirements and stresses, and the possibility of Ohmic i gnition resulting from the majority of confining fields generated by i nternal plasma currents in the spheromak, are a few of the more promin ent advantages that represent substantial improvement over conventiona l magnetic fusion reactor designs. Further, recent successes in improv ing confinement parameters (T-e similar to 400eV, T-i similar to 1 Ke V, n(e) similar to 3 x 10(14)cm(-3), B similar to 1T) have renewed the interest in advancing this concept to a proof-of-principle, reactor p rototype stage. Here we extend the initial work by Fowler, et al. [Com ments Plasma Phys. Controlled Fusion 16, 91 (1994)] indicating the pos sibility of Ohmic ignition in spheromaks, to a two fluid model that in cludes direct ion heating through turbulent Taylor relaxation mechanis ms. The contribution to direct ion heating through this non-Ohmic magn etic dissipation, and confinement scaling are quantified through compa rison with the latest results from the gun driven Compact Torus experi ment (CTX) [Phys. Fluids B 2, 1342 (1990)] spheromak. We realize good agreement between experimentally measured plasma parameters and our mo del predictions. Extrapolation to an ignition class experiment is exam ined indicating the possibility of reaching these conditions by gun dr iven Ohmic heating alone, and illustrating the merits of direct ion he ating on facilitating approach to ignition. Differences between classi cal (no direct ion heating) and direct ion heating cases are emphasize d. Conservative confinement estimates are! used throughout.