As a member of the compact toroidal class of magnetic fusion devices, the spheromak [Nucl. Fusion 19, 489 (1979)] offers substantial advantage as a fusion reactor concept over larger, more complicated, and more costly re-entrant devices like the tokamak. The compact and simply closed geometry affording high energy density, the inherent diverted nature of the magnetic topology, the force free condition μ0j(r) = ƛ(ϕ)B(r) nature of the spheromak equilibrium minimizing external coil requirements and stresses, and the possibility of Ohmic ignition resulting from the majority of confining fields generated by internal plasma currents in the spheromak, are a few of the more prominent advantages that represent substantial improvement over conventional magnetic fusion reactor designs. Further, recent successes in improving confinement parameters (Te ~ 400eV, Ti ~ 1keV, ne ~ 3 × 1014cm-3, B ~ 1T) have renewed the interest in advancing this concept to a proof-of-principle, reactor prototype stage.
Here we extend the initial work by Fowler, et al. [Comments Plasma Phys. Controlled Fusion 16, 91 (1994)] indicating the possibility of Ohmic ignition in spheromaks, to a two fluid model that includes direct ion heating through turbulent Taylor relaxation mechanisms. The contribution to direct ion heating through this non-Ohmic magnetic dissipation, and confinement scaling are quantified through comparison with the latest results from the gun driven Compact Torus eXperiment (CTX) [Phys. Fluids B 2, 1342 (1990)] spheromak. We realize good agreement between experimentally measured plasma parameters and our model predictions. Extrapolation to an ignition class experiment is examined indicating the possibility of reaching these conditions by gun driven Ohmic heating alone, and illustrating the merits of direct ion heating on facilitating approach to ignition. Differences between classical (no direct ion heating) and direct ion heating cases are emphasized. Conservative confinement estimates are used throughout.
