Performance and Stability of Near-Unity Aspect-Ratio Plasmas in the Pegasus Toroidal Experiment

The Pegasus Toroidal Experiment [1] is a mid-sized, extremely low aspect ratio spherical torus. Access to near-unity aspect ratio is achieved through use of a novel high-stress reinforced solenoid magnet assembly. It has the dual roles of exploring the limits of spherical torus plasma behavior as the aspect ratio, A, approaches 1 and examining alternate confinement concepts in the tokamak-spheromak overlap regime. Unique MHD properties arise with possibilities of strong mode coupling at very high toroidicity. Operation with ohmic heating has started to demonstrate the high- stability capabilities of operation at near-unity aspect ratio. Major parameters are R = 0.25- 0.45 m, A = 1.1 – 1.4, = 1- 3.5, Ip ~ 0.15 MA, Bt < 0.07 T, and Paux(HHFW)=1 MW. Global magnetic parameters are obtained through equilibrium reconstruction using a wide array of magnetics diagnostics. The vacuum vessel is continuous and has a skin time on the order of the plasma formation time. Induced currents in the wall are generally greater than the plasma current, and are estimated with a coupled current-filament model. These results are used as “coil ” inputs to the equilibrium code and are constrained in reconstruction by wall-mounted flux loops and B-dot coils. Equilibria are reconstructed b