Observations of High-Beta Toroidal Plasmas

A wide range of magnetohydrodynamically stable high-$\beta$ plasmas is produced in the University of Wisconsin Levitated Toroidal Octupole. Near the single-fluid regime we obtain, in the bad-curvature region, $\beta =nk(T_e+T_i)\frac{8\pi }{B^2}\approx 8$%, twice the theoretical single-fluid ballooning instability limit of 4%. We also obtain stable plasmas at $\beta =35\mathrm{}$%, nine times the theoretical limit, in a regime in which finite-ion-gyroradius (e.g., gyroviscosity) effects are important. Experimental spatial profiles of the equilibrium diamagnetism are compared with theory.