Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

Abstract

The Pegasus Toroidal Experiment [R. Fonck et al., Bull. Am. Phys. Soc. 41, 1400 (1996)] is a spherical torus designed to study the limits of plasma behavior as the aspect ratio A approaches unity. Access to near-unity A is achieved through the use of a novel high-stress reinforced solenoid magnet. High toroidal beta βt is obtained in ohmically-heated plasmas by operation at low field with densities up to the Greenwald limit. Values of βt up to 20% and normalized beta up to 5 have been obtained. The ratio of plasma current to toroidal field rod current, known as the toroidal field utilization, reaches values as large as 1 but appears to approach a “soft” boundary at that level related to both ohmic flux limitations and the onset of resistive magnetohydrodynamic (MHD) activity. The m/n=2/1 and 3/2 modes are most frequently observed, in agreement with the inferred safety factor profiles. Experiments are beginning to access the external kink stability boundary at edge safety factor q95=5, which is significantly higher than that observed in conventional tokamaks. Calculations using the DCON code [A. H. Glasser and M. S. Chance, Bull. Am. Phys. Soc. 42, 1848 (1997)] confirm instability to the ideal kink.