Abstract
Research on the stability of spherical torus plasmas at and above the no-wall beta limit is being
addressed on the National Spherical Torus Experiment @M. Ono et al., Nucl. Fusion 40, 557 ~2000!#,
that has produced low aspect ratio plasmas, R/a;1.27 at plasma current exceeding 1.4 MA with
high energy confinement (TauE/TauE– ITER89P.2). Toroidal and normalized beta have exceeded
25% and 4.3, respectively, in q;7 plasmas. The beta limit is observed to increase and then saturate
with increasing li . The stability factor bN /li has reached 6, limited by sudden beta collapses.
Increased pressure peaking leads to a decrease in bN . Ideal stability analysis of equilibria
reconstructed with EFIT @L. L. Lao et al., Nucl. Fusion 25, 1611 ~1985!# shows that the plasmas are
at the no-wall beta limit for the n51 kink/ballooning mode. Low aspect ratio and high edge q
theoretically alter the plasma stability and mode structure compared to standard tokamak
configurations. Below the no-wall limit, stability calculations show the perturbed radial field is
maximized near the center column and mode stability is not highly effected by a nearby conducting
wall due to the short poloidal wavelength in this region. In contrast, as beta reaches and exceeds the
no-wall limit, the mode becomes strongly ballooning with long poloidal wavelength at large major
radius and is highly wall stabilized. In this way, wall stabilization is more effective at higher beta
in low aspect ratio geometry. The resistive wall mode has been observed in plasmas exceeding the
ideal no-wall beta limit and leads to rapid toroidal rotation damping across the plasma core.