Resistive Stability of 2/1 Tearing Modes in DIII-D Plasmas With 1/1 Resonance

The stability of multiple coupled resistive modes is examined using reconstructions of experimental equilibria in the DIII-D tokamak, revealing the physics involved in mode onset as discharges evolve to instability. Stability analyses of two discharges are compared, one high beta and one low beta, each with m=2/n=1 tearing mode onset and a m=1/n=1 resonant surface at a small minor radius. The physics dominating the onset of tearing modes in high beta tokamak discharges is often in dispute, and only recently have diagnostics and equilibrium reconstructions approached the accuracy necessary to decipher the subtle effects involved [1]. A low beta DIII-D experiment was therefore designed specifically to test theories of tearing mode stability [2], with a 2/1 tearing mode being driven by an induced current gradient. The analysis of this discharge allows a point of comparison for the analysis of high beta discharges, where the physics becomes more complicated. Experimental attempts to access the highest beta in tokamak discharges, including hybrid discharges, are typically terminated by the growth of a large 2/1 tearing mode. In hybrid discharges the plasma current is significantly noninductive, intended to lengthen the discharge time, while sustaining the baseline dimensionless parameters of a burning plasma experimen