Simulation studies of high-β disruptions and alpha particle loss in reversed shear plasmas

The 3D nonlinear MH3D code is used to study high-β disruptions and alpha particle loss during the disruption in reversed shear(RS) plasmas. In normal shear plasmas, it had been found that toroidally localized high-n ballooning modes can be driven unstable by the local pressure steepening at the bad curvature region which arises from the evolution of low-n modes. Nonlinearly, the high-n mode becomes even more localized and produces a strong local pressure bulge which destroys the flux surfaces resulting in a thermal quench. A similar behavior is also seen in simulations of RS plasmas. However, in RS, disruptions can also be caused by low-n modes alone without a toroidally localized high-n ballooning mode. Brief descriptions of the simulation results of the two types of disruption in RS plasmas, one with high-n ballooning modes and the other without, follow. The initial equilibrium of the first case is unstable to the n=1 ballooning mode with an m=2 dominance. The nonlinear evolution of this mode results in a 3D equilibrium with two local pressure steepenings, one inside the RS core region and the other outside. A toroidally localized high-n ballooning mode grows out of the local steep pressure region outside the RS core, and eventually destroys the flux surfaces resulting in a thermal quench. The toroidally localized steep pressure gradient inside the RS core, although much stronger than the one outside, remains stable, showing that the advantage of RS configuration regarding pressure driven modes extends fax into the 3D configuration. The second example has a similar initial linear behavior, except that the saturation amplitude is smaller than in the first case. Therefore, the resulting local pressure gradient is smaller, and is not enough to drive a toroidally localized high-n ballooning mode. The low-n modes however, undergo a very large m=2 distortion and the tip of this deformation pushes out toward the plasma surface