Instability Induced Current Redistribution in a Spherical Torus

Peter Norgaard, Princeton University

The tokamak is a leading reactor concept for magnetic confinement fusion energy. Many of these devices, such as the National Spherical Torus Experiment (NSTX), use neutral beams to inject fast ions, which provide heating and drive the current necessary for stability and confinement. I am presently studying fast ion dynamics in the presence of large scale magnetohydrodynamic (MHD) instabilities. Experimental data indicates that the fast ion current redistributes during periods of high MHD mode activity. This influences the tokamak equilibrium profile and therefore affects the overall stability and confinement.

To model the system dynamics I numerically integrate the trajectories of thousands of representative test particles as they transit the equilibrium magnetic field with a superimposed time-dependent perturbation mode. Issues of computational difficulty include the relatively disparate time scales of the ion motion and the instability growth time, which motivates the use of gyro-period averaging methods. It may also be necessary to solve the fully coupled problem, whereby the fast ion current redistribution modifies the MHD mode structure. The implications of this research may be significant for larger burning plasma experiments, such as ITER, and for future fusion reactor designs.

Abstract Author(s): P.C. Norgaard, C.W. Rowley, J.E. Menard, R.B. White