Redistribution of Energetic Ions During Reconnection Events in NSTX

The motion of fast ions during a sawtooth event in the spherical torus NSTX is studied with the code GYROXY, which has been extended to simulate the perturbation of the electromagnetic field during a sawtooth crash. The dominant mechanisms of the particle redistribution are elucidated. It is concluded that the resonance interaction of particles with the perturbation and the stochasticity enhanced by the plasma diamagnetism are sufficient to account for the observed strong drops of the neutron yield provided that the pre-crash profile of the safety factor is sufficiently flat in the core.