In order to better understand the behaviour of both neutral beam injected and spontaneously generated fast ions in the Madison Symmetric Torus reversed-field pinch, we have developed the full orbit-following code random ion orbits (RIO). The low magnetic field and relatively large level of MHD activity present in MST require a full orbit code as the guiding centre assumptions are violated even for ions with modest energy. Furthermore, quasi-periodic bursts of MHD activity (sawteeth) generate large transient electric fields and significant modifications to the equilibrium magnetic fields. Understanding the full effect of these sawteeth on the spatial and velocity distribution of the fast ions is of great interest. To this end, RIO now has the ability to take the full 3D, time evolving, magnetic and electric fields produced by the visco-resistive MHD code DEBS as input. In static cases, where broad-spectrum magnetic perturbations from DEBS are input, but fixed in time, beam injected ions are found to be generally well confined with the core fast ion density profile largely unaffected by the magnetic modes while the fast ion density in the mid-radius is substantially reduced. In the dynamic case, the large amplitude magnetic fluctuations that occur at the sawtooth crash produce substantial fast ion loss. Those fast ions that are not lost are accelerated by a large, transient, parallel electric field in the co-current direction. This causes the average energy of the beam ions to increase by ∼20%, consistent with recent experimental measurements.
MHD-induced energetic ion loss during H-mode discharges in the National Spherical Torus Experiment