Fast Ion Transport in the Quasi-Single Helical Reversed-Field Pinch

At sufficiently high Lundquist number, the reversed-field pinch (RFP) transitions to a 3D-helical equilibrium known as quasi-single helicity (QSH). A helical axis forms when the innermost resonant tearing mode grows and envelops the magnetic axis while the secondary modes maintain, or decrease, their amplitudes. This new, self-organized state has been shown to create strong thermal transport barriers, making QSH a possible scenario for a low-magnetic-field ohmic fusion reactor (Lorenzini et al. Nature 2009). The behavior of fast ions in the QSH state, however, poses a critical question for the helical RFP’s fusion relevance. Current work on MST investigates the dynamics of fast ion transport during QSH using neutral beam-born ions. Energetic particle modes (EPMs) on MST upshift in frequency with increasing core-resonant mode amplitude and disappear in high current QSH plasmas. FIR interferometry has resolved electron density perturbations associated with the EPMs and shows the EPMs moving radially outward during the QSH transition. The 3D shear-Alfven continuum solver STELLGAP describes the frequency rise as an effect of the equilibrium change on the fast ion resonance. Additionally, neutral particle analysis and neutron flux measurements suggest fast ion losses at sufficient core-resonant mode strength. The particle tracking code ORBIT corroborates rapid fast ion loss times in QSH and demonstrates weak neoclassical effects. ORBIT results indicate that the rapid fast ion transport depends heavily on the presence of the secondary tearing modes. While thermal particle orbits display reduced stochasticity and improved confinement, fast ion orbits display the opposite. While this presents an obstacle in the exploitation of the QSH regime for fusion, data in Lundquist number scaling exhibits a reduction in the secondary mode strengths, lessening their impact on fast ions. This research is supported by US DOE.