Characterization of Beam-Driven Instabilities in Several RFP Equilibria

Short-lived, coherent bursts are observed in MST plasmas during 1 MW neutral beam injection (NBI). Fast hydrogen ionizes with v$_{\vert \vert }$/v $\approx $ 0.9, creating a spatially localized beam-like population of fast ions. Primary bursts have mode numbers $n$=5, $m$=1 for magnetic equilibria with edge safety q(a) = 0, and $n=$6$, m$=1 for q(a) $<$ 0 discharges. The frequencies of these modes scale with beam velocity. Secondary bursts with $n$'=$n$-1 exhibit an Alfv\'{e}nic scaling of frequencies, as well as fast down-chirping under varying plasma conditions, including low-current ($\le $ 200 kA) discharges and inductive current profile control (PPCD) cases. Burst prevalence and frequencies exhibit sensitivity to NBI parameters, the q-profile, Alfv\'{e}n speed, and to 3D effects such as the onset of a Single Helical Axis (SHAx) state. Reduced-MHD calculations for Alfv\'{e}n continuum frequencies have been performed for q(a)=0, standard q(a)$<$0, and PPCD cases. ~~The VMEC 3D equilibrium code computes both axisymmetric and 3D SHAx equilibria. We plan to use the V3FIT 3D equilibrium reconstruction~code to generate equilibria consistent with observed diagnostics. Work supported by US DoE.