Measuring the evolution of ion canonical vorticity during RFP relaxation with stencils of polyhedral Mach probe clusters

Conversions between twisted magnetic flux tubes and ion flow vorticity flux tubes provide an alternative viewpoint of self-organized processes in plasmas. In the MST RFP, tearing instabilities drive a reorganization of current and momentum. Recent two-fluid simulations indicate that there can be a change in cross helicity, interlinking of magnetic and flow vorticity flux tubes, during sawtooth events. We are developing a canonical vorticity probe to measure the magnetic and ion flow vorticity fluxes simultaneously during sawtooth relaxation in MST. The probe will consist of polyhedral clusters of B-dot and Mach probes. The 3D Mach vector is determined from the geometric addition of the logarithms of ion saturation currents collected at the vertices of a polyhedron, not necessarily 180 degrees apart. The clusters will be arranged in a finite difference stencil so that the curl of these vectors can be determined. Two prototypes of a single cluster have been constructed [1] and fielded on the MST to examine self-organization and sawtooth relaxation. This work will help to generalize our understanding of plasma relaxation to a canonical-helicity-constrained relaxation explaining multi-scale dynamics.

[1] A. M. Sellner et al. Session 10.02 Undergradute Research (2021 & 2022).