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).