A canonical vorticity probe formed by four tetrahedral clusters of Mach and magnetic probes to measure the evolution of ion canonical vorticity during RFP relaxation*

Conversions between twisted magnetic and ion flow vorticity flux tubes provide an alternative viewpoint of self-organized processes in plasmas. We have built a canonical vorticity probe capable of simultaneously measuring the magnetic field and ion flow vorticity during sawtooth relaxation in RFP plasmas on MST. The probe consists of tetrahedral clusters of 3x magnetic (B-dot) and 4x Mach probes. The three-dimensional 3D Mach vector is determined from the geometric addition of the logarithms of ion saturation currents collected at the vertices of a tetrahedron, not 180 degrees apart. The clusters are arranged in a tetrahedral finite difference stencil so that the curl of these vectors can be determined. The full probe an two prototypes of a single cluster have been constructed [1] and fielded on MST to examine self-organization and sawtooth relaxation. 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, the interlinking of magnetic and flow vorticity flux tubes, during sawtooth events. This work will help generalize our understanding of plasma relaxation to a canonical-helicity-constrained relaxation explaining multi-scale dynamics.