Deep Insertion Probe Measurements on MST

Recent measurements and numerical simulations expose the importance of multiple fluctuation-induced forces and stresses in the self-organization processes of the RFP. Probe measurements in the region $\frac{r}{a} > 0.8$ show that the MHD and Hall dynamo terms $( \langle \tilde{\bf{v}} \times \tilde{\bf{b}} \rangle _ \Vert \mbox{and} \langle \tilde{\bf{j}} \times \tilde{\bf{b}} \rangle _\Vert )$ are both large, but with opposite trends in their radial profiles. Two-fluid NIMROD simulations predict complex radial profiles for these quantities, where one dominates the other in different regions. Fluctuation measurements deeper in the plasma would be valuable, and a magnetic probe for measuring $\langle \tilde{\bf{j}} \times \tilde{\bf{b}} \rangle _\Vert$ is first in development. Aided by MST's new programmable toroidal field power supply, RFP plasmas are reliably produced at low plasma current, allowing probe insertion to $\frac{r}{a} > 0.6$. The plasma parameters (e.g., Lundquist number) are closer to simulation values, making direct comparison with simulation more straightforward. Pseudo-spectral analysis will be used to measure the radial profile of the tearing mode structure, to compare with predictions from NIMROD and DEBS (single-fluid MHD).