Experimental identification of low frequency modes in the Helically Symmetric eXperiment

Evolution of magnetohydrodynamic (MHD) instabilities on short time scales and macroscopic length scales threatens transport and plasma stability, especially during the high beta plasma operations expected in future power plants. However, the universality of MHD instabilities among stellarators is largely unknown, due to each experiment's unique geometry. Furthermore, MHD instabilities are often highly nonlinear, complicating predictive simulations. The Helically Symmetric eXperiment (HSX) is a four field-period quasihelically symmetric stellarator and is capable of operating in a variety of configurations. We present results following experiments in a magnetic hill configuration, which is characterized by proximity to a 1/1 resonance near the plasma edge, low magnetic shear, and is expected to be MHD unstable. Data analysis from this configuration identified multiple low frequency modes ranging from 300 Hz to 2 kHz. Modal data from interferometry, electron cyclotron emission (ECE), and magnetic diagnostics were compared to identify density, temperature, and magnetic activity in the experiment. Use of correlation analysis identified particularly strong modal dependencies on temperature and density, giving insight into the origins of each mode. Future work involves comparison of experiment data with M3D-C1, a high-fidelity MHD simulation code, to confirm the origins of identified modes.