Density Fluctuation Analysis of MHD Instabilities In WHAM

Density fluctuations are a sign of magnetohydrodynamic (MHD) activity in axisymmetric magnetic mirror devices, and understanding these fluctuations is essential to mitigating the underlying instabilities. Without finite Larmor radius stabilization from neutral beam injected fast ions, the most unstable modes are high-azimuthal-mode-number (high-m) flute instabilities. These modes are robustly observed in fast visible imaging of the plasma boundary in WHAM. This work compares visual observations with data from two complementary density diagnostics: a single-pass heterodyne interferometer with a 15 MHz intermediate frequency, and an array of secondary electron emission detectors that measure neutral beam shine-through. Channel-to-channel correlation analysis is used to infer spatial structure and dynamics of the flute modes. These findings inform the design of future diagnostics aimed at higher-resolution characterization of the mode spectrum and improved understanding of MHD behavior in the mirror configuration.