NSTX-U employs a coordinated set of magnetic and soft X-ray diagnostics to detect and characterize magnetohydrodynamic (MHD) modes across a broad frequency range. A key strength of this diagnostic set is its frequency and mode-number coverage: high-frequency Mirnov coils resolve MHz-scale magnetic fluctuations; high-n Mirnov arrays enable toroidal mode number identification; RWM/LM sensors target low-frequency resistive wall and locked modes; and the Ultra Soft X-Ray (USXR) array provides internal emissivity fluctuations associated with low- to intermediate-frequency MHD activity.

Together, these diagnostics enable identification of coherent modes (e.g., TAEs, fishbones, RWMs, tearing modes), determination of toroidal and poloidal mode structure, measurement of mode frequency evolution, and correlation of magnetic activity with kinetic profiles, fast-ion behavior, and confinement transitions.

High-frequency Mirnov coils measure rapid magnetic fluctuations with bandwidth extending into the MHz range. These sensors are optimized for detecting high-frequency MHD activity, including Alfvénic modes and energetic-particle-driven instabilities. By sampling dB/dt at high rate, the system captures broadband spectra as well as coherent oscillations relevant to fast-ion transport and wave–particle interaction studies.

High-frequency Mirnov data are typically analyzed using spectrograms, cross-phase analysis, and mode tracking to identify frequency chirping, mode splitting, and coupling phenomena. These measurements are particularly important in high-β spherical tokamaks, where energetic-particle instabilities play a central role.

The high-n Mirnov array consists of toroidally distributed magnetic pickup coils designed to resolve toroidal mode number (n). By comparing phase differences between sensors, the system determines the toroidal structure of MHD modes, distinguishing between low-n global modes and higher-n localized or Alfvénic activity.

High-n identification is essential for characterizing tearing modes, resistive wall modes, and energetic-particle-driven instabilities. When combined with equilibrium reconstruction and kinetic profile measurements, mode-number analysis enables interpretation of stability thresholds and mode localization.

Dedicated sensors for resistive wall modes (RWMs) and locked modes monitor low-frequency magnetic perturbations associated with global instabilities. RWMs occur when plasma pressure exceeds the ideal-wall stability limit but is stabilized by plasma rotation; locked modes represent rotating instabilities that slow and couple to the vessel wall.

These sensors provide early detection of mode growth and rotation slowing, supporting disruption avoidance strategies and control experiments. In spherical tokamaks, where high-β operation is common, RWM detection is particularly important for performance optimization and machine protection.

The Ultra Soft X-Ray (USXR) array measures line-integrated soft X-ray emissivity along multiple poloidal chords, providing sensitivity to internal low- to intermediate-frequency MHD modes. Because soft X-ray emissivity depends on electron temperature and impurity content, coherent MHD activity appears as oscillations in chord-integrated brightness.

Poloidally separated sightlines enable identification of poloidal mode structure and radial localization. USXR measurements are commonly used to study sawteeth, tearing modes, internal kink modes, and other core MHD phenomena. When combined with Mirnov arrays, USXR provides complementary internal structure information beyond purely magnetic boundary measurements.