NSTX and NSTX-U employ dedicated diagnostics to characterize high-harmonic fast wave (HHFW) coupling, near-field structure, and edge plasma conditions in the scrape-off layer (SOL) adjacent to the antenna. A key strength of this diagnostic set is its local antenna-region sensitivity: the HHFW RF Coupling Diagnostic measures RF fields, power, and spectral content in the SOL for coupling optimization, while the HHFW Reflectometer measures electron density near the antenna to quantify loading conditions and wave accessibility.

Together, these diagnostics link antenna loading, edge density evolution, and RF power deposition performance. Their combined use enables optimization of coupling efficiency, identification of parasitic RF effects (e.g., sheath-driven phenomena), and evaluation of edge density control strategies that influence wave accessibility in the spherical tokamak regime.

The HHFW RF Coupling Diagnostic measures radio-frequency electric fields, local RF power density, and spectral characteristics in the SOL region near the HHFW antenna structure. These probe-based measurements provide a direct assessment of antenna-plasma coupling and near-field wave structure. By resolving RF amplitude and frequency content, the diagnostic supports evaluation of coupling efficiency, spectral purity, and operational optimization during high-power RF heating scenarios.

Measurements from this system are used to assess changes in edge loading conditions, identify anomalous field enhancements, and correlate RF performance with density and magnetic geometry. Such measurements are particularly important in spherical tokamaks, where strong shaping and low aspect ratio can influence edge accessibility and coupling sensitivity.

The HHFW Reflectometer measures electron density (n_e) in the vicinity of the HHFW antennas using microwave reflectometry. By launching microwaves and detecting reflections at cutoff layers determined by local density, the system provides localized density measurements in the antenna-facing SOL region. These measurements are directly relevant to RF loading conditions and wave accessibility.

Because HHFW coupling efficiency depends sensitively on edge density, real-time and shot-to-shot reflectometry measurements are used to interpret variations in antenna loading and RF performance. The reflectometer complements RF probe data by providing the plasma-side boundary condition needed to model antenna-plasma interaction.