NSTX-U integrates multiple fast-ion diagnostics to characterize energetic particle phase space, resolving energy, pitch angle, radial distribution, and loss dynamics. In spherical tokamaks, fast ions produced primarily by neutral beam injection (NBI) carry a substantial fraction of the total plasma energy and contribute to current drive, heating, and stability. Because NSTX-U operates at relatively high β and low aspect ratio, energetic-particle confinement and redistribution are especially sensitive to magnetohydrodynamic (MHD) activity.
A key strength of this diagnostic set is its phase-space coverage. Fast-Ion D-Alpha (FIDA) spectroscopy provides spatially and velocity-resolved measurements of confined fast ions; Neutral Particle Analyzers (NPA) measure energy-resolved escaping neutrals and prompt loss; neutron diagnostics constrain the global fast-ion population; and magnetic fluctuation measurements link energetic-particle behavior to Alfvén eigenmodes and other MHD instabilities.
Together, these diagnostics span confined and lost populations, active beam-driven and redistributed fast ions, and time-resolved responses to MHD events. Their combined use enables reconstruction of portions of the fast-ion distribution function, quantification of instability-driven transport, identification of resonances between energetic particles and modes (e.g., TAEs, fishbones), and validation of orbit-following and transport modeling during changes in heating, confinement regime, or plasma equilibrium.
By integrating active measurements (directly viewing beam-induced fast ions) with passive measurements (sensitive to background or redistributed populations), NSTX-U fast-ion diagnostics provide a multi-perspective view of energetic-particle confinement in high-β spherical tokamak plasmas.
SSNPA provides energy- and pitch-sensitive measurements of the fast-ion population by detecting charge-exchanged fast neutrals that escape the plasma. When energetic ions undergo charge exchange with injected beam neutrals (active) or background neutrals (passive), a fraction of the resulting fast neutrals leave the confinement region ballistically. SSNPA systems measure this escaping neutral flux using compact solid-state detectors with high bandwidth, making them particularly useful for observing fast-ion redistribution and loss during MHD activity.
NSTX-U employs multiple SSNPA viewing geometries to sample different regions of velocity space and different loss/redistribution pathways. The Bay views below were designed to separate sensitivity to passing vs trapped fast ions and to distinguish active signals (beam-viewing) from passive signals (non-beam-viewing), enabling cross-checks on background charge-exchange and edge-neutral effects.
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FIDA spectroscopy measures Doppler-shifted Balmer-α emission produced when fast deuterons undergo charge exchange and subsequently emit Dα light. Because the Doppler shift depends on the fast-ion velocity component along the line of sight, FIDA provides localized and velocity-space–selective information about the confined fast-ion distribution. In practice, NSTX-U FIDA measurements are typically separated into active components (charge exchange with injected beam neutrals) and passive components (charge exchange with background neutrals), enabling interpretation of beam-driven signals and background/edge-neutral contributions.
The NSTX-U FIDA suite includes multiple geometries that emphasize different pitch-angle regions and radial locations. Vertical and tangential views provide complementary sensitivity to trapped vs passing populations and to changes driven by Alfvén eigenmodes, fishbones, sawteeth, and other fast-ion–active MHD phenomena. High-speed implementations extend this capability to rapidly evolving events by increasing the time resolution of the optical system and detectors.