Photodiode based multi-modal diagnostic for low-energy neutral beam injection in the LTX-β spherical tokamak

We present a compact photodiode-based diagnostic array developed to study low-energy neutral beam injection in the Lithium Tokamak Experiment spherical tokamak. The in-vacuum diagnostic combines filtered soft-x-ray (SXR), narrowband Lyman-α, and unfiltered absolute extreme ultraviolet (AXUV) photodiode rows with partly overlapping, nearly coincident tangential views of the plasma, including the neutral beam path. This geometry provides simultaneous sensitivity to beam-induced SXR emission; neutral-hydrogen line radiation associated with recycling, fast neutrals, and fueling; and broadband emission that can include direct neutral impacts from fast-ion charge-exchange losses. Initial measurements from 12 to 20 keV hydrogen beam operation show beam-synchronous detector responses in all three modalities. The unfiltered AXUV signals exhibit millisecond-scale rise and fall times that vary across sightlines and depend on lithium-conditioning history. Comparison with classical slowing-down time estimates indicates that charge exchange with background neutrals contributes appreciably to the measured decay. The diagnostic can potentially be used to constrain a forward model to estimate the time-resolved balance of beam heating and fueling for small tokamaks.