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

TL;DR

This paper introduces a compact, multi-modal photodiode diagnostic array for studying low-energy neutral beam injection in the LTX-$eta$ spherical tokamak, enabling simultaneous measurements of various plasma emissions.

Contribution

The development of a novel in-vacuum photodiode array that captures multiple plasma emission modalities with nearly coincident views, providing comprehensive beam and plasma diagnostics.

Findings

Initial measurements show beam-synchronous responses in all modalities.

AXUV signals exhibit slow rise and fall times influenced by lithium-conditioning.

Charge exchange with background neutrals significantly affects the decay of signals.

Abstract

We present a compact photodiode-based diagnostic array developed to study low-energy neutral beam injection in the LTX-$\beta$ spherical tokamak. The in-vacuum diagnostic combines filtered soft-x-ray (SXR), narrowband Lyman-$\alpha$, and unfiltered 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 12-20 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 are much slower than the detector response, 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.