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

A. Maan; Tosh Le; D. P. Boyle; R. Majeski; S. Banerjee; G. J. Wilkie; M. Lampert; C. Lopez Perez; R. Shousha; W. Capecchi; H. Gajani

arXiv · 2605.20993

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

A. Maan·Tosh Le·D. P. Boyle·R. Majeski·S. Banerjee·G. J. Wilkie·M. Lampert·C. Lopez Perez·R. Shousha·W. Capecchi·H. Gajani

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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.

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A. Maan, Tosh Le, D. P. Boyle, R. Majeski, S. Banerjee, G. J. Wilkie, M. Lampert, C. Lopez Perez, R. Shousha, W. Capecchi, H. Gajani. 2026-05-20. Photodiode based multi-modal diagnostic for low-energy neutral beam injection in the LTX-$\beta$ spherical tokamak. https://arxiv.org/abs/2605.20993

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