Photodiode based multi-modal diagnostic for low-energy neutral beam injection in the LTX-β spherical tokamak
Abstract
We present a compact photodiode-based diagnostic array developed to study low-energy neutral beam injection in the LTX-β spherical tokamak. The in-vacuum diagnostic combines filtered soft-x-ray (SXR), narrowband Lyman-α, 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.
Versions
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BibTeX
@misc{maan2026photodiode,
title = {{Photodiode based multi-modal diagnostic for low-energy neutral beam injection in the LTX-$β$ spherical tokamak}},
author = {A. Maan and Tosh Le and D. P. Boyle and R. Majeski and S. Banerjee and G. J. Wilkie and M. Lampert and C. Lopez Perez and R. Shousha and W. Capecchi and H. Gajani},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2605.20993},
eprint = {2605.20993},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.20993},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Photodiode based multi-modal diagnostic for low-energy neutral beam injection in the LTX-β spherical tokamak
AU - A. Maan
AU - Tosh Le
AU - D. P. Boyle
AU - R. Majeski
AU - S. Banerjee
AU - G. J. Wilkie
AU - M. Lampert
AU - C. Lopez Perez
AU - R. Shousha
AU - W. Capecchi
AU - H. Gajani
PY - 2026
DA - 2026/09/07
PB - arXiv
DO - 10.48550/arxiv.2605.20993
UR - https://arxiv.org/abs/2605.20993
AB - We present a compact photodiode-based diagnostic array developed to study low-energy neutral beam injection in the LTX-β spherical tokamak. The in-vacuum diagnostic combines filtered soft-x-ray (SXR), narrowband Lyman-α, 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.
ER -