Abstract
A new compact, multi-channel Solid State Neutral Particle Analyzer (SSNPA) diagnostic based on silicon photodiode array has been designed and is being fabricated for the National Spherical Torus Experiment-Upgrade (NSTX-U). The SSNPA system utilizes a set of vertically stacked photodiode arrays in current mode viewing the same plasma region with different filter thickness to obtain fast temporal resolution (∼120 kHz bandwidth) and coarse energy information in three bands of >25 keV, >45 keV, and >65 keV. The SSNPA system consists of 15 radial sightlines that intersect existing on-axis neutral beams at major radii between 90 and 130 cm, 15 tangential sightlines that intersect new off-axis neutral beams at major radii between 120 and 145 cm. These two subsystems aim at separating the response of passing and trapped fast ions. In addition, one photodiode array whose viewing area does not intersect any neutral beams is used to monitor passive signals produced by fast ions that charge exchange with background neutrals.
Highlights
Neutral Particle Analyzer (NPA) diagnostics measure charge exchange neutral particle flux escaping from the plasma and provide valuable information on bulk ion temperature, plasma isotope composition, and fast ion distribution function, which are important for the control of the tritium/deuterium (T/D) density ratio in ITER1 and the study of fast ion confinement and transport in Tokamak experiments
The existing Solid State Neutral Particle Analyzer (SSNPA) diagnostic was displaced from its original location and a new compact, multichannel SSNPA system based on AXUV silicon photodiode arrays has been designed
The new SSNPA system consists of a set of vertically stacked AXUV photodiode arrays in current mode with an overlapping view of the plasma near the midplane, which allows neutral particle measurements of the same plasma region with different filter thickness
Summary
Neutral Particle Analyzer (NPA) diagnostics measure charge exchange neutral particle flux escaping from the plasma and provide valuable information on bulk ion temperature, plasma isotope composition, and fast ion distribution function, which are important for the control of the tritium/deuterium (T/D) density ratio in ITER1 and the study of fast ion confinement and transport in Tokamak experiments. In contemporary magnetic confinement devices, a variety of NPAs have been developed and deployed based on different re-ionization techniques and spectrometer design, including electrostatic, E B type and time-of-flight spectrometers. conventional NPAs, for example, E B type NPA, can accurately measure the energy and distinguish incident particle mass, they are relatively large, expensive, and complicated, which make it impractical to build a compact and multi-channel system. The new SSNPA system consists of a set of vertically stacked AXUV photodiode arrays in current mode with an overlapping view of the plasma near the midplane, which allows neutral particle measurements of the same plasma region with different filter thickness. This technique evolves from the successful operation of SSNPAs14 in current mode on NSTX and DIII-D devices, and a multi-energy soft x-ray. In the rest of this paper, the design considerations and technical details are described, as well as the techniques to improve the signal to noise ratio
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