Compact and multi-view solid state neutral particle analyzer arrays on National Spherical Torus Experiment-Upgrade.

D Liu,E D Fredrickson,W W Heidbrink,Y B Zhu,G Z Hao,K Tritz

doi:10.1063/1.4959798

Abstract

A compact and multi-view solid state neutral particle analyzer (SSNPA) diagnostic based on silicon photodiode arrays has been successfully tested on the National Spherical Torus Experiment-Upgrade. The SSNPA diagnostic provides spatially, temporally, and pitch-angle resolved measurements of fast-ion distribution by detecting fast neutral flux resulting from the charge exchange (CX) reactions. The system consists of three 16-channel subsystems: t-SSNPA viewing the plasma mid-radius and neutral beam (NB) line #2 tangentially, r-SSNPA viewing the plasma core and NB line #1 radially, and p-SSNPA with no intersection with any NB lines. Due to the setup geometry, the active CX signals of t-SSNPA and r-SSNPA are mainly sensitive to passing and trapped particles, respectively. In addition, both t-SSNPA and r-SSNPA utilize three vertically stacked arrays with different filter thicknesses to obtain coarse energy information. The experimental data show that all channels are operational. The signal to noise ratio is typically larger than 10, and the main noise is x-ray induced signal. The active and passive CX signals are clearly observed on t-SSNPA and r-SSNPA during NB modulation. The SSNPA data also indicate significant losses of passing particles during sawteeth, while trapped particles are weakly affected. Fluctuations up to 120 kHz have been observed on SSNPA, and they are strongly correlated with magnetohydrodynamics instabilities.

Highlights

Fast ions play a key role in plasma heating and current drive in fusion plasmas, but their confinement and transport are susceptible to plasma instabilities.1,2 Accurate measurements of fast ion distribution in real space and velocity space are crucial for understanding and eventually controlling fast ion dynamics
Solid state neutral particle analyzer (SSNPA) based on the absolute extreme ultraviolet (AXUV) silicon photodiodes from Opto Diode Corporation4 has been successfully demonstrated in many magnetic fusion research facilities worldwide
The main limitations are that the solid state neutral particle analyzer (SSNPA) diagnostic cannot resolve the incident particle mass, and that x-ray and neutron/gamma ray induced noise could limit the performance in high temperature plasmas

Summary

INTRODUCTION

Fast ions play a key role in plasma heating and current drive in fusion plasmas, but their confinement and transport are susceptible to plasma instabilities. Accurate measurements of fast ion distribution in real space and velocity space are crucial for understanding and eventually controlling fast ion dynamics. Neutral particle analyzer (NPA) is a valuable fastion diagnostic that measures fast ions which charge exchange with beam neutrals and/or background neutrals. Compared with conventional NPA that uses parallel electric and magnetic fields as mass spectrometer, the SSNPA diagnostic is very attractive due to its compactness, cost-effectiveness, and easy handling features. The main limitations are that the SSNPA diagnostic cannot resolve the incident particle mass, and that x-ray and neutron/gamma ray induced noise could limit the performance in high temperature plasmas. In addition to NBs, NSTX-U will use high harmonic fast wave (HHFW) for plasma heating. The new SSNPA system aims at providing useful information on fast ion distribution in real space and velocity space, fast ion transport in the presence of instabilities, and fast ion acceleration by HHFW.

HARDWARE SETUP AND MAIN FEATURES

EXPERIMENTAL RESULTS AND DISCUSSION

SUMMARY AND FUTURE WORK