Performance of the solid state neutral particle analyzer array on the national spherical torus experiment

Liu, D.*; Heidbrink, W. W.*; Darrow, D. S.*; Roquemore, A. L.*; Medley, S. S.*; Shinohara, Koji

The Solid State Neutral Particle Analyzer (SSNPA) array on the National Spherical Torus Experiment (NSTX) consists of four chords with tangency radii 60, 90, 100 and 120 cm that view across the three co-injection neutral beam lines. Each chord utilizes a silicon photodiode that is coupled to fast digitizers to measure the energy distribution of charge exchange fast neutral particles (30100keV). By the end of the NSTX 2005 run campaign, the noise in the SSNPA was reduced by half and reasonable signals were obtained with good electromagnetic shielding, fast digitization of raw signals, software-based pulse height analysis and pulse shape discrimination. Energy resolution of 10 keV and time resolution of 2 ms have been achieved. Temporal evolutions of energetic neutral flux obtained with the SSNPA are in good agreement with those obtained with the EB type Neutral Particle Analyzer. With these improvements, the SSNPA enables us to study MHD instabilities and fast ion redistribution.