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

The solid state neutral particle analyzer (SSNPA) array on the national spherical torus experiment (NSTX) consists of four chords with tangency radii of 60, 90, 100, and 120cm that view across the three coinjection 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 (30–100keV). By the end of the NSTX 2005 experimental campaign, the electromagnetic 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 ∼10keV and time resolution of 2ms have been achieved. Temporal evolutions of energetic neutral flux obtained with the SSNPA are in good agreement with those obtained with the E‖B-type neutral particle analyzer. With these improvements, the SSNPA can be used to study magnetohydrodynamic instabilities and fast ion redistribution. Example data from plasma discharges are presented along with the noise reduction techniques and postshot pulse height analysis methods.