Compact multichannel neutral particle analyzer for measurement of energetic charge-exchanged neutrals in Alcator C-Mod

Abstract

A four-channel compact neutral particle analyzer (CNPA) based on operating small Si diode detectors in pulse-height analysis (PHA) mode is used to measure energetic hydrogen minority ions with energies between $\sim 50$ and $350\mathrm{keV}$ stemming from ion-cyclotron range-of-frequency heated D(H) Alcator C-Mod plasmas with both active and passive charge exchange (CX). First core minority ion distribution results from Alcator C-Mod discharges and a detailed description of the diagnostic are presented. The diagnostic employs integrated electronics and fast digitization of the shaping amplifier voltage. The digitized data are stored for postshot PHA, which removes the constraints of real-time PHA and allows for improved performance via elimination of base line shift effects and potentially relieving pileup through Gaussian fitting routines. The CNPA is insensitive to the large gamma and neutron background in Alcator C-Mod discharges but is susceptible to the plasma’s soft x-ray flux. The soft x-ray flux limits the CNPA energy resolution to $\sim 15–20\mathrm{keV}$ . A simple model is used to interpret the active CNPA data which permits rapid estimates of the core hydrogen minority temperatures and anisotropy with a time resolution of $\sim 100\mathrm{ms}$ . Hydrogenlike boron is identified as an important electron donor for the CX signal.