Initial neutral particle analyzer measurements of ion temperature in the National Spherical Torus Experiment

Abstract

The neutral particle analyzer (NPA) diagnostic on the National Spherical Torus Experiment (NSTX) utilizes a PPPL-designed E∥B spectrometer which measures the energy spectra of minority H and bulk D species simultaneously with 39 energy channels per mass species and a time resolution of 1 ms. The calibrated energy range is E=0.5–150 keV and the energy resolution varies from ΔE/E=3%–7% over the surface of the microchannel plate detector. The NPA measures thermal Maxwellian ion spectra to obtain line integrated ion temperatures, T i . For line integral electron densities below n e L∼3.5×10 19 m −2 , good agreement is observed between the line integrated NPA T i and the central T i (0) measured by the spatially localized charge exchange recombination spectroscopy (CHERS) diagnostic. However, with increasingly higher n e L the NPA T i falls below the central T i (0) measured by CHERS because the charge exchange neutral emissivity weights the line integrated NPA measurement outboard of the plasma core. An analytic neutral analysis code, DOUBLE, has been applied to the NPA data to correct for this effect and restore agreement with T i (0) measured by CHERS. A description of the NPA diagnostic on NSTX and initial ion temperature measurements along with an illustration of application of the DOUBLE code are presented.