Abstract
A 3D halo neutral code developed at the Princeton Plasma Physics Laboratory and implemented for analysis using the TRANSP code is applied to projected National Spherical Torus eXperiment-Upgrade (NSTX-U plasmas). The legacy TRANSP code did not handle halo neutrals properly since they were distributed over the plasma volume rather than remaining in the vicinity of the neutral beam footprint as is actually the case. The 3D halo neutral code uses a ‘beam-in-a-box’ model that encompasses both injected beam neutrals and resulting halo neutrals. Upon deposition by charge exchange, a subset of the full, one-half and one-third beam energy components produce first generation halo neutrals that are tracked through successive generations until an ionization event occurs or the descendant halos exit the box. The 3D halo neutral model and neutral particle analyzer (NPA) simulator in the TRANSP code have been benchmarked with the Fast-Ion D-Alpha simulation (FIDAsim) code, which provides Monte Carlo simulations of beam neutral injection, attenuation, halo generation, halo spatial diffusion, and photoemission processes. When using the same atomic physics database, TRANSP and FIDAsim simulations achieve excellent agreement on the spatial profile and magnitude of beam and halo neutral densities and the NPA energy spectrum. The simulations show that the halo neutral density can be comparable to the beam neutral density. These halo neutrals can double the NPA flux, but they have minor effects on the NPA energy spectrum shape. The TRANSP and FIDAsim simulations also suggest that the magnitudes of beam and halo neutral densities are relatively sensitive to the choice of the atomic physics databases.
Highlights
The National Spherical Torus eXperiment (NSTX) [1] operated as a midsize low aspect ratio fusion research facility with typical discharge parameters being major radius R = 0.85–0.9 m, minor radius a = 0 .67 m resulting in an aspect ratio of A = R/a ~ 1.3, plasma current Ip = 0.3–1.5 MA and toroidal field BT = 0.35–0.55 T
The TRANSP neutral particle analyzer (NPA) simulator was used to show that the 3D halo neutrals increase the NPA flux by factors ~2.3 – 2.8 above charge exchange on the beam primary neutrals alone, depending on the elected NPA sightline
The 3D halo neutral model and NPA simulator in TRANSP have been benchmarked with Fast-Ion D-Alpha simulation (FIDAsim) code
Summary
Energetic ion energy distributions are commonly measured using charge exchange neutral particle diagnostics [4] and two types of analyzers have been used on NSTX. Extensive measurements of energetic ion loss and/or redistribution including active and passive charge exchange contributions have been reported for NSTX [7, 8] Another system is a multi-sightline solid state neutral particle analyzer (ssNPA) [9, 10] utilizing silicon photodiode detectors that can be configured for pulse height analysis to measure energy resolved spectra or current analog output for energyintegrated measurements with fast time resolution [11].
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