Abstract
Spectra of passive fast-ion D-alpha (FIDA) light from beam ions that charge exchange with background neutrals are measured and simulated. The fast ions come from three sources: ions that pass through the diagnostic sightlines on their first full orbit, an axisymmetric confined population, and ions that are expelled into the edge region by instabilities. A passive FIDA simulation (P-FIDASIM) is developed as a forward model for the spectra of the first-orbit fast ions and consists of an experimentally-validated beam deposition model, an ion orbit-following code, a collisional-radiative model, and a synthetic spectrometer. Model validation consists of the simulation of 86 experimental spectra that are obtained using 6 different neutral beam fast-ion sources and 13 different lines of sight. Calibrated spectra are used to estimate the neutral density throughout the cross-section of the tokamak. The resulting 2D neutral density shows the expected increase toward each X-point with average neutral densities of at the plasma boundary and near the wall. Fast ions that are on passing orbits are expelled by the sawtooth instability more readily than trapped ions. In a sample discharge, approximately 1% of the fast-ion population is ejected into the high neutral density region per sawtooth crash.
Highlights
Fast ions are produced through neutral beam injection, radio-frequency heating, and fusion reactions
This paper shows that, in the presence of a known fast-ion source, information about the neutral density profile can be obtained from passive fast-ion D-alpha (FIDA) measurements
The measured passive FIDA signals originate from three distinct fast-ion populations that are produced by the neutral beams
Summary
Fast ions are produced through neutral beam injection, radio-frequency heating, and fusion reactions. The measured passive FIDA signals originate from three distinct fast-ion populations that are produced by the neutral beams. For a passive FIDA diag nostic, this toroidally asymmetric population is detected when fast ions traverse the FIDA sightlines on their first full orbits This population can be viewed as a known source of fast ions that probe the edge and pedestal regions. Ions born on confined orbits eventually distribute themselves toroidally These confined ions are the second population that produce passive FIDA light. Core fast ions that are expelled into the edge region by instabilities are the third fast-ion population that produces passive FIDA light These transiently increase the edge c onfined population; ions scattered onto loss orbits can contribute to the signal. The final section (section 7) presents conclusions and suggestions for future work
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