‘Beam-emission spectroscopy’ diagnostics also measure edge fast-ion light

Abstract

Beam-emission spectroscopy (BES) diagnostics normally detect fluctuations in the light emitted by an injected neutral beam. Under some circumstances, however, light from fast ions that charge exchange in the high neutral-density region at the edge of the plasma make appreciable contributions to the BES signals. This 'passive' fast-ion D α (FIDA) light appears in BES signals from both the DIII-D tokamak and the National Spherical Torus Experiment (NSTX). One type of passive FIDA light is associated with classical orbits that traverse the edge. Another type is caused by instabilities that expel fast ions from the core; this light can complicate measurement of the instability eigenfunction.

Characterization of off-axis fishbones

“…For the rapidly changing background conditions associated with these fishbone bursts, the temporal resolution of the spectroscopic FIDA diagnostics proved inadequate to detect changes in core fast-ion density; however, bursts of edge FIDA emission are observed by the filter-based f-FIDA diagnostic [28] on both 'active' sightlines that view an injected beam and 'passive' sightlines that do not intersect an injected beam. The beam-emission spectroscopy (BES) diagnostic [29] suffers from contamination by edge FIDA light for these fishbone bursts [30], complicating use of the data for internal eigenfunction measurements of density fluctuations. However, for one discharge where the neutral beams viewed by the BES diagnostic were off, the signals are entirely from passive FIDA light produced when fast ions are expelled to the high neutral density region at the plasma edge [30].…”

“…The beam-emission spectroscopy (BES) diagnostic [29] suffers from contamination by edge FIDA light for these fishbone bursts [30], complicating use of the data for internal eigenfunction measurements of density fluctuations. However, for one discharge where the neutral beams viewed by the BES diagnostic were off, the signals are entirely from passive FIDA light produced when fast ions are expelled to the high neutral density region at the plasma edge [30]. The seventh edge loss diagnostic is indirect.…”

Scrape-off layer ion acceleration during fast wave injection in the DIII-D tokamak

“…Passive generation denotes the emission of light due to charge exchange processes between energetic ions and background neutrals (active generation occurs when the neutral particle is injected by a beam). The ability of the DIII-D BES system [29] to detect the presence of energetic ions has recently been demonstrated [30]. In typical geometry, the fast-ion BES signal is dominated by ions of energy The toroidal positions of many BES sightline terminations lie on the 180-degree fast antenna.…”

“…The BES spectral range is on the blue-shifted side of the cold D α line. Following the method of [30], the phase space sensitivity of the BES measurement is calculated and shown in figure 12. This normalized phase space weighting, W BES , is calculated for the portion of the BES sightline that terminates near the 180-degree fast wave antenna.…”

Measurements of fast-ion transport by mode-particle resonances on DIII-D

“…Particles with orbits satisfying equation (2) can lose/gain energy and canonical toroidal momentum during the resonant interaction, leading to an orbit transformation into the lost region of velocity space. Off-axis fishbone-induced losses of beam ions are observed in seven independent detectors, including a novel implementation of DIII-D's FIDA and beam-emission spectroscopy (BES) systems [34]. The primary goal of the FIDA diagnostic is to measure the Doppler shift of photons emitted from atomically excited neutralized fast ions that undergo charge exchange with injected beam neutrals [15].…”