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 Dα (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.
Highlights
Beam-emission spectroscopy (BES) is a diagnostic technique that infers local fluctuations in density nfrom fluctuations in the light emitted by an injected neutral beam [1]
Fluctuations in the neutral-beam density induced by fluctuations near the plasma edge can influence the measurement [2], in practice, fluctuations in light intensity are normally proportional to the local density fluctuation at the intersection of the diagnostic sightline with the injected neutral beam
We show that passive FIDA light is detected by BES diagnostics under certain circumstances
Summary
Beam-emission spectroscopy (BES) is a diagnostic technique that infers local fluctuations in density nfrom fluctuations in the light emitted by an injected neutral beam [1]. The DIII-D [5] and National Spherical Torus Experiment (NSTX) [6] BES diagnostics measure Doppler-shifted Dα light from injected deuterium neutral beams. Another instrument, the fast-ion Dα (FIDA) diagnostic measures Doppler-shifted Dα light [7]. In the case of virulent fast-ion instabilities, bursts of passive FIDA light can be comparable to the beam emission in intensity. These bursts complicate use of BES to measure the mode structure, techniques are available to distinguish the FIDA light from the beam emission. The paper concludes with a discussion of the diagnostic implications (section 6)
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