Measurements of fast-ion acceleration at cyclotron harmonics using Balmer-alpha spectroscopy

Abstract

Combined neutral beam injection and fast wave heating at the fourth and fifth cyclotron harmonics accelerate fast ions in the DIII-D tokamak. Measurements with a nine-channel fast-ion D-alpha (FIDA) diagnostic indicate the formation of a fast-ion tail above the injection energy. Tail formation correlates with enhancement of the d-d neutron rate above the value that is expected in the absence of fast-wave acceleration. FIDA spatial profiles and fast-ion pressure profiles inferred from the equilibrium both indicate that the acceleration is near the magnetic axis for a centrally located resonance layer. The enhancement is largest 8-10 cm beyond the radius where the wave frequency equals the cyclotron harmonic, probably due to a combination of Doppler-shift and orbital effects. The fast-ion distribution function calculated by the CQL3D FokkerPlanck code is fairly consistent with the data.

Use of Fast Ion D-Alpha diagnostics for understanding ICRF effects

“…from cold D^, bremsstrahlung and impurities, and Sf is the fast ion signal [9]. Ff{E,p) is the local fast ion distribution function.…”

“…As a reference, p/ < 3 cm in DIII-D and p/ < 20 cm on NSTX. Fast ion acceleration by ICRF waves has been already observed on both machine in past years [8] [9]. Since then, a set of Fast-Ion DAlpha (FIDA) spectroscopic diagnostics has become routinely available on both devices, allowing more detailed studies of the response of fast ions to HHFW injection.…”

Charge exchange spectroscopy as a fast ion diagnostic on TEXTOR

“…No D ␣ related signal from neutrals moving towards the observer has so far been seen above ϳ15 keV during Ohmic shots or when only NBI1 is injected. A possible approach to eliminate the background is fast ͑much faster than the slowing down time͒ beam modulation, 5 for technical reasons not achievable due to the long readout time of the camera. An alternative approach is modeling of the background, possible due to the high signal-to-noise ratio of the TEXTOR CXRS spectra and the good spectral resolution, which allows us to resolve the parasitic impurity lines.…”

“…3 Recently the same diagnostic principle has been successfully applied to the notorious deuterium n =3→ 2 ͑D ␣ ͒ CX spectrum at DIII-D. 4,5 In this paper the fast ion relevant capabilities of the upgraded CXRS diagnostic on TEXTOR are described. The diagnostic setup is aimed at combining the information from the emission following charge exchange between circulating fast ions and neutral particles from a beam and the emission from excited neutral beam particles.…”

A tangentially viewing fast ion D-alpha diagnostic for NSTX

“…2 A crucial aspect is the effective average over velocity-space that is a characteristic of each diagnostic setup and is mainly determined by the viewing geometry relative to the neutral beam and magnetic field. 3 The sensitivity to different portions of the fast ion velocity-space can be expressed by the weight function W ͑E , p͒ ͑mapped in energy E and pitch angle p = v ʈ / v coordinates, where v is the fast ion velocity and v ʈ its component parallel to magnetic field͒. The measured FIDA spectrum s͑ ͒, apart from numerous background components, results from the convolution of W ͑E , p͒ with the fast ion distribution function F͑p , E͒, s͑ ͒ = ͵ W ͑p,E͒F͑p,E͒dEdp.…”