Abstract
Neutral-beam ions that are deflected onto loss orbits by Alfvén eigenmodes (AE) on their first bounce orbit and are detected by a fast-ion loss detector (FILD) satisfy the “local resonance” condition proposed by Zhang et al. [Nucl. Fusion 55, 22002 (2015)]. This theory qualitatively explains FILD observations for a wide variety of AE-particle interactions. When coherent losses are measured for multiple AE, oscillations at the sum and difference frequencies of the independent modes are often observed in the loss signal. The amplitudes of the sum and difference peaks correlate weakly with the amplitudes of the fundamental loss-signal amplitudes but do not correlate with the measured mode amplitudes. In contrast to a simple uniform-plasma theory of the interaction [Chen et al., Nucl. Fusion 54, 083005 (2014)], the loss-signal amplitude at the sum frequency is often larger than the loss-signal amplitude at the difference frequency, indicating a more detailed computation of the orbital trajectories through the mode eigenfunctions is needed.
Highlights
Neutral-beam ions that are deflected into a loss detector on their first bounce orbit act like a “light-ion beam probe” that detects wave-particle interactions between fast ions and Alfven eigenmodes (AE)
This is achieved by adjusting the plasma current so that source neutrals that ionize near the last-closed flux surface (LCFS) execute an orbit that brings them near the loss detector on their first bounce orbit
The equilibrium reconstructed by the EFIT code15 using motional Stark effect16 and magnetics data has a reversed q profile that is consistent with the observed reversed-shear AEs (RSAE) activity and preserves Te as a flux function
Summary
Neutral-beam ions that are deflected into a loss detector on their first bounce orbit act like a “light-ion beam probe” that detects wave-particle interactions between fast ions and Alfven eigenmodes (AE). other neutral beams generally inject and help to drive the AE instabilities, the plasma conditions are arranged so that one particular source acts as the dominant source of signal at the fastion loss detector (FILD). Neutral-beam ions that are deflected into a loss detector on their first bounce orbit act like a “light-ion beam probe” that detects wave-particle interactions between fast ions and Alfven eigenmodes (AE).. Other neutral beams generally inject and help to drive the AE instabilities, the plasma conditions are arranged so that one particular source acts as the dominant source of signal at the fastion loss detector (FILD). Reference 7 contains a simple calculation that shows that, in a uniform plasma, orbital deflections by two modes will produce signal oscillations at the sum and difference frequencies. The calculation predicts a larger amplitude at the difference frequency than at the sum frequency, consistent with the experimental example shown in Ref. 7.
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