Abstract
A major goal of the DIII-D program is to study `advanced tokamak' plasmas with good confinement, large normalized β, and a large fractionof self-sustained current. Many of these plasmas have large beam pressures(≲(1/3) of the total pressure) and weak magnetic shear; Alfvén instabilities with laboratory frequencies of 100-250 kHz are oftenobserved. The instabilities correlate with reductions in the neutron ratebelow the classically expected value, complicatingdetermination of the pressure and current profiles. Quantitative analysis of onecase suggests that two types of energetic particle modes are destabilized: the resonant toroidicity-induced Alfvén eigenmode and theresonant kinetic ballooning mode. The strong dependence on neutralbeam injection parameters and the variability in mode frequency arequalitatively consistent with this identification. Further analysis andmeasurements are planned.
Highlights
Beam-driven instabilities with frequencies below the frequency of toroidicity-induced Alfven eigenmodes (TAEs) but above the frequency of tearing modes and fishbones have been observed in DIII-D since 1991 [1]
Energetic particle modes are waves that do not exist in the absence of a fast-ion population; they often resemble a normal mode of the background plasma but the frequency is determined primarily by a characteristic frequency of the fast-ion distribution function [3]
Two branches of energetic particle modes are potentially involved: a resonant TAE that has a lower frequency than the standard TAE and a resonant kinetic ballooning mode (KBM) that has a higher frequency than an ordinary ballooning mode
Summary
Beam-driven instabilities with frequencies below the frequency of toroidicity-induced Alfven eigenmodes (TAEs) but above the frequency of tearing modes and fishbones have been observed in DIII-D since 1991 [1]. Subsequent analysis failed to establish conclusively the identity of this instability [2], which was called the β-induced Alfven eigenmode (BAE) in the initial publication. Two branches of energetic particle modes are potentially involved: a resonant TAE that has a lower frequency than the standard TAE and a resonant kinetic ballooning mode (KBM) that has a higher frequency than an ordinary ballooning mode. This paper reviews recent work on this topic. Plans for future work are outlined in the final section
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