Beam-driven energetic particle modes in advanced tokamak plasmas

Abstract

A major goal of the DIII-D program is to study `advanced tokamak' plasmas with good confinement, large normalized β, and a large fraction of 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 often observed. The instabilities correlate with reductions in the neutron rate below the classically expected value, complicating determination of the pressure and current profiles. Quantitative analysis of one case suggests that two types of energetic particle modes are destabilized: the resonant toroidicity-induced Alfvén eigenmode and the resonant kinetic ballooning mode. The strong dependence on neutral beam injection parameters and the variability in mode frequency are qualitatively consistent with this identification. Further analysis and measurements are planned.