Gyrokinetic simulations of near-edge turbulence and transport of NBI-only and NBI+ECRH plasmas on DIII-D

Plasma rotation is effective in suppressing turbulent transport and stabilizing MHD, but current devices rely on beam torque to generate rotation, which will become less significant in fusion reactors. The intrinsic torque generated by turbulence through residual stress holds potential for improving confinement by mitigating turbulent transport via increased radially sheared ExB flow. Measurements in DIII-D utilizing balanced beam torque have demonstrated substantial edge residual stress with distinct differences in plasmas with and without ECRH [1]. We present a gyrokinetic analysis of the near-edge region during the NBI-only and NBI+ECRH heating phases. Linear simulations show a dominant electron-directed instability with characteristics of a trapped electron mode (TEM) during the NBI-only phase and a mixture of electron-directed and ion-directed instabilities with characteristics of TEM and ion temperature gradient modes during the NBI+ECRH phase. Nonlinear simulations provide heat fluxes in good agreement with experimental power-balance estimates. A synthetic diagnostic is applied to simulated density fluctuations, enabling comparison with BES measurements of density fluctuation spectra.

[1] X. Qin et al, Measurement of Turbulence-driven Reynolds Stress and its Contribution to Intrinsic Toroidal Rotation in the DIII-D Tokamak, to be submitted