Gyrokinetic simulations of ETG and ITG turbulence

We report on the resolution of a significant discrepancy between published continuum-code simulations and subsequent global particle-in-cell (PIC) simulations of electron-temperature-gradient (ETG) turbulence. Our investigations, using gyrokinetic δf-PIC- and continuum-code simulations and analytical theory, strongly support the conclusion from the earlier continuum-code simulations that ETG turbulence can drive the electron thermal conductivity χ_e large enough to be significant in some tokamaks. A successful ETG-turbulence benchmark between δf-PIC and continuum codes for ETG turbulence has also been completed. Scans in the magnetic shear show an abrupt transition to a high-χ_e state as the shear is increased from the benchmark value of s ≤ 0.1 to above s ≤ 0.4. When nonadiabatic ions are used, this abrupt transition is absent, and χ_e reaches values consistent with transport analyses of DIII-D, JET, JT60-U and NSTX discharges. The balances of zonal-flow driving and damping terms in late-time quasi-steady phase of ITG turbulence have been unfolded using a new run-time gyrokinetic-simulation diagnostic. The zonal flow level is set by a balance of large driving and damping terms which almost cancel each other. The driving is found to be mostly by the Reynolds stress, while the dissipation is mostly by the linear (transit-time) damping terms. It is also shown that useful zonal-flow-balance information can be obtained with spatially localized samples at as few as four poloidal locations. Real-geometry simulations have been undertaken, using the nonlinear δf-PIC gyrokinetic code SUMMIT/PG3EQ_NC, of the DIII-D 'Cyclone' shot #81499 and of shot #118561, which had broad-wavenumber-range density fluctuation measurements. Real geometry is found to have a significant effect on the transport rates, even though the effect on the linear growth rates is often modest.