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 χe 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-χe 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 χe 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