Large-scale gyrokinetic turbulence simulations: Effects of profile variation

Two common computational domains used in gyrokinetic turbulence simulations are a local flux-tube and a global whole plasma volume. The effect of a radially varying pressure gradient is found to explain some of the qualitative differences between these two models. It is shown that a coherent purely radial mode is the result of profile variation. In addition, as profile variation is increased, there is a fairly sudden transition to much lower levels of heat flux. This may explain lower values found in past global simulations. The self-generated purely radial electrostatic potential is found to be 180 degrees out of phase with the flux-surface-averaged ion temperature. A theoretical relation between these two quantities is derived by relating the ExB nonlinearities for ion density and temperature for purely radial modes. This relation is used to explain the various radial mode shapes. Extending these results, a possible scheme is explored to reduce the heat flux by adding a ripple to the ion temperature profile. It may be possible to achieve similar results experimentally using ion cyclotron resonance heating. Finally, simulation results show the additional stabilizing effect of equilibrium E-r shear from profile variation in the radial force balance equation. (C) 1999 American Institute of Physics. [S1070-664X(99)92805-7].