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° out of phase with the flux-surface-averaged ion temperature. A theoretical relation between these two quantities is derived by relating the E×B 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 Er shear from profile variation in the radial force balance equation.
Electron cyclotron resonance (ECR) heating influences two of the main parameters (electron temperature and, indirectly, density) that determine the charge state of the ions produced in an ECR ion source (ECRIS). Therefore, various schemes to optimize ECR heating in the ECRIS have been pursued such as multiple-frequency heating, the radio-frequency tuning effect, volume heating, or wide-band heating. We investigate two-frequency ECR heating of electrons in a simple magnetic mirror field by right handed circularly polarized waves with infinite phase velocity. The study shows a heating barrier different from the well-know adiabatic barrier. Study also revealed a mechanism whereby multiple frequencies give improved heating. A preliminary interpretation of the study is presented.
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