Theory Weekly Highlights for August 2004

August 27, 2004

GYRO simulations were used to study the interaction between the n=0 neoclassical radial transport and high-n ExB transport from ITG turbulence. It was found that the conventional addition of independent neoclassical and turbulent flows is a good approximation. At vanishing ρ* there is no interaction. For finite ρ* global simulations, GYRO now computes the radial neoclassical flows with the addition of the neoclassical n = 0 driver, as well as number, energy, and momentum conserving Krook ion-ion collisions. In the presence of turbulence, the neoclassical flows are the local space average of the large orbit neoclassical flows. The n = 0 radial modes (zonal flows) induce corrugations in the ambipolar neoclassical flows centered about the null flow. The neoclassical driver combined with a neoclassical n=0 initial value, produces a parallel ion velocity shear which provides the Kelvin-Helmhotz drive to the ITG mode via the nonlinear interaction. The neoclassical drive and neoclassical flows have little effect on the high-n EXB turbulent flows apart from the Kelvin-Helmhotz drive. The ion-ion collisions weakly damp the zonal flows and yield slightly higher ITG transport as expected.

August 20, 2004

Extensive numerical studies have been performed using the ICRF full wave code TORIC coupled to the ORBIT-RF Monte-Calro code to model DIII-D ICRF discharges, in collaborations with J. Wright and P. Bonoli of MIT. As a first test, a set of converged TORIC ICRF wave solutions for a DIII-D discharge is passed on to ORBIT-RF to compute the wave absorbed power. Initial results indicate that using a single dominant poloidal and toroidal wave number to model the wave spectrum the computed absorbed power from ORBIT-RF is within a factor of two of the power absorption from TORIC. This difference is likely due to the approximation of the poloidal wave spectrum as a delta function, that neglects effects due to other poloidal and toroidal mode numbers. By refining the wave spectrum, we expect further improvement in agreement.

Version one of the ProfileViewer has been made available on the GA Fusion LSF cluster. This new tool allows for plotting of experimental signals versus various radial flux surface labels using EFIT reconstructed equilibria from MDSplus. ProfileViewer fully supports profile overlaying and interactive time slicing, in addition to the full array of features inherited by using the GAPlot Objects.

August 13, 2004

Recent GYRO nonlinear scans in ExB shear with adiabatic electrons have shown that two different states can exist at low transport levels yielding two different quench points. Without the destabilization effects of parallel velocity shear, simulations with ExB shear on from the beginning yield no transport above a critical shear rate. However, if the simulation is restarted from a previous one with a finite saturation level, then the transport drops to a nonzero saturation level using the same critical shear rate value. This effect has only been observed in simulations without any parallel velocity shear. When the destabilizing effect of parallel velocity shear is included, transport is not quenched by ExB shear. This will be further investigated.

August 6, 2004

The characteristics of feedback stabilization of the n=1 resistive wall mode in DIII-D geometry are studied systematically using the MARS-F code for a set of equilibria with beta varying from the no wall to the ideal wall limit, and with experimentally measured characteristics of the amplifier and feedback circuit. It is found that the limiting beta value that can be feedback stabilized is related to the inherent phase lag introduced by the amplifier circuit and to a lesser extent introduced by the time delay. The frequency range of the feedback circuit needs to be extended upward before an equilibrium with beta approaching the ideal wall limit can be stabilized. Improvements in performance by using improved feedback circuits are being investigated.

These highlights are reports of research work in progress and are accordingly subject to change or modification