Theory Weekly Highlights for October 2009

October 30, 2009

Four GA Theory group members will be presenting invited talks at the upcoming 51st Meeting of the American Physical Society Meeting in Atlanta Ga. These cover our work in the areas of gyrokinetic simulation, MHD stability, and RF physics.

“Predictive Gyrokinetic Transport Simulations and Application of Synthetic Diagnostics”, by J. Candy, in session DI3 on Monday, November 2.

“A New View of the Sawtooth Instability and its Relation to the Internal Kink”, by A.D. Turnbull, in session G13 on Tuesday November 3.

“Iterated Finite Orbit Monte-Carlo Simulations with Full-Wave Fields for ICRF Wave Heating Experiments”, by M. Choi, in session T13 on Thursday, November 4

“Gyrokinetic Simulations of Enhanced Alpha Transport by De-stabilized Alfven Turbulence”, by E.M. Bass, in session X13 on Friday November 6.

October 23, 2009

Electron energy flux attenuation in cold ablation cloud media was calculated from numerical solutions of the Fokker-Plank equation for the evolving electron distribution function as a function of energy, pitch angle, and distance along a field line, with an imposed half-space Maxwellian distribution at the boundary of the medium. Solutions for energy flux as a function of depth in ablation clouds surrounding H, He, Li, Be, B, C, Ne, Ar, Kr and Xe pellets, contemplated for disruption mitigation, were all found to be conveniently fitted to a simple form involving a Modified Bessel function that was previously only used for hydrogenic pellet ablation where the pitch angle diffusion term can be neglected and analytic solutions are possible. The only difference is that the heat flux for the other species needs to be renormalized to unity at the boundary (to adjust for backscattering at higher Z) and the argument of the heat flux involving the penetration depth normalized to the slowing down range needs to be rescaled by a numerical factor to bring it into excellent agreement with the numerical solutions.

Dr. Yueqiang Liu from Culham laboratory is visiting GA for three weeks to collaborate with DIII-D scientists on issues related to the stability of restive wall mode and the plasma response to external magnetic perturbations.

Nirmal Bisal of the Institute for Plasma Research in India is visiting GA for 1 year to collaborate on the development of the Integrated Modeling Fitting code IMFIT.

October 16, 2009

To study the importance of finite orbit width effect of fast ions on experimental observations, ORBIT-RF is coupled with AORSA and used to simulate DIII-D and NSTX HHFW heating experimental conditions. Previous zero-orbit width theory had predicted no spatial shift of fast ions whereas observations in tokamak experiments indicated a significant outward radial shift. The finite orbit simulations reproduce both the spectra and outward spatial shift of ICRF heated fast ions, qualitatively consistent with the Fast Ion D-Alpha (FIDA) measurements in both DIII-D and NSTX. The computed neutron enhancement rate is also in reasonable agreement with measurement. However, a discrepancy still remains in that ORBIT-RF/AORSA computes a larger outward shift of fast ions than FIDA. The FIDA data is averaged over a fairly long time window to get better statistics for the steady-state discharge. Further study is in progress to understand if the difference is due to this statistical averaging or possibly due to convergence issues between ORBIT-RF and AORSA.

October 09, 2009

A new interface between EFIT and NOVA-K using the GATO mapping as a stand-alone code was completed and tested for a DIII-D equilibrium. The test used an equilibrium recalculated by the PPPL q solver for discharge #96043 in order to compare the result directly with that using the built-in NOVA mapping, which can only accept q-solver equilibria. The equilibrium was unstable to an ideal internal kink mode. All quantities required by NOVA were compared by hand and found to check out to within 5% to 10%. The final eigenvalues from NOVA using the mapped input from GATO agreed with that from the built-in NOVA mapping to within a few percent. This should greatly expand the utility of NOVA and NOVA-K since the GATO mapping can equally well accept direct equilibria from EFIT and inverse equilibria fromTOQ, TEQ, and JSOLVER. In addition, the GATO mapping has the capability of mapping arbitrarily close to a diverted surface, provision for up-down asymmetry, and has considerable flexibility in options for packing the mesh at particular surfaces.

October 02, 2009

A number of large non-linear GYRO simulations of energetic particle (EP) turbulence have been run with the goal of solidifying understanding of TAE/EPM turbulence, particularly its onset as kinetic drive strength increases. Runs for a range of EP drive strengths (expressed through the density of energetic particles nEP) clearly show the transition from a pure ITG/TEM spectrum, where EPs act as tracer particles, to cases where the EPs drive TAE/EPM turbulence (see attached figure showing Energetic particle density diffusion coefficient DEP showing increase with the onset of low-n TAE/EPM turbulence) . Runs underway reveal that background ExB drift shear suppressed TAE/EPM fluctuations. Quantitative evaluation of this effect is underway.

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