Theory Weekly Highlights for May 2010

May 28, 2010

Recent GYRO simulations have provided several new insights into the physics needed for accurate validation modeling. In plasmas with Zeff > 2, carbon ions can drive a significant contribution (up to 50%) of the overall ion thermal transport, and up to 10% of this flux can be driven at kθρs > 1. A related finding is that carbon density and temperature fluctuations can have significantly different correlation lengths and amplitudes than the corresponding deuterium fields, which must be accounted for in synthetic diagnostics and future validation studies. A second key result is that multiscale turbulence simulations of actual DIII-D discharges indicate that the presence of significant long-wavelength (kθρs < 1) turbulence suppresses turbulence at shorter wavelengths. Significant is defined as being large enough to drive experimentally relevant levels of ion thermal transport. This result not only confirms theoretical expectations, but also provides useful guidance on the necessary simulation resolution for future predictive gyrokinetic modeling efforts. These findings were presented in a poster by C. Holland at the 2010 Sherwood theory meeting.

Dr. Aiping Sun of Southwestern Institute of Physics in Chengdu, China is visiting GA for 6 months to collaborate on the IMFIT integrated modeling project and ONETWO transport simulations.

May 21, 2010

The first solution of the Grad-Shafranov equation for discharges dominated by a “mature” avalanche runaway electron (RE) current was obtained in the cylindrical limit. The RE current-profile tends to be much more central-peaked relative to the initial plasma current profile, yet q0 > 1 so that the RE current channel is always stable to internal kinks, in qualitative agreement with experiments on DIII-D. The current profile displays an interesting profile consistency or “resilience”, as it is practically independent of the profile of the initial RE seed component preformed in the thermal quench phase. The numerical solution is also weakly non-stationary with a secular time dependence that can be traced to the slow dissipation, or “erosion”, of the RE plateau and associated inductive energy due to collisions on the cold background electrons. The model predicts massive RE currents could be completely eroded in ITER in a span of only 200 ms by a combination of a modest particle densification, of the order of twenty times, and negative surface loop voltage application with Esur ~ 1 V/m. The model is being used to interpret recent DIII-D experiments where this effect was first discovered.

Alan Turnbull co-organized a Town Meeting with M. Tillack of UCSD and C. Kessel of PPPL on “Edge Plasma Physics and Plasma Material Interactions in the Fusion Power Plant Regime” at UCSD on May 20 and 21. The meeting was organized to initiate discussion between physicists and materials scientists on identifying and understanding the interaction between the edge plasma physics and materials issues in the plasma facing components, in a full reactor environment beyond ITER. The meeting was well attended by approximately 30 scientists, including a half a dozen from Europe and Japan.

May 14, 2010

The GATO mapping was recently updated to read inverse equilibria specified on an arbitrary poloidal angle grid and map to an arbitrary flux surface grid specified by the Jacobian. Previously the mapping input was restricted to either direct equilibria or inverse equilibria on an equal arclength grid and the output to either equal arclength or straight field line PEST coordinates. The recent changes open up the possibility of reading tokamak equilibria generated by any equilibrium code, including 2-D equilibria from VMEC.

Vincent Chan, Lang Lao, Mickey Wade, and Clement Wong returned from a visit to Wuhan, China where they attended the 5th PRC-US Magnetic Fusion Collaboration Workshop to discuss US-China collaboration.

May 07, 2010

Studies of the effects of compressional magnetic perturbations on the linear gyrokinetic stability of unusually high-beta DIII-D H-mode plasmas using the new GYRO eigenvalue solver find a large destabilizing effect on the linear growth rate at low kθρs in the core but little effect for the higk-k ETG mode. While the perturbed parallel field, δB//. has a negligible effect for standard DIII-D plasmas, the destabilizing effect in these plasmas, which operate within 10% of the high-n ideal beta limit, is around 17%. Unlike our previously studied high-beta NSTX-like cases, here we find that the dominant low-k mode at the experimental beta is a pure ITG mode, rather than a hybrid ITG/KBM mode, despite the high pressure gradient. Furthermore, although MHD theory predicts that in the kθ=0 limit, there is a cancellation between the potentially stabilizing component of the grad B drift proportional to the pressure gradient and the δB// terms, we find that using a numerical model which includes δA// but neglects δB// and which selectively zeros the pressure gradient effect in the grad B drift (which was previously done for most electromagnetic GYRO simulations) does not recover the destabilizing effects of δB// for these finite-k modes.

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