Theory Weekly Highlights for August 2008

August 29, 2008

NIMROD simulations of the DIII-D plasma response to Resonant Magnetic Perturbation (RMP) fields were presented at the RMP workshop held at General Atomics on August 25 and 26. The presentation included recent simulations with both even parity I-coil fields and odd parity fields. For the even parity simulations, the value of Δ' was estimated at each rational surface with n = 3, m > n based on the resonant mode amplification without rotation, Ω = 0. By combining the estimated Δ' and the theoretical screening factor Σ, a theoretical prediction of the mode amplitudes for various rotation profiles could be compared directly to simulations. The comparison showed the mode amplitude was consistently under-predicted by the theory. Two simulations for the odd parity I-coil at different resistivities were similarly used to back out estimated values for Δ' and Σ(Ω), by treating these values as two unknowns in the theoretical expression. While the Δ' estimates were quite reasonable, the screening factor did not scale correctly with resistivity and rotation velocity Ω. These inconsistencies with error field theory point to the limitations of a cylindrical theory in predicting the mode amplitudes in a toroidal plasma, in which mode coupling can come into play.

August 22, 2008

The drift-kinetic neoclassical code NEO has been upgraded to include the effects of strong toroidal rotation on neoclassical transport, thus eliminating the diamagnetic ordering limit and completing the description of the second-order transport. The code has been successfully benchmarked with the Hinton and Wong analytic theory for a pure plasma. For studies of impure rotating plasmas, a general enhancement of the magnitude of the ion and impurity particle and energy fluxes is found, as expected due to the increase in the effective fraction of trapped particles as a result of the nonuniform poloidal redistribution due to the centrifugal force. While an increase in the poloidal flows is also found, even with strong rotation the effect is too weak to explain the large impurity flows observed in DIII-D experiments. The earlier studies with the diamagnetic ordering have recently been published (PPCF 2008, vol. 50, p. 095010). A journal paper describing the numerical methods used in NEO and highlighting the new results is in progress.

August 15, 2008

GA hosted a Summer Workshop for the SciDAC Gyrokinetic Simulatation of Energetic Particles (GSEP) project August 11-12 attended by over a dozen theorists, with 3 via web-cast, and 3 experimentalists. The participants were from GA, UC Irvine, ORNL, UCSD, PPPL, and the University of Texas at Austin. Zhihong Lin (UCI) is the overall SciDAC project PI. Ming Chu showed GYRO simulations demonstrating local TAE eigenmodes in s-alpha circular geometry and global eigenmodes in real DIII-D geometry. Eric Bass described plans to find subdominant modes in GYRO. A plan for verification of the GYRO and GTC gyrokinetic simulations, and for the hybrid MHD-kinetic linear simulations was agreed upon. In addition, a plan for validation of the codes against DIIID was established.

August 08, 2008

The ScIDAC gyrokinetic simulation project Center for the Study of Plasma Microturbulence (CSPM) held a three-day workshop at GA Aug 4-6 attended by 14 researchers from LLNL, PPPL, University of Maryland, University of Colorado, MIT and GA. One focus topic was on GYRO verification with the GENE and GEM codes of the sub-critical transport at about half the local high-n ideal ballooning beta limit. The beta scan, originally published in 2005 by the GYRO code, has now been confirmed by the GENE code. Further GYRO simulations will be done for a recent DIII-D discharge at about 95% of the local high-n ideal beta limit to determine if the anomalously large sub-critical transport persists in actual experiments.

August 01, 2008

Chris Holland and Vincent Chan attended the 2008 SciDAC conference in Seattle from July 13-17. Chris Holland presented a talk on synthetic diagnostics and the new TGYRO code, “Validating Simulations of Core Tokamak Turbulence: Current Status and Future Directions.”

Prof. Herb Berk from the Institute of Fusion Studies of the University of Texas at Austin is visiting GA from July 28 through August 8 to work with GA and DIII-D physicists on the interpretation of internal Alfven eigenmode measurements from Alfven cascade mode observations in DIII-D.

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