Theory Weekly Highlights for January 2004

January 30 2004

A re-analysis of published simulation predictions for the early MHD bursts in the βN = 2 L-Mode NCS discharge 87009, has shown that many of the predictions are exhibited in more recent, lower βN L-Mode NCS discharges. These were not previously understood or observed in discharge #87009. In both the simulations from the NFTC and FAR codes and the experimental data, the initially localized resistive interchange mode at the inner rational surface broadens until it reaches the magnetic axis. It then reconnects either at or just off-axis, consistent with a double tearing structure in which the innermost island is weaker than the outer island. The inner island then dissipates forming a single tearing mode. In the original simulations, the tearing was partly successful in explaining the β limit disruption. In the lower βN experiments, however, the modified profiles lead simply to a loss of high performance, which is more consistent with the simulations.

January 23 2004

The first qualitative benchmark between TWIST-R and PEST III for the linear resistive eigenmode of a Solovev equilibrium was successfully completed. This required a remapping to the PEST-III coordinate system from the straight field line system used by TWIST-R. In the process, it was found that the numerically computed mapping phase factor was not periodic. Correction of that problem appears to have eliminated other known numerical periodicity problems that were previously identified in TWIST-R. The TWIST-R code then reproduced all the main features of the stable PEST-III solution, except for some minor apparent differences in the core where it is known that TWIST-R has some remaining numerical accuracy problems. An approach to eliminating these by constructing the equilibrium directly on the stability calculation grid is being implemented. Also, work is almost complete on the algorithm to extract the coefficients of the large and small Frobenius solutions to obtain a quantitative comparison of the stability indices delta_prime and gamma_prime.

January 16 2004

John deGrassie was the lead Scientific Coordinator from San Diego on the JET experiment in England. This successful remote participation was made possible by remote collaboration technology being developed and deployed by the SciDAC National Fusion Collaboratory Project.

January 09, 2004

A new closure scheme for a six-moment gyro-fluid model of the circulating particle response function has been developed. The new closure reduces the error in the pressure and heat flux moments dramatically compared to previous closure schemes. This should improve the accuracy of the model in computing quasi-linear energy flux weights. The new six-moment model will be used as part of the upgrade to the theory based transport code GLF23.

Lang Lao, Punit Gohil, and Vincent Chan visited JT-60U to participate in the JT-60U QH-Mode and High Bootstrap Fraction Overdrive experiments and to discuss 2004 collaboration. QH-mode discharges were successfully produced in JT-60U with counter beam injection. EHO's were observed in many of these discharges.

Professor Kim Molvig from MIT visited GA for two days and gave a seminar on “Enhanced Neoclassical Polarization, Zonal Flow Suppression, and Enhanced Turbulence in Tokamaks”.

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