Theory Weekly Highlights for November 2002

November 22 2002

As part of a collaboration with the Hefei Institute of Plasma Physics in China, the EFIT and the ONETWO equilibrium reconstruction and transport codes were applied to analyze long pulse enhanced confinement discharges in the HT-7 superconducting tokamak. Long pulse, limited H-mode discharges with ITB were produced in HT-7 by combining LHCD and IBW. Transport analyses using ONETWO show that ion transport approaches the neoclassical level inside the ITB region. Analyses using the GKS gyro kinetic stability code indicate that ITG modes are unstable inside the ITB region. The results suggest that the enhanced ion confinement may be due the stabilization of the ITB turbulence by the pondermotive force from IBW.

November 15 2002

Dylan Brennan and Jeff Candy gave invited presentations at the 44th Division of Plasma Physics APS Meeting in Orlando. Brennan discussed the new model for resistive tearing stability, based on increasing delta-prime as a discharge approaches an ideal stability limit. Candy presented the recent continuum global gyrokinetic GYRO code developments and comparisons with DIII-D experiments obtained using the code.

November 08 2002

In addition to accurately predicting the onset conditions, island evolution modeling for DIII-D Tearing Mode experiments has qualitatively reproduced a previously puzzling, but routinely observed, feature in the evolution of spontaneous NTMs, namely a lull in the growth of the island. Initially, the linearly unstable island saturates at a size determined by the polarization cutoff. However, as β increases and the ideal limit is approached, Δ' increases sharply and the island begins to grow rapidly. Subsequently, finite island effects slow the growth. This is the “lull” phase. Once the island has grown sufficiently large, however, the island undergoes the transition to the usual NTM state and the growth rate increases rapidly again. Several features in the modeling are critical to reproducing this behavior. For the early phase, the parameters in the island evolution equation need to be accurately calculated from the equilibrium reconstruction, β needs to be ramped at the same rate as is observed in the experiment to obtain the correct time dependent behavior of Δ'. Also crucial is the use of a polarization model that removes the unphysically large polarization of the usual theory for vanishingly small islands. Otherwise, the onset and lull cannot be reproduced.

November 01 2002

The renormalized GLF23 and Multi-Mode (MM95) core transport models have been used together with scalings for the H-mode pedestal height to predict the profiles in H-mode discharges. Using a recently developed power dependent pedestal scaling along with the GLF23 model, an RMS error of 20% in the core stored energy is obtained for 47 H-mode discharges from DIII-D, JET, and C-Mod. In combined core and pedestal modeling of ITER, FIRE, and IGNITOR the power dependence of the pedestal height is found to be a critical issue. Results obtained using the GLF23 and MM95 core models, along with the power dependent pedestal model, are more optimistic than the results obtained using MHD limit (power independent) pedestal scalings. These results were recently presented at the IAEA Fusion Energy conference in Lyon, France.

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