Theory Weekly Highlights for April 2001

April 20, 2001

Calculations were redone for the edge instabilities in DIII-D H Mode discharges #87099 and #92001 in order to check the sensitivity of the results to the value of the normalized poloidal flux (separatrix is 100%) at which the calculating grid is cut off. In these discharges, the calculated instabilities from GATO correlated with the onset of a type I ELM and were strongly localized toward the edge. For both cases, it was shown that it is essential that the stability calculation extends even beyond 99% of the separatrix flux in order to find the observed ELM instability. This is surprising since the instabilities themselves extend significantly further toward the center than 99% poloidal flux-even up to a third of the minor radius in the case of discharge #87099. This sensitivity partly explains why it has been difficult in past calculations to find these instabilities and correlate them with the observed ELMs.

April 13, 2001

In collaboration with the FARTECH group, the Mirnov comparison for the L Mode NCS disruption instability in DIII-D discharge #87009 was repeated with higher resolution stability analysis, an improved model for the equilibrium shifts, and the latest version of the VACUUM code. The discrepancies that were reported at IAEA have now been eliminated and the new analysis shows excellent agreement between the ideal instability predicted by GATO and the observed Mirnov signal. A similar analysis for the resistive wall mode in discharge #96519 is also underway. Initial results from this are also promising.

April 06, 2001

Stability analysis of a DIII-D H-mode discharge equilibrium from the DIII-D/C-Mod edge similarity experiments, reconstructed using kinetic and MSE data, shows that the normalized edge pressure gradients are near the ideal first regime ballooning stability limit. The edge pressure gradients in these discharges are lower than those found in typical DIII-D H-mode discharges with giant ELMs. The edge bootstrap currents are also lower due to the higher edge collisionality. These results are consistent with the small ELMs sometimes observed in these discharges and are similar to those found in JT-60U discharges that exhibited small ELMs. The CMOD team has also provided a comparison shot, which will be analyzed and compared directly with the DIII-D discharge.

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