Potential Methods for Improving Pedestal Temperatures and Fusion Performance
G. W. Hammett, W. Dorland, M.A. Beer, and M. Kotschenreuther
Date of PPPL Report: October 14, 1999
Conference Paper: Presented at the Workshop on Opportunities and Directions in Fusion Energy Science for the Next Decade, (Snowmass, CO, July 11-23, 1999).The physics of the tokamak edge is very complicated, and the scaling of the H-mode transport barrier pedestal has significant uncertainties. Evidence from the largest tokamaks appears to support a model in which the H-mode pedestal width scales linearly with the poloidal gyroradius and the gradient scales with ideal MHD ballooning limits. However, there appears to be significant variability in the data from different tokamaks, including observations on DIII-D that indicate a regime where the pedestal is in second stability and the width is independent of poloidal gyroradius, which would give a more favorable scaling to reactor scales. An important question is the role of the bootstrap current in the pedestal, and another is how far can the improvements in edge stability be pushed with higher triangularity and elongation. Even with the more pessimistic model, where the pedestal width is proportional to the poloidal gyroradius, the results presented here suggest that pedestal temperatures, and thus the fusion performance, may be signifcantly improved by designs with stronger plasma shaping (higher triangularity and elongation), moderate density peaking, and higher magnetic field (and thus reduced size), such as in ARIES-RS, FIRE, and some of the new ITER-RC designs.