Examination of sawtooth stabilization by localized electron cyclotron heating near the q = 1 surface

Sawtooth oscillations in the Kyoto University WT-3 tokamak are observed to be strongly modified or suppressed by localized electron cyclotron heating near the q = 1 surface. It has been proposed that this is due to current profile modification leading to stabilization of the m/n = 1/1 resistive mode. In general, a local current density increase occurs due to ECH. This can be attributed to direct current drive and/or an enhanced local conductivity due to localized electron heating. To gain further insight into these phenomena, the authors use the TORCH code to simulate the electron cyclotron heating and current drive that occurs in WT-3. The tokamak equilibria into which the rf power is launched is generated using the PEST code. The authors describe the launched electron cyclotron power using a gaussian beam antenna model in order to establish the degree of localization of the electron cyclotron power. The total driven current resulting from the electron power deposition is computed taking into account trapped particle and relativistic effects. The modified current profile is then assessed for its stability properties using the PEST code. In this study, they first concentrate on the wave and parameter space appropriate to the WT-3 experiment to help in the understanding of the experimental results and in the design of new experiments that can distinguish between the different physics mechanisms that play a role in the stabilization of the sawtooth oscillations. They then consider the applicability of ECH stabilization of sawtooth oscillations in the BPX and ITER tokamaks. 1 ref