Modification of sawteeth by second harmonic electron-cyclotron heating in a tokamak

Abstract

Electron‐cyclotron heating, using second harmonic waves launched from the low field side, has a strong effect on sawteeth in the DIII‐D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. I, p. 159]. There are two distinct methods and mechanisms for sawtooth period extension. In the first case a factor of 6 increase in sawtooth period, compared to that during Ohmic heating, occurs with electron‐cyclotron heating (ECH) localized near the q=1 surface. The sawtooth period increases faster than linearly with increasing ECH power. Qualitatively, there is agreement with sawtooth suppression models, although details of the accompanying m/n=1/1 oscillation appear inconsistent. The second case is a technique for complete sawtooth suppression in which impurities and off‐axis heating broaden the current profile and drive q₀>1. It appears that the temperature and the current and q profiles inside q=1 are determined by the magnetohydrodynamic activity, which is modified by ECH, and not by the ECH power deposition directly.