Sawtooth stabilization studies on TFTR

Ever since the sawtooth instability was first observed in tokamaks, it has been found to have a detrimental effect on the plasma performance when qa<3, as measured by the level of stored energy, plasma beta, energy confinement time and effective Q achieved. A potentially important means of controlling sawteeth was discovered on JET and subsequently verified on TFTR. In particular, it was found that the application of high power ICRF heating resulted in the stabilization of the sawteeth, with a corresponding increase in the stored energy and confinement times by factors of 20% or more. It has been proposed that the stabilization is due to the creation of a population of very energetic minority ions whose perpendicular pressure acts to suppress the m=1 internal kink instabilities thought to be responsible for the sawtooth oscillations. An objective of the TFTR research program is to explore the hot ion sawtooth stabilization regime, particularly at high densities, to determine if it can be successfully utilized to maximize plasma performance during the D-T operations phase on TFTR and ultimately on BPX. In this paper, the key experimental observations on sawtooth stabilization will be reviewed in the context of the existing theoretical model. (author) 10 refs., 3 figs