Sawtooth studies with q-profile measurements on TFTR (abstract)a)

The q profile plays a key role for plasma equilibrium and theories of MHD instabilities. With the development of the motional Stark effect (MSE) diagnostic,b) accurate q(r,t) profiles have been measured and utilized for equilibrium and stability analysis. Data have been obtained for several conditions on TFTR including L mode, supershot, current ramps, noninductive current drive, and high βp. Sawtoothing discharges on TFTR have q(0) well below one, and it remains below one throughout the evolution of the sawtooth cycle. This is in contradiction with the widely believed Kadomtsev model. It has also been found that stabilization of sawteeth can be achieved with q(0)<1. The criteria for stabilization have been compared to several theoretical models such as resistive, ideal, and two-fluid MHD. The ω* stabilization from the two-fluid MHD model agrees well with the data from TFTR.