A study of the sawtooth instability by means of external helical windings on the Tokoloshe tokamak

Despite 17 years of research the sawtooth instability, seen on all tokamaks, is still not understood. Knowledge of the centre value of the safety factor (qo) is of importance in order to relate the experimental observations to existing sawtooth theories. On the Tokoloshe tokamak, apart from normal m/n = 1/1 oscillations observed on sawteeth, a burst of 1/1 oscillations resembling a slow sawtooth is seen immediately prior to the onset of normal fast sawteeth. During the rise phase of a normal sawtooth the 1/1 precursor has the same rotation frequency as the 2/1 Mirnov signal. Directly after a sawtooth crash the 1/1 has a lower frequency and is apparently decoupled from the 2/1 mode. The main features of the 1/1 burst actively and a plausible explanation for the observed 1/1 and 2/1 mode locking may be found with the assumption that an m = 1,n = 1 tearing mode exists in the plasma. This is equivalent to the assumption that qo < 1 during at least part of the sawtooth cycle. Application of an external resonant m = 1,n = 1 helical field, apart from showing a general effect on the sawtooth amplitude, also affects both the m = 1 burst activity and the normal sawtooth amplitude in a highly specific manner. When two diodes view the plasma along chords just inside the resonant surface on opposite sides of the plasma centre, the soft X-ray intensity signal of one diode only is reduced, starting at a specific time during the sawtooth rise phase. The effect of the l = 1 coil on the soft X-ray intensity therefore has an m = 1 symmetry. The phase of this stationary coil induced m = 1 mode depends on the direction of the plasma current (Ip) and the direction of the coil current (I1) in the l = 1 winding. Comparison with numerical field line tracing calculations for two q-profiles (qo = 0,7 and a flat q profile with qo = 1.006) indicates that the phase of the asymmetry as a function of Ip and I1 is compatible with qo < 1. The imposed asymmetry on the m = 1 burst amplitude has the same phase as for normal sawteeth, indicating that sawtoothing on Tokoloshe starts with the onset of the condition qo < 1. 71 figs., 97 refs