Measurements of the sawtooth cycle and fluctuation-induced Hall electromotive force in MST tokamak plasmas

The significance of the dynamo-like, fluctuation-induced Hall electromotive force (EMF) is investigated during the sawtooth crash in q(a) ~ 2.5 Madison Symmetric Torus (MST) tokamak plasmas (B_T = 0.14  T, R=1.5 m, a = 0.5 m) using a probe covering r/a  > 2/3. Previous experiments in MST reversed-field pinch plasmas as well as simulations of the resistive kink instability in high-performance tokamaks have shown that the Hall EMF, 〈J × B〉, could play a role during reconnection events. Here, we characterize the magnetic fluctuations generated during a sawtooth instability to evaluate the Hall EMF’s contribution in the sawtooth relaxation process. Using a generalized Ohm’s law, the parallel Hall EMF can be expressed in terms of flux-surface-averaged, correlated fluctuations. Initial results indicate that the Hall EMF is non-zero during the sawtooth crash but small compared to the inductive electric field. Magnetic fluctuations appear to be localized near the q = 2 surface, suggesting m/n = 2/1 mode activity. Significant probe/plasma interaction has prevented direct measurement of magnetic fluctuations near the q = 1 surface, and there is significant variation between relaxation events. Work supported by US DOE and WiPPL Team.