The 3/2 Magnetic Island and Its Effect on the Central Tokamak Region

In the hybrid discharge scenario in \hbox{DIII-D}, the central plasma evolves into a quasi-steady state without sawtooth. The central safety factor (q) is pegged close to 1 and correlates with the development of a rotating 3/2 magnetic island [1]. The causal relationship between the 3/2 island and the non-sawtoothing of the discharge is investigated. Equilibria modeling the discharge with different central q are analyzed using the PEST-III stability code. The 3/2 island is found to develop a 2/2 side-band with increasing amplitude as the central q approaches 1. This near resonant Alfven wave propagates with enhanced phase speed relative to the background plasma. With sufficient phase speed, the 2/2 side-band could drive currents which impedes the further decrease in q to trigger the sawtooth. The central 2/2 side-band does not lead to appreciable enhanced trapping of the plasma; but does modify the trajectory of the trapped particles and lead to additional transport.\par \vspace{0.5em} \noindent [1]~P.A.\ Politzer, et al., 32nd EPS Conf.\ on Plasma Physics, Tarragona, Spain (2005).