The Role of the $m/n=3/2$ Tearing Mode in the Hybrid Scenario

The hybrid scenario has been proposed as a robust operating scenario for high performance operation of ITER. Understanding the physics of the hybrid regime will allow more confident implementation. In these plasmas, $J (0)$ is lower and $q(0)$ is higher than in comparable conventional plasmas. A key feature in DIII-D hybrids is an $m/n=3/2$ NTM. This island structure is associated with the reduction ($q_{95}\leq4$) or elimination ($q_{95}\geq4$) of sawteeth. Decreasing the sawtooth amplitude reduces or eliminates a trigger for the deleterious $m/n=2/1$ NTM, which limits beta in the conventional H-mode scenario. The effect of the 3/2 mode on sawteeth has been shown using localized ECCD ($\leq50\,$kA) to enhance or suppress the mode amplitude. With co-ECCD the mode is suppressed and sawteeth appear. With counter-ECCD the 3/2 amplitude increases and small pre-existing sawteeth are suppressed. A variety of physical mechanisms may be involved in the regulation of $q(0)$ and the sawteeth by the 3/2 mode. Because the stationary state always has $q(0)$ close to one, it is likely that the observed 2/2 component of the 3/2 mode is playing a role.