Sawtooth Suppression by Tearing Modes in Hybrid Plasmas on DIII-D

Hybrid discharges have the remarkable property that the m/n=3/2 neoclassical tearing mode (NTM) raises the central safety factor (q$_{0})$ above unity and suppresses sawteeth. Experiments on DIII-D are trying to distinguish between several sawtooth suppression mechanisms. One mechanism that can be tested for is the redistribution of the beam ions by the 3/2 NTM, which would increase the off-axis neutral beam current drive (NBCD). The NBCD profile can be determined from the evolution of the poloidal flux measured by motional Stark effect (MSE) polarimetry. Also, the fast ion D$_{\alpha }$ (FIDA) diagnostic can measure the fast ion density profile for hybrid discharges with and without the 3/2 NTM. Another analysis tool is TRANSP simulations of the current profile evolution, which shows that diffusion of the beam ions is unlikely to raise q$_{0}$ above 1. Other possible sawtooth suppression mechanisms are hyper-resistivity, counter current drive in the plasma core via a coupling between the 3/2 NTM and a 2/2 sideband, and magnetic flux pumping.