A New Resistive Response to 3-D Fields in Low Rotation H-modes

A new resistive response to 3-D fields is identified in low rotation H-modes that are far from ideal MHD $\beta$ limits. The response increases as natural ($\Delta^\prime$) tearing stability limits are approached, either by lowering plasma rotation or by raising $\beta$. This leads to 2/1 tearing modes that degrade performance, with threshold fields to trigger modes falling to zero as the natural tearing limit is approached. These applied static 3-D field appear to act through rotation braking to decrease intrinsic tearing stability, thereby leading to formation of rotating modes at modest $\beta_N\sim 1-2$ in low torque plasmas. A formalism has been developed based on the observed physics mechanisms to account for the 3-D field threshold scaling. Further scans have been executed in toroidal field and density to determine coefficients of this scaling in torque-free H modes. These yield a more adverse toroidal field scaling for future devices than previous Ohmic studies, though the main increase in field sensitivity comes about because of proximity to the natural tearing $\beta$ limit at $\beta_N\sim 2$.