Sustaining DIII-D High Beta Discharges Without the Sawteeth That Can Lead to Disruptions*

We discuss three methods for avoiding or minimizing sawteeth (ST) in DIII-D.  Two methods rely on relatively benign internal 3D perturbations to redistribute poloidal flux, causing the minimum safety factor (q_min) to stay above unity and thus avoiding sawteeth.  The hybrid scenario on DIII-D requires the presence of an internal tearing mode (usually 3/2) for the “flux pumping”, i.e. anomalous broadening of the current profile.  During stationary intervals in hybrid discharges, the rate of poloidal magnetic energy consumption is more than the rate of energy flow from the poloidal field coils. DIII-D discharges with a helical core induced by imposed rotating n=1 3D fields have a similar anomalous consumption of poloidal flux.  A possible origin of the behavior in the hybrid scenario is that the 3/2 tearing mode and its 2/2 side band produce helical perturbations in the core velocity similar to that from the imposed n=1 magnetic field.  These perturbations in each case produce a dynamo EMF that drives the current redistribution.  The third approach is to apply electron cyclotron current drive just inside q=1 to reduce ST amplitude and increase ST frequency, keeping q on axis closer to one and thereby providing smaller seeds for NTMs.