Role of microtearing mode in DIII-D and future high- β p core plasmas

Excellent confinement has been routinely observed in DIII-D (J. L. Luxon, Nucl. Fusion 2002) high β p discharges, which are characterized by a strong large-radius internal transport barrier (ITB) in almost all kinetic channels. Typically, the ion thermal transport is neoclassical with conventional long-wavelength turbulence instabilities suppressed by α stabilization, while the mechanism for the anomalous electron thermal transport remains unclear [Garofalo et al., Nucl. Fusion 55(12), 123025 (2015)]. A new gyrokinetic analysis shows that while the large values of α in the ITB can stabilize all local electrostatic drift wave (ES-DW) instabilities as well as the kinetic ballooning mode, a new slab-like microtearing mode (MTM) with its eigenfunction mainly peaking on the high field slide is destabilized. This destabilization is shown to be more likely to happen in discharges with high safety factors. Nonlinear gyrokinetic simulations demonstrate that this MTM branch can reproduce the experimentally inferred electron thermal flux in the ITB region and, therefore, provide convincing evidence that the electron temperature profile in the ITB is regulated by the MTM. Extrapolations to the future scenarios, like ITER high β p plasmas, show that the dominant instability is likely to come back to ES-DW due to the lower density gradient and collisionality. However, even in this regime, some unusual features associated with MTMs predicted for DIII-D parameters, such as the high-field-side peaking and slab nature, may remain for the reactor ES-DW.