Microtearing Instabilities, ∇B Reversal, and Magnetic Drifts in the Pegasus Local Minimum |B| Regime

A local minimum |B| “magnetic well” region is readily accessed in high-𝛽 plasmas driven by local helicity injection in the A ~ 1 Pegasus ST. This magnetic topology may afford novel, favorable characteristics affecting turbulent transport. ∇B reversal on the low-field-side is stabilizing for drift waves, reduces the trapped particle fraction, and expands the parameter space for fast ion trapping. The magnetic configuration, however, remains net-paramagnetic with near omnigeneity (|B| ≈ |B|(ψ)) in the bad curvature region. Small banana orbit widths in an omnigeneous region reduce neoclassical transport. Here, we report on the gyrokinetic stability of microtearing modes in the Pegasus minimum |B| regime. Multiple classes of microtearing instabilities arise at k_y𝜌_s ~ 0.1-1 in the magnetic well region at ψ_N ~ 0.3-0.9 on the outboard midplane. Modes at k_y𝜌_s ~ 0.2 are insensitive to the ∇p component in magnetic drifts which is common in lower 𝛽 scenarios. Modes at k_y𝜌_s ~ 0.8, however, are highly sensitive to the ∇p component. Also, exclusion of the ion species is destabilizing for k_y𝜌_s ~ 0.8 modes in contrast to typical microtearing modes. Preliminary nonlinear simulations will also be presented.