Suppression of microtearing transport in diamagnetic well regimes for high-beta spherical torus configurations*

A diamagnetic well and local minimum |B| region was readily accessed in high-β plasmas driven by local helicity injection in the A ~ 1 Pegasus ST. ▽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 high-β plasma, however, remains net-paramagnetic with near omnigeneity (|B| ≈ |B|(ψ)) in the bad curvature region. Here, we report on the gyrokinetic stability of microtearing modes (MTM) in the Pegasus minimum |B| regime. Multiple classes of MTM at k_yρ_s ~ 0.1-1 arise in the region ψ_N ~ 0.3-0.9. Collisionless high-k modes (k_yρ_s ≈ 1) with narrow parallel mode structures are destabilized at β_crit ≈ 3%, and collisional low-k modes (k_yρ_s ≈ 0.3) with extended parallel mode structures are destabilized at β_crit ≈ 12%. Nonlinear gyrokinetic simulations for a conventional monotonic |B| equilibrium show that the low-k MTM produce electromagnetic electron thermal transport, but the transport and low-k modes are suppressed in the diamagnetic well configuration.  With finite-beta suppression of ITG and TEM, the results point to an attractive confinement regime for ST devices.