Suppression of microtearing transport in a diamagnetic well induced at high-beta in the low-aspect-ratio Pegasus spherical torus

A diamagnetic well and local minimum |B|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 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 in the Pegasus minimum |B|regime. Multiple classes of microtearing instabilities at k_yρ_s 0.1-1 arise in the magnetic well region at 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 modes produce electromagnetic electron thermal transport, but the transport and low-k instabilities are suppressed in the diamagnetic well configuration.

Work supported by the US DOE Grants DE-SC0001288, DE-FG02-96ER54375, and DE-FG02-04ER-54742.