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 \textbar B\textbar region is readily accessed in high-$\beta $ plasmas driven by local helicity injection in the $A\sim $ 1 Pegasus ST. This magnetic topology may afford novel, favorable characteristics affecting turbulent transport. $\nabla $B reversal on the low-field-side is stabilizing for drift waves, reduces the trapped particle fraction, and expands the parameter space for fast ion trapping. The high-$\beta $ plasma, however, remains net-paramagnetic with near omnigeneity (\textbar B\textbar $\approx $ \textbar B\textbar ($\psi ))$ in the bad curvature region. Here, we report on the gyrokinetic stability of microtearing modes in the Pegasus minimum \textbar B\textbar regime. Multiple classes of microtearing instabilities at k$_{\mathrm{y}}\rho _{\mathrm{s}}\sim $ 0.1-1 arise in the magnetic well region at $\psi _{\mathrm{N}}\sim $ 0.3-0.9. Collisionless high-k modes (k$_{\mathrm{y}}\rho_{\mathrm{s}}\approx $ 1) with narrow parallel mode structures are destabilized at $\beta_{\mathrm{crit}}\approx $ 3{\%}, and collisional low-k modes (k$_{\mathrm{y}}\rho _{\mathrm{s}}\approx $ 0.3) with extended parallel mode structures are destabilized at $\beta_{\mathrm{crit}}\approx $ 12{\%}. Nonlinear gyrokinetic simulations for a conventional monotonic \textbar B\textbar 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.