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First nonlinear gyrokinetic simulations of microtearing modes in the core ofa MAST case are performed on two surfaces of the high-collisionality dischargeused in Valovi\v{c} et al. Nucl. Fusion 51.7 (2011) to obtain the favorableenergy confinement scaling with collisionality, $\tau_E\propto\,\nu_*^{-1}$. Onthe considered surfaces microtearing modes dominate linearly at binormal lengthscales of the order of the ion Larmor radius. While the effect of electroncollision frequency is moderate in linear simulations, a strong dependence onthis parameter is found in nonlinear simulations at $r/a=0.5$, where $r$ and$a$ are the surface and tokamak minor radius, respectively. The dynamics ofmagnetic islands generated by microtearing modes is analysed, showing that theradial extent of the stochastic region caused by islands overlapping plays animportant role in determining the saturation level of the microtearing modedriven heat flux. Local nonlinear gyrokinetic simulations show that themicrotearing mode driven heat flux, $Q_e^\mathrm{MTM}$, is largely dominated bymagnetic flutter and depends strongly on the magnetic shear, $\hat{s}$.Comparing two surfaces, $r/a=0.5$ and $r/a=0.6$, reveals that$Q_e^\mathrm{MTM}$ is negligible at $r/a=0.5$ ($\hat{s}=0.34$), with theelectron temperature gradient driven heat flux, $Q_e^\mathrm{ETG}$, comparableto the experimental electron heat flux, $Q_e^\mathrm{exp}$, while$Q_e^\mathrm{MTM}$ is significantly larger and comparable to $Q_e^\mathrm{ETG}$and $Q_e^\mathrm{exp}$ at $r/a=0.6$ ($\hat{s}=1.1$). Microtearing modes causemore experimentally significant transport in higher $\hat{s}$ regions and mayinfluence (together with electron temperature gradient modes) the observedscaling of energy confinement time with collisionality (Valovi\v{c} et al.Nucl. Fusion 51.7 (2011)).

Nonlinear microtearing modes in MAST and their stochastic layer formation