Numerical investigation of microtearing modes in the core of experimentally relevant spherical tokamak scenarios

Linear and nonlinear gyrokinetic simulations are performed in experimentally relevant scenarios built from a MAST case where a microtearing mode instability dominates at ion Larmor radius scale (these microtearing modes are only weakly unstable and nonlinear simulations indicate they do not cause an experimentally significant level of transport across the chosen surface). This collisional microtearing mode instability appears only when a velocity dependent electron collision frequency is considered. Electrostatic potential fluctuations are found to provide a strong destabilising mechanism. The sensitivity to the electron collision frequency is investigated in both linear and nonlinear simulations. While the effect of electron collision frequency is moderate in linear simulations, a strong dependence on this parameter is found in nonlinear simulations. The effect of magnetic islands generated by microtearing modes and their interaction is analysed, showing that the radial extension of the stochastic region caused by islands overlapping plays an important role in determining the saturation level of the microtearing mode driven heat flux. Nonlinear simulations on the chosen surface find that the radial extend of the stochastic region and the mocrotearing mode heat flux both increase with decreasing electron collision frequency. The magnetic shear is found to play an important role in the formation of a stochastic layer.