Validation and predictions of reduced models for stochastic electron transport in MAST and MAST-U

Magnetic stochastic perturbations can strongly influence cross-field transport in high β tokamak plasmas. The impact of stochastic magnetic fields on electron heat transport in MAST/MAST-U is studied over a range in collisionality. The physics guided semi-empirical Rechester-Rosenbluth and Rebut-Lallia-Walkins models are separately used to describe the stochastic field contribution to electron heat transport, and to supplement TGLF reduced model predictions of the transport from electrostatic turbulence. These combined models of anomalous transport are implemented in the JINTRAC code, and applied to transport simulations of the flat-top phase in MAST/MAST-U. The different ranges of validity of the stochastic transport models are briefly reviewed, focusing on the length-scales involved in the transport process. The principal relevant length-scales have been calculated using the plasma equilibrium characteristics, and used to determine the most appropriate stochastic transport model that is then applied in each shot. This analysis strongly suggests that stochasticity is an important transport mechanism in spherical tokamaks that should be included in ST plasma scenarios where strong electron heat transport is not described by other instabilities.