Electron transport in spherical tokamaks based on stochasticity
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. Different formulae, based on the Rechester-Rosenbluth and the semi-empirical Rebut-Lallia-Walkins models, are used to describe the stochastic field contribution to electron heat transport, and these expressions are used to supplement TGLF reduced model predictions of the transport from electrostatic turbulence. This more complete anomalous transport model is 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, and that this must be included to model ST plasma scenarios where strong electron heat transport is not described by other instabilities. On the basis of obtained results the importance of stochasticity for the STEP device has been also discussed.