A quasi-linear model of electromagnetic turbulent transport and its application to flux-driven transport predictions for STEP

TL;DR

This paper develops a fully electromagnetic quasi-linear transport model based on gyrokinetic simulations, accurately predicting fluxes in STEP and enabling flux-driven transport simulations to assess steady-state conditions.

Contribution

The paper introduces a novel electromagnetic quasi-linear transport model that incorporates flow shear effects and is validated against nonlinear gyrokinetic simulations for STEP.

Findings

The model accurately predicts fluxes across various equilibria.

Flux-driven simulations show the existence of a steady-state with fusion power comparable to design.

Nonlinear simulations confirm the reduced model's flux predictions.

Abstract

A quasi-linear reduced transport model is developed from a database of high-$\beta$ electromagnetic nonlinear gyrokinetic simulations performed with Spherical Tokamak for Energy Production (STEP) relevant parameters. The quasi-linear model is fully electromagnetic and accounts for the effect of equilibrium flow shear using a novel approach. Its flux predictions are shown to agree quantitatively with predictions from local nonlinear gyrokinetic simulations across a broad range of STEP-relevant local equilibria. This reduced transport model is implemented in the T3D transport solver that is used to perform the first flux-driven simulations for STEP to account for transport from hybrid-KBM turbulence, which dominates over a wide region of the core plasma. Nonlinear gyrokinetic simulations of the final transport steady state from T3D return turbulent fluxes that are consistent with the reduced model, indicating that the quasi-linear model may also be appropriate for describing the transport steady state. Within the assumption considered here, our simulations support the existence of a transport steady state in STEP with a fusion power comparable to that in the burning flat-top of the conceptual design, but do not demonstrate how this state can be accessed.