Electromagnetic stabilization of tokamak microturbulence in a high-$\beta$ regime

TL;DR

This paper investigates electromagnetic and flow shear stabilization of ITG turbulence in high-$eta$ tokamak plasmas, showing electromagnetic effects are dominant at inner radii and can significantly improve fusion power estimates for future devices.

Contribution

It demonstrates the importance of electromagnetic stabilization in high-$eta$ regimes and extends low-$eta$ findings to high-$eta$ conditions relevant for future fusion reactors.

Findings

Electromagnetic stabilization dominates at inner radii, aligning with experimental heat fluxes.

Flow shear stabilization is more effective at outer radii, with negligible electromagnetic effects.

Electromagnetic effects could enhance fusion power by approximately 20% in ITER scenarios.

Abstract

The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-$\beta$ JET L-mode discharge illustrates the relation between ITG stabilization, and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-$\beta$ JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant, and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-$\beta$ generalization of comparable observations found at low-$\beta$ at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.