Reducing turbulent transport in tokamaks by combining intrinsic rotation and the low momentum diffusivity regime

TL;DR

This paper proposes a novel confinement improvement strategy for spherical tokamaks by combining intrinsic rotation from up-down asymmetry with the low momentum diffusivity regime, reducing turbulence without external momentum input.

Contribution

It introduces a new approach that leverages intrinsic momentum flux and flux surface shaping to enhance confinement, avoiding external momentum sources.

Findings

Flow shear can be significantly increased in the LMD regime.

Energy transport can be reduced by up to 25%.

Strategy scales well to large fusion devices.

Abstract

Based on the analysis of a large number of high-fidelity nonlinear gyrokinetic simulations, we propose a novel strategy to improve confinement in spherical tokamak plasmas by combining up-down asymmetric flux surface shaping with the Low Momentum Diffusivity (LMD) regime. We show that the intrinsic momentum flux driven by up-down asymmetry creates strong flow shear in the LMD regime that can significantly reduce energy transport, increasing the critical gradient by up to $25\%$. In contrast to traditional methods for generating flow shear, such as neutral beam injection, this approach requires no external momentum source and is expected to scale well to large fusion devices. The experimental applicability of this strategy in spherical tokamaks is addressed via simulations by considering actual equilibria from MAST and a preliminary equilibrium from SMART.