Physics of the low momentum diffusivity regime in tokamaks and its experimental applicability

TL;DR

This paper investigates a novel approach to enhance plasma flow shear in tokamaks by reducing momentum diffusivity, using gyrokinetic simulations to demonstrate its potential for improving confinement and applicability to future fusion devices.

Contribution

It introduces the Low Momentum Diffusivity (LMD) regime and shows how tilting flux surfaces can generate significant intrinsic momentum flux in tokamaks.

Findings

Low momentum diffusivity achieved at specific plasma conditions.

Tilted flux surfaces induce strong intrinsic momentum flux.

Potential for improved flow shear in future fusion reactors.

Abstract

Strong $E\times B$ plasma flow shear is beneficial for reducing turbulent transport. However, traditional methods of driving flow shear do not scale well to large devices such as future fusion power plants. In this paper, we use a large number of nonlinear gyrokinetic simulations to study a novel approach to increase flow shear: decreasing the momentum diffusivity to make the plasma ``easier to push''. We first use an idealized circular geometry and find that one can obtain low momentum diffusivity at tight aspect ratio, low safety factor, high magnetic shear and low temperature gradient. This is the so-called Low Momentum Diffusivity (LMD) regime. To drive intrinsic momentum flux, we then tilt the flux surface, making it up-down asymmetric. In the LMD regime, this intrinsic momentum flux drives strong flow shear that can significantly reduce the heat flux and increase the critical temperature gradient. We also consider the actual experimental geometry of the MAST tokamak to illustrate that this strategy can be practical and create experimentally significant flow shear. Lastly, a preliminary prediction for the SMART tokamak is made.