Effect of the Shafranov shift and the gradient of $\beta$ on intrinsic momentum transport in up-down asymmetric tokamaks

TL;DR

This study investigates how the Shafranov shift and the gradient of beta influence intrinsic momentum transport in up-down asymmetric tokamaks, revealing their competing effects on rotation profiles through analytical and gyrokinetic simulations.

Contribution

It provides a detailed analysis of the Shafranov shift's dependence on tilt angle and the impact of beta' on momentum transport, combining equilibrium calculations with gyrokinetic simulations.

Findings

Shafranov shift depends strongly on tilt angle but not on current or pressure profiles.

Shafranov shift can significantly enhance momentum transport.

Beta' reduces momentum transport, broadening the rotation profile.

Abstract

Tokamaks with up-down asymmetric poloidal cross-sections spontaneously rotate due to turbulent transport of momentum. In this work, we investigate the effect of the Shafranov shift on this intrinsic rotation, primarily by analyzing tokamaks with tilted elliptical flux surfaces. By expanding the Grad-Shafranov equation in the large aspect ratio limit we calculate the magnitude and direction of the Shafranov shift in tilted elliptical tokamaks. The results show that, while the Shafranov shift becomes up-down asymmetric and depends strongly on the tilt angle of the flux surfaces, it is insensitive to the shape of the current and pressure profiles (when the geometry, total plasma current, and average pressure gradient are kept fixed). Next, local nonlinear gyrokinetic simulations of these MHD equilibria are performed with GS2, which reveal that the Shafranov shift can significantly enhance the momentum transport. However, to be consistent, the effect of $\beta'$ (i.e. the radial gradient of $\beta$) on the magnetic equilibrium was also included, which was found to significantly reduce momentum transport. Including these two competing effects broadens the rotation profile, but leaves the on-axis value of the rotation roughly unchanged. Consequently, the shape of the $\beta$ profile has a significant effect on the rotation profile of an up-down asymmetric tokamak.