Local dependence of ion temperature gradient on magnetic configuration, rotational shear and turbulent heat flux in MAST

TL;DR

This study analyzes how ion temperature gradient scale length in MAST tokamak plasma correlates with magnetic shear, rotational shear, and turbulent heat flux, suggesting a self-regulating turbulent system near marginal stability.

Contribution

It reveals the local dependence of ion temperature gradient on magnetic configuration, shear, and turbulence, providing new insights into plasma turbulence regulation in spherical tokamaks.

Findings

R/LTi strongly correlates with rotational shear and magnetic pitch angle.

Inverse correlation between R/LTi and turbulent heat flux.

Results support a marginal stability adjustment mechanism.

Abstract

Experimental data from the Mega Amp Spherical Tokamak (MAST) is used to show that the inverse gradient scale length of the ion temperature R/LTi (normalized to the major radius R) has its strongest local correlation with the rotational shear and the pitch angle of the magnetic field (or, equivalently, an inverse correlation with q/{\epsilon}, the safety factor/the inverse aspect ratio). Furthermore, R/LTi is found to be inversely correlated with the gyro-Bohm-normalized local turbulent heat flux estimated from the density fluctuation level measured using a 2D Beam Emission Spectroscopy (BES) diagnostic. These results can be explained in terms of the conjecture that the turbulent system adjusts to keep R/LTi close to a certain critical value (marginal for the excitation of turbulence) determined by local equilibrium parameters (although not necessarily by linear stability).