Ion temperature profile stiffness: non-linear gyrokinetic simulations and comparison with experiment

TL;DR

This study uses non-linear gyrokinetic simulations to investigate the reduction in ion temperature profile stiffness observed at JET, highlighting the roles of electromagnetic effects and plasma parameters, with results aligning well with experiments.

Contribution

The paper demonstrates that non-linear electromagnetic effects can significantly reduce ion temperature profile stiffness, providing insights into experimental observations at JET.

Findings

Electromagnetic effects reduce ion heat flux and profile stiffness.

Toroidal flow shear alone does not significantly decrease heat flux.

Simulations agree with experimental ion heat flux within uncertainties.

Abstract

Recent experimental observations at JET show evidence of reduced ion temperature profile stiffness, hypothesised to be due to concomitant low magnetic shear (s) and significant toroidal rotational flow shear. Non-linear gyrokinetic simulations are performed, aiming to investigate the physical mechanism behind the observations. A comprehensive set of simulations are carried out, comparing the impact on the ion heat flux of various parameters that differ within the data-set. These parameters include q, s, rotation, effect of rotation on the magnetohydrodynamic (MHD) equilibrium, R/L_n, beta_e, Z_eff, and the fast particle content. The effect of toroidal flow shear itself is not predicted by the simulations to lead to a significant reduction in ion heat flux, due both to an insufficient magnitude of flow shear and significant parallel velocity gradient destabilisation. It is however found that non-linear electromagnetic effects due to both thermal and fast-particle pressure gradients, even at low beta_e, can significantly reduce the profile stiffness. A total of five discharges are examined, at both inner and outer radii. For all cases studied, the simulated and experimental ion heat flux values agree within reasonable variations of input parameters around the experimental uncertainties.