Reassessing the impact of megaelectronvolt ions in fusion plasmas via gyrokinetic simulations

TL;DR

This paper critically reevaluates previous gyrokinetic simulation results on megaelectronvolt ions in fusion plasmas, revealing that numerical resolution issues led to overestimated heat conductivity and questioning earlier claims of turbulence suppression.

Contribution

It identifies key numerical resolution problems in prior simulations and demonstrates their impact on heat conductivity estimates, urging a reassessment of earlier conclusions.

Findings

Numerical resolution issues caused overestimation of heat conductivity.

Correcting these issues increases heat conductivity by more than tenfold.

Previous claims of turbulence suppression due to TAEs are called into question.

Abstract

Gyrokinetic simulations conducted by Mazzi et al. reveal the suppression of turbulence in fusion plasmas through the destabilization of Toroidal Alfv\'en Eigenmodes (TAEs) by megaelectronvolt ions. Our analysis demonstrates that the authors' numerical findings are strongly influenced by the selected simulation settings, calling into question their claim that the resulting heat conductivity aligns with the TRANSP power balance. Specifically, we assert that the numerical results presented by Mazzi et al. are a direct consequence of the inadequacy of resolution employed in their numerical simulations. Notably, there are three primary factors contributing to this issue: (i) the employed radial box size is insufficient, leading to an undesirable impact of the boundary conditions on the simulations; (ii) the adoption of a higher minimum toroidal mode number fails to accurately resolve the entire range of TAEs, resulting in an underestimation of TAE drive and introducing spurious effects on wave-particle resonances; (iii) an insufficient resolution in the magnetic moment direction exacerbates these challenges. Upon addressing the aforementioned numerical issues, a significant increase in heat conductivity for each plasma species was observed by more than tenfold, diverging from the expected values derived from the TRANSP power balance calculations. Consequently, our results raise serious doubts on the authors' assertion of enhanced performance in the presence of strongly unstable TAEs, emphasizing the need for a reevaluation of their claims.