Comprehensive evaluation of the linear stability of Alfv\'en eigenmodes driven by alpha particles in an ITER baseline scenario

TL;DR

This study systematically evaluates the linear stability of Alfvén eigenmodes driven by alpha particles in ITER scenarios, revealing that higher core temperatures lead to more unstable, core-localized modes with significant growth rates despite damping effects.

Contribution

It introduces a comprehensive workflow using the CASTOR-K code to efficiently assess all possible eigenmodes' stability in ITER scenarios, considering various damping mechanisms.

Findings

Higher core temperatures increase alpha-particle density and gradient, leading to more unstable modes.

Core-localized low-shear toroidal Alfvén eigenmodes are the most unstable.

Radiative damping reduces growth rates but does not eliminate the instability.

Abstract

The linear stability of Alfv\'en eigenmodes in the presence of fusion-born alpha particles is thoroughly assessed for two variants of an ITER baseline scenario, which differ significantly in their core and pedestal temperatures. A systematic approach is used that considers all possible eigenmodes for a given magnetic equilibrium and determines their growth rates due to alpha-particle drive and Landau damping on fuel ions, helium ashes and electrons. This extensive stability study is efficiently conducted through the use of a specialized workflow that profits from the performance of the hybrid MHD drift-kinetic code $\mbox{CASTOR-K}$ (Borba D. and Kerner W. 1999 J. Comput. Phys. ${\bf 153}$ 101; Nabais F. ${\it et\,al}$ 2015 Plasma Sci. Technol. ${\bf 17}$ 89), which can rapidly evaluate the linear growth rate of an eigenmode. It is found that the fastest growing instabilities in the aforementioned ITER scenario are core-localized, low-shear toroidal Alfv\'en eigenmodes. The largest growth-rates occur in the scenario variant with higher core temperatures, which has the highest alpha-particle density and density gradient, for eigenmodes with toroidal mode numbers $n\approx30$. Although these eigenmodes suffer significant radiative damping, which is also evaluated, their growth rates remain larger than those of the most unstable eigenmodes found in the variant of the ITER baseline scenario with lower core temperatures, which have $n\approx15$ and are not affected by radiative damping.