Multi-scale analysis of global electromagnetic instabilities in ITER Pre-Fusion-Power Operation plasmas

TL;DR

This study uses gyrokinetic simulations to analyze electromagnetic instabilities across multiple scales in an ITER PFPO plasma scenario, revealing various Alfvén eigenmodes and microscale instabilities without energetic particle influence.

Contribution

It provides a comprehensive multi-scale analysis of electromagnetic instabilities in ITER plasmas, including the identification of different Alfvén eigenmodes and microscale modes.

Findings

Identification of RSAE, TAE, and EAE modes with weak damping.

Observation of BAE/AITG modes near rational surfaces.

Detection of microscale instabilities at high toroidal mode numbers.

Abstract

Global electromagnetic gyrokinetic simulations are performed with the Particle-in-Cell code ORB5 for an ITER Pre-Fusion-Power-Operation (PFPO) plasma scenario, with half-field (2.65 T) and half-current (7.5 MA). We report on a 'multi-scale' analysis of the discharge, considering eigenmodes and instabilies across three scale-lengths. Although the scenario will nominally have neutral beam heating with particles injected with 1 MeV, Alfv\'en eigenmodes are investigated in the absence of such source, and Reversed Shear (RSAE), Toroidal (TAE) and Elliptical (EAE) Alfv\'en eigenmodes are found with weak damping for moderately low toroidal mode numbers ($10 \leq n \leq 35$). At higher toroidal mode numbers ($40 \leq n \leq 70$), unstable Alfv\'enic modes have been observed close to rational surfaces and are labelled as Beta-induced Alfv\'en eigenmodes (BAE)/Alfv\'enic Ion Temperature Gradient (AITG) modes, since their frequency is associated with the BAE gap and they are driven by the bulk plasma on the Alfv\'enic continuum. These modes are unstable in the absence of energetic particles, and adding a species of energetic particles (with an isotropic 1 MeV slowing down distribution) has negligible impact on their growth rate. At higher toroidal mode numbers ($150 \lessapprox n < 220$), low frequency microscale instabilities are observed.