Linear gyrokinetic particle-in-cell simulations of Alfven instabilities in tokamaks

TL;DR

This paper uses a gyrokinetic particle-in-cell code to study the linear behavior of Alfven modes in tokamaks, including their spectrum, structure, and effects of equilibrium parameters.

Contribution

It presents a detailed gyrokinetic simulation framework for Alfven instabilities and analyzes mode structures and spectra in various tokamak configurations.

Findings

Verification of continuum modes in cylindrical geometry

Identification of phase-shift in modes touching the continuum

Radial symmetry of modes in the continuum gap

Abstract

The linear dynamics of Alfven modes in tokamaks is investigated here by means of the global gyrokinetic particle-in-cell code NEMORB. The model equations are shown and the local shear Alfven wave dispersion relation is derived, recovering the continuous spectrum in the incompressible ideal MHD limit. A verification and benchmark analysis is performed for continuum modes in a cylinder and for toroidicity-induced Alfven Eigenmodes. Modes in a reversed-shear equilibrium are also investigated, and the dependence of the spatial structure in the poloidal plane on the equilibrium parameters is described. In particular, a phase-shift in the poloidal angle is found to be present for modes whose frequency touches the continuum, whereas a radial symmetry is found to be characteristic of modes in the continuum gap.