Study of high-n modes in tokamaks using a high speed nonlocal gyrokinetic model

Abstract

Gyrokinetic theory has been used to derive a system of integral equations which nonlocally describe low frequency, short wavelength modes in a plasma of axisymmetrical toroidal geometry with low-β and circular nonconcentric flux surfaces with small Shafranov shift. The eigenmode equations contain the two potential approximation in φ and 'A' ∥ with full finite Larmor radius and trapped electron effects in the collisionless limit. The analysis makes use of the so-called "ballooning formalism" to lowest order in 1/'n' which yields a radially local calculation for the eigenfrequencies and the eigenfunctions. This representation, in conjunction with an efficient numerical algorithm, allows the eigen frequencies to be computed with sufficient accuracy and high speed for arbitrary high-' n' modes in the drift and shear-Alfven branches. This is the main accomplishment of this work. Test cases using artificial and actual tokamak experimental discharge parameters for the collisionless-trapped-electron, ion-temperature-gradient and ballooning modes have been benchmarked with the premium, comprehensive kinetic formulation of Rewoldt exhibiting favourable results.