Double adiabatic theory of collisionless geodesic acoustic modes in tokamaks

TL;DR

This paper develops a double-adiabatic fluid theory for collisionless geodesic acoustic modes in tokamaks, analyzing their properties, external excitation methods, and damping mechanisms.

Contribution

It introduces a novel application of the CGL double-adiabatic closure to model these modes and explores external driving and damping effects.

Findings

Derived the basic linear normal mode for collisionless geodesic acoustic modes.

Identified external magnetic forces and ion heating as potential resonance drivers.

Analyzed damping effects due to collisional magnetic pumping.

Abstract

Collisionless geodesic acoustic modes in tokamaks being supersonic for large "safety factor" q, the CGL (G. Chew, M. Goldberger, F. Low, 1956)1 double-adiabatic fluid closure is applied to formulate a theory for these modes. The basic linear normal mode is obtained. External means to drive these modes at resonance, as has been proposed earlier, are explored. The external drivers considered include external magnetic forces to effect flux surface displacements, as well as non-axisymmetric ion heating. Finally, the damping of these modes from collisional magnetic pumping is investigated using a model set of CGL collision-corrected equations.