Deriving the trajectory equations of gyrocenter with a multi-parameter Lie transform method

TL;DR

This paper introduces a multi-parameter Lie transform method to derive gyrocenter trajectory equations, effectively removing nonphysical gauge-dependent terms present in previous single-parameter approaches.

Contribution

The paper develops a multi-parameter Lie transform approach that accounts for all characteristic scales, improving the physical accuracy of gyrocenter trajectory equations.

Findings

Eliminates nonphysical gauge-dependent terms from trajectory equations.

Decouples gyroangle from other degrees of freedom up to second order.

Provides a more accurate and physically consistent derivation method.

Abstract

It's pointed out that the values of the generators derived by the modern gyrokinetic theory are inappropriately amplified by the pullback transform with the existence of electromagnetic perturbation, and the trajectory equations of the gyrocenter include terms containing the perturbative magnetic vector potential not in the form of its curl. These terms are not physical ones, as the perturbative magnetic vector potential could include an arbitrary gauge term. In this paper, instead of the single-parameter Lie transform method utilized by the modern gyrokinetic theory, a multi-parameter Lie transform method is utilized. All characteristic scales of the system can be taken into account. With the assistance of a kind of linear cancellation rule, the new method can decouple the gyroangle $\theta$ from the remaining degrees of freedom up to the second order of the parameter as the ratio between the Larmor radius and the transverse length scale of the equilibrium magnetic field. With this new method, those nonphysical terms don't appear anymore in the trajectory equations.