Vlasov versus reduced kinetic theories for helically symmetric equilibria

TL;DR

This paper introduces a new constant of motion for helically symmetric plasma equilibria and compares full Vlasov theory with reduced kinetic models, highlighting differences in current density predictions near the magnetic axis.

Contribution

It derives a new constant of motion in Vlasov theory for helically symmetric equilibria and assesses the limitations of reduced kinetic theories near the magnetic axis.

Findings

Vlasov current density can significantly differ from reduced models.

Reduced kinetic theories may have limitations in describing energetic particles.

Differences are notable near the magnetic axis.

Abstract

A new constant of motion for helically symmetric equilibria in the vicinity of the magnetic axis is obtained in the framework of Vlasov theory. In view of this constant of motion the Vlasov theory is compared with drift kinetic and gyrokinetic theories near axis. It turns out that as in the case of axisymmetric equilibria [H. Tasso and G. N. Throumoulopoulos, Phys. Plasmas 18, 064507 (2011)] the Vlasov current density thereon can differ appreciably from the drift kinetic and gyrokinetic current densities. This indicates some limitation on the implications of reduced kinetic theories, in particular as concerns the physics of energetic particles in the central region of magnetically confined plasmas.