Electron cyclotron resonance near the axis of the gas-dynamic trap

TL;DR

This paper analytically studies the propagation of extraordinary electromagnetic waves near electron cyclotron resonance in an open linear trap, considering magnetic field inhomogeneity, and reveals how it influences wave trajectories and plasma heating efficiency.

Contribution

It extends previous models by including magnetic field line slope effects, providing analytic ray trajectories and criteria for resonance point attraction in electron cyclotron resonance heating.

Findings

Magnetic field inhomogeneity significantly alters wave propagation.

Resonance point can act as an attractor or repeller for wave rays.

Analytic solutions are applied to a practical heating experiment.

Abstract

Propagation of an extraordinary electromagnetic wave in the vicinity of electron cyclotron resonance surface in an open linear trap is studied analytically, taking into account inhomogeneity of the magnetic field in paraxial approximation. Ray trajectories are derived from a reduced dispersion equation that makes it possible to avoid the difficulty associated with a transition from large propagation angles to the case of strictly longitudinal propagation. Our approach is based on the theory, originally developed by the Zvonkov and Timofeev [1], who used the paraxial approximation for the magnetic field strength, but did not consider the slope of the magnetic field lines, which led to considerable error, as has been recently noted by Gospodchikov and Smolyakova [2]. We have found ray trajectories in analytic form and demonstrated that the inhomogeneity of both the magnetic field strength and the field direction can qualitatively change the picture of wave propagation and significantly affect the efficiency of electron cyclotron heating of a plasma in a linear magnetic trap. Analysis of the ray trajectories has revealed a criterion for the resonance point on the axis of the trap to be an attractor for the ray trajectories. It is also shown that a family of ray trajectories can still reach the resonance point on the axis if the latter generally repels the ray trajectories. As an example, results of general theory are applied to the electron cyclotron resonance heating experiment which is under preparation on the Gas Dynamic Trap in the Budker Institute of Nuclear Physics [3].