Gap eigenmode of radially localised helicon waves in a periodic structure

TL;DR

This paper models a spectral gap and localized eigenmode in a periodic structure for helicon waves using an electromagnetic solver, demonstrating agreement with theory and identifying a defect-induced eigenmode consistent with Bragg's law.

Contribution

It introduces a numerical approach to identify and analyze gap eigenmodes in a periodic structure for helicon waves, including the effect of defects.

Findings

Computed gap frequency and width match theoretical predictions

Identified a discrete eigenmode inside the spectral gap

Eigenmode's wavelength aligns with Bragg's law

Abstract

An ElectroMagnetic Solver (EMS) [Chen et al., Phys. Plasmas, 13, 123507 (2006)] is employed to model a spectral gap and a gap eigenmode in a periodic structure in the whistler frequency range. A Radially Localised Helicon (RLH) mode [Breizman and Arefiev, Phys. Rev. Lett, 84, 3863 (2000)] is considered. We demonstrate that the computed gap frequency and gap width agree well with a theoretical analysis, and find a discrete eigenmode inside the gap by introducing a defect to the system's periodicity. The axial wavelength of the gap eigenmode is close to twice the system's periodicity, which is consistent with Bragg's law. Such an eigenmode could be excited by energetic electrons, similar to the excitation of Toroidal Alfv\'{e}n Eigenmodes (TAE) by energetic ions in tokamaks.