Plasma fibre using bright-core helicon plasma

TL;DR

The paper introduces the plasma fibre concept by drawing an analogy between bright-core helicon plasmas and optical fibres, arguing that a strong radial density gradient can support total internal reflection and guided EM propagation. It develops theoretical descriptions for step-like and Gaussian radial density profiles, deriving an explicit threshold for total reflection $\phi_{1*}$ in the step-like case and a graded-index ray-tracing formulation for Gaussian profiles. Electromagnetic simulations using Maxwell's equations with a cold-plasma dielectric tensor show waveguide-like, frequency-dependent confinement: step-like profiles yield core-dominated propagation with a cut-off near $f$ where core guidance ceases, while Gaussian profiles exhibit distinct propagation windows (e.g., $1.9$–$2.5$ GHz and $3.6$–$4.0$ GHz) with multi-mode behavior. The results establish the feasibility of dynamically reconfigurable plasma-based waveguides (plasma fibres) for communications or diagnostics, and propose experimental routes using bright-core discharges and B-dot/Langmuir probes to map axial attenuation and core confinement.

Abstract

This paper reports an innovative concept of ``plasma fibre" using bright-core helicon plasma, inspired by its spatial and spectral similarities to the well-known optical fibre. Theoretical analyses are presented for both ideal case of step-like density profile and the realistic case of Gaussian density profile in radius. The total reflection of electromagnetic waves near the sharp plasma density gradient and consequently the wave-guide feature could indeed happen if the incident angle is larger than a threshold value. Numerical computations using electromagnetic solver that based on Maxwell's equations and cold-plasma dielectric tensor yield consistent results. The experimental verification and prospective applications are also suggested. The ``plasma fibre" could be functional component that embedded into existing communication systems for special purpose based on its capability of dynamic reconfiguration.

Figures (7)

• Figure 1: Conceptual drawing of "plasma fibre" using bright-core helicon plasma.
• Figure 2: Illustration of the total reflection of wave propagation from step-like density gradient: (a) in $(r;~z)$ space, (b) in $(r;~n_e)$ space.
• Figure 3: Illustration of the total reflection of wave propagation from Gaussian density gradient: (a) in $(r;~z)$ space, (b) in $(r;~n_e)$ space.
• Figure 4: Radial profiles of plasma density employed in the EMS simulations: "Step-like" is constructed as ideal case, "Gaussian" refers to actual CSDX deviceThakur:2015aaThakur:2014aaBurin:2005aa.
• Figure 5: Typical results of computed wave field for the step-like density profile shown in Fig. \ref{['fg_density']}: (a) axial variations inside core, (b) axial variation outside core, (c) radial variations across the core, (d) dispersion relations for frequency range of $0.1-5$ GHz.