Fundamental Limits on Fiber-Based Electron Acceleration $-$ and How to Overcome Them

TL;DR

This paper investigates the fundamental physical limits of fiber-based electron acceleration using electromagnetic pulses and proposes gain-assisted dispersion engineering to overcome these constraints, enabling scalable and efficient particle acceleration.

Contribution

It derives conditions for phase velocity in hollow-core fibers and introduces gain-based methods to achieve velocity matching and dispersion control for electron acceleration.

Findings

Derives an expression for phase velocity in TM modes of hollow-core fibers.

Shows that material dispersion constraints hinder acceleration efficiency.

Proposes gain-assisted dispersion engineering to overcome fundamental limits.

Abstract

To accelerate ultra-relativistic charged particles, such as electrons, using an electromagnetic pulse along a hollow-core waveguide, the pulse needs to have a longitudinal electric field component and a phase velocity of $c$, the speed of light in vacuum. We derive an approximate closed-form expression for the wavelength at which the phase velocity of the TM$_{01}$ mode in a metal-clad hollow-core fiber with a dielectric layer is $c$. The expression is then used to derive conditions for material dispersion required of the dielectric in order to simultaneously have $c$ phase and group velocity. It is shown that the dispersion would need to be so heavily anomalous that the losses in the anomalously dispersive regime would render such a particle accelerator useless. We then propose the utilization of gain in the form of two spectral peaks in the dielectric to circumvent the otherwise fundamental limits and allow for TM$_{01}$ pulses with $c$ phase and group velocity and thus arbitrary length-scaling of fiber-based electron accelerators. In theory, the group velocity dispersion could also be made zero with further gain-assisted dispersion engineering, allowing for the co-propagation of dispersionless electromagnetic pulses with relativistic particles.