Enhancement of laser-driven electron acceleration in an ion channel

TL;DR

This paper demonstrates that laser-driven electron acceleration in an ion channel can be significantly enhanced through parametric amplification of betatron oscillations, leading to higher electron energies and broader spectra.

Contribution

It introduces a novel mechanism for energy enhancement via parametric amplification of betatron oscillations in laser-driven ion channels, with a detailed parameter analysis.

Findings

Maximum electron energy is significantly increased above a wave amplitude threshold.

Electron spectra are broadened with energy enhancement.

The mechanism is robust across different ion densities.

Abstract

A long laser beam propagating through an underdense plasma produces a positively charged ion channel by expelling plasma electrons in the transverse direction. We consider the dynamics of a test electron in a resulting two-dimensional channel under the action of the laser field and the transverse electric field of the channel. A considerable enhancement of the axial momentum can be achieved in this case via amplification of betatron oscillations. It is shown that the oscillations can be parametrically amplified when the betatron frequency, which increases with the wave amplitude, becomes comparable to the frequency of its modulations. The modulations are caused by non-inertial (accelerated/decelerated) relativistic axial motion induced by the wave regardless of the angle between the laser electric field and the field of the channel. We have performed a parameter scan for a wide range of wave amplitudes and ion densities and we have found that, for a given density, there is a well pronounced wave amplitude threshold above which the maximum electron energy is considerably enhanced. We have also calculated a time-integrated electron spectrum produced by an ensemble of electrons with a spread in the initial transverse momentum. The numerical results show that the considerable energy enhancement is accompanied by spectrum broadening. The presented mechanism of energy enhancement is robust with respect to an axial increase of ion density, because it relies on a threshold phenomenon rather than on a narrow linear resonance.