Proton Acceleration in a Laser-induced Relativistic Electron Vortex

TL;DR

This paper demonstrates that laser-induced relativistic electron vortices in plasma can accelerate protons to high energies, with potential control via laser angle and plasma density, using particle-in-cell simulations.

Contribution

It introduces a novel mechanism for proton acceleration through electron vortex dynamics in laser-irradiated plasma with density gradients.

Findings

Protons can reach energies of 100-200 MeV.

A quasi-monoenergetic proton beam with 140 MeV mean energy is achievable.

Vortex velocity and proton energy are controllable by laser angle and plasma density.

Abstract

We show that when a solid plasma foil with a density gradient on the front surface is irradiated by an intense laser pulse at a grazing angle, around 80 degrees, a relativistic electron vortex is excited in the near-critical-density layer after the laser pulse depletion. The vortex structure and dynamics are studied using particle-in-cell simulations. Due to the asymmetry introduced by nonuniform background density, the vortex drifts at a constant velocity, typically 0.2 to 0.3 times the speed of light. The strong magnetic field inside the vortex leads to significant charge separation; in the corresponding electric field initially stationary protons can be captured and accelerated to twice the velocity of the vortex (100-200 MeV). A representative scenario - with laser intensity of 10^21 W/cm^2 -is discussed: two dimensional simulations suggest that a quasi-monoenergetic proton beam can be obtained with a mean energy 140 MeV and an energy spread of about 10%. We derive an analytical estimate for the vortex velocity in terms of laser and plasma parameters, demonstrating that the maximum proton energy can be controlled by the incidence angle of the laser and the plasma density gradient.