Proton acceleration in a laser-induced relativistic electron vortex

L Q Yi,T Fülöp,B F Shen,A Pukhov,I Pusztai

doi:10.1017/s0022377819000485

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,${\sim}80^{\circ }$, 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 non-uniform background density, the vortex drifts at a constant velocity, typically$0.2{-}0.3$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$10^{21}~\text{W}~\text{cm}^{-2}$– 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${\sim}10\,\%$. 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.

Highlights

Acceleration of protons by intense lasers has attracted extensive interest due to the potential significance in several branches of science, technology and medicine (Daido, Nishiuchi & Pirozhkov 2012)
We describe a new proton acceleration mechanism that relies on a laser-induced relativistic electron vortex (EV) in a near-critical-density (NCD) plasma with a density gradient normal to the target surface
We show that when a laser pulse irradiates an NCD plasma in a grazing angle, in the presence of a sharp, pre-formed target-normal density gradient, the resulting intense electric currents form an electron vortex after the laser depletion

Summary

Introduction

Acceleration of protons by intense lasers has attracted extensive interest due to the potential significance in several branches of science, technology and medicine (Daido, Nishiuchi & Pirozhkov 2012). The physics of laser pulse propagation in NCD plasmas has been widely studied (Pukhov & Meyer-ter Vehn 1996; Nakamura & Mima 2008; Bulanov et al 2010; Liu et al 2013; Bin et al 2015; Liu et al 2016; Hilz et al 2018; Ma et al 2019), the current study focuses a new regime, where the plasma density inhomogeneity length scale is comparable to the vortex size in the direction normal to the surface of the target (y), while in the tangential directions, the density length scale is much larger. To collisionless shock acceleration, protons initially at rest can be reflected to twice the EV drift velocity (Macchi et al 2013), and a narrow energy spectrum can be obtained in the direction of the vortex propagation

EV formation and the proton acceleration

Model of EV velocity in strongly non-uniform plasmas

Findings

Discussion

Conclusions