Three electron beams from a laser-plasma wakefield accelerator and the energy apportioning question

X Yang,G H Welsh,G Vieux,M R Islam,P A Grant,S M Wiggins,M P Tooley,B Ersfeld,F Y Li,S Cipiccia,D Reboredo Gil,E Brunetti,Z M Sheng,D A Jaroszynski,D W Grant

doi:10.1038/srep43910

Abstract

Laser-wakefield accelerators are compact devices capable of delivering ultra-short electron bunches with pC-level charge and MeV-GeV energy by exploiting the ultra-high electric fields arising from the interaction of intense laser pulses with plasma. We show experimentally and through numerical simulations that a high-energy electron beam is produced simultaneously with two stable lower-energy beams that are ejected in oblique and counter-propagating directions, typically carrying off 5–10% of the initial laser energy. A MeV, 10s nC oblique beam is ejected in a 30°–60° hollow cone, which is filled with more energetic electrons determined by the injection dynamics. A nC-level, 100s keV backward-directed beam is mainly produced at the leading edge of the plasma column. We discuss the apportioning of absorbed laser energy amongst the three beams. Knowledge of the distribution of laser energy and electron beam charge, which determine the overall efficiency, is important for various applications of laser-wakefield accelerators, including the development of staged high-energy accelerators.

Highlights

Is combined with the high-energy forward bunch[35]. These low-energy beams are appealing for applications such as single-shot imaging or dosimetry, because of their high-charge and inherent capability to induce fluorescence over a large area. Knowledge of their properties is important for the laser-plasma wakefield accelerators (LWFAs) studies, as they are shown to be closely related to the injection dynamics, which are critical to the production of high-energy beams
When a short intense laser pulse travels in underdense plasma, its ponderomotive force pushes electrons away from high intensity regions, leaving approximately spherical ion cavities (“bubbles”) trailing behind the pulse[4,5]
Numerical simulations and experiments show that LWFAs emit high-charge, low-energy side-electron beams that are insensitive to the initial conditions over a large range of laser and plasma parameters

Summary

Introduction

Is combined with the high-energy forward bunch[35] These low-energy beams are appealing for applications such as single-shot imaging or dosimetry, because of their high-charge and inherent capability to induce fluorescence over a large area. Knowledge of their properties is important for the LWFA studies, as they are shown to be closely related to the injection dynamics, which are critical to the production of high-energy beams. The oblique and backward beams can produce large fluxes of bremsstrahlung radiation if not properly dumped, leading to undesirable X-ray radiation background Another important concern is the large amount of energy carried off the plasma by these high-charge beams, despite their low energies per electron, which have consequences for high energy staged accelerators. The energy apportioning issue raises additional concerns about the overall efficiency of LWFAs

Methods

Results

Conclusion