Near-threshold electron injection in the laser–plasma wakefield accelerator leading to femtosecond bunches

M R Islam,G H Welsh,R P Shanks,S Cipiccia,E Brunetti,B Ersfeld,R C Issac,S M Wiggins,M P Anania,R A Cairns,G Raj,A Noble,D A Jaroszynski D A Jaroszynski

doi:10.1088/1367-2630/17/9/093033

Abstract

The laser–plasma wakefield accelerator is a compact source of high brightness, ultra-short duration electron bunches. Self-injection occurs when electrons from the background plasma gain sufficient momentum at the back of the bubble-shaped accelerating structure to experience sustained acceleration. The shortest duration and highest brightness electron bunches result from self-injection close to the threshold for injection. Here we show that in this case injection is due to the localized charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches with little dark current are formed in the first bubble. We confirm experimentally, using coherent optical transition radiation measurements, that single or multiple bunches with femtosecond duration and peak currents of several kiloAmpere, and femtosecond intervals between bunches, emerge from the accelerator.

Highlights

The laser–plasma wakefield accelerator (LWFA)[1] produces high quality relativistic electron beams with energies currently ranging from 100ʼs of MeV to several GeV by exploiting the large electric field gradients generated by intense laser pulses interacting with plasma [2,3,4,5,6,7]
The accelerating structure of the LWFA operating in the nonlinear blowout regime, consists of a string of ion ‘bubbles’ of evacuated regions of plasma created by the combination of the ponderomotive force of an intense, ultra-short, laser pulse and the electrostatic restoring force of background ions acting on plasma electrons
The predictions of the injection model are compared with experimental measurements of the bunch structure carried out on the ALPHA-X beam line [8], shown in figure 6, by measuring coherent transition radiation (CTR) [39] emitted from the electron bunches as they traverse two thin foils placed 84 cm and 1 m from the LWFA

Summary

Introduction

The laser–plasma wakefield accelerator (LWFA)[1] produces high quality relativistic electron beams with energies currently ranging from 100ʼs of MeV to several GeV by exploiting the large electric field gradients generated by intense laser pulses interacting with plasma [2,3,4,5,6,7]. Several models have been proposed to describe injection, but most are not valid for current experimentally relevant parameters, as we show below, or do not accurately predict bunch properties, such as bunch duration and structure, or the threshold for self-injection, which has been studied experimentally [20] (in a restricted density range) and using numerical simulations [21] (but without investigating the bunch structure in either case). Its time dependence leads to a bunch structure consisting of either single ultra-short duration bunches, with low (local) dark current, or more complicated trains of very closely spaced bunches that are injected into the same bubble The formation of such bunch structure due to near-threshold injection cannot be explained by other injection mechanisms and to our knowledge has not been described before. We show that the bunch structure can be controlled by varying the plasma density

Near-threshold self-injection

Numerical simulations

Experimental results

Conclusions