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• High Power Laser Science and Engineering
• Vol. 4, Issue 3, 03000e24 (2016)
Nasr A.M. Hafz, Song Li, Guangyu Li, Mohammad Mirzaie, Ming Zeng, and Jie Zhang
Author Affiliations
• Key Laboratory for Laser Plasmas (MOE) and Department of Physics and Astronomy, Collaborative Innovation Center of IFSA, Shanghai Jiao Tong University, Shanghai 200240, China
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Abstract

Ionization-induced electron injection in laser wakefield accelerators, which was recently proposed to lower the laser intensity threshold for electron trapping into the wake wave, has the drawback of generating electron beams with large and continuous energy spreads, severely limiting their future applications. Complex target designs based on separating the electron trapping and acceleration stages were proposed as the only way for getting small energy-spread electron beams. Here, based on the self-truncated ionization-injection concept which requires the use of unmatched laser–plasma parameters and by using tens of TW laser pulses focused onto a gas jet of helium mixed with low concentrations of nitrogen, we demonstrate single-stage laser wakefield acceleration of multi-hundred MeV electron bunches with energy spreads of a few percent. The experimental results are verified by PIC simulations.

1 Introduction

Since it was proposed in 1979[1], laser wakefield acceleration (LWFA) has attracted a lot of attention of many groups around the world. Attributed to its ultra-high acceleration gradients $({\sim}1~\text{GV}~\text{cm}^{-1})$, the LWFA scheme might be considered as a basis for future compact electron–positron colliders[25] and ultra-compact x-ray free-electron lasers[6, 7]. The LWFA research has achieved a significant breakthrough in 2004[810] when self-injected quasimonoenergetic electron beams were firstly obtained in experiments under the highly nonlinear ‘bubble’ regime[11, 12]. In this regime, electrons of an underdense plasma produced by pure hydrogen or helium gas jets, are expelled radially by the ponderomotive force of a focused ultra-intense laser pulse, leading to form a nearly spherical ionic cavity (bubble) whose size corresponds to the plasma wavelength ${\it\lambda}_{p}$. The bubble propagates together with the laser pulse at a velocity close to the speed of light $c$, and experiences evolution due to the evolution of the laser pulse during its propagation in the plasma. During the variation of bubble size an electron density spike builds up at its back and eventually breaks, causing electron injection[1315] inside the bubble. The self-injected electrons witness the longitudinal accelerating field inside the bubble and are accelerated with low energy spread. After 2004, dramatic progress has been made in the LWFA research[2, 1618], including the latest experimental results by Leemans et al.[19] on the generation of self-injected monoenergetic electrons with energy up to 4.2 GeV, from 9-cm-long capillary discharge waveguide powered by laser pulses with peak power up to 300 TW. However, experimental studies[20] have shown that the self-injection process is highly nonlinear and uncontrollable as it requires large laser normalized vector potential $a_{0}$, to trigger the injection in single low-$Z$ gas plasma, where $a_{0}=\sqrt{(P\text{(TW)}\times 10^{3})/21.5}\times {\it\lambda}_{0}\;({\rm\mu}\text{m})/w_{0}\;({\rm\mu}\text{m})$, $P$ is the laser power, ${\it\lambda}_{0}$ and $w_{0}$ are the laser’s wavelength and focal spot size.

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Nasr A.M. Hafz, Song Li, Guangyu Li, Mohammad Mirzaie, Ming Zeng, Jie Zhang. Generation of high-quality electron beams by ionization injection in a single acceleration stage[J]. High Power Laser Science and Engineering, 2016, 4(3): 03000e24