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• High Power Laser Science and Engineering
• Vol. 9, Issue 1, 010000e5 (2021)
Jie Feng1、2, Yifei Li3, Jinguang Wang3, Dazhang Li4、*, Changqing Zhu3, Junhao Tan3, Xiaotao Geng3, Feng Liu1、2, and Liming Chen1、2、*
Author Affiliations
• 1Key Laboratory for Laser Plasmas (MoE), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai200240, China
• 2IFSA Collaborative Innovation Center and School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai200240, China
• 3Beijing National Research Center of Condensed Matter Physics, Institute of Physics, CAS, Beijing100190, China
• 4Institute of High Energy Physics, CAS, Beijing100049, China
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Abstract

We demonstrate an all-optical method for controlling the transverse motion of an ionization injected electron beam in a laser plasma accelerator by using the transversely asymmetrical plasma wakefield. The laser focus shape can control the distribution of a transversal wakefield. When the laser focus shape is changed from circular to slanted elliptical in the experiment, the electron beam profiles change from an ellipse to three typical shapes. The three-dimensional particle-in-cell simulation result agrees well with the experiment, and it shows that the trajectories of these accelerated electrons change from undulating to helical. Such an all-optical method could be useful for convenient control of the transverse motion of an electron beam, which results in synchrotron radiation from orbit angular momentum.

1 Introduction

The concept of laser plasma wakefield accelerators (LWFAs) was first proposed by Tajima and Dawson[1]. Over the past few decades, LWFAs have become increasingly mature and have recently exhibited stable[2], low divergence (milliradians)[3] and energy tunable[4] electron bunches with a charge at the picocoulomb level[5]. An electron beam is most efficiently produced in the ‘bubble’ regime[6], which requires laser pulses that are both intense (normalized vector potential a0 > 1) and short (pulse duration $\tau \le 2\pi c/{\omega}_{\mathrm{p}}$, where ωp is the plasma frequency). The ponderomotive force of these laser pulses propagating in an underdense plasma pushes the background electrons away from the high-intensity regions and drives a relativistic plasma wave. The wave consists of a string of ion cavities (also referred to as ‘bubbles’), and the electrons trapped inside can be accelerated by the electrostatic field set up by the separation of electrons and ions. Moreover, these accelerated electrons will also oscillate in the plasma wakefield with betatron frequency ${\omega}_{\beta }={\omega}_{\mathrm{p}}/\sqrt{2\gamma }$ and emit synchrotron radiation[7]. There are several methods of electron capture, including ponderomotive force injection[8], colliding laser pulse injection[9], plasma density gradient injection[10] and transverse self-injection[11–13]. With these methods, the injected direction of electrons is hard to control, and these injection processes are not easy to achieve in experiment. In contrast, another method is ionization-induced injection[14–16], which is used in this study. Owing to the different ionization potential levels of high Z atoms[15][17–19] (such as nitrogen), the outer shell electrons can be ionized instantaneously by the rising edge of the laser pulses (98 eV for N+5 requires an intensity of ~2×1016 W/cm2) and pushed away. The inner shell electrons (552 eV for N+6 requires an intensity of ~1×1019 W/cm2) are ionized close to the peak of the laser intensity envelope. These ionized electrons will appear at rest and slip backward relative to the laser pulses and the wake. The electrons are trapped after gaining enough energy from the longitudinal electric field of the first period of the wake to move at the phase velocity of the wake and will gain additional energy[15]. Ionization injection is a more controllable method, regarded particularly for its stability[14][20][21]. Moreover, these trapped electrons mainly oscillate along the direction of laser polarization in the ion cavity.

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Jie Feng, Yifei Li, Jinguang Wang, Dazhang Li, Changqing Zhu, Junhao Tan, Xiaotao Geng, Feng Liu, Liming Chen. Optical control of transverse motion of ionization injected electrons in a laser plasma accelerator[J]. High Power Laser Science and Engineering, 2021, 9(1): 010000e5