Injection of electron beams into two laser wakefields and generation of electron rings

Q. Chen, Dominika Maslarova, J. Wang, S. Li, and D. Umstadter

Phys. Rev. E 106, 055202 – Published 4 November 2022

Abstract

Mutual injection of electron beams into two laser plasma wakefields was observed experimentally when driving laser pulses interfered in plasma at a small crossing angle and were slightly relatively delayed, approximately by one pulse duration. Particle-in-cell simulations revealed that the mutual injection was sensitive to the spatial overlap of the laser pulses, which therefore could be used to control the mutual injection. The dual synchronized, femtosecond electron beams are potentially useful for pump-probe experiments in ultrafast science. In addition, out-of-axis ring-shaped electron beams were detected in both experiments and simulations.

Received 4 September 2022
Accepted 13 October 2022

DOI:https://doi.org/10.1103/PhysRevE.106.055202

Physics Subject Headings (PhySH)

Laser driven electron acceleration Laser wakefield acceleration Laser-plasma interactions Plasma optics

Accelerators & BeamsPlasma PhysicsNonlinear DynamicsInterdisciplinary Physics

Authors & Affiliations

Q. Chen ¹, Dominika Maslarova ^2,3, J. Wang ¹, S. Li¹, and D. Umstadter^1,*

¹Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
²Institute of Plasma Physics of the Czech Academy of Sciences, Za Slovankou 1782/3, 182 00 Prague, Czech Republic
³Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehová 78/7, 115 19 Prague, Czech Republic

^*donald.umstadter@unl.edu

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Vol. 106, Iss. 5 — November 2022

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Images

Figure 1
Experiment schematic. Laser pulses interfere in plasma and inject electron beams into the wakefield(s). Mutual injection occurs when the two pulse's arrival time to the intersection point differ by about one pulse duration. The delay was controlled by a delay line for the collimated beam in the upstream of the off-axis parabolic mirror.
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Figure 2
Experimentally observed mutual injection electron beams. (a) Typical spatial profiles. (b) The spectrum (black solid curve) and the angular divergence (red dotted curve) for the circled left beam in panel (a).
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Figure 3
Shot-to-shot varying electron beam profiles in the mutual injection experiment. Various forms of dual electron beams could have been caused by the unstable interference and injection due to the pointing jitter of laser pulses. The right-hand side beam followed the ahead laser pulse $# 1$ , and the left-hand side beam followed the delayed laser pulse $# 2$ . The ring electrons appeared more on the pulse #2 side, similar to simulations results, as shown in Fig. 5.
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Figure 4
2D simulation results. (a) Electron density profile at the simulation time of $t = 2.17$ ps. The green and black dots correspond to the positions of electrons of energy higher than 15 MeV, in the direction of pulse #1 ( $y > L_{y} / 2$ , wakefield ahead) and the direction of pulse #2 ( $y < L_{y} / 2$ , delayed wakefield), respectively. (b) Corresponding energy spectrum of the electrons of $> 15$ MeV from panel (a), corresponding to the direction of pulse #1 (green) and to the direction of pulse #2 (black). The overlap portions of the histograms are colored in dark green.
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Figure 5
Front view (charge over area $Q / A$ ) of electron beams from the 3D simulations depicting electrons of $E \geq 5$ MeV, with $Δ z_{foci} = 0$ at the simulation times of (a) $t = 0.93$ ps, (b) $t = 1.04$ ps, and (c) $t = 1.25$ ps; and with $Δ z_{foci} = 8$ $μ m$ at the simulation times of (d) $t = 0.93$ ps, (e) $t = 1.04$ ps, and (f) $t = 1.25$ ps. The laser beam ahead, #1, is moving in the direction of increasing $y$ , and the delayed beam, #2, is moving in the direction of decreasing $y$ , as indicated by the labels in the figures. The time of the focus collision corresponds to the simulation time of $t = 0.12$ ps. The center of the collision is located in the plane $y = 60$ $μ m .$
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Figure 6
2D simulation results: Dependence of charge of electrons of energy higher than 15 MeV $Q / z$ on $Δ t$ , at the simulation time of $t = 2.17$ ps. The green crosses and grey dots correspond to the charge in the direction of the pulse #1 ( $y > L_{y} / 2$ , wakefield ahead) and the direction of the pulse #2 ( $y < L_{y} / 2$ , delayed wakefield), respectively. The charge is expressed as charge per length $z$ , since 2D data are presented. The values are normalized to the highest value in the plot (at $Δ t = 1.8 τ$ ).
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