Abstract
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
Owing to the fact that the plasma wakefield has a transverse electric field of tens of gigavolts per metre and the radius of a plasma bubble is limited to several micrometres[22], it is difficult to find a strong enough external electric field or magnetic field for controlling the transverse motion of an electron beam in a bubble, especially helical motion. Moreover, Luo et al.[23] simulated and acquired the helical motion of an electron beam and elliptically polarized radiation by laser pulses incident at a skew angle to the axis of the plasma waveguide, but this method was hard to achieve in experiment. Thaury et al.[24] used one laser pulse to drive an asymmetrical plasma wakefield and another pulse of colliding injection to achieve the helical motion of an electron beam, but the process of colliding injection is not easy to control and has low repetition probability. In addition, Chang et al.[25] also generated the helical motion of an electron beam and circularly polarized radiation by using a petawatt-class circularly polarized laser pulse interaction with near-critical density plasma, but the divergences of the electron beam and radiation were very large.
In this paper, we propose a simple all-optical method to control the transverse motion of the ionization injected electron beam by changing the evolution of the plasma wakefield transversal distribution. We also use three-dimensional particle-in-cell (3D-PIC) simulation to verify our experimental results and analyze the dynamics of electron transverse motion.
2 Experimental setup and results
Figure 1.Experimental setup. (a) Laser intensity distribution measured in front of, at and behind, respectively, the focal spot in the case of a perfect focus situation. (b) Laser intensity distribution measured after adjusting the posture of the OAP mirror. (c) The top-view image of the plasma channel.
Figure 2.Electron beam spatial distribution. The first three columns are driven by the asymmetrical focus, showing three different typical shapes. All of these electron beam profiles were acquired under the same experimental conditions. The last column is driven by the symmetrical focus. All of the angles refer to the included angle between the elliptical long axis and the horizontal axis.
Figure 3.Deflected electron distributions and charges for five consecutive shots. The red numbers are the total charges for electron energy above 80 MeV. (a) Asymmetrical focus. (b) Symmetrical focus.
3 3D-PIC simulation and results
The 3D-PIC simulations were carried out using KLAPS code[3132], and the tunnel-ionization model was adopted for field ionization. The simulation box size was 50μm×60μm×60μm with 1500×450×450 cells in the x-, y- and z-directions, respectively, and one cell contained two macro particles. In addition, a third-order time interpolation for the magnetic field was used in the simulation. P-polarized (y-direction) 800 nm laser pulses with a0 = 1.7 were focused to a radius of 15 μm at x = 50 μm behind the front edge of the nitrogen gas. The pulse had a Gaussian transverse profile and sine-squared longitudinal shape with pulse duration of 30 fs (FWHM). The neutral nitrogen longitudinal profile had a 100 μm up-ramp followed by a 2 mm long plateau with uniform density of 6×1017 cm−3.
If a laser pulse with the focus situation shown in Figure 1(b) is propagating and self-focusing in the plasma, the shape of the laser spot will change from a slanted 45° ellipse to a circle and then to a slanted 135° ellipse. The process of laser spot shape change will continue until the laser pulses cannot sustain self-focusing in the plasma. Therefore, in order to study the process of this laser pulse propagation and self-focusing in the plasma, and the influence of the asymmetrical laser focus on the plasma wakefield acceleration, the asymmetrical laser intensity distribution in front of the focal plane was set according to the intensity distribution measured in the experiments (as shown in Figure 1(b)), and the electric-field intensity distribution is expressed as
Figure 4.Laser plasma wakefield acceleration in 3D-PIC simulations. (a), (b) The cross-sections of the plasma bubble in the
Figure 5.Electron beam spots in 3D-PIC simulations. (a)–(c) Electron beam spots driven by a 45° slanted elliptical laser focus at different laser propagation distances (~485 μm, 685 μm and 885 μm) in nitrogen. (d) Corresponding to the case of the circular laser spot at a distance of ~885 μm. (e), (f) The phase spaces of the P
To compare the influence of the asymmetrical plasma wakefield on the transverse motion of the accelerated electron beam in the plasma bubble, the phase spaces of Py-z and Pz-y corresponding to the electrons of Figures 5(c) and 5(d) are shown in Figures 5(e) and 5(f) and Figures 5(g) and 5(h), respectively. Electrons in the symmetrical plasma wakefield have more momentum in the y-direction than in the z-direction, as shown in Figures 5(g) and 5(h), resulting in the shape of the electron beam tending to be an ellipse, as shown in Figure 5(d). However, for the asymmetrical plasma wakefield, the maximum momentum in the y-direction is approximately equal to that in the z-direction, as shown in Figures 5(e) and 5(f), and a majority of electrons have momentum in the z-direction larger than that in the case of the symmetrical plasma wakefield, as show in Figures 5(e) and 5(g). Therefore, a majority of electrons have experienced a strong force in the z-direction in the asymmetrical transversal wakefield.
4 Discussion
Figure 6.Analysis of transverse force for the electrons in the plasma wakefield. (a), (d) The cross-sections of the plasma bubble corresponding to the circular spot and the elliptical spot, respectively. (b), (c) The transverse force in the directions of
Figure 7.Trajectories of the electrons driven by (a) circular laser focus and (b) elliptical laser focus.
5 Conclusion
In conclusion, a simple method of controlling the transverse motion of an electron beam in a plasma bubble is presented. Laser pulses with a power of 100 TW drive a plasma wakefield and accelerate an electron beam in the regime of ionization injection. The transverse motion of the accelerated electron beam can be controlled by changing the intensity distribution of the laser focus by adjusting the posture of the OAP mirror. When the shape of the laser focus changes from circular to slanted elliptical, the geometrical symmetry of the transverse force in the plasma bubble is changed, resulting in the motion of the electron beam changing from undulating to helical. Moreover, the profile of the electron beam also changes with the laser focal spot’s shape. The experimental results were verified by 3D-PIC simulations.
This method is expected to conveniently control the transverse motion of an electron beam in a plasma wakefield and to generate synchrotron radiation with orbit angular momentum.
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