Large-charge quasimonoenergetic electron beams produced by off-axis colliding laser pulses in underdense plasma

Electrons can be efficiently injected into a plasma wave by colliding two counterpropagating laser pulses in a laser wakefield acceleration. However, the generation of a high-quality electron beam with a large charge is difficult in the traditional on-axis colliding scheme due to the growth of the electron beam duration coming from the increase of the beam charge. To solve this problem, we propose an off-axis colliding scheme, in which the collision point is away from the axis of the driver pulse. We show that the electrons injected from the off-axis region are highly concentered on the tail of the bubble even for a large trapped charge, thus feeling almost the same accelerating field. As a result, quasimonoenergetic electron beams with a large charge can be produced. The validity of this scheme is confirmed by both the particle-in-cell simulations and the Hamiltonian model. Furthermore, it is shown that a Laguerre-Gauss (LG) laser can be adopted as the injection pulse to realize the off-axis colliding injection in three dimensions symmetrically, which may be useful in simplifying the technical layout of the real experiment setup.

Figure 1

Schematic diagram of off-axis colliding injection.

Figure 2

Simulation results for the off-axis case ($y_0=10{\lambda }_0$). (a) Electron density distribution at $t=3.3\text{ps}$. (b) Phase space plots of trapped electrons (right axis) and longitudinal electric field $E_x$ on the axis of the pump pulse (left axis). As a reference, results of the on-axis case ($y_0=0$) are presented in (c) and (d).

Figure 3

Electron spectra at $t=30\text{ps}$ for the on-axis case ($y_0=0$) and off-axis case ($y_0=10{\lambda }_0$), respectively.

Figure 4

Electron beam duration (left axis) and beam charge (right axis) versus the radial collision position $y_0$. The insets are electron density distribution for the $y_0=10\mu \mathrm{m}$ case and the $y_0=15\mu \mathrm{m}$ case, respectively.

Figure 5

Charge and absolute energy spread as a function of the injection laser pulse amplitude $a_1$ for on-axis case ($y_0=0$) and off-axis case ($y_0=10{\lambda }_0$), respectively.

Figure 6

Off-axis collision injection realized in three dimensions by adopting a LG laser as the injection pulse. (a) Cross section of a LG laser in the $(y,z)$ plane. (b) Cross section of laser field in the $(x,y)$ plane at $t=50\text{fs}$. (c) Cross section of electron density in $(x,y)$ plane at $t=400\text{fs}$. (d) Electron spectra at $t=400\text{fs}$ for the Gaussian case and LG case, respectively.