Nanometer-scale pre-bunched electron beams generated from all-optical plasma-based acceleration

Abstract

High-quality and prebunched electron beams can produce coherent x-rays with high intensity and narrow bandwidth, which is essential for modern light sources. An all-optical scheme based on plasma-based acceleration to produce bright electron beams that are pre-bunched on nanometer-scales is proposed. By using a density modulation created by two low intensity counter-propagating lasers, the phase velocity of the plasma wake excited by an intense driver laser in a uniform plasma can be modulated at a frequency twice that of the colliding lasers and thus turn the injection on and off. The injected electrons are micro-bunched at the Doppler shifted wavelength of the modulated wavelength using the corresponding phase velocity of the gradual expansion of the wakefield. It is demonstrated that by controlling the properties of the drive and colliding lasers, that beams with exotic pre-bunched structures can be produced, which may have critical applications in ultrafast high power x-rays. This extremely compact, all-optical scheme to produce ultra-bright pre-bunched electron beams may therefore enable novel applications for ultrafast x-ray users and arouse general interest in various fields.

Figures (4)

• Figure 1: Schematic for all-optical pre-bunched beam generation. (a) Two long and low intensity colliding lasers modulate the plasma density through the ponderomotive force when they superimpose, which can turn the injection on and off frequently in a nonlinear wake driven by an intense drive laser to form a pre-bunched electron beam (not to scale). (b) The density perturbation at $z=84.2~\micro\meter$ (solid lines) and $84.4~\micro\meter$ (dashed line). Profile 1: $t_\mathrm{rise}=t_\mathrm{fall}=56~\femto\second$, $t_\mathrm{flat}=337~\femto\second$, profile 2: $t_\mathrm{rise}=t_\mathrm{fall}=112~\femto\second$, $t_\mathrm{flat}=293~\femto\second$ and profile 3: $t_\mathrm{rise}=t_\mathrm{fall}=309~\femto\second$, $t_\mathrm{flat}=0$. (c) The density perturbation encountered by the head of a drive laser which is 280 fs behind the colliding laser. (d) The phase velocity modulation of the tail of the wake excited by a drive laser with $a_0=0.5$.
• Figure 2: The amplitude of the phase velocity modulation extracted from 1D PIC simulations with different modulation periods $\lambda_m$ (a) and different $a_0$ (b). The result predicted by Eq. \ref{['deltavphi']} is in black and the simulation results are in blue and red. The excursion of the plasma electrons under different $a_0$ is shown in (b) by the red crosses.
• Figure 3: All-optical generation of pre-bunched beams. (a) The evolution of the peak $a_0$ of the drive laser and the velocity of the wake center. (b) The charge density distribution of the injected electrons in the ($z_\mathrm{i}, \xi$) space. The blue and red lines show the $\xi$-distribution at $z_\mathrm{i}=65$ and 92 $\micro\meter$. (c) The longitudinal phase space and the current profile of the injected electrons at $t=912~\femto\second$. (d) The bunching factor for electrons with $\xi<21.4~\micro\meter$ when the polarization of the drive laser is perpendicular (blue) or parallel (red) to that of the colliding lasers. A result corresponding to the case where the drive laser is parallel to the colliding lasers and $\lambda_\mathrm{1}=1600~\nano\meter$ is shown in yellow. The plasma electron density distribution at $z=67.3~\micro\meter$ (e) and $101~\micro\meter$ (f). The insets show the enlarged density distribution near the wake tail. The black solid lines represent the density lineout at $\xi=20.2~\micro\meter$, marked by the dashed lines.
• Figure 4: The effects of the colliding lasers on the pre-bunched beams. (a) On-axis $E_z$ of the plasma wake driven by a 800 nm wavelength drive laser for different polarization and wavelengths of the colliding pulses. The lines are vertically offset. (b) The Wigner function of the density modulation during the injection when two colliding pulses with $\beta = 6.3~\femto\second^{-2}$ are used.