Multichromatic Narrow-Energy-Spread Electron Bunches from Laser-Wakefield Acceleration with Dual-Color Lasers

A method based on laser wakefield acceleration with controlled ionization injection triggered by another frequency-tripled laser is proposed, which can produce electron bunches with low energy spread. As two color pulses copropagate in the background plasma, the peak amplitude of the combined laser field is modulated in time and space during the laser propagation due to the plasma dispersion. Ionization injection occurs when the peak amplitude exceeds a certain threshold. The threshold is exceeded for limited duration periodically at different propagation distances, leading to multiple ionization injections and separated electron bunches. The method is demonstrated through multidimensional particle-in-cell simulations. Such electron bunches may be used to generate multichromatic x-ray sources for a variety of applications.

Figure 1

Schematic view of dual color lasers trigged periodic injection in LWFA. A laser with base frequency ${\omega }_1$ (the red curves) and its harmonic ${\omega }_2$ (the blue curves) propagate in a mixed gas plasma. The dashed black curves show the superposition of the two frequency laser fields at different propagation distances. Laser parameters are chosen so that ionization-induced injection can be switched on when the beating is constructive and be switched off when the beating is destructive. In the plot, $E_{\text{ith}}$ represents the effective threshold field for the high-$Z$ gas inner shell ionization.

Figure 2

Typical evolution of the dual color lasers according to Eq. (2). (a) $a_{10}=1$, $a_{20}=1/4$, ${\omega }_2/{\omega }_1=2$ and ${\omega }_p/{\omega }_1=0.01$. (b) $a_{10}=1$, $a_{20}=1/9$, ${\omega }_2/{\omega }_1=3$ and ${\omega }_p/{\omega }_1=0.01$. (c) Illustration showing the reason to use a SWBL combination. (d) Two lineouts of (b) at $s$ values indicated by the black and red lines.

Figure 3

1D PIC simulation of the SWBL and injections. (a) Electron energy at the diagnostic point vs the initial position of the injected electrons. (b) The SWBL peak amplitude evolution. The blue dotted line is the estimated inner shell ionization threshold, and the black dash-dotted line is the separation from the mixed gas to the pure helium gas. (c) Electron beam spectrum at the distance $4860\mu \mathrm{m}$, where the minimum energy spread during the phase rotation is measured to be 0.29% in FWHM. (d) The energy and energy spread in FWHM vs the initial laser phase $\varphi$.

Figure 4

2D PIC simulations of the SWBL injection scheme. (a) The density snapshot at $z=3780\mu \mathrm{m}$. The colored dots show the locations of three electron bunches. (b) The energy and space distribution of the energetic electrons. (c) The spectrum of the injected electrons, showing three monoenergetic peaks. (d) The pseudopotential ($\psi$) difference of the wake and the laser peak field evolution. The dash-dotted line is the separation from the mixed gas region to the pure helium region.

Figure 5

A 3D PIC simulation of the SWBL injections. (a) A 3D wake plot. Only a half of the bubble is plotted to show the inner structure of the bubble and the injected electrons. The electron beam has normalized emittances of $3.3\mu \mathrm{m}\mathrm{rad}$ in the laser polarization direction, and $2.3\mu \mathrm{m}\mathrm{rad}$ in the perpendicular direction. (b) Phase space of the injected charge. The white curve is the projection to the $p_z$ axis.