Optimization of electron bunch quality using a chirped laser pulse in laser wakefield acceleration

In this work, we perform particle-in-cell simulations to investigate the electron bunch quality using a chirped laser pulse in a laser wakefield acceleration mechanism. Particular attention is devoted to the electron beam quality improvement using a chirped laser pulse through group delay dispersion. By adjusting the value of group delay dispersion, the electron beam quality was controlled and improved in the range: charge (31–325 pC), energy (240–485 MeV), energy spread (2.1%–12.5%), and bunch duration (3.3–17 fs). The electron bunch charge highly depends on the chirping of the laser pulse. We anticipate that the ultrashort sub-GeV electron bunch generated in the laser wakefield acceleration mechanism using chirped laser pulses may be crucial in developing free-electron laser-based compact radiation sources.

Figure 1

Particle-in-cell simulations illustrating controlled electron injection using plasma density modulation. (a) Electron density distribution at different group delay dispersion (${\varphi }^{(2)}$, noted in the graph) of laser pulses. (b) The laser electric field distribution corresponding to ${\varphi }^{(2)}$ with the red line describing the on-axis laser field. The FWHM pulse duration of the laser pulse is indicated in the graph.

Figure 2

(a) Energy spectra for injected electron obtained with different values of the group delay dispersion (${\varphi }^{(2)}$) of the laser pulse. (b) The distribution of electron bunch is represented corresponding to each ${\varphi }^{(2)}$, where the color scheme represents the energy gain by the injected electrons.

Figure 3

The improved bunch quality of accelerated electrons is obtained for a linearly chirped laser pulse. Slice distribution of (a) average electron energy $⟨E⟩$, (b) energy spread of electrons $\mathrm{\Delta }E$, and (c) and (d) normalized rms emittance ${\epsilon }_x$ (in the $x$ direction) and ${\epsilon }_y$ (in the $y$ direction) of the injected electron bunch for different values of group delay dispersion (${\varphi }^{(2)}$) with slice length of 40 nm around $z=3.9\mathrm{mm}$.

Figure 4

Evolution of electron bunch properties over the longitudinal traveling distance $z$: (a) peak energy (E), (b) relative energy spread ($\mathrm{\Delta }E/E$ %), (c) normalized rms values of the emittance in the $x$, and (d) $y$ directions. In each figure, three curves represents different values of group delay dispersion (${\varphi }^{(2)}$) of the laser pulse.

Figure 5

Results from particle-in-cell simulations for different group delay dispersion (${\varphi }^{(2)}$) values of laser pulse showing: (a) phase space and longitudinal on-axis electric field $E_z$, and (b) longitudinal charge distribution of the injected electron bunch around $z=3.5\mathrm{mm}$. Electron bunches of different charge are injected by changing the temporal chirp (${\varphi }^{(2)}$) of the laser pulse. Higher charges decrease the accelerating fields of the wakefields due to the effect of beam loading.

Figure 6

Electron bunch properties with respect to the group delay dispersion ${\varphi }^{(2)}$ of a laser pulse. (a) Charge Q (black dots) and electron bunch duration ${\tau }_b$ (blue dots), (b) electron peak energy E (black dots) and energy spread $\mathrm{\Delta }E/E$ (blue dots), and (c) normalized rms emittance ${\epsilon }_x$ (black dots) in the $x$ direction and ${\epsilon }_y$ (blue dots) in the $y$ direction.