Achieving High Efficiency And Enhanced Beam Quality In Laser Wakefield Acceleration

Abstract

Laser wakefield acceleration, characterized by the extremely high electric field gradient exceeding 100GV/m, is regarded as a compact and cost affordable technology for the next generation of particle colliders and light sources. However, it has always been a major challenge to effectively increase the energy transfer efficiency from the laser to the accelerated beam, while ensuring the beam quality remains suitable for practical applications. This study demonstrates that the laser with shorter pulse duration allows for a two-step dechirping process of the accelerated electron beam with charge of nanocoulomb level. The electron beams with an energy spread of 1% can be generated with the energy transfer efficiency of 10% to 30% in a large parameter space. For example, one electron beam with the energy of 420MeV, the charge of 5.5nC and the RMS energy spread of 2% can be produced using an 8.3J laser pulse with 7.2fs duration.

Figures (4)

• Figure 1: The distribution of the energy transfer efficiency according to the model, (a) with plasma density $n_p$ and laser pulse duration $\tau_{\rm FWHM}$, while $w_0=\ \rm 6\ \mu m$ and $a_0=15$, (b) with laser strength parameter $a_0$ and laser pulse duration $\tau_{\rm FWHM}$, while $w_0=\ \rm 6\ \mu m$ and $n_p=6\times 10^{18}\ \rm cm^{-3}$. The practical parameters should be on the right side of the dashed lines $\tau_{\rm FWHM}=2 L_{inj}w_p^2/cw^2$, so that the laser pump-depletion length is sufficient for the acceleration.
• Figure 2: The phase-space distributions and the spectra (black curves) of the electron beam during the acceleration process for different laser duration cases. The distributions of longitudinal electric field $E_z$ are also shown as the green curves. The laser duration are (a)-(d) $\tau_{\rm FWHM}=5\ \rm fs$, (e)-(h) $\tau_{\rm FWHM}=8\ \rm fs$ and (i)-(l) $\tau_{\rm FWHM}=15\ \rm fs$.
• Figure 3: Diagram of the multi-objective Bayesian optimization. (a) The objective pairs of energy spread $\Delta E_{\rm FWHM} / \bar{E}$ vs. charge $Q_{\rm FWHM}$. (b) The beam energy $\bar{E}$ vs. charge $Q_{\rm FWHM}$. (c) The beam energy $\bar{E}$ vs. energy spread $\Delta E_{\rm FWHM} / \bar{E}$. (d) The energy transfer efficiency $\eta_{\rm FWHM}$ vs. charge $Q_{\rm FWHM}$. (e) The energy transfer efficiency $\eta_{\rm FWHM}$ vs. energy spread $\Delta E_{\rm FWHM} / \bar{E}$. (f-1) The energy transfer efficiency $\eta_{\rm FWHM}$ vs. energy $\bar{E}$. In (f-2), three clusters of dots are categorized according to the associated laser depletion length $L_{etch}$ and the legend numbers are indicative of the three types of dechirping mechanisms.
• Figure 4: The snapshots of the plasma wakefield (a)-(d) and the corresponding distribution of the LPS (e)-(h) of the injected electron beam for the simulation with plasma density $n_p=5.4\times 10^{18} \rm cm^{-3}$ and an 8.3 J laser at different positions. The line-out of the axial longitudinal electric field $E_z$ (the green curve) is in the unit of $40\ \rm GV/m$. $\rho_e$ is the charge density of the background electrons, $e$ is the elementary charge. The black curves in (e)-(h) are the energy spectrum, and the blue curve in (h) is the beam current. At $z\sim840\ \rm \mu m$, the energy transfer efficiency of $\eta_{\rm FWHM}=28\%$ is reached. And we can see the front part of the beam is in a positive chirp state, so, this situation still belongs to the second type of dechirping mechanism.