Microcoulomb-level electron beam and multi-Joule hard X-rays driven by a high-efficiency laser-plasma accelerator
We report on the production of ultrahigh-charge relativistic electron beams and the development of a laser-wakefield acceleration platform at the LMJ facility. Making use of the kilojoule-class, sub-picosecond PETAL laser pulse focused onto a supersonic helium gas jet, electron beams carrying a total charge beyond 1 $\mu$C were generated, with energies up to $\sim$500 MeV. Given the ps-scale laser pulse duration, an on-target intensity approaching $10^{19}~\mathrm{W/cm^2}$, and a plasma density reaching 2% of the critical density, electron energisation arises from a combination of self-modulated laser wakefield acceleration (SMLWFA) and direct laser acceleration (DLA). The resulting electron spectrum exhibits a Maxwellian-like distribution, characteristic of this mixed SMLWFA/DLA regime. The total energy carried by the electron beam is estimated to be up to 17 J, within a sub-ps duration. A broadband Joule-level photon beam was also produced by Bremsstrahlung, demonstrating the potential for future applications. Experimental results are supported by start-to-end numerical simulations, including 3-D particle-in-cell and Monte-Carlo particle transport calculations. These findings pave the way for applications requiring high-charge electron beams, including the generation of high-power secondary radiation or particle sources. The use of these beams to probe matter in high-energy density states driven by the nanosecond-duration LMJ beams represents another promising avenue.