Ultra-energetic electron bunches from SPW excitation in the ultra-high intensity regime*

Recent experiments [1] have demonstrated that the resonant excitation of surface plasma waves (SPW) by ultra-high intensity fs lasers impinging on a solid-density target strongly enhances the laser-plasma coupling and provides a new path for generating relativistic, high charge electron bunches emitting radiation with interesting characteristics. In this work, we show that laser wavefront rotation (WFR) [2] acts to both shorten the duration (down to very few optical cycles) and increase the intensity of SPW [3], thus favoring the production of ultra-short, energetic electron bunches. Optimal laser parameters were identified analytically and verified by means of Particle-In-Cell (PIC) simulations with the open-source code SMILEI [4]. The laser pulse with WFR was combined with a smart grating target design. In the laser-plasma relativistic regime of interaction (i.e. Iλ² = 3.4 × 10¹⁹ W/cm² μm²), we show that this set-up may produce SPW with ~3.6 cycles duration which accelerate high-charge (few 10’s of pC), high-energy (up to 70 MeV) electron bunches of few fs duration [5]. Extending this set up, or more in general exploiting the possibility of SPW excitation by forthcoming multi-petawatt laser facilities ( Iλ² > 10²¹) implies an in depth understanding of the SPW excitation conditions and lifetime in that regime. Through extensive parametric studies we identify the optimum SPW excitation angle in the ultra-relativistic regime, that coincides with the optimal angle to optimize the electron acceleration along the plasma surface [6]. The dependence on the plasma density and grating shape is also discussed. As a conclusion, we show that excitation of SPW by a grating can hold at the highest laser intensities available, opening the doors to new experiments on forthcoming multi-petawatt laser systems.