Surface Wave Electron Acceleration from Flat Foils at Parallel Laser Incidence

The acceleration of electrons by surface plasma waves (SPWs) generated during the interaction of ultrashort, linearly polarized, and contrast-enhanced laser pulses at a peak intensity of ∼6×10^{20}  W cm^{-2} with flat, noncorrugated foils at parallel incidence (with respect to the target surface) is investigated. We experimentally demonstrate the generation of a collimated electron beam (<0.6  mrad) with a non-Maxwellian spectrum, characterized by peaks at superponderomotive energies (30-36 MeV) and total charge ∼120  pC. Through particle-in-cell (PIC) simulations, we identify the J×B force at the front edge as the primary mechanism for injecting electrons into the SPW, where they are further accelerated. Furthermore, our simulation findings reveal that lateral surface contaminants influence SPW dynamics and give rise to a long-ranging plasmonic mode.