Efficient electron injection by hybrid parametric instability and forward direct laser acceleration in subcritical plasma

The efficient injection of electrons into a propagating relativistic laser pulse with normalized vector potential a₀ ~ 2 is demonstrated numerically and experimentally in a thin plasma layer with density 0.15–0.3 of the critical value. The injection is due to the wavebreaking of parametric plasma waves. The trapped particles gain multi-MeV (up to 20 MeV) energies by the direct laser acceleration in the plasma channel formed by the laser pulse in the lower density plasma tail. Numerical calculations were supported by experiments with micron-scale films pre-evaporated by an additional nanosecond laser pulse and a TW femtosecond laser facility. The experimentally observed bunch of electrons with energy above 1.6 MeV had a divergence of ~0.05 rad and charge of ~50 pC measured with photoneutron Be(g,n) reaction.