Measurements of the Critical Power for Self-Injection of Electrons in a Laser Wakefield Accelerator

A laser wakefield acceleration study has been performed in the matched, self-guided, blowout regime producing 720±50 MeV quasimonoenergetic electrons with a divergence ΔθFWHM of 2.85±0.15 mrad using a 10 J, 60 fs 0.8 μm laser. While maintaining a nearly constant plasma density (3x1018 cm-3), the energy gain increased from 75 to 720 MeV when the plasma length was increased from 3 to 8 mm. Absolute charge measurements indicate that self-injection of electrons occurs when the laser power P exceeds 3 times the critical power Pcr for relativistic self-focusing and saturates around 100 pC for P/Pcr>5. The results are compared with both analytical scalings and full 3D particle-in-cell simulations.