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

Abstract

A laser wakefield acceleration study has been performed in the matched, self-guided, blowout regime producing $720\pm 50\mathrm{MeV}$ quasimonoenergetic electrons with a divergence $\Delta {\theta }_{\mathrm{FWHM}}$ of $2.85\pm 0.15\mathrm{mrad}$ using a 10 J, 60 fs $0.8\mu \mathrm{m}$ laser. While maintaining a nearly constant plasma density ($3\times {10}^{18}{\mathrm{cm}}^{-3}$), 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 $P_{\mathrm{cr}}$ for relativistic self-focusing and saturates around 100 pC for $P/P_{\mathrm{cr}}>5$. The results are compared with both analytical scalings and full 3D particle-in-cell simulations.