High-Quality Laser-Driven Electron Beams by Ionization Injection in Low-Density Nitrogen Gas Jet

We report a laser wakefield acceleration of electron beams up to 130 MeV from laser-driven 4 mm long nitrogen gas jet. Using moderate laser intensity ( $\sim 35\times 10^{18}~{\rm W}\cdot {\rm cm}^{-2}$ ) and relatively low plasma densities ( $0.8\times 10^{18}~\mathrm{cm}^{-3} -2.7\times 10^{18}~\mathrm{cm}^{-3}$ ), we have achieved stable laser propagation and consequently stable acceleration of electron beams. We experimentally studied the dependence of the electron beam parameters on the laser beam energy. Evidence of the ionization-induced electron injection has been recognized from the characteristic long-tail beam energy spectrum, however, the majority of the electrons were contained in quasi-monoenergtic peaks.