Laser Wakefield Acceleration of Quasimonoenergetic Electron Beams in Pure Nitrogen Gas Jets*

Nitrogen has been used as an added gas at the few percent level in He gas jets to generate 100's of MeV electrons in the ionization induced seeding scheme for laser wakefield acceleration. Recent simulations (K.P. Singh~et al. Phys. Plasmas~16, 043113 (2009)) showed that quasimonoenergetic collimated GeV electrons could be also generated with pure N$_{2}$ using a chirped intense laser pulse. Here we report measurements of wakefield acceleration carried out in pure N$_{2}$ gas at the ALLS Laser facility at INRS, Varennes. Maximum energy higher than 0.5 GeV of quasimonoenergetic electron beam with a low divergence of 2.2 mrad was obtained with 80 TW, 30 fs laser pulses. Long-tail features were observed stretching from the quasimonoenergetic bunches due to continuous ionization injection. Measured peak electron energy decreased with the plasma density, which agrees with the predicted maximum electron energy gain scaling. Experiments showed a threshold density of 3x10$^{18}$ cm$^{-3}$ for self-trapping. Our experiments suggest that pure N$_{2}$ is a potential candidate gas to achieve GeV monoenergetic electrons in the ionization induced injection scheme for laser wakefield acceleration.