Laser Acceleration of Electrons in Shock Wave Enhanced Gas Jets

Controlling the gas density gradient and profile is important for electron and proton acceleration. Using an optimized gas density profile, we have demonstrated 40 times higher electron energy compared to a Gaussian gas jet without lost of charge or stability. Propagation of a shock wave through a gas jet can modify the gas density profile and create sharp density gradients [1, 2]. Using different shock waves energies and shock originating positions, we were able to modify the plasma density profile of a ``typical'' Gaussian gas jet into a variety of profiles, from thin (foil-like) structure to elongated profiles with fast rise and slow fall. We used a plasma bubble Cherenkov diagnostic [3, 4] to optimize the acceleration process. Accelerated electron energy and charge were cross-correlated with the second harmonic diagnostic signal. The optimized gas density profile generated stable 0.5 nC of 40 MeV electrons using a 10 TW laser. The shock wave modified gas jet can be used as a stand alone electron source or as an injector coupled to additional acceleration structures. We demonstrated stable injection of electrons from the shock wave modified gas jet into a lower density plasma. The results are also being studied with numerical simulations. \\[4pt] [1] D. Kaganovich et al., Physics of Plasmas \textbf{18}, 120701 (2011)\\[0pt] [2] D. Kaganovich et al., Applied Physics Letters \textbf{97}, 191501 (2010)\\[0pt] [3] D. F. Gordon, et al., Phys. Rev. Lett. \textbf{101}, 045004 (2008)\\[0pt] [4] M. H. Helle et al., Phys. Rev. Lett. \textbf{105}, 105001 (2010)