Laser Wakefield Acceleration Using Wire Produced Double Density Ramps
  A novel approach to implement and control electron injection into the accelerating phase of a laser wakefield accelerator (LWFA) is presented. It utilizes a wire, which is introduced into the flow of a supersonic gas jet creating shock waves and three regions of differing plasma electron density. If tailored appropriately, the laser plasma interaction takes place in three stages: Laser self-compression, electron injection and acceleration in the second plasma wave period. Compared to self-injection by wavebreaking of a nonlinear plasma wave in a constant density plasma, this scheme increases beam charge by up to one order of magnitude in the quasi-monoenergetic regime. Electron acceleration in the second plasma wave period reduces electron beam divergence by approximately 25 %, and the localized injection at the density downramps results in spectra with less than a few percent relative spread. 
M. Burza, A. Gonoskov, K. Svensson, F. Wojda, A. Persson, M. Hansson,   G. Genoud, M. Marklund, C.-G. Wahlstr\"om, O. Lundh
physics.plasm-ph