  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.
