Characteristics of the Shock Structure for Transition Injection in Laser Wakefield Acceleration

In order to develop a stable and efficient laser wakefield accelerator by transition injection, the characteristics of the plasma density generated in a gas medium by shock was experimentally measured. The density difference between before and after the shock, and the density after the shock are important parameters for determining the charge and energy of the electron beam in the transition injection scheme. Based on experiments, we developed an adjustment method of these parameters and measured the characteristics of the electron beam by using the shock structure. The shock structure in the gas medium was generated by inserting a blade in a supersonic gas nozzle, and plasma was generated by using a 16-TW laser. Before the shock, the density increases and after that, the density is the same as the density without the shock. The ratio of the density difference was in the range of ~ 0.1–0.6, which could be adjusted according to the position of the blade. The density after the shock was adjustable by varying the distance between the laser and the nozzle, and it was in a suitable range of ~ 5 × 1018 cm −3–1.2 × 1019 cm −3 for the electron acceleration. The measured electron beam indicates that the position of the shock needs to be in the range of ~ 0.3–0.5 mm before the laser focal position in the vacuum because of faster laser focus due to self-focusing.