Controllable high-quality electron beam generation by phase slippage   effect in layered targets
  The bubble structure generated by laser and plasma interactions changes in size depending on the local plasma density. The self injection electrons position with respect to wakefield can be controlled by tailoring the longitudinal plasma density. A regime to enhance the energy of the wakefield accelerated electrons and improve the beam quality is proposed and achieved using layered plasmas with increasing densities. Both the wakefield size and the electron bunch duration are significantly contracted in this regime. The electrons remain in the strong acceleration phase of the wakefield while their energy spread decreases because of their tight spatial distribution. An electron beam of 0.5GeV with less than 0.01 energy spread is obtained through 2.5D PIC simulations. 
Q. Yu, Y. J. Gu, X. F. Li, S. Huang, F. Zhang, Q. Kong, Y. Y. Ma, and   S. Kawata
physics.plasm-ph