  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.
