Lawrence Berkeley National Laboratory

Plasma density down-ramps are proposed as a method for improving the performance of laser injection schemes. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield. Also, a decrease in density leads to a decrease in wake phase velocity, which lowers the trappingthreshold. The specific method of two-pulse colliding pulse injector was examined using a 3D test particle tracking code. A density down-ramp of 30% led to an order of magnitude enhancement of the trapping fraction compared to the uniform plasma case, without degrading other bunch properties.