eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Peer Reviewed Title: Improvement of electron beam quality in optical injection schemes using negative plasma density gradients Author: Fubiani, G. Esarey, E. Schroeder, C.B. Leemans, W.P. Publication Date: 07-26-2005 Publication Info: Lawrence Berkeley National Laboratory Permalink: http://escholarship.org/uc/item/9hk0h4p7 Keywords: plasma density laser wakefield accelerator Abstract: Enhanced electron trapping using plasma density down ramps as a method for improving the performance of laser injection schemes is proposed and analyzed. 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, and a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two-pulse colliding pulse injector was examined using a three-dimensional test particle tracking code. A density down-ramp with a change of density on the order of tens of percent over distances greater than the plasma wavelength led to an enhancement of charge by two orders in magnitude or more, up to the limits imposed by beam loading. The accelerated bunches are ultrashort (fraction of the plasma wavelength, e.g., ~; 5 fs), high charge (> 20 pC at modest injection laser intensity 10^17 W/cm^2), with a relative energy spread of a few percent at a mean energy of ~; 25 MeV, and a normalized root-mean square emittance on the order 0.5 mm mrad.