Tapered plasma channels to phase-lock accelerating and focusing forces in laser-plasma accelerators

Tapered plasma channels are considered for controlling dephasing of a beam with respect to a plasma wave driven by a weakly relativistic, short-pulse laser. Tapering allows for enhanced energy gain in a single laser-plasma accelerator stage. Expressions are derived for the taper, or longitudinal plasma density variation, required to maintain a beam at a constant phase in the longitudinal and/or transverse fields of the plasma wave. In a plasma channel, the phase velocities of the longitudinal and transverse fields differ and, hence, the required tapering differs. The length over which the tapered plasma density becomes singular is calculated. Linear plasma tapering as well as discontinuous plasma tapering, which moves beams to adjacent plasma wave buckets, is also considered. The energy gain of an accelerated electron in a tapered laser-plasma accelerator is calculated and the laser pulse length to optimize the energy gain is determined. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3430638]