Beat wave injection of electrons into plasma waves using two interfering laser pulses.

An electron injector concept that uses a single injection laser pulse colliding with a pump laser pulse in a plasma is analyzed. The pump pulse generates a large amplitude laser wakefield (plasma wave). The counterpropagating injection pulse collides with the pump laser pulse to generate a beat wave with a slow phase velocity. The ponderomotive force of the slow beat wave is responsible for injecting plasma electrons into the wakefield near the back of the pump pulse. Test particle simulations indicate that significant amounts of charge can be trapped and accelerated ( approximately 10 pC). For higher charge, beam loading limits the validity of the simulations. The accelerated bunches are ultrashort ( approximately 1 fs) with good beam quality (relative energy spread of a few percent at a mean energy of approximately 10 MeV and a normalized root-mean-square emittance on the order 0.4 mm mrad). The effects of interaction angle and polarization are also explored, e.g., efficient trapping can occur for near-collinear geometries. Beat wave injection using a single injection pulse has the advantages of simplicity, ease of experimental implementation, and requires modest laser intensity < 10(18) W/cm(2).

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