Lawrence Berkeley National Laboratory

A method is proposed to generate femtosecond, ultra-low emittance (~;;0.01 micron rad), electron beams in a laser-plasma accelerator using two lasers of different colors. A long wavelength pump pulse, with large ponderomotive force and small peak electric field, excites a wake without fully ionizing a high-Z gas. A short-wavelength injection pulse, with small ponderomotive force and large peak electric field, co-propagating and delayed with respect to the pump laser, ionizes a fraction of the remaining bound electrons at a trapping wake phase, generating an electron beam that is accelerated in the wake.