Abstract

Plasma accelerators [1] can produce extremely large accelerating gradients, enabling compact sources of high-energy beams. To improve the quality of laser-plasma-accelerated electron beams, controlled injection methods are being pursued, including ionization injection [2]. The beam emittance may be reduced by using a laser to ionize electrons at a trapped phase in a plasma wake that is independently excited by a particle beam [3] or an intense laser [4]. The all-optical method [4] relies on two lasers of different colors: a long wavelength pulse, with large ponderomotive force and small peak electric field, excites a plasma 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 wake drive laser, ionizes a fraction of the remaining bound electrons at a trapping phase of the wake, generating an electron beam. For example, a 10-µm CO₂ laser driving a plasma wake and a frequency-doubled (0.4 µm) Ti:Al₂O₃ laser for ionization injection are well suited to this injection method.

© 2015 IEEE