Optimum angle for side injection of electrons into linear plasma wakefields

A unified model of electron penetration into linear plasma wakefields is formulated and studied. The optimum angle for side injection of electrons is found. At smaller angles, all electrons are reflected radially. At larger angles, electrons enter the wakefield with superfluous transverse momentum that is unfavorable for trapping. Separation of incident electrons into penetrated and reflected fractions occurs in the outer region of the wakefield at some `reflection' radius that depends on electron energy.