Evolution of ultra-relativistic hollow electron beams during their propagation in plasmas

Abstract

Ultra-relativistic hollow electron beams can drive plasma wakefields (∼GV/m) suitable for positron acceleration. Stable propagation of hollow electron beams for long distances in plasmas is required to accelerate positrons to high energies by these plasma wakefields. In this work, we show by quasi-static kinetic simulations using the code WAKE that an ultra-relativistic azimuthally symmetric hollow electron beam with zero emittance propagates in a plasma by developing a fish-bone like structure and shifting its bulk, differentially along its length (rear part fastest), towards its axis due to the decrease in the betatron time period of beam electrons from the beam-front to beam-rear. Hollow electron beams with a small radius collapse into their axis due to the pull by the secondary wakefields generated by some of the beam electrons reaching the axis. Hollow beams with the radius equal to or larger than a minimum value, however, can propagate stably in plasmas for several meters. The minimum beam radius for the stable beam propagation in plasmas depends very weakly on the peak beam density with which the magnitude of the positron-accelerating electric field increases. Thus, the peak beam density can be used as a control parameter to achieve high acceleration gradients for positrons without affecting the minimum beam radius.