Plasma Wakefields in the Quasi-Nonlinear Regime

Abstract

It has long been noted that the nonlinear “blowout” regime of the PWFA has certain critical aspects for producing high quality beams that are owed to the elimination of electron density and current inside of the beam‐occupied region: time‐independent, linear ion‐focusing, and acceleration independent of transverse position. Unfortunately, in applying this scheme to a linear collider, efficiency considerations strongly encourage use of pulse trains, in which one superimposes the wakes of driving and accelerating beams in turn. This implies that one needs to maintain stable wakes with the ability to maintain a resonant response which, given the presence of wave‐breaking and amplitude dependent frequency, is not straightforward in the nonlinear regime. Here we propose a solution to this problem: operation in the quasi‐nonlinear regime, where one uses beams with relatively low charge and transverse beam size much smaller than a plasma skin‐depth. In this case, the beam density may exceed that of the plasma, producing blowout, but due to the small total charge, producing a disturbance that behaves in many ways as linear, having frequency essentially that of linear plasma oscillations. To illustrate the salient characteristics of this regime we present the results of single and multi‐pulse simulations, and give a theoretical analysis for the preservation of the plasma frequency. We discuss a proposal for testing this regime at the BNL ATF.