Accelerating ultra-short electron/positron bunches in field ionization produced plasmas

The acceleration of ultra-short electron/positron bunches in field-ionization produced plasmas is studied through particle-in-cell (PIC) simulations. The 3D fully relativistic parallel PIC code QuickPIC[1] is able to model afterburner relevant plasma wakefield acceleration (PWFA) parameters due to the implementation of the quasi-static approximation. Several new physics packages, including field-ionization and radiation reaction, were added into QuickPIC so that it could be used to simulate frontier PWFA experiments. The wakes created by a short electron bunch in the blow-out regime when its self electric field ionizes a gas to produce the plasma is studied by comparing them with wakes created in pre-ionized plasmas. A criteria for the wake amplitude to scale similarly with drive beam parameters is obtained. A detailed simulation study of the energy doubling experiment at Stanford Linear Accelerator Center (SLAC) suggests that the much faster beam head erosion process in a field-ionized plasma is what causes the maximum energy gain to saturate as the plasma length is increased (as published in [2]). Other observations from the simulation such as beam scalloping, loss of charge from the defocusing field, azimuthally asymmetric wakes due to emittance asymmetry are also discussed. A theoretical model that provides scalings for the head erosion rate in terms of beam/plasma parameters is described. The predictions from the model are in good agreement with simulation results. The first simulation study of a positron beam propagating in an extended length of field-ionized plasma is presented. Beam head erosion as well as the dynamic focusing followed by a stabilization of the beam envelope is observed. This simulation, which has beam parameters in the same range as those can be produced at SLAC, achieved multi-GeV energy gain for the tail of the positron beam in tens of centimeters of a field-ionized plasma.