Studies of driver bunch stability for Beam-Quality conservation in Plasma Wakefield Acceleration
Creators: name: Marocchino, Alberto
Publisher: name: figshare
Year: 2018
Subjects: subject: 20204 Plasma Physics; Fusion Plasmas; Electrical Discharges
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: An ideal PWFA scenario consists of a driver followed by a trailing bunch that maintain its quality, i.d. its brightness B=varepsilon_x varepsilon_y over traveled distance (with I the beam r.m.s. current and varepsilon the normalized emittance along the x,y axes). It is also desirable to keep the beam energy spread as small as possible. To achieve these goals both the driver and the trailing bunch need to be finely manipulated, it is necessary to control with great accuracy their charge, shape and dimensions; i.d. it has been shown that the trailing bunch need to have a trapezoidal shape to avoid beam loading. No geometrical conditions are given on the driver. In this paper we discuss the characteristics the driver have to meet in order to generate a stable bubble, that in turns allows for a stable trailing bunch propagation with no quality loss. We observe that the driver into a plasma channel naturally evolves toward a stable configuration; by taking advantage of this natural behavior we discuss how the driver can be injected at different dimensions without influencing the trailing bunch evolution. We can rephrase the previous sentence by saying that for PWFA the driver can be injected into the plasma also at un-matched conditions allowing for some beneficial flexibility. Such a flexibility is beneficial when considering a realistic case where a single photo-injector creates and transport both bunches up to the plasma accelerating section. The practical difficulties reside in tuning the photo-injector optics for two significantly different bunches at the same time. A single photo-injector can be tuned only around the requirements of one of the two: this is particularly true for plasma acceleration where the beam parameters result to be quite "extreme" in terms of dimensions and a slight initial mismatch can lead to disastrous results. The simplest strategy to overcome this technological limitation is to employ two different accelerator lines. This, however, poses a number of issues on the correct alignment and synchronization of the beams and is far from being cost effective. In this paper we present the effectiveness of the PWFA flexibility: the photo-injector is tuned for the trailing bunch and we partially ignore the driver geometrical conditions since we show that by mean of a passive plasma lens self-stabilizing effect it naturally reach a stable condition with little influence on the trailing bunch.<br>The identification of the geometrical thresholds that the driver has to meet at the plasma entrance are a non trivial problem that require a numerical approach with a systematic scan. We perform this study with a big-data approach exploited with hundreds of simulations with the hybrid-code Architect. Since Architect is a serial code we have performed the whole scan in one single parallel run by running the hundreds of simulations in parallel, overnight, on a HPC cluster computer. Driven by analytical models we have data-mined the results by bringing to a definition of a robustness threshold that identify the maximum longitudinal and transverse bunch extent at which the driver needs to be delivered at the plasma.