Efficient Acceleration of High-Quality GeV-Electron Bunches in a Hybrid Laser- and Beam-Driven Plasma Wakefield Accelerator

Abstract

Plasma-based accelerators are compact and provide high gradients, yet their practical use has been limited by energy gain, stability, beam quality, and energy transfer efficiency. Here, we address several of these challenges simultaneously using a hybrid scheme in which an electron bunch from a laser wakefield accelerator (LWFA) drives a subsequent plasma wakefield accelerator (PWFA) stage with internal witness injection. Close to driver depletion in the PWFA stage, we obtain witness bunches with higher electron energy, reduced energy spread and divergence, and higher angular-spectral charge density compared to LWFA alone. We report energy transformer ratios approaching~2, and about 20\% of the initial energy in the drive beam was transferred to the witness bunch, thereby achieving a driver-to-witness energy transfer efficiency that largely surpasses that of all previous PWFA experiments.

Figures (4)

• Figure 1: Schematic setup for the experimental generation of high-energy, high-quality witness bunches in a hybrid LWFA-PWFA. In the LWFA stage, electron bunches are generated via self-truncated ionization injection (STII). After a vacuum gap, these electron bunches (light blue) drive the PWFA stage. A wire-generated shock (orange) enables the density down ramp injection (DDI) of witness electrons (violet) into the PWFA stage. Insets (a)-(c) show the longitudinal phase space of driver and witness retrieved from an exemplary PIC simulation at different positions in the PWFA stage Note1. (d) shows the plasma density distribution of the PWFA together with the evolution of the driver-to-witness energy transfer efficiency along the length of the PWFA stage.
• Figure 2: Energy spectra of electron driver and witness for various conditions of hybrid LWFA-PWFA. (a) LWFA-generated drive beam. (b) Decelerated drive beam after the PWFA stage, without internal injection. (c) Decelerated driver and a typical internally injected witness after the PWFA stage. (d) Witness with the highest driver-to-witness energy transfer efficiency. (e) Witness with highest peak electron energy. Gray boxes highlight the spectral feature recognized as the witness. See Note1 for detailed beam parameters.
• Figure 3: Tuning the witness energy by variation of the injection position. (a) Interferometric measurement of the unperturbed plasma density distribution and definition of injection positions. (b) Mean peak witness electron energies and their distribution as a function of acceleration length (violet). The zero position of the x-axis is chosen to coincide with the zero-crossing of the linear fit and corresponds to a position close to the end of the PWFA plasma target. For comparison, the drivers' mean peak electron energy and its min/max deviation are plotted in light blue.
• Figure 4: Comparison of published PWFA results in terms of electron energy gain of the witness normalized to the peak electron energy of the respective driver, driver-to-witness energy transfer efficiency, and relative energy spread of the witness. The error bars for this work correspond to the uncertainties discussed in the supplemental material Note1. Note the logarithmic scale of all three categories.