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High-efficiency acceleration of an electron beam in a plasma wakefield accelerator

Abstract

High-efficiency acceleration of charged particle beams at high gradients of energy gain per unit length is necessary to achieve an affordable and compact high-energy collider. The plasma wakefield accelerator is one concept1,2,3 being developed for this purpose. In plasma wakefield acceleration, a charge-density wake with high accelerating fields is driven by the passage of an ultra-relativistic bunch of charged particles (the drive bunch) through a plasma4,5,6. If a second bunch of relativistic electrons (the trailing bunch) with sufficient charge follows in the wake of the drive bunch at an appropriate distance, it can be efficiently accelerated to high energy. Previous experiments using just a single 42-gigaelectronvolt drive bunch have accelerated electrons with a continuous energy spectrum and a maximum energy of up to 85 gigaelectronvolts from the tail of the same bunch in less than a metre of plasma7. However, the total charge of these accelerated electrons was insufficient to extract a substantial amount of energy from the wake. Here we report high-efficiency acceleration of a discrete trailing bunch of electrons that contains sufficient charge to extract a substantial amount of energy from the high-gradient, nonlinear plasma wakefield accelerator. Specifically, we show the acceleration of about 74 picocoulombs of charge contained in the core of the trailing bunch in an accelerating gradient of about 4.4 gigavolts per metre. These core particles gain about 1.6 gigaelectronvolts of energy per particle, with a final energy spread as low as 0.7 per cent (2.0 per cent on average), and an energy-transfer efficiency from the wake to the bunch that can exceed 30 per cent (17.7 per cent on average). This acceleration of a distinct bunch of electrons containing a substantial charge and having a small energy spread with both a high accelerating gradient and a high energy-transfer efficiency represents a milestone in the development of plasma wakefield acceleration into a compact and affordable accelerator technology.

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Figure 1: Three-dimensional particle-in-cell simulation of beam-driven plasma wakefield interaction.
Figure 2: Energetically dispersed beam profiles.
Figure 3: Spatially integrated electron beam spectra from the data set.
Figure 4: Energy-transfer efficiency dependence on wake loading.

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Acknowledgements

The FACET E200 plasma wakefield acceleration experiment was built and has been operated with funding from the United States Department of Energy. Work at SLAC was supported by DOE contract DE-AC02-76SF00515 and also through the Research Council of Norway. Work at UCLA was supported by DOE contracts DE-FG02-92-ER40727 and DE-SC0010064. Simulations were performed on the UCLA Hoffman2 and Dawson2 computers and on Blue Waters through NSF OCI-1036224. Simulation work at UCLA was supported by DOE contracts DE-SC0008491 and DE-SC0008316, and NSF contracts ACI-1339893 and PHY-0960344. The work of W.L. was partially supported by NSFC 11175102, the Thousand Young Talents Program and the Tsinghua University Initiative Scientific Research Program.

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All authors contributed extensively to the work presented in this paper.

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Correspondence to M. Litos.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 FACET experimental area schematic.

Electron beam line features: a, beam notching device, b, transverse deflecting structure, c, initial spectrometer, d, final-focus quadrupole magnets, e, lithium plasma ionization laser, f, lithium vapour column, g, spectrometer imaging quadrupole magnets, h, spectrometer dipole magnet, and i, Cherenkov and phosphor screens. Bend dipole magnets in the ‘W’-shaped chicane are each labelled ‘D’. The arrow beneath the e symbol indicates the electron beam’s direction of motion (left to right).

Extended Data Figure 2 Measured longitudinal profile of two-bunch beam.

Image of a typical two-bunch beam streaked onto a profile monitor screen by the transverse deflecting radio-frequency structure (Extended Data Fig. 1b). The drive bunch appears on the right-hand side. Overlaid on the image is the projected longitudinal profile (red line). The left (x) and top (y) axes show the transverse dimensions of the streaked beam on the profile monitor screen, while the colour axis indicates the charge density of the transverse profile. The bottom (z) axis shows the streaked dimension (y) with the appropriate scaling factor applied to give the corresponding longitudinal coordinate. The right axis shows the linear charge density corresponding to the projected longitudinal profile.

Extended Data Figure 3 Lithium vapour column density profile.

The profile of the neutral vapour pressure density of the lithium vapour column deduced from the measured temperature profile (temperature versus relative distance of insertion of a thermocouple probe) along the heat pipe oven is shown as the blue line. The simple fit used to describe the density profile in our model is shown as the red line.

Extended Data Figure 4 Fit to accelerated charge.

The blue line is the spectral projection of the same data shot shown in Fig. 2c and d. The green line is a fit to the data using a half-Gaussian tail (cyan line) to account for the diffuse, high-angular-divergence accelerated charge plus a full, asymmetric Gaussian (red) used to describe the core of the accelerated trailing bunch after subtracting the half-Gaussian tail.

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Supplementary information

Simulation of beam-driven plasma wakefield acceleration

The video shows 120 successive frames from a simulation of the electron beam‐driven plasma wakefield acceleration process using the 3D particle-in-cell code QuickPIC9,10. The input beam and plasma parameters are based on those of the experiment. The upper frame of the movie shows a slice through the center of electron beam and plasma wake structure, where x is the dimension transverse to the motion, ξ = z-ct is the dimension parallel to the motion, and Ez is the on-axis longitudinal electric field. The plasma electron density is represented in blue, while the beam density is represented in red. The ion density (now shown) is uniform. The lower frame of the video shows the evolution of the longitudinal phase space of the electron beam, using the color scale along the bottom for the beam charge density. The video depicts the beam-plasma interaction over the full length of the plasma source, including propagation before and after the 10cm plasma density ramps on either end of the 26cm flat-top density region. (MOV 9133 kb)

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Litos, M., Adli, E., An, W. et al. High-efficiency acceleration of an electron beam in a plasma wakefield accelerator. Nature 515, 92–95 (2014). https://doi.org/10.1038/nature13882

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