Electrical breakdown sets a limit on the kinetic energy that particles in a conventional radio-frequency accelerator can reach. New accelerator concepts must be developed to achieve higher energies and to make future particle colliders more compact and affordable. The plasma wakefield accelerator (PWFA) embodies one such concept, in which the electric field of a plasma wake excited by a bunch of charged particles (such as electrons) is used to accelerate a trailing bunch of particles. To apply plasma acceleration to electron–positron colliders, it is imperative that both the electrons and their antimatter counterpart, the positrons, are efficiently accelerated at high fields using plasmas1. Although substantial progress has recently been reported on high-field, high-efficiency acceleration of electrons in a PWFA powered by an electron bunch2, such an electron-driven wake is unsuitable for the acceleration and focusing of a positron bunch. Here we demonstrate a new regime of PWFAs where particles in the front of a single positron bunch transfer their energy to a substantial number of those in the rear of the same bunch by exciting a wakefield in the plasma. In the process, the accelerating field is altered—‘self-loaded’—so that about a billion positrons gain five gigaelectronvolts of energy with a narrow energy spread over a distance of just 1.3 metres. They extract about 30 per cent of the wake’s energy and form a spectrally distinct bunch with a root-mean-square energy spread as low as 1.8 per cent. This ability to transfer energy efficiently from the front to the rear within a single positron bunch makes the PWFA scheme very attractive as an energy booster to an electron–positron collider.
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The FACET E200 plasma wakefield acceleration experiment was built and has been operated with funding from the United States Department of Energy (US DOE). Work at SLAC was supported by DOE contract DE-AC02-76SF00515 and also through the Research Council of Norway. Work at the University of California Los Angeles (UCLA) was supported by DOE contract 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 11425521, 11175102, and the National Basic Research Program of China Grant No. 2013CBA01501. We are grateful to P. Muggli for many discussions regarding plasma wakefield acceleration.
The authors declare no competing financial interests.
Extended data figures and tables
a–d, Various examples of the accelerated portion of the spectrum of the positrons after the interaction with the plasma, with Eimage at 25.35 GeV (a, b) and 27.85 GeV (c, d). The charge density of the dispersed positron beam profile is shown in colour, and the spectral charge density is represented by the black solid line (right-hand scale). The spectrum is peaked with a relatively large energy spread in a and c, is continuously decreasing in b and is flat in d for the highest maximum energy gain of ∼10 GeV. Particles that do not participate in the interaction appear to be saturated at the initial beam energy of 20.35 GeV.
a, Final positron spectrum from QuickPIC simulations as σz and σr are varied. b, Final positron spectrum as the bunch longitudinal asymmetry is varied. The red (green) curve corresponds to a longitudinally symmetric (asymmetric) bunch. c, Final positron spectrum as the normalized transverse emittance is varied, holding σr constant.
a, Electron plasma density and positron beam density in the x–ξ plane. b, Final spectrum of the positron bunch after its interaction with the plasma. The inset shows the projected density in the x–ξ plane of the positrons with energies greater than 24 GeV. c, Transverse force Fx ≈ e(Ex − cBy) experienced by the positrons. The dashed lines represent the Fx = 0 contour. F and D indicate, respectively, the focusing and defocusing regions of the wake. The inset shows a lineout of the pseudo potential ψ (given implicitly by F⊥ = −∇⊥eψ) taken at ξ = −75 μm. d, Longitudinal electric field Ez. In a and d, the on-axis density profile of the positron bunch is represented in red dashed line. In comparison to the y–ξ plane shown in Fig. 3, here the transverse force is slightly defocusing close to the axis in the region −85 μm < ξ < −70 μm. The emittance being larger in x, the beam evolves towards a larger spot size along this dimension. As a result, the on-axis plasma electrons that provide the focusing force take a marginally longer time to reach the x = 0 plane, which leads to a small region where the force is locally slightly defocusing. Nevertheless, the positrons remain confined and guided by the pseudo potential well, as seen in the inset of c.
Positrons are created in the positron source of Sector 19, transported to the start of the linear accelerator by the Positron Return Line, and cooled in the South Damping ring. They are then accelerated in the linear accelerator and compressed in the ring-to-linear accelerator section, in Sector 10 and in Sector 20, and are focused by the final focus system to the entrance of the plasma in Sector 20.
Extended Data Figure 5 Piecewise reconstruction of the decelerated part of the average positron spectrum.
Each piece of the reconstructed spectrum is the average of ∼35 spectra measured at a given quadrupole energy set point Eimage. Eimage = 10.35 GeV is represented by the blue solid line, Eimage = 12.85 GeV by the red solid line, Eimage = 15.35 GeV by the green solid line, Eimage = 17.85 GeV by the black solid line and Eimage = 20.35 GeV by the magenta solid line. Except for the two extreme energy set-points, each piece of the reconstruction covers a 2.5-GeV energy range centred around its energy set-point. Single shot spectra are linearly extrapolated for energies below 9 GeV, and this extrapolation is accounted for in the blue curve. The energy set-points are represented by the dashed lines.
The electron plasma density and the beam density in the (y–ξ) plane are shown on the upper part of the video for 790 successive frames, depicting the evolution of the bunch and the plasma wake as the beam propagates through the plasma. y is the dimension transverse to the direction of motion of the bunch, and ξ = z - ct is the dimension parallel to the motion. The plasma electron density is represented in blue, while the beam density is represented in red. The on-axis longitudinal electric field Ez is in red solid line. The lower part of the video shows the evolution of the longitudinal phase space of the positron bunch. In this simulation, the bunch contains 1.4 × 1010 positrons and has transverse and longitudinal spot sizes of σr = 70 μm, σz = 30 μm. The bunch energy is 20.35 GeV and the plasma has a uniform density of 8 × 1016 cm-3 over a 1.2-m-long region, with 15-cm-long linear up- and down-ramps on either side. These linear density ramps correspond to the first 15 cm and the last 15 cm of the simulation. The simulation was carried out using the 3D particle-in-cell code QuickPIC20,21. (MOV 14222 kb)
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Corde, S., Adli, E., Allen, J. et al. Multi-gigaelectronvolt acceleration of positrons in a self-loaded plasma wakefield. Nature 524, 442–445 (2015). https://doi.org/10.1038/nature14890
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