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.
Extended data figures
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.
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Capturing relativistic wakefield structures in plasmas using ultrashort high-energy electrons as a probe
Scientific Reports (2016)