Plasma waves generated in the wake of intense, relativistic laser1,2 or particle beams3,4 can accelerate electron bunches to gigaelectronvolt energies in centimetre-scale distances. This allows the realization of compact accelerators with emerging applications ranging from modern light sources such as the free-electron laser to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavolt-per-metre wakefields can accelerate witness electron bunches that are either externally injected5,6 or captured from the background plasma7,8. Here we demonstrate optically triggered injection9,10,11 and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This ‘plasma photocathode’ decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical11 density down-ramp injection12,13,14,15,16 and is an important step towards the generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness17. This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultrahigh-brightness beams.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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The FACET ‘E210: Trojan Horse’ plasma wakefield acceleration experiment was built and operated with support from UCLA (US Department of Energy (DOE) contract no. DE-SC0009914), RadiaBeam Technologies (DOE contract no. DE-SC0009533), the FACET E200 team and DOE under contract no. DE-AC02-76SF00515, H2020 EuPRAXIA (grant no. 653782), Helmholtz VH-VI-503, EPSRC (grant no. EP/N028694/1) and the Research Council of Norway (grant no. 230450). R.Z. and M.C.D. acknowledge support from DOE grant no. DE-SC0011617 and US NSF grant no. PHY-1734319. B.H. acknowledges support from the DFG Emmy–Noether programme. This work used computational resources of the National Energy Research Scientific Computing Center, which is supported by DOE DE-AC02-05CH11231, JURECA (project hhh36), HLRN and Shaheen (project k1191). D.L.B. acknowledges support from the US DOE Office of High Energy Physics under award no. DE-SC0013855. J.R.C. acknowledges support from the National Science Foundation under award no. PHY 1734281.
G.A., A.M., D.L.B. and J.R.C.’s primary or secondary affiliations are with companies who supported the experimental and computational work, and a patent has been filed based on related work17 (PCT/GB2017/052942) by the University of Strathclyde, supported by RadiaBeam Technologies.
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Supplementary discussion, Supplementary Figs. 1–4 and Supplementary Information on Supplementary videos.
Particle-in-cell simulation video of electron bunch generation from plasma torch injection with a laser pulse energy of 5 mJ.
Particle-in-cell simulation video of electron bunch generation from plasma photocathode injection with a laser pulse energy of 0.5 mJ.