Generation and acceleration of electron bunches from a plasma photocathode

Plasma waves generated in the wake of intense, relativistic laser(1,2) or particle beams(3,4) 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 injected(5,6) or captured from the background plasma(7,8). Here we demonstrate optically triggered injection(9-11) 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 optical(11) density down-ramp injection(12-16) and is an important step towards the generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness(17). This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultrahigh-brightness beams.