Ultra-compact plasma photocathode in a hybrid wakefield accelerator.

Plasma-based particle accelerators, realized by driving plasma waves with either an intense laser pulse or particle beam, offer accelerating fields orders of magnitude higher than conventional accelerators. This enables sources of highly relativistic electron beams with significantly reduced spatial and economic footprints. However, optimizing beam quality and brightness remains the key challenge for demanding applications such as free-electron lasers. The plasma photocathode is a method for generating high-quality beams directly within plasma waves - but hitherto was exclusively realizable using the high-current drive beams from the kilometer-scale conventional accelerator at SLAC. Here we show implementation of a plasma photocathode in a millimeter-scale, purely plasma-based hybrid wakefield accelerator, powered by a 150 TW laser system. Its operation at high plasma densities enables exploitation of micrometer-scale source sizes and rapid acceleration through high-field gradients, which increases the potential output beam quality substantially. This high-density plasma photocathode can be implemented at state-of-the-art laser-plasma accelerators worldwide, and opens the door for high-quality beam production.