Tunable laser plasma accelerator based on longitudinal density tailoring

Laser-driven particle accelerators have the potential to be much cheaper than conventional accelerators. But so far, the reliability and energy spread of the beams they produce has been poor. A technique that decouples the particle-injection and acceleration stages of these devices could improve their performance. Laser plasma accelerators 1 have produced high-quality electron beams with GeV energies from cm-scale devices 2 and are being investigated as hyperspectral fs light sources producing THz to γ -ray radiation 3 , 4 , 5 , and as drivers for future high-energy colliders 6 , 7 . These applications require a high degree of stability, beam quality and tunability. Here we report on a technique to inject electrons into the accelerating field of a laser-driven plasma wave and coupling of this injector to a lower-density, separately tunable plasma for further acceleration. The technique relies on a single laser pulse powering a plasma structure with a tailored longitudinal density profile, to produce beams that can be tuned in the range of 100– 400 MeV with per-cent-level stability, using laser pulses of less than 40 TW. The resulting device is a simple stand-alone accelerator or the front end for a multistage higher-energy accelerator.