1. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA
2. University of California, Berkeley, California 94720, USA
3. Technische Universiteit Eindhoven, Postbus 513, 5600 MB Eindhoven, the Netherlands
4. Tech-X Corporation, 5621 Arapahoe Ave. Suite A, Boulder, Colorado 80303, USA
5. University of Colorado, Boulder, Colorado 80309, USA

Correspondence to: W. P. Leemans¹ Email: wpleemans@lbl.gov

Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m^-1 (refs 1–3). These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators^{4, 5} as compact next-generation sources of energetic electrons and radiation. To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread^{1, 2, 3}, which limits potential applications. Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 10⁹ electrons above 80 MeV). Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance. The results open the way for compact and tunable high-brightness sources of electrons and radiation.

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