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Particle accelerators are usually associated with the discovery of fundamental particles, but they also have a long history of powering light sources. One such source is the free-electron laser, in which a high-energy beam of electrons from a linear accelerator generates ultrashort X-ray laser pulses by travelling through a series of magnets. However, conventional accelerators are expensive and unwieldy, needing up to one kilometre of space under Earth’s surface, and a smaller, cheaper accelerator based on plasma (ionized gas) might be capable of doing the job. The plasma in such a device needs to settle before each new interaction with the electron beam, but the interactions must be repeated at a high rate to power a free-electron laser that has sufficient average brilliance. Writing in Nature, D’Arcy et al.1 report that the maximum repetition rate of a plasma-based accelerator could be as high as one million times per second — or even higher, putting it comfortably in the realm of nearly all potential applications.