Dephasingless laser wakefield acceleration of electrons using a flying focus

Laser–plasma accelerators are able to sustain electric fields that are orders of magnitude stronger than conventional radio-frequency cavities, offering a path towards ultracompact, high-energy particle accelerators. It is predicted that electron energies exceeding 100 GeV can be achieved using metre-scale accelerators, which would match the highest-energy electrons ever produced at CERN’s Large Electron–Positron Collider. However, electron energy gain remains limited by dephasing, where the electrons eventually outrun the accelerating field driven by a subluminal laser pulse. Flying-focus laser pulses can eliminate dephasing by continuously focusing incoming light along the accelerator axis to drive a plasma wave at the vacuum speed of light. Here we demonstrate that a flying focus can be used to accelerate electrons well beyond the traditional dephasing limit, exceeding it by a factor of two. This establishes a proof of concept for spatiotemporal control of laser-driven wakefields and the generation of tera-electronvolt-class electron beams using near-future multi-petawatt lasers. A factor that limits the energy gain in laser–plasma accelerators is dephasing between the accelerated electrons and the laser pulse. Now a proof-of-concept experiment shows that the dephasing limit can be overcome with a flying focus.