Phase-locked laser-wakefield electron acceleration

Independent velocity and intensity control of photons for driving a laser-plasma accelerator may enable a dephasing-free regime. Subluminal and superluminal light pulses have attracted considerable attention in recent decades(1-4), opening perspectives in telecommunications, optical storage and fundamental physics(5). Usually achieved in matter, superluminal propagation has also been demonstrated in vacuum with quasi-Bessel beams(6,7)or spatio-temporal couplings(8,9). Although, in the first case, the propagation was diffraction free, but with hardly controllable pulse velocities and limited to moderate intensities, in the second, high tunability was achieved, but with substantially lengthened pulse durations. Here we report a new concept that extends these approaches to relativistic intensities and ultrashort pulses by mixing spatio-temporal couplings and quasi-Bessel beams to independently control the light velocity and intensity. When used to drive a laser-plasma accelerator(10), this concept leads to a new regime that is dephasing free, where the electron beam energy gain increases by more than one order of magnitude.