Direct laser acceleration in arbitrary-velocity wakefields

Electrons accelerated in the plasma wake of a high-intensity laser pulse can reach higher energies through simultaneous interaction with a second, delayed pulse. The energy gain is determined by the phase velocity of the wake as well as the phase and group velocities of the pulses. With conventional pulses, these velocities are fixed by the plasma density, which limits the maximum energy gain. Here, we show that flying-focus pulses can enhance the energy gain in synergistic laser wakefield and direct laser acceleration by enabling control of both the wake and pulse velocities independent of the plasma density. Calculations demonstrate that this enhancement is achievable with experimentally realizable flying-focus pulses on 100 TW-class laser systems.