Electron acceleration by ultra-intense helical laser beams injected during transmission through relativistically transparent targets

The concept of electron acceleration by a laser beam in vacuum is attractive due to its seeming simplicity, but its implementation has been elusive, as it requires efficient electron injection into the beam and a mechanism for counteracting transverse expulsion. We show how a specific configuration of a helical laser beam, such that the transverse field profiles are hollow while the longitudinal fields are peaked on central axis, can accelerate dense bunches of electrons to high energies. Electrons accelerated within these fields have an extremely high acceleration gradient, achieving their maximum energy within a few Rayleigh lengths (~100 microns). These electrons are collected into dense bunches, with short (~100 as) bunch duration, by the longitudinal magnetic fields at early times. Electrons injection into the central accelerating region of the helical beam is demonstrated as the helical beam passes through a strongly transparent low-density target. This mechanism is explored using three-dimensional particle-in-cell simulations. Using these simulations we demonstrate that a 3 PW helical laser can generate a 50 pC low-divergence electron beam with a maximum energy of 1.5 GeV.