Injection dynamics of direct-laser-accelerated electrons in a relativistically transparent plasma

The dynamics of electron injection in the direct laser acceleration (DLA) regime is investigated using three-dimensional particle-in-cell simulations and theoretical analyses. It is shown that, as an ultraintense laser pulse propagates into a near-critical density or relativistically transparent plasma, the longitudinal charge-separation electric field excites ion density spikes, which modulate the local electric field. The corresponding electric field acts as a series of potential wells to guide the electrons on the edge of the plasma channel into its center where the DLA can take place. On the other hand, an azimuthal magnetic field is self-generated, and it can deflect the injected electrons from the intense laser-field region. Understanding these physical processes paves the way for further optimizing the properties of direct-laser accelerated electron beams and the associated x- and $\ensuremath{\gamma}$-ray sources.