On the particle resonances and trapping of direct laser acceleration *

As one of the leading acceleration mechanisms in laser-driven underdense plasmas, direct laser acceleration (DLA) is capable of producing high-current electron beams in a plasma channel for several applications. However, the mechanism relies on highly nonlinear particle-laser resonances, rendering its modeling and control to be very challenging. Here, we report on novel physics of the particle resonances and, based on that, define a potential path toward more controlled DLA. Key findings are acquired by treating the electron propagation angle independently within a comprehensive model. This approach uncovers the complete particle resonances over broad propagation angles, the physical regimes under which paraxial/nonparaxial dynamics dominates, a unified picture for different harmonics, and crucially, the physical accessibility to these particle resonances. These new insights can have important implications where we address the basic issue of particle trapping as an example. We show how the uncovered trapping parameter space can lead to better acceleration control. More implications for the development of this basic type of acceleration are discussed.