  As one of the leading acceleration mechanisms in laser-driven underdense
plasmas, direct laser acceleration (DLA) is capable of producing
high-energy-density electron beams in a plasma channel for many 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/non-paraxial 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.
