This work proposes and numerically demonstrates a novel vacuum-based scheme for generating attosecond-scale, hundred-MeV electron beams via vacuum-based direct laser acceleration (VDLA) using an ultra-intense, radially polarized first-order Bessel beam. By leveraging the unique structured field that provides simultaneous longitudinal acceleration and transverse focusing, relativistic electrons injected off-axis are efficiently trapped, phase-locked, and compressed into ultra-thin, high-density micro-bunches with durations down to 90 as. Particle-in-cell simulations reveal a transverse-stratified acceleration mechanism: electrons injected near r ≈ 2.5λ achieve the highest energy gain (exceeding 400 MeV), while those originating from slightly larger radii exhibit superior collimation. The acceleration dynamics are clearly divided into a trapping stage and a phase-locking stage, confirming a resonant, high-gradient interaction. This all-optical, plasma-free approach offers a promising route toward compact, stable attosecond electron sources for ultrafast imaging, coherent radiation generation, and advanced accelerator applications.
Ye et al. (Wed,) studied this question.