In the study of compact laser plasma accelerators, generating electron beams with high energy, high charge, and controllable structures has long been a goal pursued by scientists. Recently, the team led by Longqing Yi at Tsung-Dao Lee Institute proposed a new electron acceleration scheme based on the interaction between high-power vortex lasers and micro-plasma waveguides. For the first time, they demonstrated the precise control capability of high-order waveguide modes over the helical electron beam micro-bunching. The electron cutoff energy can reach the GeV level, with a high charge exceeding one hundred nC and a beam divergence angle of approximately 2°.

In this study, a circularly polarized Laguerre-Gaussian (LG) laser was used as the driving source and injected into a plasma waveguide with a diameter of a few micrometers. The interaction between the laser and the waveguide walls excites high-order waveguide modes, which extract electrons from the plasma wall and accelerate them to high-energies, as illustrated in Figure 1(a). Through theoretical derivations and three-dimensional particle-in-cell (PIC) simulations, the research team discovered that the topological charge and polarization state of the driving laser jointly determine the azimuthal order of the waveguide modes, thereby controlling the helical structure of the electron beam as shown in Figure 1(b).

Furthermore, traditional theory predicts that acceleration by high-order waveguide modes leads to rapid electron energy saturation due to dephasing caused by substantially superluminal phase velocity. However, this study reveals that electrons undergo transverse migration (Figure 2) during the acceleration process, which mitigates the dephasing effect and allows the helical electron bunch to reach much higher energy.