Helical Electron Beam Micro-Bunching by High-Order Modes in a Micro-Plasma Waveguide

Electron acceleration by a high-power Laguerre-Gaussian pulse in a micro-plasma waveguide is investigated. When the incident laser travels in the waveguide, electrons on the wall are extracted into the vacuum core and accelerated by the longitudinal field of the waveguide mode. Using 3D particle-in-cell simulations, we demonstrate that circularly polarized Laguerre-Gaussian laser pulses can excite high-order waveguide modes that exhibit helical longitudinal electric fields. The 3D profile of this accelerating field is imprinted into the high energy electron beam, leading to customized helical micro-bunching of high-energy ( $\sim$ GeV) electron beams with extremely high charge (100s nC), and small divergence ( $\sim 2^{\circ }$ ). In particular, we demonstrate that when co-propagating with high-order waveguide modes, the electron can migrate transversely to keep up with accelerating brackets, leading to significant boost of the acceleration energy. This work paves the way towards the generation of high-charge, relativistic electron beams with controlled helicity, which holds great potential for advances in fundamental science and a variety of applications.