Coherent Attosecond Pulses Generated by a Relativistic Electron Beam Interacting with an Intense Laser at a Grazing Angle.

The interaction between relativistic electron beams and intense laser fields has been extensively studied for generating high-energy radiation. However, achieving coherent radiation from such interactions needs to precisely control the phase matching of the radiating electrons, which has proven to be exceptionally challenging. In this Letter, we demonstrate that coherent attosecond radiation can be produced when a laser pulse interacts at a grazing angle with a relativistic electron beam. The electrons oscillate in the laser field and are modulated with a superluminal phase, and coherent ultrashort pulse trains are produced in the far field at the Cherenkov angle. This is verified by theoretical modeling and numerical simulations, including three-dimensional particle-in-cell (PIC) simulations and far-field time-domain radiation simulations. It is further demonstrated that the scheme exhibits robustness across a wide range of beam parameters, even for a large-size, rarefied electron beam, making it experimentally feasible. Based on our proposed scheme, high-repetition-rate, compact, and high-energy attosecond pulse sources are available.