Femtosecond microbunched electron beam - A new tool for advanced accelerator research

We employed periodic trains of femtosecond electron bunches for testing several novel concepts of acceleration. A microwave-driven linac sends a 45-MeV electron beam (e-beam) through a magnetic wiggler wherein the e-beam energy is modulated via the inverse free electron laser (IFEL) technique by interacting with a 30-GW CO₂ laser beam, so creating 3 fs long microbunches separated by a 30 fs laser period. We show several examples of utilizing such a femtosecond bunch train in advanced accelerator and radiation source research. We demonstrated that microbunching improves the performance of the laser acceleration process compared to the previously investigated single-bunch technique. Specifically, microbunches were phased to the electromagnetic wave of the CO₂ laser beam inside a matched tapered wiggler where ∼80% of electrons gained energy as an ensemble while maintaining a narrow energy spread (i.e., monoenergetic). Another plasma wakefield acceleration (PWFA) experiment explored resonant wakefield excitation in an electric discharge plasma with the plasma frequency matched to that of the CO₂ laser. Simulations predict orders-of-magnitude enhancement in the wakefield's amplitude compared with that attained with single bunches. In the Particle Acceleration by Stimulated Emission of Radiation (PASER) experiment, we tested a prediction that an active laser medium can produce particle acceleration by stimulating the emission of radiation. The process benefits from the action of a periodic train of microbunches resonating with the laser transition. Finally, we analyze prospects for using partially coherent x-ray sources based on Thomson backscattering from the electron microbunch train.