Coherent Optical Signatures of Electron Microbunching in Laser-Driven Plasma Accelerators

We report observations of coherent optical transition radiation interferometry (COTRI) patterns generated by microbunched similar to 200-MeV electrons as they emerge from a laser-driven plasma accelerator. The divergence of the microbunched portion of electrons, deduced by comparison to a COTRI model, is similar to 9x smaller than the similar to 3 mrad ensemble beam divergence, while the radius of the microbunched beam, obtained from COTR images on the same shot, is <3 mu m. The combined results show that the microbunched distribution has estimated transverse normalized emittance similar to 0.4 mm mrad.