The dynamics of electrons forming the boundary layer of a highly nonlinear laser wakefield driven in the so called bubble or blowout regime is investigated using particle-in-cell simulations. It is shown that when the driver pulse intensity increases or the focal spot size decreases, a significant amount of electrons initially pushed by the laser pulse can detach from the bubble structure at its tail, middle, or front and form particular classes of waves locally with high densities, referred to as the tail wave, lateral wave, and bow wave. The tail wave and bow wave correspond to real electron trajectories, while the lateral wave does not. The detached electrons can be ejected transversely, containing considerable energy, and reducing the efficiency of the laser wakefield accelerator. Some of the transversely emitted electrons may obtain MeV level energy. These electrons can be used for wake evolution diagnosis and producing high frequency radiation.
By using three-dimensional particle-in-cell simulations externally injected electron beam acceleration and radiation in donut-like wakefields driven by a Laguerre-Gaussian pulse have been investigated. Studies show that during the acceleration process, the total charge and azimuthal momenta of electrons can be stably maintained for a few hundreds of micrometers distance. Electrons experience low frequency spiral rotation and high frequency betatron oscillation, which leads to synchrotron-like radiation. And the radiation spectrum is mainly determined by the electrons' betatron motion. The far field distribution of radiation intensity shows axial symmetry due to the uniform transverse injection and spiral rotation of electrons. Our studies suggest a new way to simultaneously generate hollow electron beam and radiation source from a compact laser plasma accelerator.
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