Ring-shaped electron beams from laser-wakefield accelerator

Using analytical methods and computer simulations, we investigate physical processes which lead to the formation of ring-shaped electromagnetic and electron structures in laser-plasma interaction. We observe that as the intense laser pulse excites a nonlinear Langmuir wave in an underdense plasmas, a significant portion of the pulse is refracted outwards the propagation direction due to the interactions with thin, high-density electron walls surrounding the wave cavities. Because of the radial symmetry, the refracted light forms a distinct electromagnetic ring that encircles the driver pulse. The efficiency of the energy transfer to the electromagnetic ring is relatively high, so that the ring can generate its own Langmuir wave and trigger the electron self-injection, which results in a ring-shaped beam of high-energy electrons. The properties of the ring-shaped electromagnetic and electron beams depend on the parameters of the Langmuir wave cavity walls, thus they can be controlled by tuning the parameters of the laser and plasma. The ring structures could be applied as a drivers for acceleration of positively charged particles, or as a diagnostic to determine regimes and the overall efficiency of the laser-wakefield accelerator.