Electron reflux dynamics in relativistically transparent plasma

The electron re-injection effect has revealed the great probability rate of photon generation due to the head-on collision between relativistic electrons and laser. We study the electron re-injection dynamics when the ultra-intense laser irradiates the near-critical-density plasma and successfully controls the photon radiation by means of the transversely tailored plasma. Starting from the relativistic corrected ponderomotive force, the critical strength of the laser field required by the refluxing effect is theoretically obtained. Then, the theoretical description of the wavefront formed by electron refluxing is given via plugging in the difference in the transverse phase velocity of the plasma wave. Simulation results display a curved surface of the refluxing electrons, which are in good agreement with the calculation results stemming from the physics model. The re-built phase space of the refluxing electrons illustrates that they gain energy mainly from the longitudinal electrostatic field on the re-injection path. Despite the energy of the refluxing electron being relatively low, it could radiate more photons via more efficient non-linear Compton scattering than the electron being accelerated in the positive direction. Furthermore, we employ a transverse density profile in the plasma and successfully achieve control of the electron re-injection effect and the properties of the resultant photons as well. Simulation results exhibit that overcritical electron beams are successively re-injected from the plasma density peaks. These backward electrons emit photons along the two maximal plasma densities as they collide with the laser pulse. Although the quality of the photons is not improved, their spatial distribution is changed, which is a big step toward manipulating light sources.