Simulation studies on enhancement of electron beam energies and betatron radiation energies in laser wakefield acceleration

Abstract

The laser wakefield acceleration (LWFA) method using an intense laser pulse and plasma has attracted much attention as a substitute for conventional radio frequency (RF)-based accelerators because it can provide a large accelerating electric field up to 100 GV/m. This method can be used not only for obtaining a high energy electron beam within a short acceleration distance but also for ultrafast research fields because the pulse duration of the accelerated electron beam is short to several fs. The fs X-ray pulses are generated due to the transverse oscillation motion of electrons during the acceleration process, which is called ‘betatron radiation’. We have studied several methods for increasing the electron beam energies and betatron radiation energies in LWFA using two-dimensional (2D) particle-in-cell (PIC) simulations. First, we studied a method for improving the energy of an electron beam by using a weak laser pulse following the intense driving laser pulse for electron acceleration. The electrons in the accelerated electron beam have a larger momentum in the transverse direction due to the weak modulating pulse, thereby decreasing the longitudinal velocity. Therefore, we can get a longer acceleration distance by using this method by delaying the time when the dephasing occurs. In our simulation results, the energy of the electron beam increases as the peak power of the modulating pulse increases, but the total charge of the electron beam decreases. And the energy increase of the electron beam was greatly reduced at a certain intensity. The energy of the betatron radiation is also enhanced when the transverse oscillation amplitude is increased by the modulating pulse. We show that the energies of the betatron radiation increase in proportion to the intensity of the modulating pulse by using 2D PIC simulations. In addition, the energy of the betatron radiation tends to increase as the frequency of the modulating pulse increases. Finally, we studied an increment of the betatron radiation energy caused by injecting strong driving laser pulses at the off-axis position in the preformed parabolic density plasma. The laser pulse injected at the off-axis position oscillates in the transverse direction and the accelerated electrons in the ion cavity oscillate with larger amplitude due to the influence of the oscillating cavity. Therefore, the betatron radiation energy increases as the radial distance of the focused laser beam from the axis increases.