Positron generation and acceleration by injecting two ultra-relativistic intense electron beams upon high-Z target*

We numerically study the plasma acceleration of trailing positron bunch as a result of injecting two ultra-relativistic incident electron bunches upon high-Z target [X. Wang, et. al., PRL 101, 124801 (2008)]. The experiment is capable at a new 10 GeV electron beam facility, Facilities for Accelerator Science and Experimental Test (FACET) II, which is under construction for the ultra-relativistic beam-driven plasma wakefield accelerator. We demonstrate the drive positrons and the trailing electrons experience defocusing force due to the excitation of nonlinear wake inside plasma, while trailing positrons are trapped and accelerated when two bunches are properly spaced. The effects of beam parameters and geometries are explored with the aim of optimizing the positron acceleration. We support our findings with the 3D Monte Carlo simulation EGS5 and the 3D particle-in-cell code QuickPIC [W. An et al., JCP 250, 165 (2013)]. Since the beam density at FACET II reaches the order of ~10²⁰ cm⁻³, we also discuss the effect of strong magnetic field generation inside the high-Z target which enhances the gamma-ray flux and the number of positrons beyond the expectation of EGS5 result.