Ionization injection of 'inception' beams in plasma wakefield accelerators

Future plasma wakefield accelerators (PWFAs) will only become a compact alternative to standard accelerator technologies when they deliver beams with competing ultra-low energy spread. Beam-loading of the fields in PWFAs can enable the production of relativistic electron beams with ultra-low energy spread. Ionization injection technique is one primary method for injecting trailing beams with the required charge to achieve beam-loading. In this paper we describe a novel phenomenon that occurs when the charge of the trailing beams approaches the amount required to beam-load the plasma wakefields. The space-charge fields of the beams trigger the ionization and injection of an additional, high-energy-spread 'inception' beam. Simulations using particle-in-cell code OSIRIS show that the inception beam can have higher current than that of the trailing beams. The charge in the inception beam is shown to scale with the increased charge, and hence space-charge fields, of the trailing beams.