Resonant excitation of very high gradient plasma wakefield accelerators by optical-period bunch trains

Abstract

The use of a periodic electron beam bunch train to resonantly excite plasma wakefields in the quasinonlinear (QNL) regime has distinct advantages over employing a single, higher charge bunch. Resonant QNL excitation can produce plasma electron blowout using a small charge per pulse if the beam emittance beams are very low. The local density perturbation in such a case is extremely nonlinear, achieving total rarefaction, yet the resonant response of the plasma electrons at the plasma frequency is preserved. The needed electron beam pulse train with interbunch spacing equal to the plasma period can be produced via inverse free-electron laser bunching. As such, in achieving resonance with a laser wavelength of a few microns, a high plasma density is employed, with the attendant possibility of obtaining extremely large wakefield amplitudes, near 1 TV/m for FACET-II parameters. In this article, we use particle-in-cell (PIC) simulations to study the plasma response, the beam evolution including density modulation, and the instabilities encountered when using a bunched-beam scheme to resonantly excite waves in a dense plasma.