Periodic self-injection of electrons in a few-cycle laser driven oscillating plasma wake

Abstract

Millijoule-energy few-cycle laser pulses excite the plasma wakefield and accelerate electrons at kilohertz repetition rate, generating mega-electron volt (MeV) electron bunches with femtosecond temporal duration for ultrafast electron diffraction applications. By simulating few-cycle laser pulses interacting with the underdense nitrogen plasma, we have studied the mechanism of periodic electron self-injection, which manifests the laser carrier envelop phase (CEP) effect in few-cycle laser wakefield acceleration. A few-cycle laser pulse experiencing a significant wavelength redshift induces the transverse oscillation of the plasma bubble at the same frequency as the laser CEP changes by 2π. The oscillation of the plasma bubble periodically injects free electrons into the bubble at a doubled frequency, broadening the electron energy spread.