Particle-in-cell simulations of guided mode evolution and ionization injection in a multi-GeV laser wakefield accelerator*

Recent experiments [1] have demonstrated quasi-monoenergetic laser wakefield acceleration (LWFA) of electron bunches to 5 GeV in a 20 cm optically generated plasma waveguide [2]. This talk presents a three-stage model of intense pulse propagation, ionization injection, and electron acceleration applicable to our experiments. Our understanding of the process is based on extensive 3D WarpX [3] particle-in-cell simulations. During early propagation (stage I), beating between different low order waveguide modes results in large amplitude intensity variations, deformation of the transverse fields, and sparse injection. This behavior is suppressed as the higher order modes undergo group velocity walk-off from the fundamental and fall back in time. Ponderomotive modification of the channel by the envelope of the fundamental mode then causes small amplitude variations in pulse intensity correlated with enhancement and suppression of ionization injection (stage II). Stage I and stage II both contribute to striated electron energy spectra. Eventually, the accumulation of nonlinear effects results in a collapse of the pulse intensity and dephasing of the striated energy spectrum of the accelerating electrons (stage III). This model points to the ability to longitudinally manipulate pulse intensity to isolate and induce injection at arbitrary axial locations. Recent experimental results confirm this level of control [4].