Ionization injection and acceleration in a dephasingless laser wakefield accelerator*

The viability of laser wakefield accelerators for use in next-generation x-ray free-electron lasers or linear colliders is limited by dephasing, diffraction, and pump depletion. Spatiotemporally structuring the laser pulse using an axiparabola and echelon can overcome these limitations by producing an intensity peak that travels at or near the vacuum speed of light in a plasma over many Rayleigh ranges [J. P. Palastro, et al., PRL, 124(13), 134802 (2020)]. Here, we present particle-in-cell simulations that show ionization injection and acceleration of a 25-pC electron beam to a maximum energy of 2.1 GeV over 20 dephasing lengths (1.3 cm) using a 6.2-J laser driver in the nonlinear bubble regime. Stable propagation is obtained by masking the inner portion of the axiparabola, accelerating the focus to compensate for the changing spot size, and offsetting the density profile to mitigate unwatned self-focusing. Scaling these results to a 500-J laser pulse could produce a 125-GeV electron beam in under 1 m.