Description
Laser-wakefield accelerators (LWFAs), which accelerate electrons in plasma waves driven by short intense laser pulses, have demonstrated the ability to produce high-quality electron beams with energies up to several GeV. One of the fundamental limitations for reaching ever higher energies is dephasing, when ultrarelativistic electrons outrun the wakefield and slip into its decelerating phase. A promising approach to overcoming this limitation is the use of structured light pulses to control the on-axis intensity propagation velocity of the laser pulse [1, 2].
Here, we report on the first experimental observation of electron acceleration in a tunable-velocity wakefield and mitigation of dephasing using structured light pulses [3]. In the experiments, a 1.5 J, 27 fs laser pulse is focused using an axiparabola, a specialized mirror that produces quasi-Bessel beams with an extended focal line. The on-axis intensity propagation velocity is further modified by applying pulse-front curvature (PFC) before reflection. Through its interaction with a gas jet, we demonstrate that this structured light pulse creates coherent plasma wakefields capable of injecting and accelerating electrons to high energies. We further show that changing the PFC value induces a statistically significant shift in the cutoff energy of the accelerated electron spectrum, demonstrating experimental control over the wakefield velocity and partial mitigation of dephasing. The experimental results are supported by comprehensive simulations including FLUENT simulations of the gas density profile, axiprop simulations of the reflected laser pulse propagation from the axiparabola to the focal region, followed by particle-in-cell simulations of the structured light pulse with the obtained gas density and laser profiles.
[1] C. Caizergues et al. “Phase-locked laser-wakefield electron acceleration,” Nat. Photonics. 14, 8 (2020)
[2] J.P. Palastro et al. “Dephasingless laser wakefield acceleration,” Phys. Rev. Lett. 124, 134802 (2020)
[3] A. Liberman et al., “First Electron Acceleration in a Tunable-Velocity Laser Wakefield,” arXiv:2509.21098