Producing high-brilliance gamma rays with flying focus laser pulses

Some of the highest-energy photon beams realised experimentally rely on a fundamental quantum electrodynamics process: nonlinear Compton scattering between laser photons and ultra-relativistic electrons. We discuss how the energy lost by electrons and the yield of photons emitted in this process can be substantially increased by replacing a stationary-focus laser pulse with an equal-energy flying-focus (FF) pulse. The moving focal point of a flying focus forms an intensity peak that can travel at any velocity, independent of the laser group velocity, over distances much longer than a Rayleigh range. This enables co-propagation of ultra-relativistic particles with the laser focus, so that they stay in the region of peak field intensity for prolonged interaction times, allowing accumulation of strong field effects at orders of magnitude lower peak laser powers and intensities. Since the energy loss in the quantum regime and the photon yield scale more favorably with the interaction time than the laser intensity, utilizing FF pulses provides an outright advantage over stationary-focus setups. Analytic estimates and simulations show that GeV-scale electrons colliding with 1-10 J laser pulses can increase up to five times the yield of 1-20 MeV photons using a FF pulse. If realized, this would represent the brightest laboratory gamma source per initial electron charge in this energy range.