Over the past decade, progress in laser wakefield acceleration (LWFA) has led to the production of multi-GeV electron beams from cm-scale targets. High quality electron beams and high intensity laser pulses are now simultaneously available at multi-petawatt facilities, allowing the study of all-optical laser-electron beam collisions. Such configurations open the possibility to test strong-field quantum electrodynamics (SFQED), in order to understand the behavior of charged particles under the influence of an intense laser field. In particular, experiments on nonlinear Compton scattering can reveal nonlinear features in the high-energy gamma emission spectra.
We present measurements of high-energy gamma photon beams generated from nonlinear Compton scattering experiments at the CoReLS 4PW facility. The gamma beams were produced during the collision of LWFA-accelerated electrons (E<3.5 GeV) and an ultrashort laser pulse (25fs) of intensity I>1020 W/cm2, achieving a quantum nonlinearity parameter χ≈0.4. The unprecedented properties of the gamma beams required the development of novel detection techniques, relying on LYSO scintillation detectors. Using these in-house detection methods, we observed photon spectra that can be parametrized by a critical energy >150 MeV, extending over hundreds of MeV. The beams have a low divergence (≈1 mrad), small source size and ultrashort duration, thus exhibiting an ultrahigh brilliance. Such high energy gamma beams open up new research possibilities in fundamental physics and nuclear photonics.