Broadband hard X-ray attosecond pulses from extremely chirped electron beams

Attosecond pulses from free-electron lasers have opened the doors to atomic site-specific pumping and probing of quantum systems. Key to their success has been electron beam shaping techniques enabling the generation of sub-femtosecond current spikes with peak currents on the order of 10 kA. We demonstrate in an RF linac the generation of current spikes with extreme chirps on the order of 350 MeV/micron, competitive with the chirps expected from beam-driven plasma wakefield accelerators. Leveraging chirp-taper compensation, we use these highly chirped beams to generate single spike hard X-ray attosecond pulses with bandwidths exceeding 30 eV, a factor of two beyond earlier single spike hard X-ray demonstrations. Such large chirps can be further compressed downstream of lasing, enabling subsequent superradiant light emission or direct excitation with the beam's intense space charge field for attosecond pump-probe experiments.