Ultrarelativistic electron beams accelerated by terawatt scalable kHz laser

We show the laser-driven acceleration of unprecedented, collimated ($ 2 \ \mathrm{mrad} $ divergence), and quasi-monoenergetic ($ 25 \ \% $ energy spread) electron beams with energy up to $ 50 \ \mathrm{MeV} $ at $ 1 \ \mathrm{kHz} $ repetition rate. The laser driver is a multi-cycle ($ 15 \ \mathrm{fs} $) $ 1 \ \mathrm{kHz} $ optical parametric chirped pulse amplification (OPCPA) system, operating at $ 26 \ \mathrm{mJ} $ ($ 1.7 \ \mathrm{TW} $). The scalability of the driver laser technology and the electron beams reported in this work pave the way towards developing high-brilliance x-ray sources for medical imaging, innovative devices for brain cancer treatment, and represent a step towards the realization of a kHz GeV electron beamline.