Ultrarelativistic electron beams accelerated by terawatt scalable kHz laser

We show the laser-driven acceleration of unprecedented, collimated ($2\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}$ divergence), and quasi-monoenergetic ($25\mathrm{\%}$ energy spread) electron beams with energy up to $50\mathrm{M}\mathrm{e}\mathrm{V}$ at $1\mathrm{k}\mathrm{H}\mathrm{z}$ repetition rate. The laser driver is a multi-cycle ($15\mathrm{f}\mathrm{s}$) $1\mathrm{k}\mathrm{H}\mathrm{z}$ optical parametric chirped pulse amplification (OPCPA) system, operating at $26\mathrm{m}\mathrm{J}$ ($1.7\mathrm{T}\mathrm{W}$). 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.