Cascaded high-gradient terahertz-driven acceleration of relativistic electron beams

Terahertz-driven acceleration has recently emerged as a route for delivering ultrashort bright electron beams efficiently, reliably and in a compact set-up. Many working schemes and key technologies related to terahertz-driven acceleration have been successfully demonstrated and are being developeds 1 – 10 . However, the achieved acceleration gradient and energy gain remain low, and the potential physics and technical challenges in the high-energy regime are still underexplored. Here we report whole-bunch acceleration of relativistic beams with an effective acceleration gradient of up to 85 MV m –1 in a single-stage configuration and demonstrate a cascaded terahertz-driven acceleration scheme of relativistic beams with an energy gain of 204 keV. These proof-of-principle results represent a critical advance towards high-energy terahertz-driven acceleration of relativistic beams, are scalable and have great potential to provide high-quality beams, with implications for future terahertz-driven electron sources and related scientific discoveries.