“…However, in fact, the difficulty of producing high‐density electron beams will limit the radiation frequency. For example, to reach high‐frequency THz radiation, it needs an electron beam with a density of 10 22 m −3 , which seems unpractical for common experiments and compact devices . To overcome this problem, we proposed a scheme in this preliminary experimental study, where the electron beam is formed by the plasma electrons driven by a high‐voltage pulse.…”