We observed coherent Cherenkov radiation matched to a circular plane wave (CCR-MCP) for the first time using a hollow conical dielectric made of a high-density polyethylene. The refractive index and the absorption coefficient of the dielectric were evaluated to be 1.537 ± 0.004 and 0.006 ± 0.028 by measuring the pulse formed by the interference between the CCR-MCP and the coherent diffraction radiation. These values were consistent with the values shown in a reference for the high-density polyethylene. In accordance with the theory of the Cherenkov radiation, the spectrum of the CCR-MCP shifted towards higher wavenumbers compared to that of the coherent diffraction radiation. The intensity of the CCR-MCP beam was proportional to the height of the hollow conical dielectric and was 3 times the intensity of the coherent diffraction radiation. The CCR-MCP technique can produce broadband terahertz-wave sources with unprecedented power at compact accelerator facilities.

We developed an intense coherent transition radiation (CTR) at the Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University. A rectangular titanium screen with dimensions of 80 × 60 mm ² was used as the target, and the backward CTR generated on the screen was extracted through a crystal quartz window to air. We observed an intense terahertz (THz) radiation and confirmed it to be the CTR by measuring the spatial distribution and the dependence of the power on the electron charge. The CTR spectrum measured using a Martin–Puplett-type interferometer showed a maximum at a frequency of 0.3 THz and was emitted up to 1.6 THz. The CTR energy and peak power of a micropulse were approximately 80 nJ and 100 kW, respectively. The CTR energy during a 4.5 µs macropulse was approximately 1 mJ, indicating that this broadband THz light source is one of the most powerful coherent radiation sources in normal conducting linac facilities.