Influence of Vacuum Window on the Measurement of Coherent Smith–Purcell Radiation at t‐ACTS

In this article, we study classical radiation from relativistic electrons at a facility, a test accelerator as a coherent terahertz source (t‐ACTS), at the Research Center for Accelerator and Radioisotope Science (RARiS), Tohoku University. We have previously studied coherent Smith–Purcell radiation from a metallic grating. In the previous setup, a Z‐cut quartz crystal window was used to extract the radiation generated by the electron beam inside the vacuum chamber. In each experiment, the incident radiated light did not travel perpendicular to the window. To correct the transmittance of the vacuum window, the refractive index and transmittance of the window are measured using terahertz (THz)‐time‐domain spectroscopy, which assumes normal incidence. Considering that a Z‐cut quartz exhibits birefringence, these measured values may be incorrect under oblique incidence conditions; thus, they are not suitable for correcting experimental results. This article presents the development of a new vacuum window utilizing arc‐shaped high‐density polyethylene to ensure that the THz light is incident perpendicularly on the window material, regardless of the approach angle of the radiation. We discuss the differences in measurement results between the conventional Z‐cut quartz window and proposed arc‐shaped vacuum window.