Design of the optical cavity for the enhancement of coherent Cherenkov THz radiation

Since Cherenkov radiation travels at an angle depending on the refractive index of the medium and the velocity of the charged particles, coherent radiation cannot be obtained in an electron bunch with a long Gaussian distribution in the direction of travel. Therefore, coherent Cherenkov radiation is generated by controlling the tilt angle of the electron beam with high precision using an rf deflection cavity and matching the Cherenkov angle with the angle of the electron beam. The frequency of coherent radiation depends on the beam size of the electron bunches, and we focus on the THz band in this study. We also design an optical cavity with two mirrors to amplify the intensity of the generated coherent THz pulse. Since the rf gun at Waseda University can operate up to 150 bunches at 119 MHz, all the pulses can be overlapped in the same phase by matching the cavity length to the period of the electron bunches. We plan to use amorphous polymer TOPAS as the target medium, which has a Cherenkov radiation angle of about 49 degrees, so that two mirrors can be installed without interference with the beamline. In this presentation, we report the experimental results of the generation of THz-band coherent Cherenkov radiation, the design of the optical cavity, the calculation of the enhancement rate, and the future prospects. (author)