Time variation in polarization of 0.1-THz high-power Cherenkov radiation from surface-wave oscillator

A relativistic electron beam passing above a slow-wave structure emits high-power Cherenkov radiation (CR). When the beam is magnetized by an axial magnetic field, the cyclotron motion of the beam may affect the polarization of the radiation. In this study, we measure a degree of circular polarization (CP) of 0.1-THz CR generated from a magnetized electron beam where the cyclotron frequency is sufficiently lower than the radiation frequency. Right-hand (RH) and left-hand (LH) CP components of the radiation power are observed separately by using a CP divider. The RH-CP component, which rotates in the same direction of the cyclotron rotation, is dominant in the elliptical polarization of CR when the observed power is relatively low. On the other hand, the LH-CP component tends to increase as the observed power increases and becomes dominant in the polarization. The change in the handedness of the elliptically polarized CR may be associated with Faraday rotation and the effect of superposed modes with different azimuthal indices. Our results contribute to the development of Cherenkov-type high-power sources in terms of polarization control as well as the evaluation of the electrons in Cherenkov interaction.