We analyze radiation produced by an ultrarelativistic charge as it exits the open end of a cylindrical waveguide with a dielectric lining. The end of the waveguide can be either orthogonal to the structure axis or skewed. To obtain terahertz radiation from waveguides with centimeter or millimeter radii, we consider high order TM(0m) modes driven by the beam. We obtain an integral representation which describes the radiation produced by a single waveguide mode in the Fraunhofer zone. We perform a series of numerical calculations for structures which look promising for generation of THz radiation. It is shown that for a mode with large mode number, the aperture of the vacuum channel gives the main contribution to the field if the skew angle of the waveguide aperture is not too small. Simple expressions for the angle of the main pattern lobe maximum are obtained.
We analyze the radiation from a charged particle crossing the boundary between an ordinary medium and a "left-handed" metamaterial. We obtain exact and approximate expressions for the field components and develop algorithms for their computation. The spatial radiation in this system can be separated into three distinct components, corresponding to ordinary transition radiation having a relatively large magnitude, Cherenkov radiation, and reversed Cherenkov-transition radiation (RCTR). The last one is explained by reflection and refraction of reversed Cherenkov radiation at the interface. Conditions for generating of RCTR are obtained. We note properties of this radiation that have potential applications in the detection of charged particles and accelerator beams and for the characterization of metamaterial macroscopic parameters (epsilon, mu).
We report on a dielectric target that concentrates Cherenkov radiation into a small spatial area. In contrast to traditional devices, this target can focus almost all of the radiation without using additional lenses or mirrors. We consider the case where radiation is produced by a point charge moving along the axis of a cylindrical channel inside an axially symmetrical target. The specific form of the target is determined using the laws of ray optics. The field is calculated using an aperture integration method that can determine the field near the focus. Typical field plots and the spatial distribution of the field outside the target are presented. We demonstrate that at terahertz frequencies, this concentrator can increase the field magnitude by up to at least 2 orders of magnitude relative to that on the surface of the target.
We proceed with investigation of specific dielectric target that effectively concentrates Cherenkov radiation from a charged particle bunch into a small vicinity of the focus point located at the symmetry axis of the target. The case of "non-ideal concentration" where the charge velocity differs from the designed one was considered theoretically in our previous paper. In particular, we have noted that geometrical rays form caustics in this case, and areas of radiation concentration are shifted with respect to the designed focus point. Since the direction of this shift relates to the sign of the charge velocity deviation, this effect can be used for diagnostics of bunch velocity. Here we perform numerical simulations in COMSOL using frequency domain solver and compare simulated results with theoretical ones. In particular, we show that simulated focus area is indeed shifted with change in charge velocity and the position of this area correlate very well with analytical predictions.
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