Far-Infrared Transition and Diffraction Radiation Part I: Production, Diffraction Effects and Optical Propagation

Transition radiation is emitted when a relativistic charged particle crosses the boundary between two media of different dielectric properties. Radiation is also produced if the particle does not cross the discontinuity directly but passes it at such a close distance that its self-field is modified by the sudden change in the dielectric constant. In this case one speaks of diffraction radiation. For a planar boundary of infinite extent, the transition radiation process was calculated analytically by Ginzburg and Frank [1]. In most practical cases, however, the Ginzburg-Frank equation is not applicable because two basic pre-assumptions of the analytic derivation are not fulfilled: the radiation screens used in an accelerator are of limited size, and the radiation is usually observed in the near-field and not in the far-field regime. In this paper, we describe a method which allows to compute the transition or diffraction radiation from a screen of arbitrary shape and size and at any distance from the source. The only condition is that the dimensions of the screen and the distance to the observer must be large compared to the wavelength of the radiation. The method is therefore capable of treating realistic experimental setups.