Effects of diffraction and screen size on CTR based bunch length measurements

Coherent transition radiation (CTR) is now well established as one of basic tools to measure electron bunches on a subpicosecond scale. However, a series of experiments has demonstrated that suppression of CTR spectra at low frequencies, occurring in practice, leads to a great uncertainty in the bunch length determination. In addition to known sources of suppression, it was recently found that the size of a screen used to produce transition radiation (TR) can be a factor strongly affecting the spectrum. In this paper we calculate TR spectra emitted by a relativistic electron from a finite-size metallic screen and influenced by diffraction on apertures in environment and collecting optics