Coherent backward transition radiation from sub-fs "pancake-like" bunches as a tool for beam diagnostics

Coherent transition radiation (CTR), produced by short electron bunches, is a well-known radiation mechanism which is applied for bunch length monitoring. By analyzing the CTR spectrum it is possible to reconstruct the longitudinal bunch shape. Such a technique is widely used for accelerators with electron beams possessing bunch lengths σ_z much larger than their transverse beam sizes σ_x,y (with σ_x,y,z the rms transverse and longitudinal beam sizes). New developments in the field of accelerator technology allow to produce and accelerate bunches with a duration of less than 1 fs with a "pancake-like" bunch shape, i.e. with σ_z ll σ_x,y. Diagnostic schemes of this kind of bunches give rise to interesting effects, for example connected with a transverse asymmetry (σ_x σ_y), with a tilting of the bunches (i.e. the bunch axis does not coincide with the velocity direction), and even more. Spectral-angular distributions of coherent backward transition radiation (BTR) were simulated taking into account a possible bunch tilt. It is shown that spectral intensity measurements of coherent BTR emitted in backward direction allow to obtain information about the bunch length.