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

Abstract

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 σz much larger than their transverse beam sizes σx,y (with σx,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 σz ≪ σx,y. Diagnostic schemes of this kind of bunches give rise to interesting effects, for example connected with a transverse asymmetry (σx ≠ σy), 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.