Direct visualization of relativistic Coulomb field in the near and far field ranges

Coherent emission coming from relativistic charged bunches is of great interest in a wide range of user-oriented applications and high-resolution diagnostics. The complete characterization of such emission is therefore important in view of a complete understanding of its potential. Here we present a complete temporally-resolved characterization of the radiation emitted by ultra-short relativistic electron bunches using a temporal diagnostic based on electro-optical sampling with a few tens fs of temporal resolution. We have characterized, for the first time to our knowledge, the evolution of the radiation (in THz range) both in amplitude and direction of propagation by varying the detection (i.e. the observer) position from the near to the far field (FF) range. Results show that in the near-field regime the emitted radiation propagates collinearly with the electron beam; while, approaching the FF regime, the radiation behaves as the classical Cherenkov radiation.