The TeraFERMI Project

Transition Radiation is produced when a thin metallic target screen is inserted in the electron trajectory. The high energy electrons travel across the target while an intense current builds up on the metallic screen which is then radiated both in forward and backward directions. The emission properties can be calculated thanks to the Ginzburg-Frank formula. Transition Radiation displays circular polarization and cylindrical symmetry along the propagation axis. In the case of an ultrashort electron bunch one should apply the coherent enhancement factor discussed in the previous paragraph. One then talks about Coherent Transition Radiation (CTR). 

We simulate here the emission from a relativistic electron beam of 1.2 GeV, with a bunch shape modeled by a rectangular bunch density distribution of variable duration Δt, and constant charge Q=1 nC. Full lines refer to the far-field case, while dashed lines account for the near-field. For a 1 ps long bunch, the THz emission is limited below 1 THz. By decreasing the bunch length (while keeping constant the total charge Q), the energy cut-off is progressively shifted to higher frequencies, with a strong enhancement of the overall intensity. As shown in the picture, under the above described conditions, the calculated energy per pulse ranges from 150 µJ to more than 5 mJ for pulse lengths of 50 fs.