## 1 Introduction  
Terahertz radiation (30 GHz-15 THz, 20-1000 \(\upmu\)m; 1-60 meV) is a portion of the electromagnetic (e.m.) spectrum, which lies in the gap between Microwave and Infrared. THz-based technologies and research applications have seen a rapid increment in the last years due to the development of new radiation sources based both on femtosecond lasers and sub-picosecond electron bunches [1; 2].  
Intense single cycle pulses are increasingly used to explore the nonlinear light-matter interaction at THz frequency spanning from a coherent control of quantum states to induce and externally modify chemical and/or biological processes [3; 4]. Recently, laser-based sources have reached pulse energies on the level of tens of \(\upmu\)J by optical rectification of mJ laser pulses [2; 5; 6]. However, photon down-conversion process suffers of limitations to generate high power THz pulses, like the damage threshold of the nonlinear crystals and saturation conversion efficiency. Accelerator-based sourcesmay provide instead a viable way to produce highly intense THz pulses without the limitations of optical conversion processes.  
At the SPARC_LAB test facility [7], highly intense broadband THz radiation is routinely produced as Coherent Transition Radiation (CTR) emitted by ultra-short (\(\sim\)100 fs) high-brightness electron bunches [8]. These bunches are produced through the velocity bunching compression technique [9; 10; 11]. The main feature of this source relies on the possibility of generating an e.m. spectrum in the THz range of different bandwidth, centered at different frequencies spanning from 0.5 up to 5 THz, by simply acting on laser pulse shaping and longitudinal phase space manipulation techniques. This method makes the SPARC_LAB THz source flexible, allowing both broadband and narrowband THz generation, taking advantage from the laser comb technique integrated in the velocity bunching regime [12]. This versatility is not possible in both undulator-based and dielectric waveguide THz sources [13; 14], where the broadband emission cannot be achieved. In addition, unlike other ways of bunch train generation, for instance chirped-pulse beating [15] and dispersive mask techniques [16], the laser comb scheme permits a full control of both the temporal bunch inter-distance and the amplitude pulse shape. In particular, a custom train of more than 100 pC charge/bunch can be generated at the cathode and transported with no losses up to the THz station. Therefore, in case of a 4 bunches train with 100 pC/bunch, the THz radiation energy per pulse would be equivalent to that generated by a 400 pC single bunch. So, our laser technique allows to control the laser pulse amplitude, _i.e._, bunch charge, and the temporal delay of each pulse with respect to the other directly at the cathode, resulting in a customized THz emission spectrum.  
In this paper, we will resume the main results achieved at SPARC_LAB in the generation of CTR-THz radiation, due to the peculiar possibility of longitudinally manipulating ultra-short (sub-ps) high-brightness electron beams.  
The measured THz figures of merit render the SPARC_LAB source one of the most performant sources worldwide for nonlinear and pump-probe THz spectroscopy. From this point of view, the opportunity of proper tailoring the THz spectral shape by the bunch train is very compelling because it would allow to selectively pump excitations in a condensed matter systems [17; 18]. Furthermore, we show that the characterization of coherent THz radiation spectrum is a valid and useful tool for longitudinal diagnostics of exotic sub-ps electron beams in new Free-Electron Laser sources [19] and for future plasma accelerated femtosecond scale, pC electron bunches [20].