## 3 THz Source Experimental Layout  
The SPARC_LAB photoinjector (INFN-LNF, Frascati, Italy), which consists in a RF Gun and 3 S-Band travelling wave accelerating sections, is able to provide high brightness electron beams boosted up to 170 MeV at 10 Hz repetition rate. A schematic layout is shown in Figure 2a. Ultra-short electron bunches, mandatory for the generation of broadband high intense THz pulses, are generated at SPARC_LAB through RF compression in the so-called velocity bunching (VB) regime. The electron beam is injected in the first accelerating section at the zero crossing RF field phase. Since the beam is slightly slower than the phase velocity of the RF wave, a longitudinal phase space rotation occurs, based on a time-velocity chirp in the electron bunch, for which electrons in the tail are faster than those in the bunch head. As consequence, the electron beam slips back to phases where the field is accelerating, but it is simultaneously chirped and compressed. The so generated ultra-short, \(\sim\)100 fs, and highly charged, up to \(\sim\)600 pC, electron bunches reach the THz station placed at the end of the linac, as reported in Figure 2a. A diagnostic transfer line, right after the THz station, and equipped with a RF deflecting cavity (RFD), quadrupoles and a dipole magnet, allows a full 6D characterization of the electron beam. Figure 2b reports sketch of the THz source chamber. In our case, TR is produced on a 30 \(\times\) 30 mm aluminum coated silicon plate placed at 45\({}^{\circ}\) with respect to the electron beam direction, which both generates radiation and reflects it out from the beamline. The backward-generated radiation, passing through a quartz window, is collected and collimated by a 90\({}^{\circ}\) off-axis parabolic mirror with a focal length of 152 mm (Edmund Optics, Barrington, NJ, USA) and carried to the experimental table by a flat mirror. The coherent radiation spectrum is measured using a hand-built Michelson interferometer with a 24 \(\upmu\)m Mylar pellicle beamsplitter coupled with a room-temperature pyroelectric detector. The transverse spatial distribution of CTR has been observed by means of a THz camera (Spiricon Camera III, Ophir, North Logan, UT, USA) placed in the focal plane of an off axis parabolic mirror (APM2) with a focal length of 50 mm (Edmund Optics, Barrington, NJ, USA). The THz total energy per pulse was measured by a second pyroelectric detector (THz-I-BNC, GENTEC-EO, Suite Quebec, Canada) with a sensitivity calibrated with 970 GHz through a Schottky Diode (Virginia Diodes Inc, Charlottesville, VA, USA) emitting a power of 1 mW. THz filters (0.3, 0.5, 1, 3 and 5 THz), with approximately a 15% bandwidth, have been used to select different frequencies. Figure 2: (a) Simplified cartoon of SPARC_LAB ; (b) Experimental layout of the THz source chamber placed at the end of the linac. Narrow spectral bandwidth and tunable THz radiation can be produced by a train of sub-ps electron bunches. An active method for tailoring an adjustable train of electron bunches with a sub-picosecond length and picosecond spacing has been demonstrated at the SPARC_LAB test facility [12; 23], combining the velocity bunching regime, and properly shaped trains of UV laser pulses hitting the photocathode (comb laser beam). The technique used to generate laser comb pulses relies on a birefringent crystal, where the input pulse is decomposed in two orthogonally polarized pulses with a time separation proportional to the crystal length. If more birefringent crystals are inserted in the laser beam path, it is possible to produce multi-peaks. A comb laser pulse illuminating a metallic photocathode in a RF gun generates a train of short electron bunches. Downstream the photoinjector, the beam acquires an energy modulation because of the space charge forces and, if injected in a RF-compressor operating in the over-compression regime, the energy modulation can be transformed back into a density modulation.