## 4 Results and Discussion
### Spectral-Shaped THz Generation  
High charged trains of sub-ps bunches with THz repetition rate have been successfully manipulated and transported down to the linac exit to reach the THz station. We report the characterization of THz spectral shapes generated from different configurations of 5-bunch trains, in particular the inter-distance and the bunch charge balanced by UV laser shaping. The first studied bunch consists of a four bunches beam, each with a mean charge of \(\sim\)50 pC and an inter-distance of 1 ps. This train is followed by a further bunch with charge of \(\sim\)20 pC, which has an inter-distance of 1.6 ps from the fourth. The longitudinal phase space (time-energy) was measured by combining the RFD and the dipole spectrometer. The average energy of the whole train is \(\sim\)110 MeV with an energy spread of 0.2%. The bunch train current is reported in the inset of the Figure 5, where each bunch in the train has been labeled (1-5), while the measured beam parameters are reported in Table 1.  
Figure 4: Low charge regime: Spectral distribution intensity measured by Michelson interferometer for different bunch charge Q. Inset: generated THz energy per pulse as a function of bunch charge Q. The curve for the highest bunch charge (500 pC) has been scaled by a factor of 100. Experimental data (blue dots), Q\({}^{2}\) dependence (red line). The integrated THz intensities per pulse shown in the inset are affected by a systematic uncertainty due to the missing calibration of the THz-I-BNC GENTEC-EO pyroelectric detector at low frequencies (below 0.6 THz). Figure 3: Broadband THz generation: (**a**) spectral distribution sampled by THz filters. The error bars represent the bandwidth of the bandpass filters; (**b**) Transverse intensity distributions of CTR in the focus of the parabolic mirror for different wavelength. Since the used power meter (THz-I-BNC GENTEC-EO) is not calibrated for frequencies below 0.61 THz, the measured energy for frequencies below this value is a qualitative assessment. The generated spectral distribution measured by means of a Michelson interferometer is reported in Figure 5a. As previously discussed, respect to a gaussian-like shape emitted by a single bunch, a comb-like beam generates a peaked THz spectrum. The central frequency of the main peaks corresponds to the inverse of the inter-distance between the bunches: the 1 THz peak is related to the mean inter-distance of \(\sim\)1 ps of the first 3 bunches, while the low frequency peak at 0.6 THz is due to the inter-distance of 1.6 ps between the last two bunches. The remaining components of the high frequency in the spectrum result from the harmonics properly enveloped by the single bunch spectrum as observed in Figure 1. Figure 5b shows the expected, calculated THz spectral emission for the measured bunch train distribution reported in the inset of Figure 5a. As one observes, the spectral calculation reproduces well the measured one. The reduction of the inter-distance between the bunches leads to a displacement of the THz spectrum towards high frequencies, as one can see in Figure 6a. The considered bunch train is approximately identical to the previous one, as shown by the beam parameters in Table 1, but the inter-distance between the last two bunches is reduced from 1.6 ps to 0.5 ps. A narrower peak emerges in the measured THz spectrum at about 2 THz decreasing the contribution at 0.6 THz as shown by green arrows in Figure 6a. While the remaining part of the spectrum results exactly unchanged. Also the numerical calculation of the generated spectrum, blue line in Figure 1, evaluated from measured current profile shows the rise of a narrow peak around 2 THz while the contribution at 0.6 THz decreases. In order to investigate the production of narrow-band THz radiation, a train of four bunches with a ramp of charge has been generated directly at the cathode. The resulting electron bunch train, as measured at the end of the linac, is shown in the inset of Figure 7, while beam parameters are reported in Table 1 (third column).