# Synchronised TeraHertz Radiation and Soft X-rays Produced in a FEL Oscillator
## 6 Conclusions  
The dual source we presented here, simultaneously delivering THz and X-ray radiation, has a compact footprint and can therefore be installed in medium-size laboratories, hospitals or university campuses. It is based on a relatively simple upgrade of a THz FEL oscillator, consisting in the addition of a telescopic system surrounding the Compton interaction point, placed in the centre of the cavity. A possible geometry of the cavity, a bow-tie configuration with four mirrors, was discussed. The undulator should occupy part of the region between the telescopic system and the front mirror. The dimension and cost of the device are almost the same as those of a THz oscillator. The THz radiation generated in an FEL oscillator propagating backward in the cavity hits a successive electron bunch producing soft X-rays via Thomson/Compton back-scattering. In this condition, THz FEL intracavity pulses with mJ-class energy at 15-50 microns of wavelength, driven by 20-100 MeV energy electron bunches, can deliver up to few \(10^{5}\) soft X-ray photons per shot by Compton back-scattering at a rate of 10-100 MHz, synchronised with the THz radiation. Two possible undulator choices at \(\lambda_{w}\) = 3.5 and 4.5 cm were described. The first one produces higher levels of THz radiation energy, with larger bandwidths and less regular power distributions. The second one, conversely, has smoother power density and narrower spectra, but the radiation yield features lower energy values. More than \(10^{17}\) THz photons per shot are emitted intra-cavity by the FEL oscillator, meaning more than \(10^{15}\) per shot to the extra-cavity users, and more than \(10^{5}\) X-ray photons being driven per shot. These numbers, multiplied by the repetition rate of the source, namely 10-100\(\times 10^{6}\) Hz, give \(10^{22}\)-\(10^{23}\) THz photons/s, coupled with more than \(10^{12}\) X-ray photons/s. Open technological challenges are the implementation and the optimisation of the cavity elements, such as mirrors, lenses and windows, issues which should be studied within a work of conceptual and technical design. Lenses could also be made by artificial dielectric material [44]. For windows, instead, materials with maximum transparency in both THz and soft X-rays domains should be used. The problems of the transport and focusing of radiation pulses are, however, common to THz sources of all kinds and currently under study with increasing interest in the whole THz radiation community.  
Conceptualisation, V.P.; methodology, all authors; formal analysis, all authors; investigation, all authors; resources, all authors; data curation, all authors; writing--original draft preparation, all authors; writing--review and editing, all authors. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Figure 14: Number of photons of the collimated X-ray radiation for bandwidth at 10% as a function of the complementary angle \(\alpha\) between the electron beam and THz radiation (head-to-head interaction \(\alpha=0\).) \(\lambda_{w}\)=4.5 cm, \(\lambda_{THz}=15\) μm. \(\lambda_{X}=0.5\) nm, (**a**): \(L_{c}=2\)%, (**b**): \(L_{c}=4\)%, (**c**): \(L_{c}=6\)%.  
Data Availability Statement: All data are available and could be asked to the Authors. Conflicts of Interest: The authors declare no conflict of interest.