# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 5 Conclusions  
Inverse Compton scattering sources deliver sequences of picosecond long X or gamma-ray radiation pulses. The polarization of the emitted pulses can be linear, circular or elliptical, and can be completely controlled. The radiation is temporally shorter than synchrotron light sources, with a small and round size in the range of 10-80 \(\upmu\)m, allowing to exploit in phase contrast techniques the edge enhancement effect associated to the transverse coherence of the radiation beam. This is one of the key issues for advanced radiological imaging applications. Three main paradigms have been described: (i) RF Photo-injector generating high charge electron beams scattering a high energy density laser pulse, driving collisions at low repetition rate. (ii) Moderately high charge electron beam, generated at high repetition rate in Compact Storage Rings with m)-class laser pulses stored in an optical Fabry-Perot Cavity, tightly focused. (iii) Low charge electron bunch delivered at very high repetition rate by a Super-Conducting RF Photo-Injector colliding with a mJ-class laser pulse stored in an optical Fabry-Perot Cavity (up to 1 MW stored laser power), focused at collision. The advantages of paradigm (i) are that it is possible to focus the electron beam below 20-30 \(\upmu\)m, due to the small beam emittance. Main drawback of paradigm (i) is the limitation in the repetition rate achievable, not larger than 100 Hz, in the present state of the art. Analogous limitations hold for the laser pulse, except for the possibility, not yet routinely exploited, of the use of a suitable recirculator. The electron energy, on the other hand, can be easily enhanced just by adding more RF accelerating structures allowing to exceed 20 MeV of gamma photon energy. The advantage of paradigm (ii) is the high repetition rate allowed by storing the electron beam in a ring. Limitations in focusing, mainly due to beam instabilities arising at such low energies (50 MeV), are intrinsic in this device. Energy upgrade is also an issue, since it involves the complete re-design of the machine. The implementation of pulsed operation is also difficult. The appealing characteristic of paradigm (iii) is the possibility to merge the operations typical of linacs such as dumping the beam after each collision at IP, with the high repetition rate typical of a storage ring (tens up to hundreds of MHz) thanks to the use of super-conducting RF cavities used to accelerate the electron beam. Another important advantage is the large sustainability permitted by the option of the energy recovery. The aim is to reach the largest radiation flux, with the lowest cost per photon. All the ICS projects has been conceived in the context of a broad spectrum inter-disciplinary large research infrastructure aiming at delivering advanced beams of electrons, photons, neutrons and positrons for basic and applied physics. Conceptualization, I.D., M.R., L.S. and V.P. ; methodology, all authors; investigation, all authors; writing--original draft preparation, L.S. and V.P. ; 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.