# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 3 Discussion of Different Paradigms: Low Energy Sources Operating in Classical Regime
### (i) Warm Linac + Single Shot Laser  
The first group of ICSs is constituted by RF Photo-injector producing a high charge up to 1-2 nC electron bunch against a \(J\)-class laser pulse delivered by an amplified laser system (Ti:Sa, Yb:Yag or CO\({}_{2}\)), colliding at a repetition rate of 1-100 Hz, with the beams tightly focused down to 10-40 \(\upmu\)m. The possibility of reaching such small spot sizes is due to the low beam emittance and energy spread achievable thanks to high gradient RF fields in the accelerator and to the fact that the electron beam is not recirculated but dumped after each collision. The same holds for the laser operated in single shot pulses, with energy values up to 5-7 J. The main drawback of this archetype is the low repetition rate achievable, approaching 100 Hz in the present state of the art, limited by both RF and by collision laser capabilities. Laser recirculators, that could enhance the repetition rate by a factor 30-40 [69], has been developed but are still to be diffused and routinely used. The enhancement to 40 Hz of the low repetition rate CO\({}_{2}\) laser has been performed at BNL [70]. This paradigm has been widely explored theoretically and exploited experimentally with tens of sources commissioned or still operating and tens of projects. After first pioneering attempts [3], best examples of this model are ATF at BNL [2], a pivotal ICS with more than tens years of operation and applications spread in all fields; Sparc [5], commissioned at LNF-Frascati; TTX [7] at Tsinghua and Aist [6] in Japan, infrastructures with a long cursus of experiments and currently still working; as well as STAR (Southern Europe Thomson source for Applied Research) [35], a dedicated user facility in construction at the University of Calabria (Italy). Figure 1 shows the typical scheme of such machines. In particular, the figure refers to the STAR source [36]. The device is hosted into a 2.5 m wall bunker and consists of an RF S-band photo-injector, which generates 5 MeV electron bunches, at 100 Hz repetition rate; two electron lines, low energy and high energy, alimented by an S-band linear accelerator (LINAC) which accelerates the electron bunches up to 60 MeV, both driven by a 55 MW RF pulsed modulator; focusing devices (quadrupoles and steering magnets); diagnostic devices (beam current monitors, beam position monitors, YAG screens); two lasers (one UV photocathode laser and a IR interaction laser), hosted into a dedicated clean room. A dogleg introduces an offset between the linac and the interaction point rerouting the background due to the dark current. The quasi-monochromatic polarized radiation will be tunable with continuity from 10 to 140 keV, just increasing the electron energy. At the phase I of the project, an X-ray flux of about 10\({}^{9}\) photons per second within a 10% bandwidth is expected, with picosecond-class pulse length. In Table 1 the electron beam and laser characteristics, the photon energy and the fluxes achieved by several of these devices (measured or predicted) are summarized. In the hard X-ray range, in situations where the photon energy is roughly between ten and few hundreds keV, 10\({}^{5}\)-10\({}^{8}\) photons per shot are generated, a number that can mean 10\({}^{7}\)-10\({}^{10}\) photons/s, taking into account the repetition rate of the scattering. Since this paradigm can provide intense single radiation shots not affected by beam fluctuations, it is particularly suitable for studying the radiation characteristics. Much of the run time of these machines has been dedicated to these tests. The energy-angle correlation, described in Equation (1), has been validated at HDMZ by shifting horizontally the detector [13], using