# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 3 Discussion of Different Paradigms: Low Energy Sources Operating in Classical Regime
### (iii) Superconducting Linac + Laser in Cavity  
This paradigm relies on Super-Conducting RF Photo-Injector delivering a low charge electron bunch (tens of pC) at a very large repetition rate (up to 100 MHz), colliding with a mJ-class laser in cavity. The laser pulse, stored in an optical Fabry-Perot Cavity, with up to 1 MW stored laser power, is focused to 20-30 \(\upmu\)m spot size at collision. Linacs based on superconducting cavities can present the extremely interesting option of recovering the electron energy in a stage downstream the ICS interaction point. While several accelerators based on SuperConducting Linacs (with or without the energy recovery) are already operating and other projects are in various stages of commissioning, Compton sources driven by SC linacs are so far not yet completed, apart for some experimental tests of X-ray production made by cERL at KEK [31]. There are however several projects, estimating ICS performances. The achieved maximum photon energy is 200 keV. The maximum achievable fluxes is estimated to be 3.5\(\cdot 10^{12}\) photons/s without energy recovery with an average electron beam current 1 mA, while in excess of an impressive \(10^{15}\) with energy recovery at an average electron current of about 100 mA. Proposals based on this paradigm are UH-FLUX [33] in development in UK and presenting the option of the energy recovery, Cbeta [29], bERLinPro [34], CLXS [32], an ongoing project at MIT (without energy recovery), BriXS [37] based on two injectors similar to the original scheme by [PERSON] [87] and BriXSinO [38], that is instead a demonstrator for BriXS and is in the building phase at LASA INFN Laboratory at Milan. Combining the typical operation made with a linac, where the beam is dumped after the collision at IP, with the high repetition rate typical \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**ICS** & **Electron** & **\(\mathbf{\lambda_{las}}\) (\(\upmu\)m)** & **Laser Energy** & **X Energy** & **X Flux** & **Bandwidth** & **Rep. Rate** (Hz)** \\ \hline CLS [8] & 25–45 & 1 & 5 & 8–42 & 5\(\cdot 10^{2}\)–5\(\cdot 10^{3}\) & 3\% & 65\(\cdot 10^{6}\) \\ MUCLS [9] & 25–45 & 1 & 5 & 8–42 & 5\(\cdot 10^{2}\)–5\(\cdot 10^{3}\) & 3–5\% & 65\(\cdot 10^{6}\) \\ ThomX [28] & 50 & 1.03 & 1.4–2.6\(\cdot 10^{-3}\) & 45 & 10\({}^{5}\) & 10\% & 65\(\cdot 10^{6}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Storage ring+ laser in Fabry-Pérot cavity. The data of CLS and MUCLS are measured, the ThomX data are estimated. of a storage ring (from tens to hundreds MHz) constitutes the appealing characteristic of paradigm (iii), This is possible thanks to the operation with the super-conducting RF cavities accelerating the electron beam. In case of energy recovery, the average electron beam current can arrive to exceed 20 mA, a value representing the maximum level for storage rings of paradigm (ii) when operated at low electron energy (about 50 MeV typically for an ICS, compared to 1-5 GeV of synchrotrons), approaching 100 mA, comparable to synchrotron light sources. For this paradigm one challenge is the electron beam injector, in particular the reachable average electron beam current associated the overall good beam quality, such as low emittance and energy spread. As an example of paradigm (iii), Figure 3 shows a rendering with the scheme of BriXSinO. After the injector, a dogleg introduces an offset between the photocathode and the superconducting cavities. Downstream the linac, a ring brings back the electron beam to the end of the linac for the second passage. The arc, constituted by a suitable number of Double Bending Achromats (DBA) is also able to compress the electron beam, a feature useful in the FEL operation. The zero dispersion zones between the achromats host insertion devices. In one branch a FEL oscillator delivering Terahertzs radiation is foreseen, in another one the ICS takes place. A Laser in Fabry-Perot cavity will permit one or two color operation [60]. The line is equipped with focusing systems and diagnostics.