# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 3 Discussion of Different Paradigms: Low Energy Sources Operating in Classical Regime
### (ii) Storage Ring + Laser in Cavity  
The second paradigm relies on Compact Storage Ring for the electron beam, colliding a mildly high charge electron bunch with a mJ-class laser pulse stored in an optical Fabry-Perot Cavity, at the large repetition rate of up to 25 MHz, (connected to an average beam current of 15 mA), with a focus of 50-150 \(\upmu\)m spot size at collision. This paradigm is at the basis of the most productive ICS actually existing [8; 9]. Figure 2 shows the typical scheme of a source based on it. The overall length is of order of ten meters with a surface size of about \(10^{2}\) m\({}^{2}\).  
An injection system drives the electron beam produced by a gun and accelerated by a short linac into a storage ring. The beam is necessarily coupled with a laser pulse in cavity in a region of zero dispersion. The laser in Fabry-Perot cavity is characterized by pulses carrying low energy in each passage in the cavity and produce therefore few X photons per interaction. The advantage of this paradigm is the high repetition rate connected to the storage of the electron beam in a ring, making it particularly suitable to the applications requiring high statistics. On the other hand, the beam instabilities developing  
Figure 2: The typical scheme of source based on Paradigm (ii). Size is about 100 m\({}^{2}\). Overall length about 12 m.  
in the propagation in the ring at low energies (50 MeV) impede the tight focusing of the beam at IP. Pulsed operation is also quite hard to implement. The best example of this category is CLS, which has been commercially developed and manufactured by Lyncean Technologies, Inc (Palo Alto, CA, USA). It was commissioned first at Palo Alto (named there CLS), while now is operating at Munich as MUCLS. Its best performances are a maximum photon flux of \(5\cdot 10^{10}\) with a maximum photon energy of 35 keV. At Palo Alto, insects and seeds have been first detected with a grating interferometer for differential phase-contrast imaging, producing contemporary absorption, phase contrast and dark field images [8]. Then, slices of mice lungs have been studied for diagnosing and mapping the pulmonar emphysema [77; 78]. Benchmark tests of X-ray diffraction data collection with hen egg white lysozyme, and successful high-resolution X-ray structure determination of the Glycine cleavage system protein H from Mycobacterium tuberculosis using diffraction data has also been done [79]. Great attention was also payed to the development of tomographic techniques [80]. Then the device was shifted at Munich, where, as MUCLS, was operated for producing in-vivo images. Pig chests [81] or mouse lungs [82] have been detected, paving the road to human lung in-vivo observation [83]. X ray absorption spectroscopy of a Ag foil has been performed [84], developing an ad hoc technique which exploits the large repetition rate of the source. Demonstration of more sophisticated imaging techniques have also been performed [85], as well as the microbeam radiation therapy [86]. Another example of storage ring based ICS is ThomX [28], in commissioning at Orsay-LAL by a collaboration IN2P3-Universite' de Paris Sud, that will be dedicated to applications in the field of the conservation of the cultural heritage in cooperation with the Museum Louvre. Table 2 summarizes typical parameters of these sources.