# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 1 Introduction  
Thomson/Compton Sources generate energetic and collimated photons through the homonym back-scattering process taking place between high brightness electron beams and high intensity radiation pulses. They basically consist in miniature electron-photon colliders and exploit the large blue-shift imparted to the pump input photons by double relativistic Doppler effect for promoting the photon energy to the X-ray range of about \(E_{X}=4\gamma^{2}E_{L}\) /(1 + \(\gamma^{2}\theta^{2}\)), where \(\theta\) is the observer angle, \(E_{L}\) is the input pump photon energy delivered in most cases by a laser and \(\gamma\) the electron Lorentz factor. When the parameter \(4E_{L}/mc^{2}<<1\), the recoil of the electrons in their own rest frame is unessential. In this case, the Compton elastic scattering can be treated within the classical electrodynamics, neglecting quantum effects. The production of Compton radiation falls in the examples of interaction of charged particles with external electromagnetic fields, like synchrotron or undulator radiation. In this classical case, the Inverse Compton Sources (ICSs) should be referred to as Thomson Sources. When the input pump photons are in the optical range, this regime occurs typically for electron energies below about 100 MeV (\(\gamma\simeq 200\)), with X-ray output energies lower than 200 keV. However, following the usual lexicon adopted in the literature, we will use ICS (Inverse Compton Source) for both regimes, even if the device operates in the classical range. Such an useful and promising \"photon accelerator\" scheme is unfortunately limited by the low efficiency of the back-scattering process in the transformation of the laser photons into X-ray photons. The cross section of Thomson/Compton back-scattering is indeed very small, about 6\(\cdot 10^{-29}\) m\({}^{2}\), so a high luminosity collider is needed for generating intense X-ray beams: the luminosity needed to achieve an operating regime where almost each electron converts to X-ray at least one laser pump photon should exceed the one of large colliders, like LHC! This entails the necessity to drive in the interaction region extremely dense laser and electron beams. The production of intense laser pulses and high brightness electron beams with these tight requirements is now state of the art. In ICS sources, therefore, the most important challenge is to synchronize the electron-photon collisions in a controllable and stable manner, in order to deliver to the users tunable, reliable and stable X-ray beams, as it is routinely achieved in the operation of synchrotron light sources. Synchrotrons deliver to users a flux whose intensity ranges between \(10^{14}\) and \(10^{19}\) photons/(m\({}^{2}\)s 0.1% bw) with a spatial coherence length of 1-50 \(\upmu\)m. The comparison in performances between various kinds of X-rays sources [1] shows thatmicrofocus X-ray tubes deliver 2\(\cdot\)10\({}^{2}\)-2\(\cdot\)10\({}^{6}\) photons/(m\({}^{2}\)s 0.1% bw) with a spatial coherence length of 10\({}^{-1}\)-5 \(\upmu\)m, while metal jet X-ray tubes produce 5\(\cdot\)10\({}^{5}\)-2\(\cdot\)10\({}^{8}\) photons/(m\({}^{2}\)s 0.1% bw) with a corresponding spatial coherence length of 4\(\cdot\)10\({}^{-1}\)-5 \(\upmu\)m. A larger flux between 10\({}^{5}\) and 10\({}^{9}\) photons/(m\({}^{2}\)s 0.1% bw) is achieved with static anode X-ray tubes, while the rotating anode X-ray tubes provide 10\({}^{6}\)-10\({}^{10}\) photons/(m\({}^{2}\)s 0.1% bw), both with 2 \(\cdot\) 10\({}^{-3}\)-10\({}^{-1}\)\(\upmu\)m of spatial coherence length. Inverse Compton Scattering sources generate a flux with intensity from 10\({}^{10}\) to 10\({}^{13}\) photons/(m\({}^{2}\)s 0.1% bw), intermediate between tubes and synchrotron sources, with the largest spatial coherence lengths of 5-100 \(\upmu\)m. Respect to these last huge facilities, ICS sources guaranty high energetic sustainability and compact footprint, permitting the lowest cost per single X-ray photon.