# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 1 Introduction
Tens of devices of this kind have been commissioned in the last twenty-five years [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12], several of them operating nowadays as test and user facilities. Most of the existing Compton devices are actually devoted either to experiments on the radiation characterization [13; 14; 15; 16; 17; 10], test of imaging techniques [18; 19; 20; 21] or to radiological applications on biological, animal and human samples [6; 22; 23; 24; 25]. \(\upgamma\) rays have been detected at [26] and routinely produced at the Hi\(\gamma\)s facility [12]. Moreover, several advanced projects have been developed in the last years [27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38]. An additional advantage of Compton sources is the large degree of freedom due to the operation with laser pulses, instead of magnetic elements. The optical technology of pulse manipulation permits in principle to adapt the laser signal to the needs of the users, stimulating at the same time various theoretical research lines [39; 40; 41; 42; 43; 44; 45; 46; 47]. The versatility in shaping the frequency laser pulse distribution has been exploited for compensating the non-linear spectral deterioration with chirping [42; 43; 44] or with polarization gating, for mitigating the bandwidth broadening due to energy spread by chirping [46; 47], for the control of higher harmonics [48; 49], while a temporal modulated electron bunch has been shown to produce coherent emission [50]. Proposals for using the Compton light as diagnostics for the electron beam are also ongoing [51; 52]. Regarding the operational possibilities, different possible schemes there exist. Since accelerated electrons can be obtained with storage ring as well as in warm or energy recovery linacs, and, on the other hand, there are two categories for high power lasers, resonators and chirped pulse amplification (CPA), the ICS sources can be grouped in few main paradigms: (i) RF Photo-injector producing a high charge electron bunch against a \(J\)-class laser pulse, strongly focused, running collisions at low repetition rate [2; 5; 6; 7; 27; 35]. (ii) Moderately high charge electron beam, generated at high repetition rate in a Compact Storage Ring with a \(mJ\)-class laser pulse stored in an optical Fabry-Perot Cavity, tightly focused [8; 9; 26; 28; 30; 53]. (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 [29; 33; 34; 37; 38]. In addition, several exotic schemes have been conceived or implemented, including plasma accelerated beams for ultra-compact devices [11; 54], Free-Electron lasers [12; 55] or Terahertz radiation [56] as input pump instead of lasers, sophisticate or extreme laser structures allowing coherent radiation generation or two frequency operation [57; 58; 59; 60]. In this paper, after a section where we discuss generalities on the Compton interaction, we will analyse the main paradigms of ICS sources, operating either in the classical or in the quantum regime; then we will give an overview of the state of the art, going into details in few specific examples.