# State of the Art of High-Flux Compton/Thomson X-rays Sources
## 3 Discussion of Different Paradigms: Low Energy Sources Operating in Classical Regime  
As a reference value for applications in the medical field, radiological images are so far currently performed by Roentgen X-ray tubes with fluxes from at least \(10^{11}\) up to \(10^{12}\) X-ray photons/\(s\), with the typical broad-band spectrum (\(bw>40\%\)) of the bremstrahlung radiation. The objective for Compton devices voted to imaging experiments is to achieve similar fluxes but with the quasi-monochromatic X-ray spectrum (\(bw\simeq 10\%\)) of the ICSs. Nuclear photonics applications, on the other hand, require photon energies in the \(MeV\)range and relative bandwidths of the order of few \(10^{-3}\). The basic criteria for reaching such large fluxes and high spectral densities can be deduced from Equations (3) and (4). We can distinguish the ICS configurations in two classes according to the value of \(f_{RF}\), namely \(f_{RF}\) up to 100 Hz, as occurring in warm linacs, and \(f_{RF}\) of the order or larger than 1 MHz as in storage rings or Energy Recovery Linacs (ERLs). In the first case of low repetition rates, the flux should be maximized by increasing the number of electrons, namely the charge, the number of photons, i.e. the laser energy, and by focusing tightly the beams. In this case, it is clearly evident the need of high brightness electron beams, carrying high charge bunches at low emittance and energy spread, combined with lasers delivering high energy, picosecond long pulses with small bandwidth and high mode purity (\(M^{2}\simeq 1\)). In this way, the X-ray bandwidth will take advantage of the beam quality and could reach particularly low values (relative values down to \(5\times 10^{-3}\)), thus making this paradigm suitable even for nuclear photonic applications. The former requirements point clearly to electron accelerators similar to those projected for alimenting Free Electron Lasers. If the repetition rate is larger by a factor \(10^{6}\) with respect to the previous case of warm linacs, large photon fluxes can be reached also if the single shot electron number is not as large as the case before, if the pump energy is low or the focusing is not so strict. This latter situation requires laser systems capable of high repetition rate and large average power, combined to short pulses and mode-locking to assure synchronization at the sub-picosecond level between the electron beam and the laser pulse at the Interaction Point. The large repetition rate can be obtained by electron beams generated both with storage ring or Super-Conducting (SC) linacs, with or without energy recovery, colliding with laser pulses stored in Fabry-Perot cavities. Electron beams in storage rings are characterized by unexceptional quality, limiting the bandwidth narrowness and the spectral density that can be achieved. Super-Conducting linacs, on the other hand, generate low charge beamlets. In both cases, the focusing of the laser pulse is dominated by the cavity geometry and the spot in the IP could be not so reduced as in free space.  
Presently there are therefore three main paradigms for high performance ICS working in a low energy regime (electron energy less than 100 MeV):