• H.K. Avetissian, S.S. Israelyan, G.F. Mkrtchian, Kh.V. Sedrakian
  YSU, Yerevan

If in the spontaneous radiation processes of charged particles the medium with refractive index n>1 plays only a role of the third body, the presence of a medium at the interaction with external electromagnetic wave causes the new threshold phenomena which have an essentially nonlinear character. In the present work the case of strong laser field is considered within the scope of the relativistic quantum theory for electron-laser interaction in a medium. Using Strong Coupled State approximation the spectrum of eigenvalues of particle energy (in the rest frame of the wave) and the wave function is found. Due to the nonlinear resonant interaction with given laser field the equidistant levels of longitudinal motion are formed and as a consequence the spontaneous transitions from the upper levels take place with the emission of corresponding quanta. In the scope of Quantum Electrodynamics the spectral intensity of laser-assisted Cherenkov radiation is obtained. The first-order Feynman diagram is calculated where the electron lines correspond to the wave functions in the strong laser field and the resonant interaction is discussed.

• H.K. Avetissian, G.F. Mkrtchian
  YSU, Yerevan

For the X-ray FEL if the amplifying photon energy is larger or comparable to resonance widths caused by an electron beam spreads or by the finite interaction length then the quantum effects may play a significant role [1] and the generation process shifts from the classical to the quantum regime. The recent advancement of high brightness particle and laser beams technology makes achievable the fulfillment of these conditions in the scheme of X-ray Backscatter Compton laser. In this work we consider in general the SASE X-ray FEL in the quantum regime of amplification arising from the self-consistent set of the Maxwell and relativistic quantum kinetic equations. It is shown a considerable increase in start-up and narrowing of the spectrum of the SASE power for amplifying X-ray frequencies compared with the classical SASE regime.

[1] H.K. Avetissian, G.F. Mkrtchian, Phys. Rev. E 65, 046505 (2002).