• S.G. Oganesyan, G.S. Oganesyan
  LT CSC, Yerevan

We have studied dispersion properties of an electron beam in FEL. The case of a long undulator and small electron density was analyzed on the basis of the Klein-Gordon and wave equations. We have obtained that the FEL operating bandwidth is directly proportional both to the electromagnetic pulse carried frequency and the electron beam relative energy spread width. That made it possible to introduce the definition of a long pulse (its spectral width is less then operating bandwidth) and a short pulse (on the contrary). We have studied the long pulse phase and group velocity when the carried frequency wave is under maximum amplification and maximum absorption. It has been discovered that in both regimes the phase velocity exceeds the light velocity in a vacuum, i.e., c. Hence the beam is like plasma. We find that the pulse group velocity however depends on the FEL regime. In the amplification regime its quantity exceeds the c, while in the absorption one it is less then the c. From viewpoint of this result the electron beam is already similar to a resonant atomic medium.

• S.G. Oganesyan, G.S. Oganesyan
  LT CSC, Yerevan

We have studied propagation of a Gaussian electromagnetic pulse through a long FEL. Our analysis is based on the Maxwell and Klein-Gordon equations and takes into account both the active and dispersive properties of an electron beam. The laser output in (a) linear and (b) nonlinear complex phase regimes has been considered. We obtained expressions for FEL operative bandwidth and characteristic time, and introduced conception of long and short pulses. We have discovered a stabilization effect in the case (a). Namely, short pulses with random initial lengths have the same ones after coupling. The Rayleigh regions caused by dispersion and amplification (absorption) are obtained for long pulse in the case (b). If the carried frequency is under maximum gaining, then one observes only the pulse expansion. The most interesting effects have been discovered in the absorption regime. Here the FEL can operate as a slight compressor or as an explosive stretcher. Besides, FEL device makes it possible the chirping of pulses. Thus FEL might be employed for controlling over superintense ultra short pulses, which damage ordinary media