BUNCH COHERENCE IN PARAMETRIC X-RAY RADIATION

摘要

In radiation source using bunches of relativistic electrons, strong enhancement by a many-electron coherence effect occurs if the bunch is shorter than the emitted wavelength or if it is modulated with a period close to this wavelength. In the first case, the radiation intensity is proportional to the sc uare of the number Nb of electrons in the bunch, instead of being linear in N_b. Thus we have enhancement by a factor N_b. In the second case, one may consider that the "macro" bunch is made of M_b micro-bunches, each having n_b electrons (N_b = M_b · n_b). If the microbunches are shorter than the wavelength and longitudinally separated by λ/v (v is the beam velocity in units c = 1), we expect an enhancement factor n_b for each microbunch, times an enhancement factor M_b coming the interference between all the microbunches, thus an overall enhancement factor N_b again. In the general case, the amplitude per bunch is proportional to the Fourier transform of the spatial charge distribution of the bunch. Thus microbunching (MB) is a way of obtaining more radiation from a given beam current. It is a basic ingredient of free-electron lasers (FEL) at optical wavelenght and "stimulated amplification of spontaneous emission" (SASE) in the X-ray domain (see [1,2] and refs therein). Other potential applications are coherent transition radiation (CTR) in the microwave and X-ray (CXTR) regions, and possibly coherent radiation of particles channeled in crystalline and nanotube undulators.