OPTICAL SPIKES AND SUPERRADIANT PULSE-PROPAGATION IN A FREE-ELECTRON LASER

With one-dimensional nonlinear pulse propagation equations, we show numerically that the spiking behavior of free-electron lasers can be described by the superradiant pulse evolution. Two distinct dynamical regimes are defined by the characteristic length scales; slippage length (L(s)) and slippage distance (L(syn)) during one synchrotron period. In the superradiant regime (L(syn) > L(s)), the superradiant pulse (FWHM congruent-to 1/2L(Syn)) is generated in the trailing slippage region of the electron pulse. In the spiking regime (L(syn) less-than-or-equal-to L(s)), the continuous pulse modulation (FWHM congruent-to 1/2L(syn)) and the long-wavelength sidebands are observed. To compare with the experimental results, the realistic axial current profile and the shot noise are considered.