Superradiance in the high-gain free-electron laser

In this paper we describe the effects of slippage on the single-pass high-gain free-electron laser (FEL) amplifier. We use a one-dimensional computational code to show the existence of two new dynamical regimes characterized by a dimensionless parameter K, which is a measure of the slippage in one gain length. We define the long-pulse limit to be when K{}1 or the electron pulse length Lₑ is much greater than a properly defined ``cooperation length'' Lc (Lₑ{}Lc). In this case we find that only the leading region of the propagating radiation pulse exhibits the usual steady-state behavior, with peak power proportional to nₑ4/3 (where nₑ is the electron-beam density). The trailing (slippage) region exhibits a spiking behavior with peak intensities reaching many times the saturated intensity predicted by steady-state theory. We define the short-pulse regime to be when K{}1 (Lₑ{}Lc). In this regime the peak power emitted by the electrons does not scale as nₑ4/3, as predicted by steady-state theory, but scales as nₑ², which is typical of superradiant behavior. Fur- thermore, energy is extracted from the electrons in a continuous way, with no steady-state synchro- tron oscillatory-type behavior.