Decoherence in a chirped-pulse free-electron maser

Summary form only given. Theoretical studies (Gover et al., 1994, Hartemann et al., 1994, 1996) have shown that the relativistic subpicosecond electron bunches produced by high-brightness RF photoinjectors can be used to generate coherent synchrotron radiation in a free-electron maser (FEM). Furthermore, in a waveguide structure, the group velocity of the radiation can be matched to the axial bunch velocity in the helical wiggler, thus minimizing slippage in the device and yielding extremely short pulse of coherent millimeter-wave radiation, with instantaneous bandwidths exceeding 50%. Because of group velocity dispersion, the output pulses are chirped. The transition from coherent to incoherent synchrotron radiation in the FEM has first been studied within the context of a relativistic fluid model (Hartemann et al., 1994, 1996); we will discuss the limitations of such a model, and propose a new hybrid model capable of describing the electron bunch behavior both in the electron fluid and stochastic gas limits. In particular, there is a continuous transition from the exponential coherence factor of the fluid model, N/sub /spl theta///sup 2/ exp[-((/spl omega//spl Delta/z)/(2/spl beta//sub "/c))/sup 2/]. To the incoherent radiation scaling, N/sub e/. Here, N/sub e/ is the number of electrons in the bunch, /spl omega/ is the Doppler-shifted radiation frequency, /spl Delta/z is the bunch duration, and /spl beta//sub "/ is its axial velocity. This will be discussed both in the context of fast-wave FEMs, and for short wavelength, laser-driven Compton scattering sources.