RF Parasitic Oscillations in High Power FELs

Summary form only given. The FEL dispersion equation always has two solutions. Radiation at the low frequency (RF) is always excited concurrently with the desired high frequency (IR) radiation. At high beam current levels the low frequency radiation may develop into parasitic oscillation, that would accelerate/decelerate the e-beam bunches in the wiggler, and disrupt the normal FEL lasing. Such oscillations can take place whenever there is a feedback mechanism at the RF frequency. This can be caused by reflections at apertures along the e-beam line, or when the FEL design parameters permit lower frequency (fundamental or harmonic) solutions that correspond to "backward wave interaction" or near cut-off conditions of the e-beam tube. In these cases absolute instability parasitic oscillations will always develop above some threshold beam current even in a smooth waveguide. While in (Steinberg et al., 1986) we have studied this kind of oscillation processes in the context of a continuous e-beam, the present design of high power FELs requires analysis for a continuous train of intense e-beam bunches (tp~pSec). Since the emitted radiation satisfies the Superradiance (Gover, 2005) (Coherent Synchrotron) emission condition (omega t_p <<2pi), the radiative emission processes will be substantially enhanced. A new theoretical analysis and numerical simulations will be presented for quantifying this phenomena and setting conditions and design criteria to evade them.