Dispersive Nature of the FEL Amplifiers in the Whistler Mode

Abstract

The analytical formalism for whistler-pumped FEL amplifiers in collective Raman regime is developed. Compton regime (CR) has also feasible for the low-current gain; however, in practical, it does not exist for reasonable growth rate due to require of extremely high magnetic fields density (i.e., 10–15 T) to operate up to 200–250 GHz frequencies; hence, Raman regime plays an important role with the finite space charged mode only. The dispersive nature of the whistler-pumped FEL amplifiers is sensitive to frequency of electron cyclotron, plasma frequency, and magnetic fields of the amplifiers. The simultaneously of the pumped frequency with strong magnetic fields and plasma frequency should be synchronized for electron cyclotron frequency, which can rapidly increases the wiggler wave number to the radiation of amplification in the slow-whistler mode for high frequencies from millimeter to the sub-millimeter ranges. It is also clear that the background plasma should be lesser than the beam density for the charge neutralization and guiding of the signal into waveguides; hence, the plasma density can also improve the stability of high frequencies. In Raman regime, the growth rate is larger while it decreases as increases the frequency of operations and vice versa. The tapering of an axial field also plays a typical role to raise the efficiency as well as reduction in the length of interaction with axis.