Longitudinal Electric Field Effect on Multiharmonic Space Charge Wave Formation in Two-Stream Superheterodyne FEL

We performed the analysis of the axial electric field effect on the space charge wave (SCW) dynamics in multiharmonic klystron-type two-stream superheterodyne free- electron laser (TSFEL) in the framework of cubic nonlinear theory. We demonstrated that the multiharmonic SCW saturation level increases and the width of the frequency spectrum decreases in the case of a retarding electric field presence in TSFEL transit section. The saturation length also decreases in this case. The accelerating longitudinal electric field leads to the opposite effect: the width of the frequency spectrum of the multiharmonic SCW increases. We proposed the use of retarding electric fields in monochromatic TSFELs, which operate at the maximum gain mode. The use of accelerating electric field is preferable in multiharmonic TSFELs that are used to generate a powerful signal with a wide frequency spectrum.