SPECTRAL DYNAMICS OF A FREE-ELECTRON MASER WITH A STEP-TAPERED UNDULATOR

The spectral behavior of a high-power, high-gain free-electron maser ( FEM) is investigated. The maser has a step-tapered undulator consistin g of two sections with different strengths and lengths and equal perio dicities. The sections are separated by a field-free gap. The configur ation is enclosed within a low quality cavity. The millimeter wave bea m is guided within a rectangular corrugated waveguide. The purpose of this undulator setup is to enhance the efficiency at high output power . The associated high gain in the Linear as well as in the nonlinear r egime provides a unique oscillator. The spectral dynamics of this devi ce is analyzed with a multipass, multifrequency code. The radiation fi eld of the code is described as a sum over discrete frequency componen ts. The linear gain curve of the step-tapered undulator is not the sum of the curves of two single undulators and has a completely different spectrum. The gain of the FEM is so high that nonlinear interaction o ccurs within a few passes. In the fully nonlinear regime the gain is s till relatively high. The power spectrum evolves towards a state in wh ich the power at the resonance of the second undulator section is supp ressed. In the final state, where the frequency spectrum hardly change s from pass to pass, the power spectrum exhibits two peaks at frequenc ies that are determined by the first section of the undulator. The mai n peak is related to its resonance frequency, while the second peak is a lower sideband. The dependence of the sideband on the gap length, t he relative polarization of both sections, and the reflection coeffici ent is investigated.