COUPLING BETWEEN 2 RESONANT WAVES IN A WAVE-GUIDE FREE-ELECTRON LASER

The coupling between two resonant branches of a waveguide free-electro n laser is studied. It is shown that the induced electron energy sprea d, which is produced along with the bunching by the radiation, strongl y influences this coupling. In a low-gain oscillator, the growth of a lower frequency branch results in large induced energy spread, which s tops the development of the upper frequency branch. On the other hand, in a high-gain amplifier, a strong signal can be obtained at high fre quency by injecting a signal at low frequency that bunches the electro n beam without large induced energy spread. In this way, the contradic tion about the nonlinear interaction of two resonant waves in a wavegu ide free-electron laser is explained.