MODE SELECTION IN A WAVE-GUIDE FEL

The use of waveguides in low voltage FELs introduces many new features, among which, that Of two Positive gain peaks is of most interest. One of the problems encountered in designing a waveguide FEL oscillator is to suppress the lower frequency peak, which may have a higher gain than the more useful higher frequency peak. We propose a technique using a dispersive wiggler to select a desired peak from the various participating cavity modes. The principle of this cavity mode selection technique is discussed, and its feasibility tested with a simple example. It is shown that this technique can have many advantages over alternative techniques.