Experimental study of the spatial evolution of the spectrum of a Raman FEL

For a specific set of parameters, corresponding to an operating point near magneto-resonance, the evolution of the spectrum generated by a Raman FEL was investigated. It is shown that in several frequency bands radiation is emitted. These different frequencies are identified as belonging to the FEL instability and another mechanism, probably the gyrotron instability. The measurements show apparently a competition between these mechanisms. The experimentally determined growth rate is somewhat larger than the theoretical one, indicating that the FEL does not operate as a true amplifier.