The effect of a Bragg reflector on the spectral stability of the Twente Raman free-electron laser

The spectral distribution of the Twente Raman FEL has been studied as a function of the interaction length for an amplifier configuration. A stable spectrum was found for the minimum required interaction length necessary for the RF signal to be detected. Large variations in total emitted energy are observed however. With increasing interaction length the spectral distribution not only evolves but deviations are also observed, i.e. for some shots distributions are found which do not conform to the average distribution. For even longer interaction lengths the spectra can be grouped in a few different patterns. The influence of the feedback on the spectral distribution has been studied by changing the configuration to an oscillator using a Bragg reflector. For all settings investigated, the oscillator showed a more stable spectrum, i.e., less spread in total emitted energy as well as less spread in spectral distribution. For some settings operation on a single frequency in the Ka band was observed.