The backward mode in an electromagnetically pumped free electron lasers

A detailed analysis of the backward mode of an electromagnetic standing wave wiggler for FELs (free-electron lasers) is conducted. The analysis results in a clear understanding of the backward mode. It is indicated that the mode, for its absolute instability, should be suppressed in experiments for forward mode radiation to avoid beam disruption. On the other hand, it can be used as a self-excited high-power source with high efficiency in the low-frequency region and can saturate on a much higher level than the conventional forward mode