Free‐electron laser operation in a sheath helix configuration

A sheath helix loaded waveguide supports a slow electromagnetic wave which can act as a wiggler for a millimeter‐wave free‐electron laser (FEL). A nonlocal theory of FEL interaction in the Compton regime reveals that the radiation frequency and growth rate are sensitive functions of the phase velocity of the pump (wiggler) wave and increase with increasing pump frequency. The FEL radiation frequency can be tuned by an additional parameter, viz., the pitch angle of the helix in this configuration.