Design of a low-voltage annular beam Ku-band free-electron laser

A nonlinear analysis of an annular beam free-electron laser (FEL) with a helical wiggler and axial guide field is presented. An annular beam has the advantage over a solid beam of reduced dc self-fields, facilitating beam transport in short period wigglers. The annular beam geometry also permits use of a central structure to enhance the wiggler field, which tends to decrease as the period is decreased. Two configurations of the waveguide and wiggler were theoretically investigated. First, an annular beam interacting with a TE₁₁ cylindrical waveguide mode in the presence of a helical wiggler was studied. Next, the interaction with a coaxial TE₁₁ mode in the presence a novel coaxial wiggler with both inner and outer bifilar helical current windings was examined. The beam under consideration is nominally 55 kV and 5 A and the wiggler period is 0.9 cm. An axial guide field up to 4 kG was used. The beam and wiggler parameters correspond to grazing incidence with the TE₁₃ mode in Ku-band. Nonlinear slow-time-scale simulations show that for an ideal beam, efficiencies greater than 10% can be achieved with instantaneous bandwidths in excess of 20%. The effect of axial energy spread on interaction efficiency is examined