Designs for W-band free-electron masers

Theoretical analyses of high power W-band (i.e., ≈94 GHz) free-electron maser amplifiers are presented for a helical wiggler/cylindrical waveguide configuration using the three-dimensional slow-time-scale ARACHNE simulation code (Freund and Antonsen, 1996). The geometry treated by ARACHNE is that of an electron beam propagating through the cylindrical waveguide subject to a helical wiggler and an axial guide magnetic field. Two configurations are discussed. The first is the case of a reversed-guide field geometry where the guide field is oriented antiparallel to the helicity of the wiggler field. Using a 330 kV/20 A electron beam, efficiencies of the order of 7% are calculated with a full width at half maximum (FWHM) bandwidth of 5 GHz. The second example employs a strong guide field of 20 kG oriented parallel to the helicity of the wiggler. Here, efficiencies of greater than 8% are possible with a FWHM bandwidth of 4.5 GHz using a 300 kV/20 A electron beam. A normalized emittance of 95 mm mrad is assumed in both cases, and no beam losses are observed for either case. Both cases assume interaction with the fundamental TE₁₁ mode, which has acceptably low losses in the W band