Planar Slow-Wave Structure With Parasitic Mode Control

A planar sheath-like slow-wave structure with rectangular geometry is considered for use in a traveling-wave amplifier driven by a sheet electron beam. The sheath like structure provides a low-dispersion operating mode and hence potential for broad bandwidth. However, due to the large transverse dimension of the structure multiple backward-wave modes can interact with the beam. Both the operating mode and the parasitic modes are analyzed using field theories with the planar sheath approximation. These solutions are then compared with finite element computations. Suppression of backward waves is then considered by designing the structure to preferentially absorb these waves. The results show a good control of mode competition and high-primary mode gain.