AXIAL FREE-ELECTRON LASER INTERACTION BETWEEN AN ANNULAR ELECTRON-BEAM AND AN AXISYMMETRICAL TM MODE

Free-electron lasers (FEL's) operating with helical wiggler fields and axial guide fields are subject to orbit instability which causes addi tional particle loss as saturation is reached. A new type of FEL inter action possible between an annular electron beam and a TM(om) mode is described here. This interaction is between the axial electron motion and an RF mode that is radially ''wiggled'' a in periodically rippled waveguide. The net transverse force integrated over a wiggle period on each of the particles in this type of device can be made to vanish by choosing the proper waveguide radius. The resulting particle orbits a re more stable than those in conventional FEL's with helical wiggler f ields.