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Paper   IPM / Physic / 6654
School of Physics
  Title:   Ecitation of EH Mode in A Free Electron Laser with A Partially Filled Waveguide
1.  B. Maraghechi
2.  S. Mirzanejhad
  Status:   Published
  Journal: Physica Scripta
  No.:  96
  Vol.:  T75
  Year:  1998
  Pages:   229-231
  Supported by:  IPM
A three-dimensional theory for a free electron laser with a positive or reversed axial-guide magnetic field in the collective regime is presented. A relativistic electron beam partially fills a cylindrical and metallic waveguide. A one-dimensional helical magnetic wiggler field, in the beam frame, decays into a forward-scattered space-charge wave and a backscattered EH waveguide mode. With relativistic mass correction in the beam frame a nonlinear wave equation is derived which describes the interaction between the wiggler field and the excited waves. Dispersion characteristics of the eignmodes are studied. A formula for the lab-frame spatial growth rate of the parametrically excited waves is derived for a positive and reversed axial-guide magnetic field. The growth rate for the reversed axial-guide magnetic field was found to be around one order of magnitude smaller than for the positive field. Figures are presented to show the variation of the growth rate with axial-guide field and the ratio of the beam radius to the waveguide radius.

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