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Title: Unstable coupled-mode structures in a one-dimensional Raman free-electron laser

Abstract

The dispersion relation of a free-electron laser (FEL) with a one-dimensional helical wiggler and an axial guide magnetic field, in the collective regime, is used to study interactions among all possible waves. In group II orbits, with relatively large wiggler induced velocities, new couplings between the negative and positive-energy space-charge waves as well as between the right and left circularly polarized electromagnetic waves are found. These instabilities are found to be distinct from the usual FEL resonance. The space-charge wave was not found to be intrinsically unstable when it couples to the electromagnetic wave and the coupling to the radiation was found to be the only way that makes it unstable.

Authors:
; ;  [1];  [2]
  1. Department of Physics, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran (Iran, Islamic Republic of)
  2. (Iran, Islamic Republic of)
Publication Date:
OSTI Identifier:
20975002
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2731318; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; DISPERSION RELATIONS; ELECTROMAGNETIC RADIATION; FREE ELECTRON LASERS; INSTABILITY; INTERACTIONS; MAGNETIC FIELDS; ONE-DIMENSIONAL CALCULATIONS; RESONANCE; SPACE CHARGE

Citation Formats

Mohsenpour, T., Maraghechi, B., Mirzanejhad, S., and Department of Physics, Faculty of Basic Sciences, Mazandaran University, Babolsar. Unstable coupled-mode structures in a one-dimensional Raman free-electron laser. United States: N. p., 2007. Web. doi:10.1063/1.2731318.
Mohsenpour, T., Maraghechi, B., Mirzanejhad, S., & Department of Physics, Faculty of Basic Sciences, Mazandaran University, Babolsar. Unstable coupled-mode structures in a one-dimensional Raman free-electron laser. United States. doi:10.1063/1.2731318.
Mohsenpour, T., Maraghechi, B., Mirzanejhad, S., and Department of Physics, Faculty of Basic Sciences, Mazandaran University, Babolsar. Tue . "Unstable coupled-mode structures in a one-dimensional Raman free-electron laser". United States. doi:10.1063/1.2731318.
@article{osti_20975002,
title = {Unstable coupled-mode structures in a one-dimensional Raman free-electron laser},
author = {Mohsenpour, T. and Maraghechi, B. and Mirzanejhad, S. and Department of Physics, Faculty of Basic Sciences, Mazandaran University, Babolsar},
abstractNote = {The dispersion relation of a free-electron laser (FEL) with a one-dimensional helical wiggler and an axial guide magnetic field, in the collective regime, is used to study interactions among all possible waves. In group II orbits, with relatively large wiggler induced velocities, new couplings between the negative and positive-energy space-charge waves as well as between the right and left circularly polarized electromagnetic waves are found. These instabilities are found to be distinct from the usual FEL resonance. The space-charge wave was not found to be intrinsically unstable when it couples to the electromagnetic wave and the coupling to the radiation was found to be the only way that makes it unstable.},
doi = {10.1063/1.2731318},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • The effects of an arbitrary beam thickness and a conducting wall in the Raman-type free-electron laser are investigated. To simplify the problem, a two-dimensional model of a solid nonmagnetized relativistic electron beam enclosed with a parallel plate waveguide is considered. With the aid of the fluid theory for electron beams, the coupled mode equation relating the scattered wave (TE mode) and the electron plasma wave (TM mode) under the influence of the pump wave (TE mode) is derived. By using the solution to the coupled mode equation, together with the boundary conditions on the beam surface, the dispersion relation andmore » the spatial growth rate for the coupled scattered and electron plasma waves are found. From detailed numerical analysis for the properties of the scattered wave, several interesting results are obtained. First, in order to get an appreciable magnitude of the growth rate, the beam width and the separation between the conducting walls of a parallel plate waveguide are required to be considerably greater than the wavelength of the scattered wave. Second, the growth rate becomes maximum for particular values of the ratio of the beam thickness to the separation between the conducting walls. Third, the growth rate can be greater for the pump and scattered waves with even symmetry than for those with odd symmetry.« less
  • An analytical three-dimensional model is presented for free-electron lasers (FELs) operating in the small-signal linear regime. The excitation of radiation and space-charge waves is found by expanding the total electromagnetic field in terms of transverse eigenmodes in a waveguide of arbitrary cross section and solving the evolution of their amplitudes from a set of coupled excitation equations. Coupled-mode theory is employed to derive dispersion relations for the space-charge waves and for the gain. The eigenmodes of the FELs (``supermodes``) and the gain for each of them are derived after diagonalization of the coupled-mode system. It is found that for themore » case of degenerate coupled modes (equal axial wave numbers), the normal modes satisfy the well known FEL gain dispersion equation with a modified gain parameter. The gain of the supermode, calculated according to the presented coupled-mode theory, is higher than the gain of the individual modes if calculated on the basis of a single-mode model. We demonstrate the formalism by finding the gain of the TE{sub 01}, and the coupled TE{sub 21} and TM{sub 21} modes excited simultaneously in a rectangular waveguide.« less
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