Self-field effects on electron dynamics in a three-dimensional helical wiggler free-electron laser with axial magnetic field |
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Authors: | R El-Bahi and M N Rhimi |
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Institution: | Thomas Jefferson National Accelerator Facility, Newport News, VA, USA |
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Abstract: | An analytic linear theory of the electron dynamics in a three-dimensional helical wiggler free electron laser (FEL) with
axial magnetic field is presented. Orbits are obtained by perturbing the steady state-trajectories in order to determine the
characteristic frequencies Ω± of the FEL. The effect of the self-fields on electron dynamics is studied and modified
steady-state orbits and their stabilities have been analysed considering variation of electron energy and density. Among the
features
encountered is that in both group-I and group-II, one of the characteristic frequencies may have either signs affecting then
the
stability of the motion, while in group-II operation a repulsion of the frequencies at a pseudocrossing leads to highly perturbed
trajectories when the wiggler frequency is approximately half the cyclotron frequency. Self-fields effects can significantly
impair
the stability of the electron orbits. For group-I orbits, they are more important for higher wiggler frequencies and lower
beam energies.
For group-II orbits, they remain less important for higher wiggler frequencies and lower beam energies before reaching the
inversion
zone, then they behave as for group-I orbits. It should be remarked that self-fields shift the inversion zone towards higher
cyclotron
frequencies the thing that is obtained by either decreasing the wiggler frequency or increasing the beam energy. It is shown
that the
axial velocity-induced self-magnetic field has a diamagnetic effect for both groups orbits, while the wiggler-induced self-magnetic
field has a diamagnetic effect for group-I orbits and a paramagnetic effect for group-II orbits. The paramagnetic and diamagnetic
effects are more important for higher beam energies and densities. |
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Keywords: | |
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