Journal of Applied Physics · 1985 · 253 citations · 5 references
PhotonicsFree-electron LasersEngineeringPhysicsElectron BeamGeometrical OpticRelativistic Laser-matter InteractionApplied PhysicsLaser-plasma InteractionWiggle PlaneFree-electron LaserElectron Cloud EffectsCurved PoleElectron OpticFree Electron LaserBeam Optic
External focusing required to confine the electron beam in a long, linear wiggler can severely degrade free‑electron laser performance. The study aims to eliminate longitudinal velocity modulation by applying parabolically curved pole faces that focus in the electron’s wiggle plane. Analytical calculations and numerical simulations show that curved pole faces provide transverse focusing and resonance effects that counteract nonadiabatic phase shifts in a linear wiggler. The simulations demonstrate that preventing phase changes of order unity over a betatron period with curved pole faces markedly improves gain by reducing detrap and debunching.
In a free-electron laser with a long, linear wiggler, the external focusing required to keep the electron beam from dispersing can seriously degrade the performance of the laser. The transverse focusing modulates the longitudinal velocity of each electron, periodically and nonadiabatically changing the phase of the electron with respect to the electromagnetic wave. Phase changes of order unity over a betatron period can strongly detrap or debunch electrons and greatly reduce the gain of a linear wiggler amplifier. The modulation of the electron’s longitudinal velocity can be prevented if focusing in the plane of the electron’s wiggle motion is provided by parabolically curved magnet pole faces. The focusing and resonance effects of curved pole faces are analytically calculated and numerically confirmed. Numerical simulations of linear wiggler amplifiers are presented to illustrate the effect of the curved pole faces on amplifier performance.
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