A METHOD OF SUPPRESSING VELOCITY SPREADS OF AN ELECTRON-BEAM IN COMBINED HELICAL WIGGLER AND AXIAL-GUIDE MAGNETIC-FIELD

A new method is proposed to suppress the velocity spreads of the elect ron beam by employing a tapered, reversed axial guide magnetic field. Three-dimensional analysis and nonlinear simulation show that this met hod can substantially suppress both transverse and longitudinal veloci ty spreads and evidently weaken the divergence in the configuration sp ace of the electron beam. Compared to a positive guide field and unifo rmly reversed guide field, this method could be expected to enhance th e gain and efficiency of a free-electron laser/maser.