Magnetic field entrance conditions in a circularly polarized FEL wiggler

It is shown that adiabatically increasing not only the dimensionless vector potential of the wiggler magnetic field a(z) in a free-electron laser (FEL), but also its first derivative on the axial coordinate da/dz, leads to less disturbance of the beam electron velocity components in comparison with the well investigated case when only a(z) changes adiabatically. This takes place both in the case of the axial guide magnetic field being less than its critical value and in the opposite case.