  Coherent Harmonic Generation (CHG), and in particular Nonlinear Harmonic
Generation (NHG), is of importance for both short wavelength Free-Electron
Lasers (FELs), in relation with the achievement of shorter wavelengths with a
fixed electron-beam energy, and high-average power FEL resonators, in relation
with destructive effects of higher harmonics radiation on mirrors. In this
paper we present a treatment of NHG from helical wigglers with particular
emphasis on the second harmonic. Our study is based on an exact analytical
solution of Maxwell's equations, derived with the help of a Green's function
method. In particular, we demonstrate that nonlinear harmonic generation (NHG)
from helical wigglers vanishes on axis. Our conclusion is in open contrast with
results in literature, that include a kinematical mistake in the description of
the electron motion.
