  Terawatt x-ray free-electron lasers (XFELs) require high current densities
with strong transverse focusing. The implications on harmonic generation are
discussed using the MINERVA simulation code which self-consistently includes
harmonic generation. We consider helical and planar undulators where the
fundamental is at 1.5 Angstrom and study the associated harmonic generation.
While tapered undulators are needed to reach TW powers at the fundamental, the
taper does not enhance the harmonics because the taper must start before
saturation of the fundamental, with the harmonics saturating earlier.
Nevertheless, the harmonics reach substantial powers and enable enhanced
applications.
