  The terahertz FEL ferromagnetic undulator has been created. The length of the
undulator period is 9cm, number of the periods is 27. By means of selection and
redistribution of magnetic elements the spread of amplitudes of the magnetic
field was reduced to 7%. Additional windings of magnetic elements were used to
compensate for residual spread. Required focusing gradient of magnetic field
was obtained as a result of relative displacement of alternating poles with
opposite charge of magnetic field along the x axis. Parameters of undulator
including focusing properties in the horizontal plane were investigated.
