An analytical representation of the longitudinal Lorentz force describing t
he electromagnetic interaction of two identical undulator emitters moving a
long the axis of a helical undulator is obtained under the dipole approxima
tion. A functional dependence of the coherence area volume of the emitters
on the external parameters of the system is determined. The applicability o
f the results and the main aspects of their physical meaning are discussed.
(C) 2000 MAIK "Nauka/Interperiodica".