  In Fermilab's electron cooler, a 0.1A, 4.3MeV DC electron beam propagates
through the 20 m cooling section, which is immersed in a weak longitudinal
magnetic field. A proper adjustment of 200 dipole coils, installed in the
cooling section for correction of the magnetic field imperfections, can create
a helix-like trajectory with the wavelength of 1-10 m. The longitudinal cooling
force is measured in the presence of such helixes at different wavelengths and
amplitudes. The results are compared with a model calculating the cooling force
as a sum of collisions with small impact parameters, where the helical nature
of the coherent angle is ignored, and far collisions, where the effect of the
coherent motion is neglected. A qualitative agreement is found.
