  The idea and principles of volume free electron lasers were proposed in
[1-4]. It was shown there that volume distributed feedback (VDFB) can
essentially reduce the threshold current of generation and provide the
possibility of smooth frequency tuning. The present work considers an undulator
VFEL with multiwave VDFB. It is shown that dependence of threshold current on
the interaction length changes in the point of roots degeneration. This leads
to the sharp decrease of start current if condition of dynamical diffraction is
fulfilled. The dependence of amplification coefficient changes, too. So the
interaction length for generation appears shorter. The proposed scheme can be
used for generation in wide spectral range from microwaves to X-rays. The
operating features of undulator VFEL is considered.
