  We report on the observation of the self-excited dust density waves in the dc
discharge complex plasma. The experiments were performed under microgravity
conditions in the Plasmakristall-4 facility on board the International Space
Station. In the experiment, the microparticle cloud was first trapped in an
inductively coupled plasma, then released to drift for some seconds in a dc
discharge with constant current. After that the discharge polarity was
reversed. DC plasma containing a drifting microparticle cloud was found to be
strongly non-uniform in terms of microparticle drift velocity and plasma
emission in accord with [Zobnin et.al., Phys. Plasmas 25, 033702 (2018)]. In
addition to that, non-uniformity in the self-excited wave pattern was observed:
In the front edge of the microparticle cloud (defined as head), the waves had
larger phase velocity than in the rear edge (defined as tail). Also, after the
polarity reversal, the wave pattern exhibited several bifurcations: Between
each of the two old wave crests, a new wave crest has formed. These
bifurcations, however, occurred only in the head of the microparticle cloud. We
show that spatial variations of electric field inside the drifting cloud play
an important role in the formation of the wave pattern. Comparison of the
theoretical estimations and measurements demonstrate the significant impact of
the electric field on the phase velocity of the wave. The same theoretical
approach applied to the instability growth rate, showed agreement between
estimated and measured values.
