Obliquely Propagating Dust-Density Plasma Waves in the Presence of an Ion Beam

Self-excited dust-density waves are experimentally studied in a dusty plasma under microgravity. Two types of waves are observed: a mode inside the dust volume propagating in the direction of the ion flow and a new mode propagating obliquely at the boundary between the dusty plasma and the space-charge sheath. A model for dust-density waves propagating at an arbitrary angle with respect to the ion-flow direction is presented, which explains the preference for oblique or parallel modes as a function of ion velocity.

Figure 1

Side view of the IMPF-K device. The rf discharge has segmented electrodes and is operated in push-pull mode between pairs of electrodes. The two ring electrodes are electrically connected.

Figure 2

Still image of dust-density waves. The framed arrows indicate the wave propagation direction. The arrow in region 1 indicates the ordinary DDW and the arrow in region 2 the OPDDW. The electric field direction is perpendicular to the sheath boundary. The electric field $E$ near the electrode sheath is perpendicular to the boundary of the dust cloud. The dust cloud shows a weak top-bottom asymmetry, which is caused by residual gravity.

Figure 3

(a) Real part of the wave frequency as a function of the wave number $k$ for different orientation angles between wave propagation and ion beam. Ion velocity $v_0=v_B$. The observed wave in region 2 is indicated with error bars. (b) Imaginary part of the wave frequency. Positive values indicate wave growth. (c), (d) Same as (a), (b), but at reduced ion velocity $v_0=0.3v_B$. The experimental point is from region 1. (e) Growth rates for different wave numbers as a function of orientation angle at $v_0=v_B$. (f) Same as (e), but for $v_0=0.3v_B$.