Vertically elongated three-dimensional Yukawa clusters in dusty plasmas

Yukawa clusters in dusty plasmas have been investigated experimentally with respect to their structural properties when they become elongated. These elongated clusters can be formed by creating an anisotropic confinement potential with changing plasma power. It is observed that increasing deformation of the cluster is accompanied by a vertical alignment of the particles although the alignment is neither seen nor expected for spherical clusters. Simulations of elongated clusters with and without the action of an ion focus have been performed and compared to the experiment. It has been revealed that the structure of spherical clusters is unaffected by the presence of a weak ion focus whereas elongated clusters visibly react to the presence of a weak ion focus.

Figure 1

Scheme of the experimental setup. The dust particles, which are confined inside the cuvette, are illuminated by a Nd:YAG laser and observed by three orthogonal cameras.

Figure 2

Comparison between (a–c) experimental and (d–f) simulated elongated clusters for different values of the vertical confinement deformation $\alpha$. From (a) to (c) the plasma power has been slightly reduced from $P=3.3$ W to $P=3.2$ W.

Figure 3

Cluster of 18 particles with different deformation parameters: (a) almost spherical cluster with ${\alpha }_{\text{geom}}=1$ at a plasma power of $P=4.2$ W, (b) slightly elongated cluster with ${\alpha }_{\text{geom}}=0.75$ at $P=3.8$ W, and (c) stronger deformation: ${\alpha }_{\text{geom}}=0.56$ at $P=3.4$ W.

Figure 4

Cluster of 16 particles with different deformation parameters: (a) almost spherical Yukawa ball with ${\alpha }_{\text{geom}}=1.26$ and (b) strongly deformed cluster with ${\alpha }_{\text{geom}}=0.46$.

Figure 5

Clusters of $N=27$ particles: (a) almost spherical experimental cluster at $p=80$ Pa with ${\alpha }_{\text{geom}}=1.20$, $\mathit{VOP}=0.01$; (b) simulated cluster without ion focus, ${\alpha }_{\text{conf}}=1.0$, ${\alpha }_{\text{geom}}=1.07$, $\mathit{VOP}=0.011$; (c) simulated cluster with ion focus of strength $\gamma =0.15$, ${\alpha }_{\text{conf}}=1.1$, ${\alpha }_{\text{geom}}=1.05$, $\mathit{VOP}=0.027$.

Figure 6

Simulated clusters of $N=18$ particles with ${\alpha }_{\text{geom}}\approx 0.6$ (a) without ion focus ($\gamma =0$), ${\alpha }_{\text{conf}}=0.7$; (b) with small ion focus ($\gamma =0.15$), ${\alpha }_{\text{conf}}=0.8$; and (c) with moderate ion focus ($\gamma =0.30$), ${\alpha }_{\text{conf}}=0.9$.

Figure 7

Yukawa clusters with MF particles at low gas pressure $p<10$ Pa: (a) nearly spherical cluster of $N=32$ particles with ${\alpha }_{\text{geom}}=1.12$, $\mathit{VOP}=0.082$; (b) slightly elongated cluster of $N=27$ particles with ${\alpha }_{\text{geom}}=0.83$, $\mathit{VOP}=0.092$; and (c) strongly deformed cluster of $N=27$ particles with ${\alpha }_{\text{geom}}=0.50$, $\mathit{VOP}=0.130$.

Figure 8

Cluster of 22 hollow glass particles at low gas pressure ($p=7$ Pa) with the associated angle distribution histogram.

Figure 9

(a) Experimental clusters in a plane spanned by ${\alpha }_{\text{geom}}$ and the $\mathit{VOP}$. (b) Simulation results added for comparison.