from ≈ 0 to 38 mm. Here d can be adjusted while the plasma is on, and without losing the dust particles, by mean of a lead screw which is rotated from outside the vacuum. The lead screw is made of joined left-handed and right-handed threaded rods [unified national coarse (UNC) thread size 6-32] that move the two confining bars simultaneously closer together or farther apart a distance of 1/16 inch ( ≈ 1 . 59 mm) per revolution, thus keeping the minimum of the potential well centered with respect to the disk electrode. This arrangement allows us to continuously vary the anisotropy parameter α 2 while maintaining nearly the same plasma discharge parameters. The dust particles used were melamine formaldehyde resin spheres with a nominal diam- eter of 9 . 62 ± 0 . 09 µ m. As noted previously [18], we believe that the particle diameter is actually 8 . 94 µ m, giving m = 5 . 65 × 10 − 13 kg. Confined dust particles are illuminated by a red diode laser sheet. A top-view camera mounted about 30 cm above the electrode is used to acquire dust particle position data. For this experiment, the resolution of the camera and associated optics was 16.47 µ m/pixel. Sequences of 2048 frames were recorded at 27.9 frames/s. A side-view camera was used to confirm that there were no out-of-plane particles. We acquired 20 data sets with the same n = 6 particles for trap widths d = 12 . 8 to 36.6 mm. Representative measured configurations are shown in Fig. 2 for increasing d . As seen in Fig. 2(a), the particles are in a straight line configuration when d is small, or equivalently, α 2 is large. As we increase d the cluster goes through two structural transitions, straight line to zigzag [Figs. 2(b) and (c)], and then zigzag to elliptical [Fig. 2(d)]. The transition to the zigzag state occurs at d ≈ 23 mm and the transition to the elliptical state occurs at d ≈ 30 mm. In the elliptical state all six particles lie on the convex hull of the cluster. To analyze the experimental data, we first find the dependence of the anisotropy param- eter α 2 on d . This is done by measuring the frequencies ω$_{x}$ and ω$_{y}$ of the c.m. modes from the particles’ Brownian motions [8, 12, 16]. The power spectra of the time histories of the c.m. coordinates, x$_{cm}$ = (1 /n ) ∑ x$_{i}$ and y$_{cm}$ = (1 /n ) ∑ y$_{i}$ , were fitted with the expression for a driven damped harmonic oscillator to determine the corresponding normal mode fre- quencies ω$_{x}$ and ω$_{y}$ . For each d , the dust particle configuration was characterized by the root-mean-squared values x$_{rms}$ and y$_{rms}$ averaged over all frames, where x 2 rms = 1 n n ∑ i =1 ( x$_{i}$ − x$_{cm}$ ) $^{2}$, y 2 rms = 1 n n ∑ i =1 ( y$_{i}$ − y$_{cm}$ ) $^{2}$. (6) 5