Probing a dusty magnetized plasma with self-excited dust-density waves

A cloud of nanodust particles is created in a reactive argon-acetylene plasma. It is then transformed into a dusty magnetized argon plasma. Plasma parameters are obtained with the dust-density wave diagnostic introduced by Tadsen et al. [Phys. Plasmas 22, 113701 (2015)]. A change from an open to a cylindrically enclosed nanodust cloud, which was observed earlier, can now be explained by a stronger electric confinement if a vertical magnetic field is present. Using two-dimensional extinction measurements and the inverse Abel transform to determine the dust density, a redistribution of the dust with increasing magnetic induction is found. The dust-density profile changes from being peaked around the central void to being peaked at an outer torus ring resulting in a hollow profile. As the plasma parameters cannot explain this behavior, we propose a rotation of the nanodust cloud in the magnetized plasma as the origin of the modified profile.

Figure 1

Side view sketch of the experiment with field of view accessible by the DDW camera to cover the whole cloud (FOV I) or for a central stripe (FOV II) to record a video at a high frame rate.

Figure 2

Top view sketch of the optical diagnostics used for the experiment. Electrodes and Faraday shields are shown as two circles in the center. The extent of the plasma is visible in fading violet. Viewing directions of different diagnostics are plotted as dashed lines. The DDW camera, ellipsometer, and photodiodes are shielded against plasma emission with $532-\mathrm{nm}$ interference filters. Since all diagnostics use green light, they can only be used one after another.

Figure 3

Ellipsometric angles $\mathrm{\Psi }$ and $\mathrm{\Delta }$ of the scattered light measured during a particle growth process (circles) and theoretical black curve calculated with the CRAS-Mie algorithm [23]. The deviation between theory and experiment during the late growth stage is due to multiple scattering within the dust cloud [24]. A second run of the process is terminated by shutting off the acetylene at $t=50\mathrm{s}$. Shortly after, $\mathrm{\Psi }$ and $\mathrm{\Delta }$ were measured (red plus). The resulting parameters from measurement 2 are shown in Table 1. A detailed description of the procedure is given in Sec. 2b.

Figure 4

Density distribution for the 0-, 20-, and $100-\mathrm{mT}$ cases. The $x$ axis is centered at the middle of the void. Within the void a nonzero density results from uncompensated multiple scattering. The total number of particles is not conserved but decreases. The sequence of measurements started with $100\mathrm{mT}$ and then the magnetic induction was reduced.

Figure 5

(a) Wave frequency $\omega$ and (b) wave number $k$ as calculated from the video data. The data are obtained from the central line of the FOV II. For both diagrams, the error of $k$ and $\omega$ (standard deviation of the mean that follows from the frame-to-frame instantaneous values) is smaller than the diameter of the drawn circles.

Figure 6

(a) Ion density $n_{\mathrm{i}}$, (b) ion drift velocity $v_{\mathrm{i}}$ as calculated from the DDW-D in units of the Bohm velocity $v_{\mathrm{B}}=\sqrt{k_{\mathrm{B}}{T}_{\mathrm{e}}/m_{\mathrm{i}}}$ with an estimated electron temperature of $T_{\mathrm{e}}=5\mathrm{eV}$, and (c) dust charge $q_{\mathrm{d}}$. Error bars follow by error propagation of the errors in $\omega$ and $k$, which are described in the caption of Fig. 5.

Figure 7

Optical depth $\tau (x,z)$ as calculated with Eq. (8). (a) In the unmagnetized case, the cloud is weakly confined in the horizontal direction, leaping over the FOV of the telecentric lens. Therefore, the image is extrapolated using an exponentially decaying function beginning at the vertical dashed line. (b) With a magnetic field, the cloud is completely trapped within the FOV.

Figure 8

Dust density as calculated with Eq. (10). The color axis is adjusted from (a) to (e) as the total number of particles $N$ decreases: (a) $N=3.0\times {10}^9$ with $B=100$ mT, (b) $N=1.9\times {10}^9$ with $B=50$ mT, (c) $N=1.6\times {10}^9$ with $B=20$ mT, (d) $N=1.6\times {10}^9$ with $B=10$ mT, and (e) $N=1.4\times {10}^9$ with $B=0$ mT.