Dust-void formation in a dc glow discharge

Experimental investigations of dusty plasma parameters of a dc glow discharge were performed in a vertically oriented discharge tube. Under certain conditions, dust-free regions (voids) were formed in the center of the dust particle clouds that levitated in the strong electric field of a stratified positive column. A model for radial distribution of dusty plasma parameters of a dc glow discharge in inert gases was developed. The behavior of void formation was investigated for different discharge conditions (type of gas, discharge pressure, and discharge current) and dust particle parameters (particle radii and particle total number). It was shown that it is the ion drag force radial component that leads to the formation of voids. Both experimental and calculated results show that the higher the discharge current the wider dust-free region (void). The calculations also show that more pronounced voids are formed for dust particles with larger radii and under lower gas pressures.

Figure 1

Sketch of the experimental setup.

Figure 2

The photo of horizontal cross sections of a dc discharge tube with dust particle clouds. The outer lighting ring is a scattered light of the laser sheet on the gas discharge tube wall (radius of the ring is equal to the tube inner diameter $D_{in}=4.5\mathrm{cm}$). The inner ring is a scattered light on dust particles. Helium pressure $p=0.38\mathrm{Torr}$. (a) $I_d=4\mathrm{mA}$; (b) $I_d=8\mathrm{mA}$; (c) $I_d=10.4\mathrm{mA}$; (d) $I_d=13.4\mathrm{mA}$.

Figure 3

Sketch of the dust particle cloud and forces acting on it.

Figure 4

Dust particle density radial distributions. (a) $p=0.38\mathrm{Torr}, N_{\mathrm{tot}}=2N_0, a=2\mu \mathrm{m}$; (b) $p=0.38\mathrm{Torr}, N_{\mathrm{tot}}=2N_0, a=0.5\mu \mathrm{m}$; (c) $p=0.3\mathrm{Torr}, N_{\mathrm{tot}}=2N_0, a=2\mu \mathrm{m}$; (d) $p=0.38\mathrm{Torr}, N_{\mathrm{tot}}=0.5N_0, a=2\mu \mathrm{m}$. Solid lines for $I_d=8\mathrm{mA}$, dashed lines for $I_d=10.4\mathrm{mA}$, dashed dotted lines for $I_z=13.4\mathrm{mA}$.

Figure 5

Radial distributions of charged particle densities: ions $n_i\left(r\right)$ (solid line), electrons $n_e\left(r\right)$ (dashed line), charge density of dust particles $N_d\left(r\right)Z_d\left(r\right)$ (dotted line), bulk charge $\mathrm{\Delta }N\left(r\right)$ (dash-dotted line). (a) $I_d=8\mathrm{mA}$; (b) $I_d=10.4\mathrm{mA}$; (c) $I_z=13.4\mathrm{mA}$.

Figure 6

Radial distributions of the product of dust particle density and the forces acting on dust particles: solid line for the ion drag force $F_{id}{N}_d$, dashed line for the electrostatic force $F_E{N}_d$, dashed dotted line for the interparticle repulsive force $F_{dd}{N}_d$. (a) $I_d=8\mathrm{mA}$, (b) $I_d=10.4\mathrm{mA}$, (c) $I_z=13.4\mathrm{mA}$.

Figure 7

(a) Electric potential radial distributions. (b) Electric field radial distributions. Solid lines for $I_d=8\mathrm{mA}$, dashed lines for $I_d=10.4\mathrm{mA}$, dashed dotted lines for $I_z=13.4\mathrm{mA}$.

Figure 8

Axial electric field dependencies on discharge current. Calculated results: (a) $p=0.38\mathrm{Torr}, N_{\mathrm{tot}}=2N_0, a=2\mu \mathrm{m}$; (b) $p=0.38\mathrm{Torr}, N_{\mathrm{tot}}=2N_0, a=0.5\mu \mathrm{m}$; (c) $p=0.38\mathrm{Torr}, N_{\mathrm{tot}}=2N_0, a=2\mu \mathrm{m}$. (d) Experimental measurements for pristine plasma.