Probe induced voids in a dusty plasma

Dusty plasmas are susceptible to the formation of a ‘‘void,’’ a centimeter-size region within the plasma that is free of dust particles. The boundary between the dust-free plasma ~void! and the dusty plasma is typically quite sharp. Praburam and Goree first reported the appearance of a spoke-shaped, dust-free region in a cloud of 100-nm carbon particles in a radio-frequency discharge formed between parallel-plate graphite electrodes. This dust-free region ~called the great void! rotated azimuthally in the discharge and only appeared when the dust particles had grown to a sufficiently large size. Voids have also been an unexpected occurrence in dusty plasmas produced under microgravity conditions. In laboratory devices, dust particles must be levitated against gravity by the relatively strong electric fields at the sheath above the electrodes. This precludes the formation of large threedimensional ~3D! crystal structures, since the particles are relegated to relatively thin layers just above the lower electrode. Voids formed in dusty plasmas under microgravity conditions were observed in the PlasmaKristall experiments ~PKE-Nefedov! onboard a sounding rocket and recently on the International Space Station. Recent ground-based experiments have simulated these microgravity observations by applying a temperature gradient across the plasma volume of an rf glow discharge plasma. This creates an upward thermophoretic force that counteracts the role of the gravitational force, allowing the size of the cloud to increase and also facilitating the formation of a large oval-shaped void. The spontaneous formation of voids has been attributed to an instability that develops if a local depletion of negatively charged dust particles occurs within a spatially uniform dusty plasma. The dust density perturbation produces a positive potential with respect to the surrounding plasma and thus an electric field that points outward from the region of the dust depression. This electric field has two effects: it produces an inward electric force FE on the negative dust, and an outward ion drift. The ion drift causes an outward ion drag force on the dust particles FD that tends to expel them from the region. If FD.FE , the initial density depression grows into a void. Thermophoretic forces have also been invoked to explain void formation, although recent work suggests that the ion drag force is more likely the mechanism responsible for the formation of voids in dusty plasmas under microgravity conditions. Voids in dusty plasmas have also been produced by inserting a probe that is negatively biased with respect to the surrounding plasma into a dusty plasma. A cylindrical void with a diameter of ;1 cm was produced when an electrically floating tungsten rod of 1.6 mm diameter was inserted into a dusty plasma formed in a nitrogen discharge plasma. Observations were also made of the effect of moving the floating rod through the dusty plasma at speeds either below or above the dust acoustic speed. When the rod was moved through the dust cloud on a time scale long compared to the time for a dust-acoustic perturbation to move across the cloud, a moving void was formed. The present paper describes the results of a similar experiment in which a void was formed in an argon glow discharge plasma by inserting a negatively biased probe into the dusty plasma. Unlike the previous experiment, however, we were now able to study the effect that changing the probe bias has on the size of the void. The experimental results are compared to a model in which a stable void is maintained by the balance of the outward electrostatic force on the negative dust by the electric field of the probe and the inward ion drag force due to ions that are drawn into the void and collected by the probe. It is important to note that in the voids generated by a negatively biased probe described here, the roles of the electric and ion drag forces are reversed compared to voids formed under microgravity conditions ~Refs. 2 and 3!. Thus the present study provides a complimentary view of void formation in dusty plasmas. The first experiment where the balance of ion drag and electric force was proposed to explain the confinement of particles in the field of a negatively biased wire ~in 2D particle clouds! was reported by Samsonov et al. The experimental setup and methods are described in Sec. II and the results are given in Sec. III. The theoretical Electronic mail: [email protected] Electronic mail: [email protected] PHYSICS OF PLASMAS VOLUME 11, NUMBER 5 MAY 2004