OSCILLATORY PARTICLE MOTION OBSERVED IN DUSTY PLASMA UNDER MICROGRAVITY CONDITIONS

In an experiment aboard the International Space Station, performed using the PK-3 Plus instrument, a 13.56 MHz capacitively-coupled RF discharge was formed between parallel-plate electrodes. Neon gas at 0.2 Torr was partially ionized by applying 40 Volt peak-to-peak RF potentials to the symmetrically-driven electrodes. A mechanical shaker introduced 6.8-micron diameter polymer microspheres, or dust, into the plasma. The microspheres became highly charged. In this way a dusty plasma (also known as a complex plasma) was formed. Using video cameras and illumination by a sheet of laser light, the suspension of microparticles was imaged in situ. The suspension filled most of the inter-electrode region, except for a central void. Between the two electrodes and the void, the suspension was thin; it was only a single layer thick on one side, for the particular experiment that we analyze here. Tracking the motion of particles from the video images, we measured the particle velocities. We observed oscillatory particle motion that is transverse, or out-of-plane, for the single layer. The frequency spectrum of these oscillations was calculated, yielding a resonance frequency and a damping rate. These oscillations are an indication of confinement of the charged microspheres. The confinement is interpreted as being due to a combination of two forces acting on the charged microspheres: a Coulomb force that is directed toward the void and an opposing iondrag force directed away from the void.