Diffusive Transport of Microparticles in an RF Glow Discharge Plasma

Abstract

Summary form only given. RF glow discharge plasmas are used for dusty plasma experiments. Microspheres introduced into the plasma become electrically charged to the floating potential by collecting electrons and ions. The sheath electric field above a horizontal lower electrode levitates these microparticles against the downward force of gravity. Ion flow in the sheath can excite electric fields that accelerate these charged microparticles, while neutral gas opposes this acceleration by a frictional damping effect. As a result, microspheres have a random motion, which we study. We characterize the random motion of the microparticles by tracking their motion using video microscopy. Particle tracking allows us to use diagnostics comparable to those used in particle simulations of plasmas, except that here we are analyzing experimental data. By tracking their motion, we can measure the kinetic temperature of the microspheres, and we attempt to measure their diffusion coefficient as well. For some parameters we find the motion obeys the random walk typical of diffusion, but for other parameters it exhibits superdiffusion. Diffusion is characterized by a particle mean-square displacement (MSD) that increases linearly with time, while for superdiffusion the MSD increases more rapidly. In addition to the MSD, we also apply other diagnostics for random particle motion, including the probability distribution function (PDF) for particle displacements, and the particle velocity autocorrelation function. To enhance the kinetic temperature of the microparticles, and to vary it independently of all plasma parameters, we apply a newly developed laser-heating method. A powerful cw laser beam is rastered rapidly through the plasma, giving microparticles kicks at nearly random times. We have improved on some of the limitations of this heating method by achieving a more uniform temperature profile and a reduced non-random component to the particle motion. The result of this heating is that the microspheres move randomly almost the same way as molecules in a liquid. In other words, the suspension of microspheres is a strongly-coupled plasma in a liquid state. We characterize the transport in this state, and report diffusion coefficients as a function of kinetic temperature.