PARTICLE THERMOPHORESIS IN LOW-PRESSURE GLOW-DISCHARGES

Particles in glow discharges are charged negatively and are therefore often suspended electrostatically since the plasma potential is usually more positive than the potential of surfaces bounding the plasma. However, in addition to responding to electrostatic and gravitational forces, particles are sensitive to forces associated with gradients in neutral gas temperature. A series of experiments were performed varying the temperature of water circulated through planar, parallel electrodes. When particles are present and are suspended in the discharge, they move away from a heated electrode and towards a cooled electrode. In the absence of particles, the discharge optical emission spatial profile and electrical characteristics did not change significantly for any combination of electrode heating or cooling. Particles remaining in the interelectrode gap after the discharge is extinguished appear to be uncharged. With particles present, the discharge electrical properties and time-and space-resolved optical emission take on characteristics associated with discharges in electron attaching gases. We report on a simple particle-scavenging arrangement based on thermophoresis that can be used to remove particles from a discharge.