Void Structure in Complex Plasmas

Dust particles imbedded within a plasma will acquire an electric charge from collisions with free electrons in the plasma. If the ratio of the inter-particle potential energy to the average kinetic energy is sufficient, the particles can form either a ``liquid'' structure with short range ordering or a crystalline structure with longer range ordering. Dust free regions (voids) inside complex plasmas have also been observed to form under microgravity conditions, in sputtering chamber experiments and in a variety of other experimental situations. Experimental observations reveal a stable dust free state often embedded within a dust crystalline structure or liquid state and exhibiting a sharp boundary. A new mechanism for void formation employing an external DC bias on a GEC rf reference cell will be discussed. Experimental results will be shown to be in good agreement with theoretical predictions for this strongly coupled complex plasma system.