This disclosure relates to a plasma processing system for controlling plasma density near the edge or perimeter of a substrate that is being processed. The plasma processing system may include a plasma chamber that can receive and process the substrate using plasma for etching the substrate, doping the substrate, or depositing a film on the substrate. This disclosure relates to a plasma processing system for controlling plasma density near the edge or perimeter of a substrate that is being processed. In one embodiment, the plasma density may be controlled by reducing the rate of loss of ions to the chamber wall during processing. This may include biasing a dual electrode ring assembly in the plasma chamber to alter the potential difference between the chamber wall region and the bulk plasma region.