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 that may be configured to enable non-ambipolar diffusion to counter ion loss to the chamber wall. The plasma processing system may include a ring cavity coupled to the plasma processing system that is in fluid communication with plasma generated in the plasma processing system. The ring cavity may be coupled to a power source to form plasma that may diffuse ions into the plasma processing system to minimize the impact of ion loss to the chamber wall.