A plasma reactor for processing a workpiece includes a vacuum enclosure, including a wall, defining a vacuum chamber, the vacuum chamber having a main chamber portion on one side of the wall and a plenum on another side of the wall, the plenum communicating with the chamber portion through at least one opening in the wall, a workpiece support within the main chamber portion and facing the wall. A gas distribution plate is adjacent the wall and faces the workpiece support and is coupled to a reactive process gas supply for injecting reactive process gases directly into a process region adjacent the workpiece support. A gas injection port at the plenum is coupled to a diluent gas supply for injecting diluent gases into the plenum. A coil antenna adapted to accept RF power is inductively coupled to the interior of said plenum, and is capable of maintaining a plasma in a reentrant path through the plenum and across the process region.