A vacuum plasma processor includes a chamber having a grounded wall and an outlet port. Plasma is excited at a first RF frequency in a chamber region spaced from the wall and outlet port. A structure confines the plasma to the region while enabling gas to flow from the region to the outlet port. RF electric power at a second frequency connected to the confining structure causes the confining structure to be at a potential different from ground to increase the size of a sheath between the plasma and confining structure and increase the confining structure effectiveness. The region includes an electrode connected to ground by a circuit that is series resonant to the first frequency and includes capacitance of the sheath.
