A plasma chamber having improved controllability of the ion density of the extracted ribbon ion beam is disclosed. A plurality of pairs of RF biased electrodes is disposed on opposite sides of the extraction aperture in a plasma chamber. In some embodiments, one of each pair of RF biased electrodes is biased at the extraction voltage, while the other of each pair is coupled to a RF bias power supply, which provides a RF voltage having a DC component and an AC component. In another embodiment, both of the electrodes in each pair are coupled to a RF biased power supply. A blocker may be disposed in the plasma chamber near the extraction aperture. In some embodiments, RF biased electrodes are disposed on the blocker.