Workpiece support for a plasma reactor having a controlled rf power distribution to the process kit ring

The electrostatic chuck, RF bias power to the workpiece, and the process kit collar surrounding the workpiece, applied separately. At least one variable impedance element is controlled by a system controller, for example, for optimum electric field uniformity under varying plasma conditions, allows dynamic adjustment of the plasma sheath electric field at the end edge of the workpiece, adjusting the distribution of RF bias power between the workpiece and the process kit collar.