A plasma reactor or vacuum processing apparatus is provided with an orifice plate assembly. The orifice plate assembly includes an upper plate and a lower plate. Each plate is configured with through holes. The upper and lower orifice plates are independently rotatable with respect to each other. The plates are arranged within the vacuum chamber a discharge reactor such that the chuck assembly is disposed within an opening in the orifice plate assembly. The orifice plate assembly is further configured to have a perimeter shape that substantially matches the interior wall shape of vacuum chamber.