A guard aperture is described to control the ion angular distribution in plasma processing in one example a workpiece processing system has a plasma chamber, a plasma source to generate a plasma containing gas ions in the plasma chamber, the plasma forming a sheath above the workpiece, the sheath having an electric field, a workpiece holder in the chamber to apply a bias voltage to the workpiece to attract ions across the plasma sheath to be incident on the workpiece, a control aperture between the sheath and the workpiece, the aperture being positioned to modify an angular distribution of the ions that are incident on the workpiece, and a guard aperture between the sheath and the control aperture to isolate an electrical field of the control aperture from the plasma sheath.