An ion source has an arc chamber having first and second ends and an aperture plate to enclose a chamber volume. An extraction aperture is disposed between the first and second ends. A cathode is near the first end of the arc chamber, and a repeller is near the second end. A generally U-shaped first bias electrode is on a first side of the extraction aperture within the chamber volume. A generally U-shaped second bias electrode is on a second side of the extraction aperture within the chamber volume, where the first and second bias electrodes are separated by a first distance proximate to the extraction aperture and a second distance distal from the extraction aperture. An electrode power supply provides a first and second positive voltage to the first and second bias electrodes, where the first and second positive voltages differ by a predetermined bias differential.