An ion source includes an arc chamber having an extraction aperture, and a plasma sheath modulator. The plasma sheath modulator is configured to control a shape of a boundary between plasma and a plasma sheath proximate the extraction aperture. The plasma sheath modulator may include a pair of insulators positioned in the arc chamber and spaced apart by a gap positioned proximate the extraction aperture. A well focused ion beam having a high current density can be generated by the ion source. A high current density ion beam can improve the throughput of an associated process. The emittance of the ion beam can also be controlled.