Ion source and extraction plate

An ion source having an extraction plate with a variable thickness is disclosed. The extraction plate has a protrusion on its interior or exterior surface proximate the extraction aperture. The protrusion increases the thickness of the extraction aperture in certain regions. This increases the loss area in those regions, which serves as a sink for ions and electrons. In this way, the plasma density is decreased more significantly in the regions where the extraction aperture has a greater thickness. The shape of the protrusion may be modified to achieve the desired plasma uniformity. Thus, it may be possible to create an extracted ion beam having a more uniform ion density. In some tests, the uniformity of the beam current along the width direction was improved by between 20% and 50%.