A method of tuning the uniformity of a plasma with a large sheath potential by locally affecting the density of a plasma is provided. The method comprises illuminating a body exposed to the plasma with electromagnetic radiation from a source, wherein the body and the source are cooperatively configured such that the body will generate photoelectrons upon exposure to the radiation from the source. An example of such electromagnetic radiation is vacuum ultraviolet light, and an example of such a body is the edge ring surrounding a semiconductor substrate. Photoelectrons emitted from the edge ring, captured by the plasma, and accelerated into the plasma with sufficient energy to cause ionization, locally increase plasma density. The source of radiation can be a plurality of discrete sources or one or more extended sources. The source can be arranged to provide substantively uniform illumination, or can illuminate according to a non-uniform intensity distribution to compensate for existing non-uniformities in the plasma density or in the plasma process. Such sources can be embedded in the inner or outer electrode part of a multi-piece showerhead electrode assembly, or elsewhere in the chamber.