A partially-insulated cathode for exciting plasma in a plasma chamber is provided. The partially-insulated cathode includes a conductive structure enclosing a cavity having a cavity surface and an insulating material contiguously covering a portion of the cavity surface from the cavity opening up to an insulation height that is less than a cavity height. Cross-sections of the cavity in X-Y planes have at least one respective cavity-width. A cavity opening has a diameter less than a minimum cavity-width of the at least one cavity-width.