A beam delivery apparatus is used with a laser produced plasma source. The beam delivery apparatus comprises variable zoom optics (550) operable to condition a beam of radiation so as to output a conditioned beam having a configurable beam diameter (b) and a plurality of mirrors (530a, 530b) operable to direct the conditioned beam of radiation to a plasma generation site. The beam delivery apparatus enables control of the axial position of the beam where the beam has a particular diameter, with respect to the beam's focus position (570). Also, a method is used to control the axial position of the location at a plasma generation site where a beam has a particular diameter, with respect to the beam's focus position.