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1 AN EUV RADIATION SOURCE, A TARGET MATERIAL SUPPLY SYSTEM FOR USE IN AN EUV RADIATION SOURCE, AN EXPOSURE APPARATUS COMPRISING SUCH A RADIATION SOURCE, AND A METHOD OF SUPPLYING TARGET MATERIAL TO A RADIATION SOURCE 5 BACKGROUND [0001] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses include a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and 10 even more specifically an actinic mask inspection apparatus. [0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a 15 pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm. 20 [0003] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses a reflective surfaces instead of a lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be alternatingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. 25 Therefore, mask inspection to check whether a defect is present is considered a requirement for a mass- production process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the 30 phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask...