also prolongs the lifetime of electrodes 30 if the insulating medium is air the main sputtering product is alumina the same material as spacers 40 or 42 other electrode materials such as brass tend to produce a conductive coating on spacers 40 or 42 in general an electrode stack of the present invention includes n independent interleaved substacks staggered with respect to each other the electrodes of any particular substack are n discharge gaps apart fig5 a illustrates the usually preferred case of n 2 n gt 2 with the concomitant longer spacers 40 or 42 and longer and narrower lobes 36 is useful if electrodes 30 are particularly thin or if the gaps between electrodes 30 are large fig5 b illustrates the case of n 3 with three interleaved substacks of electrodes 30 u2032 each substack with its own set of spacers 40 u2032 let g represent the axial gap width between two electrodes and let d represent the thickness of an electrode as a rule the axial length s n g d u2212 d of the portion of spacers 40 or 42 that separates two electrodes 30 of a particular substack should be at least 3 times the corresponding cumulative axial gap width g ng for example if electrodes 30 are 2 mm thick and the gaps between electrodes 30 are 0 5 mm wide then with n 2 cumulative gap width of 1 mm the spacers are 3.