and the inner diameter of the anode housing to prevent the formation of stray plasma on the outside of the pipe fig7 illustrates an alternative system 200 in which the coupling heads 16 18 are each mounted on a linear track arrangement 202 by means of a rise and fall mechanism shown schematically at 204 the details of the coupling heads 16 18 and the sealing mechanism remain as described above except for the deletion of the coupling arms 22 24 26 fig8 illustrates the gas supply and electrical connections by way of a schematic layout the workpiece 12 is connected to the gas supply by means of connections 220 222 which in turn are connected to the various gas supply arrangements shown generally at 224 226 and 228 the vacuum pumps 42 44 are shown connected to cross linking duct 40 while it will be appreciated that various forms of gas supply may be employed the readers 39 attention is drawn to an evaporator arrangement at 224 in which an evaporator 230 is provided to evaporate a source of liquid diamond like component from injection mechanism 232 this injection mechanism supplies a small amount of liquid which is completely vaporized in the heated flash evaporator 224 an inert carrier gas such as argon or nitrogen.