shown in fig1 b plasma chamber 17 comprises a cylindrical portion and tapering transitional portion 31 in those embodiments transitional portion 31 essentially bridges the cylindrical portion of plasma chamber 17 and high pressure chamber 25 transitional portion 31 of plasma chamber 17 tapers downstream from the diameter d ch of the cylindrical portion of plasma chamber 17 to the diameter d ch of high pressure portion 25 transitional portion 31 can be formed in a number of alternative ways in the embodiment shown in fig1 b transitional portion 31 is formed as a beveled edge other transitions such as concave or convex transitions are possible it should be noted however that the cylindrical portion of plasma chamber 17 and high pressure chamber 25 can be arranged in direct contact with each other without transitional portion 31 transitional portion 31 facilitates favorable heat extraction for cooling of structures adjacent to plasma chamber 17 and plasma channel 11 plasma generating device 1 can be a part of a disposable instrument for example an instrument may comprise plasma generating device 1 outer shell tubes coupling terminals etc and can be sold as a disposable instrument alternatively only plasma generating device 1 can be disposable and be connected to multiple use devices other embodiments and variants are also contemplated for example the number and shape of the intermediate electrodes 9 9 u2032 9 u2033 can be varied according to the type of plasma generating gas used and the desired properties of the generated plasma flow in use the plasma generating gas such as argon is supplied to.