An essential feature of the invention is the original combination of a plasma process and a sealing system. This is achieved with external tubes (4, 6), which are connected to each other so as to be impermeable to gas, but galvanically insulated, and of which at least one is electrically conductive, such that the external tubes form an antechamber between the two external tubes (4, 6) and a treatment chamber between the external tube (6) and the internal tube (1).