also fig3 and 8 in that region the internal diameter d 10 8 over the length l 10 8 of the cooling tube 10 is smaller than the internal diameter d 10 9 of the internal portion 10 9 of the cooling tube 10 facing backwards and the external diameter d 10 10 over the length l 10 10 of the cooling tube 10 is larger than the external diameter d 10 11 of the external portion 10 11 of the cooling tube 10 facing backwards this thus gives rise to a bead like thickening 10 18 of the wall 10 19 of the cooling tube facing inwards and outwards this ensures that the flow cross section available to the coolant is only constricted in the front internal portion 10 8 and front external portion 10 10 in which a high flow velocity of a coolant is required for good heat dispersal and the greatest possible flow cross section is available in the rear region in order to keep the pressure drops in the rear internal portion 10 9 and rear external portion 10 11 as tow as possible a coolant first flows in the flow path through wv 1 water supply line 1 into the interior of the cooling tube 10 and encounters the inwardly extending region 7 5 of the electrode 7 before flowing back via the flow path wr 1 water return line 1 in the space.