Laser welding of plasma facing units for ITER divertor Dome manufacturing

The ITER divertor is a modular structure consisting of 54 cassettes. Each cassette consists of the cassette body, inner and outer vertical targets and Dome. The Russian Federation is responsible for manufacturing and delivery of 60 Domes for ITER. Each Dome consists of the steel support structure with 34 plasma-facing units of Umbrella, Inner Particle Reflector Plate and Outer Particle Reflector Plate (OPRP). The steel support structure includes three manifolds. Each plasma-facing unit part from 316L(N)-IG steel is supposed to be welded by laser beam welding. During laser beam welding of stainless steels more than 3 mm in thicknesses numerous metallic droplets appeared close to the weld area. The plasma-facing units are part of the divertor cooling system. Thus, droplets from root weld side might detach, appear in the cooling system and contaminate the cooling pumps. In the common practice of laser and e-beam welding both surfaces are machined. The main problem with of laser welding used for manufacture of the Dome are ITER requirement of full penetration and at the same time inaccessibility of weld root for machining after welding. The attempts to select different welding regimes, to use the transversal beam scanning or removable catchers from different materials did not keep droplets from appearing on the weld root. One of the most successful attempts was to use ESAB OK10.69 flux. The flux is placed on the root side of the weld, catches all droplets during the welding process and facilitates weld forming. The LS-15 fiber laser with Motoman HP50 robot in FlexLase laser cell was used for welding at full power up to 15 kW. An industrial videoscope was used for visual examinations of the weld root inside the closed volume of the mock-up. At first, samples and closed-like boxes structure with wall thicknesses about 7 mm were welded during development of welding technology. Then, laser beam welding was applied for manufacturing of the plasma-facing units of mock-up of Dome OPRP. The laser welding conditions and parameters for mock ups welded with ESAB OK10.69 flux are described. The visual and radiography examinations results of welded mock-ups are presented. These examinations allow for control of weld root and full-penetration weld of closed mock-ups without their cutting. The examinations have demonstrated a good quality of weld.