A significant R&D effort was carried out by EU Associations and Industries during the ITER EDA (Engineering Design Activities) period to develop suitable technologies for high heat flux components. This joint effort culminated in the successful manufacturing and testing of a medium-scale vertical target prototype. On the basis of this experience, the manufacturing of a near full-scale prototype was then launched and completed during the ITER CTA (Coordinated Technical Activities) period. The high heat flux parts of this component were fabricated by the Austrian company Plansee and the steel supporting structure and the final integration of the high heat flux parts onto it were carried out by the Italian company Ansaldo Ricerche.

The main function of the ITER divertor is to exhaust the major part of the alpha particles power as well as helium and impurities from the plasma. The divertor must tolerate high heat loads and contribute in providing neutron shielding for the vacuum vessel and magnet coils. Most of the plasma-facing surface has tungsten armour whereas carbon fibre reinforced carbon is envisaged for the strike point area.

The prototype consists of four units having a full monoblock geometry, obtained by drilling a hole into each plasma facing tile (the “armour”). The cooling tube is then inserted and the parts are joined. The lower part of the prototype has a carbon fibre reinforced carbon (CFC) armour, grade NB31, supplied by the French company Snecma Propulsion Solide. This composite material is the outcome of an extensive R&D effort carried out by EFDA aimed at developing suitable CFC materials with a three-directional fibre structure, a high thermal conductivity, and an appropriate balance of the mechanical properties. The upper part of the prototype has a tungsten armour, as required by the ITER design. The main critical issues were:

• the large thermal expansion mismatch between the armour and the cooling tube made of copper alloy CuCrZr;

• the requirement for two different armour materials on the same component; and

• the preservation of the thermo-mechanical properties of the CuCrZr during the whole process.

The joint interface-stress between the armour and the heat sink was mitigated by the introduction of a thin (1-2 mm) pure copper (Cu) interlayer. The development of the CFC/Cu joint was one of the most challenging R&D efforts carried out withinthe divertor area. “Active Metal Casting”, developed by Plansee, which involves casting Cu onto the CFC surface, was used to obtain this joint. The CFC is first “structured” by a laser beam to improve the joint strength, and the surface is then “activated” to facilitate wetting. The Cu/CuCrZr joint was accomplished using a so-called “low temperature” hot isostatic pressing technique also developed by Plansee. Solution annealed, water quenched CuCrZr is used as a starting point for the manufacturing process. Both the joining of the CuCrZr onto the Cu and its required “ageing” is combined into one single step carried out at 550 °C.

The full-scale prototype was tested in the high heat-flux FE200 electron beam facility at Le Creusot in France. The CFC part was successfully tested up to 1000 cycles at 20 MWm-2 followed by an additional 1000 cycles at 23 MWm-2. This is well beyond the ITER design target of 300 cycles at 20 MWm-2. The W monoblock section endured up to 1000 cycles at 10 MWm-2 of absorbed heat flux (that is about half of the actual incident heat flux). This value is one order of magnitude higher than the ITER design target for the upper part of the ITER vertical target.

“The manufacturing of this worldwide unique component and the achieved results are the reward for several years of dedicated effort by EU Associations and Industries.”, said Mario Merola, Responsible Officer for divertor technology at EFDA-CSU Garching (Germany). “It clearly demonstrates that the EU already possesses the technologies to contribute to the most critical components of the ITER divertor.” However, these results, while certainly encouraging, cannot represent the end of the story. Work still needs to be done to bridge the gap between the demonstrated capabilities at the prototypical level and the required series production for ITER. Important issues needs to be addressed, such as the definition of practical “acceptance criteria” for the manufacturing of these plasma-facing components, the development of suitable repairing procedures in order not to have to scrap all flawed components during manufacture, the integration of diagnostics into these delicate parts, as well as their final assembly onto the cassette body. “We at EFDA are eagerly awaiting the day ITER becomes a reality and we can put into practice all the knowledge and experience that we have gained in the past years”, said Merola.

FE200 is a high heat flux testing facility located at Le Creusot (France) and is operated by AREVA – Framatome ANP and Association Euratom-CEA Cadarache. It is used to simulate the cyclic thermal loads, which act on the plasma-facing components of a fusion reactor. The heat flux is obtained by a 200 kW electron beam gun and can be varied over a wide range of pulse durations and power densities.