It is not only changing of carbon for beryllium and tungsten tiles. ITER, in particular, and the whole fusion community in general look at JET’s ITER-Like Wall project. With the participation of 19 industrial companies, the contribution of 11 European fusion Associations and a budget of over 30 Million Euros, replacing the wall and subsequently operating JET is going to be a learning exercise critical for the success of ITER.

“It’s impossible to give the exact number of people involved in this project, but I estimate that it is around one hundred”, says Guy Matthews, Leader of the ITER-Like Wall project. First discussions about the scope started in 2004 and the Project Board held its first meeting in early 2005. “The challenge was not only that we had less than a year’s time to go from the concept to a design that could be tended. On top of that we had to redesign everything in the machine”.

The ITER-Like Wall Project is not the responsibility of any particular European Fusion Association but is administered within the same framework under which UKAEA acts as operator of the JET machine. Two reasons are responsible for this arrangement: The exchange of more than 4,500 plasma facing components requires detailed knowledge of the vessel interior and of the Remote Handling systems. Guy explains “You have got to use the existing support structures within the vessel and to understand exactly how Remote Handling works.”

The project management is supported by the EFDA Close Support Unit (CSU) at Culham which supervises the contracts. David Fraboulet, Project Responsible Officer in the CSU explains his role as follows: “CSU has an interface role between the client, which is the European Commission, and the project”.

The contribution of 11 European Fusion Associations has been essential to conduct in particular the research and development activities necessary to design and qualify the various components. The Romanian Fusion Association (MEdC) was given an EFDA task to develop a new process for coating carbon fibre tungsten composite tiles with tungsten. This has proven superior to the available industrial processes and essential for the success of the project.

Beryllium and tungsten replace carbon

picture of Guy Matthews

Guy Matthews, Leader of the ITER-Like Wall project

During plasma operation the interaction between hot plasma particles and the vessel wall components leads to the wall components experiencing high thermal loads. Therefore a proper choice of materials forming the plasma facing components is needed in order to accommodate the heat loads. So far, JET has been operating with carbon tiles in its first wall. Carbon and hydrogenic species (hydrogen, deuterium, and tritium) form compact molecules that can be trapped in remote areas within the vessel. Guy explains:”The reason for the change of materials is to eliminate all this chemistry which produces very high erosion and migration rates of hydro carbon.”The choice of materials for the ITER-Like Wall is, indeed, the one foreseen for the activated phase of ITER. The lightest metal beryllium was chosen for the main chamber. It is useful in engineering applications and well tolerated by the plasma. The twenty times heavier metal tungsten is one of the best in high temperatures and therefore the material of choice for the hottest parts on the bottom of the vessel, the divertor.

The variety of material combinations is quite impressive: bulk beryllium, beryllium-coated Inconel, beryllium-coated carbon fibre composite (CFC), bulk tungsten, tungsten-coated CFC.

In spite of its advantages this solution bears some challenge for the engineers. They have to cope with lower electrical resistivity of beryllium leading to high forces on tile support structures. Engineering Design and Manufacture Team Leader Eric Villedieu explains:”We had to recover for acceptable forces and the only solution was, to split each beryllium or tungsten tile in up to twenty pieces or use thin coatings. A lot of work has had to be done adapting the new components to existing machine constraints, so we could mimic the planned material configuration of ITER.” In order to keep the production of the tiles as simple as possible the project team opted for a casting process, which is “quite innovative for the fusion community”, as Eric adds.

The new divertor will be made of tungsten. In the given temperature range for fusion experiments this metal shows excellent thermal properties comparable to those of carbon, and low erosion rates. Guy Matthews stresses that JET will operate after the refurbishment as a all metal machine:”Carbon is to all intended purposes indestructible. So, by putting a meltable wall plus an upgraded heating system in JET we have to develop control and protection procedures. With the metal wall, running JET is very much more like running ITER”.

Precise answers for ITER

ITER will operate with long pulse duration and low core impurities in order to achieve its objectives. Carbon is no longer the material of choice for the first wall armour because it would trap a considerable amount of deuterium and tritium in the material of the inner wall. Making the right choice for the wall materials could be the difference between success and failure. Beside the scientific aspects Guy adds another one:”It is less expensive to make mistakes in JET rather than in ITER”.

Richard Pitts, acts as Senior Scientific Officer at ITER and is in charge of Plasma Edge Physics and Plasma-Wall Interactions. He will be looking at JET’s ITER-Like Wall project very carefully:”An experimental test with the materials mix foreseen for ITER, in a device large enough to approach some of the plasma edge conditions expected on ITER, will increase confidence that the ITER strategy is reasonable. JET should be able to establish that reliable tokamak operation at high power with acceptably low retention of hydrogenic species is indeed possible. The ITER-Like Wall project at JET truly is a defining experiment of unique importance to ITER.”

Petra Nieckchen