The ITER-Like Wall, now being installed tile by tile in the JET vessel has produced many challenges. When the project “ITER-Like Wall” started, no reliable procedure was available to add tungsten or beryllium coatings to a porous Carbon-fibre composite (CFC) surface. Industry could not fulfil this need. Cristian Ruset and his colleagues from the National Institute for Laser, Plasma and Radiation Physics (NILPRP) of the Romanian Association MEdC, attended the very first project meeting. He and his team had been working in this research field for many years and they immediately recognised the opportunity to add their expertise to the project. Because of the time pressure the project had to face, action was taken straightaway: Cristian and his colleagues proposed a method of coating CFC tiles with tungsten layers. With his proposal accepted, all that was needed at this stage was to find a test stand. With the help of Hans Maier and the IPP test stand GLADIS in Germany the first Romanian-made coatings passed the test easily. Encouraged by the positive results Cristian was asked to produce coatings twice as thick, which he did in a very convincing manner. Undoubtedly the “coating story” is a success story of the advantage of working in a strong community. JET Insight talks to Cristian Ruset and Hans Maier.

The physicist Cristian Ruset is the Responsible Officer for tungsten coating of CFC tiles for JET. He also worked in the field of plasma surface engineering and plasma nitriding.

Which challenges did you have to face in order to coat the tiles?

JET was looking for the best technology to be applied at industrial scale for tungsten coating of approx. 2,000 CFC tiles with layers of 10 micrometres and 200 micrometres. These coatings had to survive without delamination to intense thermal loading when the surface temperature reached 2,000 degree Celsius. This was a real challenge even for the 10 micrometre coatings. The reasons are the strong anisotropy of the CFC material and the big difference in the thermal expansion coefficients between tungsten and substrate. These problems have been solved for the 10 micrometre coatings by using a technique called Combined Magnetron Sputtering and Ion Implantation (CMSII) which has been developed in my institute since 2001 for producing nano-structured coatings. In 2009 we faced a another challenge, when the 200 micrometres tungsten coating for divertor tiles had to be replaced by 20-25 micrometres deposited by CMSII.

You started your research at a bench scale. How difficult was it to change later on to an industrial scale?

It was not easy. The experimental coating unit had a small chamber and one magnetron. In order to handle tile dimensions and productivity we designed a new chamber approximately ten times as large equipped with 24 magnetrons and the corresponding power supply. Our team designed, built and commissioned the new industrial coating unit in approx. 18 months, which was a real record! That was possible with a great team of dedicated specialists in plasma physics, material science, mechanical and electrical engineering, vacuum technique and electronics. Talking about people, I have to mention the particular support from Hans Maier from IPP who deals with the high heat flux tests and Guy Matthews, ITER-Like Wall project leader at JET.

Do you think that these excellent results have any impact on other projects?

Yes. The CMSII technology was applied and successfully tested for tungsten coating of approximately 350 tiles for the ASDEX Upgrade tokamak at IPP Garching. As far as ITER is concerned, the current strategy for the divertor includes only bulk tungsten. It is possible that the future strategy will be influenced by the results obtained with the ITER-Like Wall at JET.

Hans Maier, technical leader in the project, has been gathering experience on tungsten coatings at IPP since 1996. Since early 2005 he has been working on tungsten coatings on CFC for the ITER-Like Wall Project.

What did you see as the ingredients, for these successful results?

There was no technical solution available because of the thermo-mechanical incompatibility between tungsten and CFC. So the necessary activities were a combination of technology development and scientific work. Therefore experience in both science and technology was necessary. The whole scope of our abilities was required – from high heat flux testing via electron microscopy to analysis even with more specialised tools like X-ray photoelectron spectroscopy or nuclear reaction analysis.

What were your thoughts when you received the first sample of the tiles?

That depends on what you consider to be the “first sample“. Actually the first tile material I received 5 years ago was a batch of 100 kilograms of CFC to be machined into test tiles at IPP for the research and development phase. We were under extreme pressure due to lack of time, the whole initial R&D phase had to be rushed through in just a few months. When we received the first “real” JET tiles with coatings applied for high heat flux testing, we had already gone a long way together with our colleagues in Bucharest and also at JET. At that time we were really confident.

Do you think that these excellent results have any impact on other projects?

In fact coatings from our Romanian colleagues are already in use in ASDEX Upgrade. Also their performance in the ITER-Like Wall at JET is crucial for allowing useful predictions for ITER. I assume that there may be a need for tungsten coatings in future fusion reactors: Coating the plasma-facing surface of breeding blanket modules with 1-2 millimetres of tungsten deposited by vacuum plasma spraying or chemical vapour deposition could be a very economic solution.
A lesson learned for ITER may be that up-scaling from prototypes to full-size components may reveal surprises, so a team with scientific experience should accompany such activities. Flexible high heat flux testing and advanced analysis tools will be indispensable.