Looking for the right material for ITER´s wall

The Sun shows us how and releases energy by fusing atomic nuclei. Nuclear fusion could also become a climate friendly source of energy on Earth as well. The experimental reactor “ITER” (Latin for “path”) aims to demonstrate the feasibility of this technology. Countries from around the world are collaborating on building the future test reactor ITER in Cadarache, southern France. The aim is to achieve 500 megawatts of fusion power. The running of the test reactor is being prepared in numerous projects. For example, scientists from more than twenty institutions of the European Fusion Programme are working together to study how the fusion fuel will interact with the wall of the plasma chamber.

The scientists have to succeed in confining the fuel – a thin ionised hydrogen gas in a plasma state – in a magnetic cage and then in heating it up to ignition temperatures of over 100 million degrees. The interplay between the extremely hot fuel and the wall of the surrounding vessel represents one of the greatest challenges for the research.

“The plasma chamber wall for ITER must be resistant against the high thermal load that can occur in cases of plasma instability,” explains Dr. Rudolf Neu from the Max Planck Institute for Plasma Physics (IPP) in Garching, deputy head of the project group. “Furthermore, it has to store the smallest amounts possible of the radioactive fuel component tritium, produce as little material dust as possible and be resistant to material mixes that occur when wall material is eroded by plasma particles and is later deposited on other parts of the wall.”

Most of the problems could be avoided by using tungsten as a wall material. The IPP fusion device ASDEX Upgrade in Garching is the only one in the world that can experiment with a wall completely covered in tungsten – and the first results are highly promising.

The background: Energy rich plasma particles can knock atoms out of the wall, which can then penetrate the plasma and contaminate it. In contrast to light hydrogen, the heavy atoms from the wall are not fully ionised, not even at the high fusion temperatures. The more electrons that are still bound to the nuclei, the more energy they withdraw from the plasma and radiate this as ultraviolet or X-ray light. This causes the plasma to cool, dilutes it and so reduces the fusion output. If contamination by light atoms is still tolerable in concentrations of just a few per cent, the limit for heavy particles such as iron, chromium or even tungsten is much lower. This is why present-day plants all use light materials, such as beryllium or carbon.

These materials are also planned for the wall of the test reactor ITER. In the large-scale ITER plant however, this is no longer that easy. For example, the sputtering of carbon or beryllium is relatively high under bombardment with hydrogen. In the case of the high hydrogen flows from the large ITER plasma, this would result in strong material erosion. A tungsten wall would avoid the problems of the light elements.

The metal possesses advantageous thermal properties, low sputtering levels through hydrogen and reveals no long-term depositing of tritium. However, the question remains of how many heavy tungsten particles are able to penetrate into the plasma core. Their number must not exceed a share of several hundred thousand parts in the ITER plasma.

The Garching experiment ASDEX Upgrade is a pioneer in testing tungsten as a wall material. Despite poor experiences gained elsewhere, Garching had already begun to coat special parts of the wall with tungsten in 1996, instead of otherwise fully covering the wall with carbon tiles. In so doing, the scientists counted on the cold plasma edge of ASDEX Upgrade, which acts much like the later ITER. Two years ago, experiments using a pure tungsten wall began – with success.

The tungsten concentration remained below the critical threshold and the desired plasma states could be reached with only minor quality losses. Now the task is to carefully check and verify the tungsten’s compatibility in the ITER relevant plasma states. To do this, comparative experiments are planned on the JET plant, the Joint European Torus in Britain, which is twice the size of ASDEX Upgrade, in order to be able to transfer the results with reliability to the even larger ITER.