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Interaction of beryllium with hydrogen plasma - nuclear fusion first wall investigations

Beryllium is one of the most promising candidates as material for the first wall of future nuclear fusion devices. However, the processes that take place on the wall surface in such an environment are not fully investigated yet.
We are concentrating on the question whether an oxide layer will develop on the beryllium surface under these conditions and if so, how it influences the interaction with the plasma, especially the retention of hydrogen in the bulk.

Previous work

The first step in this project was the determination of the oxidation kinetics of beryllium in air. This consisted of determining the thickness of the oxide layer of beryllium samples that were heated for durations between 1 and 50 hours at temperatures up to 600 °C.
The samples were heated in our tubular furnace, measurement of the oxide layer thickness was done by sputter depth profiling in our Auger spectrometer. For this purpose, a fine polished sample surface is needed. As beryllium dust is highly toxic, we set up a glove box with a polishing machine including a closed water circuit for wet polishing.
According to our results, the oxide growth of beryllium in air follows a square root law, as is expected for diffusion limited oxidation, with an activation energy of 1,16 eV.

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Current investigations

We attached a hydrogen supply and an R.F. plasma generator to the tubular furnace to simulate the conditions in the plasma edge region of a fusion reactor (which is possible only to a certain degree). The first heating cycles (up to 400 °C) in hydrogen plasma showed that previously thick oxide layers are reduced by a certain amount, but oxygen-containing impurities in the furnace (water vapour) cause previously thin oxide layers to grow, resulting in a constant medium oxide thickness for all samples at the end of these experiments.
The next important step will be to establish a relation between the amount of impurities in the furnace and the final oxide thickness. After that, the amount of hydrogen that the beryllium samples take up during these plasma treatments will be measured by outgasing experiments. The final goal (for the time being) is to find a dependence of this hydrogen retention on the thickness of the oxide layer that develops during plasma exposure.