The influence of a beryllium containing plasma on the evolution of a mixed-material surface

Experiments at UC San Diego PISCES-B, in collaboration with EFDA, are investigating the influence of beryllium impurities on deuterium plasma erosion of graphite material. The experiments are designed to reduce uncertainties in the prediction of tritium retention in redeposited mixed-materials expected in future burning plasma devices. Earlier PISCES-B experiments hinted that small amounts of beryllium surface impurities on graphite could reduce the rate of chemical erosion. In the present experiment, beryllium is seeded into a deuterium plasma to simulate the flow of the scrape-off layer plasma from the first wall to the divertor in ITER. Plasma containing low levels of Be impurities have so far been investigated and a greater reduction of carbon erosion is found. For beryllium concentrations above similar to 0.1%, a thin coating of beryllium is observed to form on the surface of the graphite. This beryllium layer suppresses chemical and physical erosion of the underlying carbon. The ITER divertor plasma is expected to contain percent levels of beryllium impurities, so similar beryllium deposition may occur on the ITER divertor plates. Future experiments at PISCES will quantify the formation rate of redeposited carbonaceous films and their suppression, and aim at better simulating ITER conditions including higher operating temperature ranges for the graphite targets, and larger beryllium impurity fractions.