Mixed-material layer formation on graphite exposed to deuterium plasmas containing beryllium

Mixed-materials formation and properties are examined for graphite-target surfaces exposed to deuterium plasma containing small amounts of ionized beryllium. Targets are exposed to plasma in the PISCES-B divertor plasma simulator under conditions relevant to the operation of the graphite-strike plates in ITER. X-ray photoelectron spectroscopy (XPS) is used to analyze targets following exposure and reveals chemical reaction of the surface graphite with the incident-plasma beryllium flux. Partial surface carbidization is observed for a target exposure temperature of 450 K and full surface carbidization with Be2C at temperatures higher than this up to 1000 K. Rutherford backscattering spectrometry (RBS) data reveal a mixed-material layer of Be/C of varying-elemental concentration up to ∼1 μm thick. Hydrogen-isotope retention in targets is measured using thermal-desorption spectrometry (TDS) and D-3He nuclear-reaction analysis (NRA). Targets exposed to deuterium plasma exhibit retention consistent with values in the literature but extrapolated to the higher ion fluences possible in PISCES simulators. In contrast, targets with a mixed-material Be/C layer are found to have increased retention by a factor of ∼4 at low temperature ∼300 K and ∼2 at higher temperature ∼1000 K. Simulation of NRA spectra reveals that most of the increased inventory is accumulated in the mixed layer.