Assessment of Erosion and Surface Tritium Inventory Issues for the ITER Divertor

February 11, 1997

J. Nuclear Materials, 241-243, 294-298 (1997).

Brooks, J. N., Causey, R., Federici, G., Ruzic, D. N.

We analyzed sputtering erosion and tritium codeposition for the ITER vertical target divertor design using erosion and plasma codes (WBC/REDEP/DEGAS+) coupled to available materials data. Computations were made for a beryllium, carbon, and tungsten coated divertor plate, and for three edge plasma regimes. New data on tritium codeposition in beryllium was obtained with the tritium plasma experiment (TPE) facility. This shows codeposited H/Be ratios of the order of 10% for surface temperatures ≤ 300°C, beryllium thereby being similar to carbon in this respect. Hydrocarbon transport calculations show significant loss (10–20%) of chemically sputtered carbon for detached conditions (Te ≈ 1 eV at the divertor), compared to essentially no loss (100% redeposition) for higher temperature plasmas. Calculations also show a high, non-thermal, D-T molecular flux for detached conditions. Tritium codeposition rates for carbon are very high for detached conditions (not, vert, similar 20 g T/1000 s discharge), due to buildup of chemically sputtered carbon on relatively cold surfaces of the divertor cassette. Codeposition is lower (not, vert, similar 10X) for higher edge temperatures (not, vert, similar 8–30 eV) and is primarily due to divertor plate buildup of physically sputtered carbon. Peak net erosion rates for carbon are of the order of 30 cm/burn yr. Erosion and codeposition rates for beryllium are much lower than for carbon at detached conditions, but are similar to carbon for the higher temperatures. Both erosion and tritium codeposition are essentially nil for tungsten for the regimes studied.

Measurement and Modeling of the Angular-Resolved Sputtering Yield of D-Soaked Be by 100, 300, 500 and 700 eV D+

February 11, 1997

J. Nuclear Materials, 241-243, 1170-1174 (1997).

Ruzic, D. N., Smith, P. C., Turkot Jr., R. B.

The angular-resolved sputtering yield of Be by D+ was predicted and then measured. An ion beam at 100, 300, 500 and 700 eV from a Colutron ion source was focused onto S-65 C grade Be samples. The sample was exposed in situ to a 350 V dc D plasma to remove oxide, load the surface with D and more-nearly simulate the surface which would be found during steady-state fusion device operating conditions. The angular distribution of the sputtered atoms was measured by collection on a highly ordered pyrolytic graphite witness plate. The areal density of Be (and BeO, after exposure to air) was then measured using a scanning Auger spectrometer. Total deposition was measured by deposition onto a quartz crystal oscillator placed alongside the witness plate. A three-dimensional version of vectorized fractal TRIM (VFTRIM3D), a Monte-Carlo computer code which includes surface roughness characterized by fractal geometry, was used to predict the angular distribution of the sputtered particles and a global sputtering coefficient. One-quarter million trajectories were simulated to determine the azimuthal and polar angle distributions of the sputtered atoms. A fractal dimension of 2.05, and a surface binding energy of 3.38 eV, both standard values for Be, were used. Results show reasonable agreement between the code and experimental values for total yield with the experimental yields somewhat lower. The measured angular distribution is broader (less forward peaked) than predicted by the computer simulation.