Deuterium and impurity distributions on plasma eroded carbon materials for the NET first wall

Abstract

High energy ion beam analysis techniques (nuclear reaction analysis and Rutherford backscattering spectroscopy) have been applied for a quantitative and qualitative determination of the surface composition of four types of graphite based substrates exposed to high fluence, (l.l–31.8)×1025 m−2, of 50-200 eV deuterium ions in the PISCES-A plasma facility. Targets at temperatures of 350-980°C were exposed to the plasma under erosion and/or redeposition conditions. Deuterium and impurity atoms have been found on the analysed surfaces. The concentration of deuterium ranges from 4 × 1019 to 3.5 × 1022 m−2, mainly depending on the substrate temperature during the exposure but also on the impurity content on the surface. High concentrations and broad depth profiles of deuterium are found on substrates which have accumulated high levels of impurity atoms from the plasma. Boron (1 × 1019−l×1021 m−2), stainless steel components and copper (2×l018−5×l019 m−2) and lanthanum (4×1017−3.7×l019 m−2) are the most pronounced impurities detected on the samples. Boron and lanthanum originate from the LaB6 cathode of the plasma generator, whereas other metals are coming to the plasma from the anode (Cu) and PISCES wall (Fe, Ni, Cr). In many cases the areal distributions of deuterium and impurity atoms are observed to be non-uniform, varying by a factor of two, or more from point to point, even on surfaces affected by only one exposure to the plasma. A comparison between the collected amount of impurity atoms and the total deuterium fluence (CI(col)/FD) shows that the impurity fluence is in the range 2×l0−6−1.5×l0−5 of the deuterium fluence, and no dependence on the sample temperature is observed in this case.