Hydrogen retention in graphite tiles exposed to hydrogen discharges at the JT-60 open divertor has been investigated by means of thermal desorption spectroscopy (TDS). Most of the plasma facing area was covered with re-deposited layers of maximum thickness of about 70 µm appearing at the inner divertor region. Major parts of retained hydrogen were thermally desorbed as hydrogen molecules with a peak temperature of around 970 K. Almost all the hydrogen atoms were retained homogeneously in the re-deposited layers with an averaged hydrogen concentration of ∼0.03 in H/C, which is much smaller than the saturated hydrogen concentration (H/C = 0.4-1.0). Since the saturated hydrogen concentration in carbon materials decreases with increasing temperature, the re-deposited carbon layers are very likely subjected to higher temperatures during the discharges, which are supported by the higher release temperature of hydrogen in TDS. This result suggests that hydrogen retention can be significantly reduced with higher wall temperatures.

The elucidation of the trapping and detrapping mechanisms of hydrogen isotopes in SiC is one of the most critical issues for future fusion reactors if SiC is used as the first wall and structure material. In this study, 1 keV deuterium (D 2 þ ) ions were implanted into SiC and the chemical states of C and Si were evaluated by X-ray photoelectron spectroscopy (XPS). The deuterium desorption and retention were also analyzed by thermal desorption spectroscopy (TDS). The deuterium desorption behavior for SiC was compared to that for Si and graphite, and it was found that deuterium is preferentially trapped by C and, after the saturation of the C-D bond, it is trapped by Si in SiC. Deuterium desorption was found to consist of two stages, namely deuterium desorptions bound to Si and C. Their trapping mechanisms were influenced by the damaged structures produced by the D 2 þ ion implantation. Finally, deuterium retention in SiC at temperatures above 700 K was higher than that in graphite, indicating that tritium retention in SiC may be high compared to that in graphite during plasma operation.

In order to apply the dual-energy technique to material identification, a new computed tomography scanning system was proposed using a conventional X-ray tube and a CdTe detector. This system can provide information of projection data at two distinct energy bands for scanned materials. After introducing an approximation, the measured projection data were reconstructed to obtain the distributions of the X-ray linear attenuation coefficients of the materials at two different energies. Then, the corresponding atomic number and electron density can be derived with the dual-energy X-ray computed tomography (DXCT) method adopted. By comparing the obtained results with theoretical ones, the feasibility of using this system for identifying low-Z materials was demonstrated in this study.

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