T. Shibahara

Hydrogen retention of JT-60 open divertor tiles exposed to HH discharges

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??m 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.

Hydrogen retention and carbon deposition in plasma facing components and the shadowed area of JT-60U

In JT-60U, erosion/deposition analyses of the plasma facing wall have shown that local carbon transport in the inboard direction was appreciable in addition to long-range transport. The total deposition and erosion rates in the divertor region were ~1 × 1021 C atoms s−1 and ~−6 × 1020 C atoms s−1, respectively. About 40% of the deposition in the divertor region likely originates from the main chamber wall. At the plasma facing surfaces of the divertor region, the highest hydrogen concentration in the (H + D)/C ratio and the retention amount were found to be ~0.13 and ~1 × 1023 atoms m−2, respectively. In the plasma-shadowed area underneath the divertor region with a vacuum vessel baking temperature of 420 K, redeposited layers of ~2 µm thickness were found with a high hydrogen concentration of ~0.75 in (H + D)/C, which was nearly the same level as that observed in JET. Large deuterium retention was also observed at the main chamber wall covered with boron layers. Their H + D retention and (H + D)/C were ~1 × 1023 atoms m−2 and ~0.16, respectively, for the vacuum vessel temperature of 570 K. Such a high deuterium retention is most likely caused by D retained in the boron layers. Nevertheless, the integration of this retention over the whole main chamber wall results in significant inventory and needs further investigation.

Retention of Hydrogen Isotopes in Divertor Tiles Used in JT-60U

Retention characteristics of deuterium and hydrogen retained in graphite tiles placed in the divertor region of JT-60U were investigated by thermal desorption spectroscopy (TDS). The deuterium retained in the near surface of all graphite tiles was mostly replaced by hydrogen due to exposure to hydrogen plasma at the final stage operations, resulting in main deuterium retention in the deeper region. The dominant species desorbed from the divertor tiles were H2, HD, D2 and CH4. The smallest retention of hydrogen isotopes (H+D) was observed in the outer divertor tile which was eroded with maximum of 20 μm depth. The amount of H+D retained in the inner divertor tiles covered by the re-deposited layers increased with the thickness of the re-deposited layers. Hydrogen isotopes concentration ((H+D)/C) in the re-deposited layers was ~0.02, which was much smaller than those observed in JET and other devices.

Application of electron stimulated desorption for hydrogen removal from graphite

We have applied an electron stimulated desorption (ESD) technique for hydrogen removal from graphite. Hydrogen was charged into graphite by either absorption or ion implantation. The initial ESD yield was about 0.2 atoms/electron, agreeing with the literature values. The desorption rate decreased with succeeding electron irradiation. However, the desorption continued for a long time and more than half of the total H retention was released after 100 s of irradiation. The desorption rate also increased with the incident electron energy, reaching a maximum at around 850 eV and decreased at higher energies. Thus ESD was found to be effective for hydrogen release from graphite.