Uptake of deuterium in partially D-depleted residual codeposits left on DIII-D tiles after laboratory thermo-oxidation experiments

Abstract

Recent evidence has shown that tokamak carbon-based codeposits may become partially or fully depleted of hydrogen through thermo-oxidation, as the hydrogen content of the codeposits is removed more rapidly than the carbon content. In this study we examine the ability of such partially-depleted residual DIII-D divertor codeposits to uptake deuterium upon subsequent exposure to deuterium gas or deuterium plasmas. The partially D-depleted specimens used here were obtained from a previous study where DIII-D codeposits were oxidized for 2 h at 623 K (350 °C) and 267 Pa (2 Torr) O 2 [J.W. Davis et al., Thermo-oxidation of DIII-D codeposits on open surfaces and in simulated tile gaps, J. Nucl. Mater. 415 (2011) S789–S792]. In the present study some of these specimens, having undergone prior oxidation, were exposed to D 2 glow discharge plasmas or D 2 gas at 20 kPa (150 Torr) at 300 or 523 K. In the case of plasma exposure, no uptake of D was observed, while an increase in D content was seen following D 2 gas exposures. When the gas exposure took place at 300 K, heating the specimens in vacuum to 623 K for 15 min led to the release of all of the increased D content. For the gas exposure at 523 K, the increase in D content was found to require longer (8 h) vacuum baking to remove. However, in a reference codeposit specimen (from a closeby location on the tile), which had not been previously oxidized, there was a similar increase in D content following D 2 exposure at 523 K, but it could not be released even following 8 h vacuum baking at 623 K.