In investigations of hydrogen retention in first wall components the influence of the conditions of the implanted target storage prior to analysis and the storage time is often neglected. Therefore we have performed a dedicated set of experiments. The release of hydrogen from samples exposed to ambient air after irradiation was compared to samples kept in vacuum. For air-exposed samples significant amounts of HDO and D 2 O are detected during TDS. Additional experiments have shown that heavy water is formed by recombination of releasing D and H atoms with O on the W surface. This water formation can alter hydrogen retention results significantly, in particular -for low retention cases. In addition to the influence of ambient air exposure also the influence of storage time in vacuum was investigated. After implantation at 300K the samples were stored in vacuum for up to one week during which the retained amount decreased significantly. The subsequently measured TDS spectra showed that D was lost from both the high and low energy peaks during storage at ambient temperature of ~300K. An attempt to simulate this release from both peaks during room temperature storage by TMAP 7 calculations showed that this effect cannot be explained by conventional diffusion/trapping models.Keywords: tungsten, deuterium, hydrogen, retention, time, oxide, heavy water
1.IntroductionThe conventional way to study hydrogen retention in plasma facing components (PFC's) is to perform a laboratory experiment, where deuterium implantation is done by plasma or ion beam exposure of a sample. Then the amount of retained deuterium is investigated by nuclear reaction analysis (NRA) and thermal desorption spectroscopy (TDS). In the case of tungsten NRA is limited to a maximum depth of ~10 µm [1], which means in the case of extensive diffusion (large irradiation time or high temperature) the full amount of deuterium can be detected only by TDS analysis. In most cases D is released in the form of D 2 and HD, however depending on how the experiment is conducted the ion currents of HDO and D 2 O in mass-spectrometer can be considerably larger than D 2 and HD (see Fig. 1). This obscures evaluation of total retained D amount since there is no conventional way to calibrate setup sensitivity for water (mass 19 and 20). Therefore the formation of heavy water should be avoided.
Abstract.Deuterium retention in plasma-sprayed tungsten (PSW) was investigated by means of the thermodesorption technique. . The observed retention in PSW is four times higher than in polycrystalline tungsten (PCW). An additional high-temperature peak was found in the thermodesorption spectra of PSW (at 1050 K). The nature of the high-temperature peak and the mechanism of higher retention in PSW are discussed.
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