B.M.U. Scherzer

Trapping, detrapping, and replacement of keV hydrogen implanted into graphite

Trapping and detrapping of low energy hydrogen isotopes in graphite is relevant to the collection of fuel particles in graphite probes for diagnostics of particle fluxes and energies in the plasma boundary layer, and to the recycling of fuel particles. The measurement of saturation concentrations by depth profiling of D may be influenced by detrapping of deuterium by the analyzing ion-beam. Detrapping yields have been measured for 1 and 8 keV deuterium implanted to saturation concentrations in papyex and pyrolytic graphite at room temperature by 790 keV H+, 400–2500 keV 3He+, and 400–1600 keV 14N+ ions. Initial detrapping yields of 1 keV implanted deuterium by 790 keV 3He+ and 14N+ are 4 and 56 D-atoms/ion, respectively. The detrapping yiled for 14N+-ions increases with increasing energy similar to the electronic stopping power of the ions indicating the influence of electronic excitation on the detrapping mechanism. Detrapping by 790 keV H+ was found to be negligible. Papyex and pyrolytic graphite show similar behavior. Careful determination of the deuterium saturation concentration from the D concentration profile by D(3He, α)H nuclear reaction method and taking detrapping into account yields a value of 0.4 D-atoms/C-atom. Furthermore, the replacement of hydrogen by deuterium at equal ranges (8keV H+, 7keV D+) has been measured in pyrolytic graphite with elastic recoil detection (ERD) by 2.6–2.8 MeV 4He ions. The H/D isotopic replacement behavior in saturated layers is similar to that found in metals at low temperatures and in low-Z coatings as TiC at room temperature.

Blistering of Niobium Due to 0.5_keV to 9_keV Helium and Hydrogen Bombardement

Abstract Blistering of well-annealed niobium single crystals due to 0.5 to 9 keV helium and hydrogen ion bombardment at temperatures between −110°C and 1000°C has been investigated by Rutherford backscattering (RIBS) in double alignment and with a scanning electron microscope (SEM). For He bombardment blistering was observed by RIBS in the temperature range investigated for all energies above 1 keV. The critical dose at which blisters first appear is about 1 to 2 × 10 17 incident He ions per cm 2 . It increases slightly with increasing ion energy and with decreasing target temperature. Blisters of 500 to 5000 A in diameter were found. The depth at which the blisters develop increases from ≈ 180 A for 1 keV to 1100 A for 9 keV He ions. It is a factor of ≈ 3 larger than the theoretical mean range of the ions in amorphous material. Above ≈ 600°C grain boundaries develop extending also into the unbombarded region. For hydrogen bombardment no blistering could be observed at room temperature up to doses of 2 × 10 19 ions per cm 2 .

Trapping and Replacement of 1_keV - 14_keV Hydrogen and Deuterium in 316 Stainless Steel

Abstract The trapping of deuterium with energies between 1 and 14 keV implanted into 316 stainless steel at 150 K has been studied using the D( 3 He, p) 4 He nuclear reaction. Initially the trapping is 100% up to fluences of about 2 × 10 17 –1 × 10 18 D / cm 2 , depending on the ion energy. Saturation is reached at fluences of 5 × 10 17 –3.5 × 10 18 D / cm 2 . The depth profiles obtained from the α-energy spectra show that at low temperature a deuterium to metal atom ratio of 0.9 to 1.0 is reached at saturation. There is no obvious diffusion of the implanted D into the bulk. Subsequent bombardment of the deuterium saturated surface by protons of the same energy results in an exponential decrease of the trapped deuterium. The opposite process, i.e. the release of protons previously implanted up to saturation by deuterium bombardment, shows the same behavior. The replacement may be described by a two-term inverse exponential function, which represents an easily replaced component of trapped deuterium as well as a replacement resistant component. Depth profiles in the advanced stages of replacement show that near surface deuterium is replaced with greater efficiency than deuterium trapped beyond the mean projected range. The replacement mechanism is found to be independent of isotopic mass and insensitive to implant energy in the range 1–14 keV. The replacement process is believed to involve detrapping as the rate limiting step.

Deuterium retention and re-emission from tungsten materials

Deuterium retention in five types of tungsten samples (W single crystal, W produced by hot pressing, chemical vapor and plasma sprayed deposited W coatings) has been investigated during and after implantation with 1.5 keV D-ions at 300, 600, and 900 K by means of re-emission, thermal desorption spectroscopy and nuclear reaction analysis measurements. Deuterium inventory and its spontaneous and thermal release depend strongly on the structure of the material. At 300 K, the saturated amount of D retained in the samples is in the range 1 × 1017 to 4 × 1017 D/cm2. For higher implantation temperature, 900 K, the retained amount is smaller by a factor of about 50 than for 300 K. During implantation D is retained far beyond the implantation zone. An amount of ≅ 3 × 1016 D/cm2 is trapped in the near-surface layer during implantation at 300 K. More than 50% of the implanted D diffuses into the bulk and is captured by lattice imperfections.

Temperature dependence of trapping and depth profiles of 6 to 15 kev deuterium in carbon

Abstract Depth profiles of 30 keV D + 2 and 20 keV D + 2 implanted into edge and basal-oriented pyrolytic graphite have been measured by means of the D( 3 He,α)H nuclear reaction in the temperature range of 300 to 800 K. At room temperature deuterium concentrations up to 30 at.% are found in a surface layer corresponding to the range of the ions. The measured depth profiles do not fully agree either with calculated range profiles or with the damage profiles, but are determined by the two together. At higher temperatures the deuterium concentrations decrease and the profiles broaden. At room temperature the amount of trapped deuterium increases linearly with dose below 10 18 deuterons/cm 2 . The trapping coefficient is roughly 60%. At 5 × 10 18 deuterons/cm 2 the amount of trapped deuterium in the probed layer (~4000 A) reaches saturation and the trapping coefficient becomes zero. The saturation value decreases with increasing temperature and increases with increasing energy.

Thermal desorption of D2 and CD4 from bulk-boronized graphites

Abstract Thermal desorption spectra of D2 and CD4 from bulk-boronized graphites containing 3 to 38 wt% B as well as from pure graphite and stoichiometric B4C were measured. The samples were implanted with 3 keV D+3 ions at temperatures between 130 and 700°C to fluences between 2.3 × 1016 and 2.3 × 1019 D/cm2 prior to the desorption. An enhancement of D2 release is observed with increasing boron content, while the CD4 release decreases correspondingly. The desorption peak maximum of D2 is shifted towards lower temperatures with increasing boron concentrations. Apparently, it is the boron in solid solution which mainly influences the trapping and release behaviour of deuterium. The formation of CD4 is suggested to proceed via a precursor state. The formation of the complete CD4 molecule takes place only close to the desorption temperature. The reduction of binding energy for D has only a minor influence on ion-beam-induced detrapping.

Damage and deuterium trapping in highly‐oriented pyrolytic graphite

In this work the buildup of damage due to deuterium implantation in highly‐oriented pyrolytic graphite (HOPG) is investigated. HOPG was implanted with 10–30 keV D3+ at different target temperatures between room temperature and 773 K with fluences from 1014 to 1018 D/cm2. Subsequently, the damage due to the implantation and the retained deuterium were measured by Rutherford backscattering (RBS) in a channeling direction (RBSc) and by the D(3He, p)α nuclear reaction analysis (NRA), respectively. The damage of selected samples was additionally observed with transmission electron microscopy (TEM). The initial trapping efficiency is unity in the whole temperature and energy range. The maximum retention of the deuterium, however, depends on the temperature and implantation energy. The damage in HOPG measured with RBSc starts to saturate at 5×1015 D/cm2 (295 K) and 1.3×1017 D/cm2 (773 K). Both fluences are well below the fluence at which amorphization is observed in TEM.

Spatial distribution of limiter material and impurities on the first wall of TFR 400

Abstract After two years of operation the tokamak TFR 400 was shut down. About 50 samples of the stainless steel liner were cut out at the outer circumference and at several places on the small circumference and analyzed by elastic backscattering of 2.5 MeV protons, by ion induced X-rays and by high resolution scanning electron microscopy combined with X-ray analysis. Deposits of all materials are found which have been used for limiters and probes positioned close to the plasma boundary during the history of the machine, i.e. B, C, Mo and W. Additionally oxygen is present. On the side of the limiter upon which the ion current is incident more impurities were deposited on samples with an azimuthal position between two main toroidal coils than on samples positioned in line with the coils indicating deposition as ions. On the small circumference the deposits varied in magnitude up to a factor of ten but no systematic positional dependence was found.

Evaluation of silicon doped CFCs for plasma facing material

Abstract To improve the CFCs characteristics of chemical erosion, tritium retention, and H2O/air resistivity, silicon doped CFCs were developed in the framework of European Technology programme. The tritium retention, H2O reaction kinetics, outgassing behaviour and depletion of silicon in doped CFCs were investigated as a function of silicon concentration and temperature. The results show that silicon in bulk of CFCs decreases the tritium bulk retention and the H2O reactivity. However, it has been observed that the concentration of silicon is slightly decreased at elevated temperature. This paper presents the experimental results and the consequence is discussed.

Trapping and mutual release of D and 3He in molybdenum

Abstract Trapping of 4, 8, 16 keV 3 He and 4, 8 keV D up to saturation and release of implanted 3 He by D bombardment and vice versa was investigated in polycrystalline Mo targets at 160 K using the 3 HeCd, p) 4 He nuclear reaction. In a saturated sample the initial release does not depend on whether the range of the first implant is smaller, nearly equal, or larger than the second implant. This behaviour cannot be explained by a replacement process. In unsaturated samples the release of the first implant occurs only after the second implant has reached saturation.