Deuterium Retention and Physical Sputtering of Low Activation Ferritic Steel

Low activation materials have to be developed toward fusion demonstration reactors. Ferritic steel, vanadium alloy and SiC/SiC composite are candidate materials of the first wall, vacuum vessel and blanket components, respectively. Although changes of mechanical-thermal properties owing to neutron irradiation have been investigated so far, there is little data for the plasma material interactions, such as fuel hydrogen retention and erosion. In the present study, deuterium retention and physical sputtering of low activation ferritic steel, F82H, were investigated by using deuterium ion irradiation apparatus. After a ferritic steel sample was irradiated by 1.7 keV D^ ions, the weight loss was measured to obtain the physical sputtering yield. The sputtering yield was 0.04, comparable to that of stainless steel. In order to obtain the retained amount of deuterium, technique of thermal desorption spectroscopy (TDS) was employed to the irradiated sample. The retained deuterium desorbed at temperature ranging from 450 K to 700 K, in the forms of DHO, D2, D2O and hydrocarbons. Hence, the deuterium retained can be reduced by baking with a relatively low temperature. The fiuence dependence of retained amount of deuterium was measured by changing the ion fiuence. In the ferritic steel without mechanical polish, the retained amount was large even when the fluence was low. In such a case, a large amount of deuterium was trapped in the surface oxide layer containing O and C. When the fluence was large, the thickness of surface oxide layer was reduced by the ion sputtering, and then the retained amount in the oxide layer decreased. In the case of a high fluence, the retained amount of deuterium became comparable to that of ferritic steel with mechanical polish or SS 316 L, and one order of magnitude smaller than that of graphite. When the ferritic steel is used, it is required to remove the surface oxide layer for reduction of fuel hydrogen retention. Ferritic steel sample was exposed to the environment of JFT-2M tokamak in JAERI and after that the deuterium retention was examined. The result was roughly the same as the case of deuterium ion irradiation experiment.     

Implanted Deuterium Retention and Release in Carbon-Coated Beryllium

Deuterium implantation experiments have been conducted on samples of clean and carbon-coated beryllium. These studies entailed preparation and characterization of beryllium samples coated with carbon thicknesses of 100, 500, and 1000 Å. Heat treatment of a beryllium sample coated with carbon to a thickness of approximately 100 Å revealed that exposure to a temperature of 400°C under high vacuum conditions was sufficient to cause substantial diffusion of beryllium through the carbon layer, resulting in more beryllium than carbon at the surface. Comparable concentrations of carbon and beryllium were observed in the bulk of the coating layer. Higher than expected oxygen levels were observed throughout the coating layer as well. Samples were exposed to deuterium implantation followed by thermal desorption without exposure to air. Differences were observed in deuterium retention and postimplantation release behavior in the carbon-coated samples as compared with bare samples. For comparable implantation conditions (sample temperature of 400°C and an incident deuterium flux of approximately 6 × 10¹⁹ D/m²-s), the quantity of deuterium retained in the bare sample was less than that retained in the carbon-coated samples. Further, the release of the deuterium took place at lower temperatures for the bare beryllium surfaces than for carbon-coated beryllium samples.     

Deuterium trapping by irradiation damage in tungsten induced by different displacement processes

The deuterium trapping behaviors in tungsten damaged by light ions with lower energy (10 keV C⁺ and 3 keV He⁺) or a heavy ion with higher energy (2.8 MeV Fe²⁺) were compared by means of TDS to understand the effects of cascade collisions on deuterium retention in tungsten. By light ion irradiation, most of deuterium was trapped by vacancies, whose retention was almost saturated at the damage level of 0.2 dpa. For the heavy ion irradiation, the deuterium trapping by voids was found, indicating that cascade collisions by the heavy ion irradiation would create the voids in tungsten. Most of deuterium trapped by the voids was desorbed in higher temperature region compared to that trapped by vacancies. It was also found that deuterium could accumulate in the voids, resulting in the formation of blisters in tungsten.     

Deuterium retention in SiC coated graphite by D2+ implantation

The doped graphite tiles bolted to the active cooling heat sink, made of GBST1308 (1% B₄C, 2.5% Si, 7.5% Ti) coated with SiC, are now being used as the only plasma facing material (PFM) for the EAST device since the campaign of 2008. From the plasma density and fueling point of view, it is important to study thoroughly the hydrogen isotope retention in this kind of SiC-coated doped graphite. D₂⁺ implantations into the SiC coated doped graphite were performed at Shizuoka University. The chemical states of Si and C were studied by means of X-ray photoelectron spectroscopy (XPS), and the thermal desorption behavior of deuterium was analyzed by thermal desorption spectroscopy (TDS). It was found that deuterium was trapped by both C and Si in the SiC coatings. In the previous studies, Oya et al. reported the deuterium retention behavior in polycrystalline β-SiC. In this paper, difference of retention behavior in β-SiC and SiC coating will be also discussed.     

Ion-induced desorption of surface contaminants

We report measurements of mean desorption areas per incident ion for hydrocarbon contaminated surfaces of different solids bombarded with 30 keV H₂⁺, O⁺₂, and Ar⁺ and 15 keV O⁺ ions. The results for heavy ions are consistent with a momentum transfer mechanism for desorption. A molecular effect is observed for O₂⁺ ions which desorb significantly more than two separate O⁺ of the same velocity. Our results for hydrogen ions; together with results of other workers for low-energy light ions, lead us to the conclusion that desorption occurs efficiently in inelastic collisions.     

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

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(³He,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 10¹⁸ deuterons/cm². The trapping coefficient is roughly 60%. At 5 × 10¹⁸ deuterons/cm² the amount of trapped deuterium in the probed layer (~4000 Å) reaches saturation and the trapping coefficient becomes zero. The saturation value decreases with increasing temperature and increases with increasing energy.     

Enhancement of hydrogen isotope retention capacity for the impurity deposited tungsten by long-term plasma exposure in LHD

The stress relieved tungsten samples were placed at three positions, PI (sputtering erosion dominated area), DP (deposition dominated area) and HL (Higher heat load area) during 15th plasma experiment campaign in Large Helical Device (LHD) at National Institute for Fusion Science (NIFS), Japan and were exposed to ～ 6700 shots of hydrogen plasma in a 15th long-term experiment campaign in LHD. Thereafter, the additional deuterium ion implantation to these tungsten samples was performed to evaluate the change of hydrogen isotope retention capacity in the samples by long-term plasma exposure. It was found that the carbon-dominant mixed-material layer with more than 100 nm thickness was formed on a wide area of the tungsten surface. The thicker mixed-material layer was formed on the DP sample, where the deuterium retention was about 21 times as high as that for pure W. The major desorption temperature of deuterium was shifted toward higher temperature side, which was comparable to the trapping characteristic of carbon or irradiation damages.     

250 keV and 100 keV argon ion induced X-ray and optical emissions from a Cr/Ni sandwich on Si and Al samples

During 250 keV Ar⁺ ion bombardment of an Cr/Ni sandwich on Si samples [1] X-ray and optical emissions have been recorded. The optical spectrum showed pronounced Cr I lines from the first Cr layer of the bombarded sample. The X-ray spectra showed Ar K. lines. The Al foils bombarded by 100 keV Ar⁺ ions showed the optical line spectra with pronounced Al I optical lines as well as Al II and Al III optical lines, while the X-ray spectra obtained by 250 keV Ar⁺ ion bombardment showed pronounced K X-ray Al lines and less pronounced K X-ray argon lines. The correlation diagram for the Ar⁺→ Cr collisions shows that the Ar 1s electron can be promoted to the 2p levels of the united atom with a probability of vacancy creation in the Ar K shell. The Al K_α lines appeared as a consequence of the Al Is electron promotion to the 2p level of the united atom. The promoted Al Is electron left a vacancy in the Al K shell. Deexcitation in both the Ar and Al excited atom occurs when the K vacancy is filled by an electron from the upper shells.     

AFM surface investigation of polyethylene modified by ion bombardment

Polyethylene (PE) was irradiated with 63 keV Ar⁺ and 155 keV Xe⁺ ions to fluences of 1 × 10¹³ to 3 × 10¹⁵ cm⁻² with ion energies being chosen in order to achieve approximately the same penetration depth for both species. The PE surface morphology was examined by means of atomic force microscopy (AFM), whereas the concentration of free radicals and conjugated double bonds, both created by the ion irradiation, were determined using electron paramagnetic resonance (EPR) and UV–VIS spectroscopy, respectively. As expected, the degradation of PE was higher after irradiation with heavier Xe⁺ ions but the changes in the PE surface morphology were more pronounced for Ar⁺ ions. This newly observed effect can be explained by stronger compaction of the PE surface layer in the case of the Xe⁺ irradiation, connected with a reduction of free volume available.     

Comparative study of ion implantation caused damage depth profiles in polycrystalline and single crystalline silicon studied by spectroscopic ellipsometry and Rutherford backscattering spectrometry

Damage created by ion implantation of Ar⁺ ions into polycrystalline (p-Si) and single-crystalline silicon (c-Si) was characterized using Spectroscopic Ellipsometry (SE), Rutherford Backscattering Spectrometry (RBS), and Transmission Electron Microscopy (TEM). To create buried disorder, Ar⁺ ions with an energy of 100 keV were implanted into the samples. Ion doses were varied from 5×10¹³ to 6.75×10¹⁴ cm⁻². The parameters of the implantation were kept identical for both p-Si and c-Si. Damage depth profiles have been investigated using SE, RBS, and TEM, in case of c-Si, and SE and TEM in case of p-Si. The results prove the applicability of spectroscopic ellipsometry for characterizing ion implantation caused damage even in polycrystalline silicon, where the RBS method cannot be applied. The RBS and TEM results basically supported the optical model of SE.     

Nuclear reaction analysis of deuterium in deuterided Zr-2.5 wt% Nb alloy

The deuterium concentration of homogeneous bulk levels in Zr-2.5 wt% Nb alloy has been measured by nuclear reaction analysis. The accuracy is ± 10% which is comparable to the accuracy of gas analytical techniques. A large surface peak is seen in all the samples due to a deuteride rich layer less than 20 nm thick. The deuterium concentration in this surface layer is more than 10 times the bulk value.At high ion beam fluences, significant effects due to the analysing beam are seen in the deuterium depth distribution. These are ascribed to deuterium trapping in the ³He implanted region, deuterium diffusion and dissolution of deuterides.These samples are not recommended as general deuterium standards for ion beam analysis experiments because of ion beam effects and the presence of a surface peak. However, they still serve as a useful reference when care is taken to avoid effects due to high fluences.     

Temperature dependence of surface morphology and deuterium retention in polycrystalline ITER-grade tungsten exposed to low-energy,high-flux D plasma

Surface topography and deuterium retention in polycrystalline ITER-grade tungsten have been examined after exposure to a low-energy (38 eV/D), high-flux (10²² D/m² s) deuterium plasma with ion fluences of 10²⁶ and 10²⁷ D/m² at various temperatures. The methods used were scanning electron microscopy equipped with focused ion beam, thermal desorption spectroscopy, and the D(³He,p) ⁴He nuclear reaction at ³He energies varied from 0.69 to 4.0 MeV. During exposure to the D plasma at temperatures in the range from 320 to 815 K, small blisters of size in the range from 0.2 to 5 μm, depending on the exposure temperature and ion fluence, are formed on the W surface. At an ion fluence of 10²⁷ D/m², the deuterium retention increases with the exposure temperature, reaching its maximum value of about 10²² D/m² at 500 K, and then decreases below 10¹⁹ D/m² at 800 K.     

Suppression of sputtering of nickel by coverage with self-sustaining thin segregated carbon layers

Sputtering of two-layered films composed of nickel (~5000 Å) and nickel carbide (~1500 Å) at 600° C by 5 keV Ar⁺ bombardment on the nickel side has been studied using Rutherford backscattering of 1.3 MeV H⁺ ions. It is found that the removal rate of nickel atoms from specimens is dependent on ion current density and that the removal rate of nickel atoms is very much smaller than that of carbon atoms when the ion current density is low. During ion bombardments at a low current density carbon segregation by a thickness of nearly two monolayers is observed at the nickel surface. Thus suppression of the removal rate of nickel atoms is ascribed to coverage of the nickel surface with segregated carbon atoms which are continuously supplied by diffusion through the nickel film from the carbide layer.     

High energy primary knock-on process in metal-deuterium systems initiated by bombardment with noble gas ions

An experimental study confirms the possibility of nuclear fusion reactions initiating in metal-deuterium targets by bombarding them with ions that are not the reagents of the fusion reaction, in particular, with noble gas ions. The yields of (d,d) and (d,t) reactions were measured as functions of energy (0.4-3.2 MeV) and mass of incident ions (He⁺, Ne⁺, Ar⁺, Kr⁺ and Xe⁺). Irradiation by heavy ions produced a number of energetic deuterium atoms in the deuteride and deuterium + tritium metal targets. At ion energies of ∼0.1-1 MeV the d-d reaction yields are relatively high. A model of nuclear fusion reaction cross-sections in atomic collision cascades initiated by noble gas ion beam in metal-deuterium target is developed. The method for calculation tritium or deuterium recoil fluxes and the yield of d-d fusion reaction in subsequent collisions was proposed. It was shown that D(d,p)t and D(t,n)⁴He reactions mainly occur in energy region of the recoiled D-atom from 10 keV to 250 keV. The calculated probabilities of d-d and d-t fusion reactions were found to be in a good agreement with the experimental data.     

The angular distribution of sputtered silver atoms

Angular distributions of sputtered atoms have been determined for a Ag target under bombardment with 20 and 30 keV ²⁰Ne⁺, ⁴⁰Ar⁺, ⁸⁴Kr⁺ and ¹³²Xe⁺ ions both at normal and oblique angles of incidence. At normal ion incidence the distribution is symmet with respect to the target normal, while at oblique ion incidence the distribution is asymmetric in the plane containing the ion beam and the surface normal and symmetric in the transverse plane. Scanning electron microscopy of the sputtered surface shows the development of a high density array of cones in the bombarded area. The results are discussed from the viewpoint of sputtering from a very rough surface.     

Influence of surface condition on deuterium release from Li2TiO3 pebble

Lithium titanate (Li₂TiO₃) pebbles were irradiated with D₃⁺ ions with energy of 5.0 keV, and the amounts of retained deuterium in the pebbles were measured by thermal desorption spectroscopy. In this research the irradiation/heating cycles were carried out repeatedly in order to investigate the influence of surface condition on deuterium release from Li₂TiO₃. The composition ratio of Li decreased with the increase of the number of the irradiation/heating cycle. Then, the desorption peaks of the gases contained deuterium atoms were shifted to higher temperature region, and the amount of desorbed gases in forms of water tended to increase. In addition, we carried out other experiments for the comparison. Comparing these results, we considered that the increase of the defects created by the irradiation was more responsible for the change in the desorption behavior by the irradiation/heating cycles than the lithium depletion. These results suggest that the tritium recovery efficiency would decrease with the increase of the defects and the damages especially at the low temperature region during the operation.     

Angular distributions of Ni and Ti atoms sputtered from a NiTi alloy under He and Ar ion bombardment

The angular distributions of sputtered components were measured for NiTi polycrystalline alloy under 9 keV Ar⁺ and He⁺ ions bombardments with various fluences in ultrahigh vacuum. Combination of Rutherford Backscattering Spectrometry (RBS) and Auger Electron Spectrometry (AES) techniques allowed us to observe enhanced concentration of Ni over a layer with thickness comparable to a primary He⁺ ions penetration depth due to selective sputtering of Ti atoms and radiation-induced diffusion processes. A preferential emission of Ni atoms towards the surface normal was observed during bombardment by both He⁺ and Ar⁺ ions. More forward-peaked “over-cosine” angular distributions of sputtered Ni in comparison with those for Ti atoms have been measured. Nonstoichiometric sputtering of NiTi alloy dependent on emission angle was observed for bombardment fluence of He⁺ well below that needed for the steady-state altered layer formation. To explain the peculiarities of NiTi sputtering, an interpretation is discussed in terms of sputtering due to backscattered He⁺ ions.     

Helium and deuterium implantation in tungsten at elevated temperatures

High temperature helium and deuterium implantation on tungsten has been studied using the University of Wisconsin inertial electrostatic confinement device. Helium or deuterium ions from a plasma source were driven into polished tungsten powder metallurgy samples. Deuterium implantation did not damage the surface of the specimens at elevated temperatures (∼1200 °C). Helium implantation resulted in a porous surface structure above 700 °C. A helium fluence scan, ion energy scan, and temperature scan were all completed. With 30 keV ions, the pore formation started just below 4 × 10¹⁶ He⁺/cm². The pore size increased and the pore density decreased with increasing fluence and temperature. The energy scan from 20 to 80 keV showed no consistent trend.     

The influence of projectile charge state on ionization probabilities of sputtered atoms

The ionization probability of atoms sputtered from a clean polycrystalline metal surface was measured for different charge states of the projectile used to bombard the sample. More specifically, a polycrystalline indium surface was irradiated with Ar⁺ and Ar⁰ beams of energies between 5 and 15 keV, and In⁺ secondary ions and neutral In atoms emitted from the surface were detected under identical experimental conditions regarding the sampled emission angle and energy. The resulting energy integrated ionization probability of sputtered In atoms is consistently found to be smaller for neutral projectiles, the difference decreasing with decreasing impact energy. The observed trends agree with those measured for kinetic electron emission, indicating that secondary ion formation is at least partly governed by kinetic substrate excitation.     

A thermal desorption study of the surface interaction between tritium and type 316 stainless steel

The thermal desorption of tritium from a type 316 stainless steel exposed to gaseous tritium (HT-gas) was studied. In the desorption spectra of tritium, four distinct peaks appeared at about 430 (peak HT-1), 540 (peak HTO), 750 (peak HT-II) and 970 K (peak HT-III). The activation energies for the peaks HT-I, HTO and HT-II were determined as 20 ± 4, 27 ±7 and 55 ±9 kJ/mol, respectively. It is presumed that the tritium leading to the peak HTO forms tritoxyl ion (OT^?) by combining with surface O^2? ion or by the isotopic exchange with the protium of surface OH^? ion on the sorption process. The OT ion combines with the OH^? ion or the dissolved protium into HTO at the desorption process. The tritium leading to both peaks HT-II and HT-III is sorbed and dissociates into atomic species which diffuse into the bulk, while that leading to the peak HT-I is very weakly held on the topmost surface of stainless steel in the form of a molecule or HT⁺ ion.