Influence of temperature and plasma composition on deuterium retention in refractory metals

Refractory materials are being considered potential candidates to build the first wall of the fusion reactor chamber. This work reports on the results of the study of tungsten and molybdenum metals exposed to high flux densities (～10²⁴ D/m² s) and low temperature (T_e ～ 3 eV) deuterium plasmas in Pilot-PSI irradiation facility.The hydrogenic retention in poly-crystalline W and Mo targets was studied with ³He nuclear reaction analyses (NRA). The NRA results clearly show a two-dimensional radial distribution of the deuterium with a minimum at the center and a maximum close to the edge. These distribution correlates well with the thermal profile of the sample surface, where a maximum of ～1600 K was measured at the center decreasing to ～1000 K in the edges. A maximum deuterium fluence retention of 5 × 10¹⁵ D/cm² was measured. The values of the retained fractions ranging from 10^?5 to 10^?6 D_retained/D_incident were measured with thermal desorption spectroscopy (TDS) and compares well with IBA results. Moreover, the presence of C in the plasma and its co-deposition increases the D retention in the region where a C film is formed. Both NRA and TDS results show no clear dependence of retention on incident fluence suggesting the absence of plasma related traps in W under these conditions.     

Carbon film growth and hydrogenic retention of tungsten exposed to carbon-seeded high density deuterium plasmas

Tungsten (W) targets have been exposed to high density (n_e ? 4 × 10¹⁹ m^?3), low temperature (T_e ? 3 eV) CH₄-seeded deuterium (D) plasma in Pilot-PSI. The surface temperature of the target was ～1220 K at the center and decreased radially to ～650 K at the edges. Carbon film growth was found to only occur in regions where there was a clear CII emission line, corresponding to regions in the plasma with T_e ? 2 eV. The maximum film thickness was ～2.1 μm after a plasma exposure time of 120 s. ³He nuclear reaction (NRA) analysis and thermal desorption spectroscopy (TDS) determine that the presence of a thin carbon film dominates the hydrogenic retention properties of the W substrate. Thermal desorption spectroscopy analysis shows retention increasing roughly linearly with incident plasma fluence. NRA measures a C/D ratio of ～0.002 in these films deposited at high surface temperatures.     

The effects of gas transport properties on blister formation in He+-implanted glass

Blister formation in He⁺-implanted glasses is correlated with the measured helium gas diffusivity. A scries of glasses with diffusivities from ～3 × 10^?7 to ～5 × 10^?12cm²sec^?1 was implanted under nearly identical conditions with 150 keV He⁺ ions at a flux of 15 μA cm^?2 and a nominal sample temperature of 110°C. Glasses with D less than ～1 × 10^?9cm²sec^?1 were fully blistered, whereas those with D greater than ～3 × 10^?8cm²sec^?1 showed no surface deformation. Glasses with diffusivities between ～3 × 10^?8 and ～1 × 10^?9cm²sec^?1 had local regions with low density coverage of relatively large blisters. The critical concentration of implanted helium, estimated by comparing experimental data with results from a simple theoretical model, is ^{～1 × 1019 cm?3}, consistent with high pressure solubility measurements. Reemission data at low fluence are qualitatively in agreement with analytical calculations. Implications for CTR technology are discussed.     

Erosion behavior of soft,amorphous deuterated carbon films by heat treatment in air and under vacuum

The erosion of soft a-C:D films by heat treatment in air and under vacuum is studied by ion-beam analysis. When the films are heated in air above 500 K, the film thickness and the areal densities of C and especially D decrease, and oxygen is incorporated in the films. The initial atomic loss rates of carbon and deuterium from the films are 2.6 × 10¹⁷ C atoms cm⁻² h⁻¹ and 4.8 × 10¹⁷ D atoms cm⁻² h⁻¹ at 550 K. However, after D depletion the films show a resistivity against further erosion due to annealing in air. When the films are heated under vacuum erosion starts at about 600 K and all components including D decrease proportionally to the film thickness. Thermal desorption spectroscopy of the films reveals the evolution of C_xD_y type hydrocarbons. Infrared analysis shows that the incorporated oxygen is chemically bonded to carbon. The thermally-activated decomposition of the soft a-C:D films is compared to that of hard a-C:D films and a reaction scheme is suggested.     

Coating of Titanium Nitride on Stainless Steel Targets by a 4?kJ Plasma Focus Device

Titanium nitride thin films were deposited on stainless steel (SS316L) targets by using a 4?kJ plasma focus device. The corresponding energy flux delivered to SS316L surface is estimated to be 2.69?×?10¹³?kev?cm^?3?ns^?1. X-ray diffraction analysis reveals the formation of a nanocrystalline titanium nitride coating on the surface of targets. Thickness of the elements found on the surface of treated samples which are obtained by Rutherford backscattering spectrometry analysis (RBS) were (×10¹⁵ at/cm²) .45% Ti, 50% N and 5% Fe. Scanning electron microscopy was used to indicate changes in surface morphology. Existence of grains in different size confirms the formation of TiN crystals on the surface of targets.     

Tritium diffusion in zircaloy-2 in the temperature range −78 to 204° C

Tritium diffusion measurements in Zircaloy-2 were carried out over the temperature range ?78 to 204 °C by direct measurement of tritium diffusion gradients. The ⁶Li (n, α)³H reaction was used to inject tritium into the specimens and to produce initial tritium concentration in the range 0.0065 ppm to 0.013 ppm ³H by weight. Two diffusion components were identified from the concentration profiles: a surface trapping region approximately 5 μm thick and a normal diffusion profile characteristics of bulk diffusion. Surface release measurements of tritium verified the existence of a surface trapping layer. The bulk diffusion component was consistent with classical diffusion solutions and was given by: D = 0.00021_?0.00018^+0.005 exp?(8500 ± 200 cal/RT) cm² · sec^?1.The surface trapping was attributed to oxide films formed on the Zircaloy-2 at room temperature. The apparent diffusion coefficients for the surface region were consistent with: D = 4.0_?3.3^+19.7 × 10^?14 exp?(7200 ± 1500 cal/RT) cm² · sec^?1 over the temperature range 25 to 411°C.     

Depth profiles of deuterium in titanium from gas emission during sputtering

A simple technique for the determination of the depth distributions of deuterium and tritium in the near-surface region (0–1 μm) of materials has been demonstrated. In particular, the depth distribution of deuterium in a layered, deuterided titanium film has been determined by monitoring the HD and D₂ gas emissions during sputtering by 2 keV argon ions. For the nonoptimized conditions employed the detection sensitivity was $\text{D/Ti}\text{\$}\text{?}\text{0.02}$ and the fractional depth resolution was ~0.24 (i.e., ~240Å for a 1000Å film). Even for these conditions, this technique has a near-surface depth resolution superior, or equal, to that of other techniques for profiling hydrogen and a more than adequate sensitivity for the detection of deuterium and tritium in CTR-related studies, hydride studies and studies of tritided targets for neutron production.     

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.     

In search of nuclear fusion in electrolytic cells and in metal/gas systems

It has been reported recently in the literature that unexpected thermal and nuclear effects (production of excess heat, neutrons, γ-rays, and tritium) can occur during the electrolysis of heavy water at palladium or titanium electrodes, or during temperature and pressure cycling of the titanium/deuterium gas system. We have attempted to reproduce some of these experiments. A variety of electrochemical cells having palladium cathodes in the form of wires, tubes, sheets, and rods have been used to electrolyze heavy water containing 0.1 mol.dm⁻³ LiOH, 0.1 mol.dn⁻³ LiOD or 0.5 mol.dm⁻³ D₃PO₄. Current densities of up to 200 mA.cm⁻² were applied. The mass of the palladium cathodes covered the range from 1–40 grams and the surface area varied from 8–140 cm². Neutron detection systems with low constant backgrounds were used to search for neutron emission during electrolysis. These included³He- and¹⁰BF₃-based detectors. After running some of the cells for more than 30 days, no neutron emission above background could be detected. This puts upper limits of 0.5 s⁻¹ and 2×10⁻²³ fus. D-D.s⁻¹ on the neutron emission and the fusion rate, respectively. A sensitive and accurate heat-flow calorimeter was built and used to monitor the energy balance of some of the cells during electrolysis. No unexpected heat effects were observed. This puts an upper limit of 0.13 W.cm⁻³ on the specific excess power. No enrichment of the electrolyte in tritium was evident after electrolysis. Experiments were also performed with the titanium/ deuterium gas system. These consisted of exposing titanium metal to a deuterium gas pressure of 40 atmospheres, lowering the temperature to −196°C, releasing the pressure and gradually warming the titanium to room temperature. No neutron emission above background was observed during these experiments, which puts upper limits of 0.5 s⁻¹ and 4×10⁻²⁵ fus.D-D.s⁻¹ on the neutron emission and fusion rate, respectively. Submitted toJournal of Fusion Energy as part of the Proceedings of the Workshop on Cold Fusion Phenomena held in Santa Fe in May 1989.     

Nuclear tracks in garnets studied by means of Rutherford backscattering and X-ray diffraction

(Y, La)₃(Fe, Ga)₅O₁₂ epitaxial garnet films on (111) Gd₃Ga₅O₁₂ substrates irradiated with ²³⁸U ions of 1.4 MeV/u specific energy in the dose range 10¹⁰ cm^?2 to 3 × 10¹¹ cm^?2 were measured by means of Rutherford backscattering and double-crystal X-ray diffraction before and after thermal annealing in oxygen. The nuclear track diameter of 10 nm confining a cylindrical volume of highly disordered material caused by each ion impact has been deduced from the comparison of the backscattering spectra of the irradiated and unirradiated film areas. The fraction of randomly backscattered ions due to the irradiation-induced damage as well as the lattice expansion perpendicular to the crystal surface caused by irradiation-induced lateral compressive stress are proportional to the ion dose. After thermal annealing the comparison of the almost identical backscattering yield of the irradiated areas and the unirradiated film regions demonstrates a nearly perfect recrystallization of the damaged track volumes.     

Thermal desorption of D2 and CD4 from bulk-boronized graphites

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.     

Argon ion impact desorption cross sections of chlorine and carbon from molybdenum

Total desorption cross sections have been measured for Cl (σ_Cl) and C(σ_C) on molybdenum by argon ion bombardment for an incidence angle of 60° from the surface normal. For the bombardment an ion gun with low current density (i₀ ~ 1 × 10 ^?7 A cm^?2) at low system pressure (~10^?9 Torr) was used. The detection was performed by AES and the data were sensitivity factor corrected. The AES analysis of the surface after adsorption showed that Mo, C and Cl contributed to more than 94% of the atomic composition. With known i₀, it is possible to obtain σ from the adsorbate signal vs ion bombardment time curve. For ion energies between 0.2 keV to 1.0 keV the measured value for σ_Cl and σ_C are 0.5?3 × 10^?15 cm² and 0.2?4 × 10^?15 cm², respectively. The possible effects of the surface roughness due to prebombardment are discussed.     

Influence of tungsten–carbon mixed layer and irradiation defects on deuterium retention behavior in tungsten

The D₂⁺ fluence dependence on deuterium (D) retention was studied to clarify the D retention mechanism in tungsten. The additional D desorption stage was observed around 660 K in the TDS spectrum for a sample implanted with D₂⁺ up to the fluence of 10²³ D⁺ m^?2, which desorption stage was not observed the D₂⁺ implanted sample with the fluence less than 10²² D⁺ m^?2. The TEM observation showed that the highly dense voids were formed in tungsten by D₂⁺ implantation with the fluence of 10²³ D⁺ m^?2, considering that the D would be trapped by voids. To understand the D trapping by voids in C⁺ implanted tungsten, C⁺–D₂⁺ sequential implantation experiments at various C⁺ implantation temperatures were performed. It was found that the amount of D desorbed around 560 K was increased by increasing the C⁺ implantation temperature. The formation of the voids was observed with increasing the C⁺ implantation temperature by TEM, indicating that the increase of D desorption around 560 K was caused by the formation of voids. However, the desorption temperature of D trapped by voids in C⁺ implanted sample was lower than that in D₂⁺ implanted one. TEM observation and XPS measurement indicated that this difference was caused by the increase of void size and/or the presence of implanted carbon.     

A simple coincidence method of deuterium profiling using the DHe, α)H reaction

A simple standard coincidence arrangement is described for D(³He, α)H depth profiling experiments of deuterium in solids. Both reaction products are detected in coincidence, the high-energy protons being observed in transmission through a foil target. The energy of the α-particles is converted into the corresponding depth scale, whereas the local deuterium concentration is calculated from the yield of the α-particle spectrum. The measurement of α-particles in coincidence with protons allows a reduction of background arising from Rutherford scattering of ³He and other reaction products. For samples of deuterium implanted into Ta₂O₅ and Er, the method allows a reduction of the backscattering yield by a factor of more than 10³. The residual background is due to accidental coincidences. It can be even more reduced using an accurate experimental geometry. The background reduction is largest for samples with a low content of deuterium, allowing measurements of deuterium profiles of 9 × 10¹³ D⁺/cm² implanted into Ta₂O₅ at an energy of 8 keV. This corresponds to a maximum deuterium concentration of about 5 × 10⁻⁴ D per Ta₂O₅. This method is not restricted to thin films, but it allows measurements of deuterium profiles in a thick sample, e.g. of an implantation profile in a 0.4 mm silicon wafer.     

Investigation of plasma exposed W–1% La2O3 tungsten in a high ion flux,low ion energy,low carbon impurity plasma environment for the International Thermonuclear Experimental Reactor

Previous investigations of tungsten for the International Thermonuclear Experimental Reactor (ITER) were focusing on using energetic ion beams whose energies were over 1 keV. This study presents experimental results of exposed W–1% La₂O₃ in high ion flux (10²² m^–2), low ion energies (about 110 eV) steady-state deuterium plasmas at elevated temperatures (873–1250 K). The tungsten samples are floating during plasma exposure. Using a high-pressure gas analyzer, the residual carbon impurities in the plasma are found to be about 0.25%. No carbon film is detected on the surface by the EDX analysis after plasma exposure. An infrared pyrometer is also used as an in situ detector to monitor the surface emissivities of the substrates during plasma exposure. Using the scanning electron microscopy, microscopic pits of sizes ranging from 0.1 to 5 μm are observed on the plasma exposed tungsten surfaces. These pits are believed to be the results of erupted deuterium gas bubbles, which recombine underneath the surface at defect locations and grain boundaries, leading to substrate damage and erosion loss of the substrate material. Low temperature plasma exposure of a tungsten foil indicates that deuterium gas (D₂) is trapped inside the substrate. Macroscopic blisters are observed on the surface. The erosion yield of the W–1% La₂O₃ increases with temperature and seems to saturate at around 1050 K. Scattered networks of bubble sites are found 5 μm below the substrate surface. High temperature plasma exposure appears to reduce the population as well as the size of the pits. The plasma exposed W–1% La₂O₃ substrates, exposed above 850 K, retain about 10¹⁹ D/m², which is two orders of magnitude less than those retained by the tungsten foils exposed at 400 K.     

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.     

Analysis of Tritium Behavior in Very High Temperature Gas-Cooled Reactor Coupled with Thermochemical Iodine-Sulfur Process for Hydrogen Production

The tritium concentration in the hydrogen product in Japan's future very high temperature gas-cooled reactor (VHTR) system coupled with a thermochemical water-splitting iodine-sulfur (IS) process (VHTRIS system), named GTHTR300C, was estimated by numerical analysis. The tritium concentration in the hydrogen product significantly depended on undetermined parameters, i.e., the permeabilities of a SO₃ decomposer and a H₂SO₄vaporizer made of SiC. Thus, the estimated tritium concentration in the hydrogen product for the conservative analytical condition ranged from 3.4 × 10^?3 Bq/cm³ at STP (38 Bq/g-H₂) to 0.18 Bq/cm³ at STP (2,000 Bq/g-H₂). By considering the tritium retained by core graphite and the reduction in permeation rate by an oxide film on the heat transfer tube of the IHX and the HI decomposer, the tritium concentration in the hydrogen product decreased to the range from 3.3 × 10^?5 Bq/cm³ at STP (0.36 Bq/g-H₂) to 5.6 × 10^?3 Bq/cm³ at STP (63 Bq/g-H₂), which were smaller than those for the conservative analytical condition by factors of about 3.2 × 10^?2 and 9.6 × 10^?3, respectively. The effectof the helium flow rate in the helium purification system on the tritium concentration in the hydrogen product was also evaluated.     

Contaminative Behavior of Radioactive Materials on Protein Film Surface

The mechanism of skin contamination with radioactive materials was studied using protein film to simulate the human skin. The protein films were contaminated with ³⁵S(H₂SO₄), ²⁰⁴Tl (TlNO₃) and ¹⁴⁷Pm(PmCl₃), and many factors such as soiling time, radioactive concentration, specific activity and acidity of the soiling solutions, and the concentration of coexisting electrolytes were changed to examine their effect.

It is concluded that the protein film is a useful material for studying the mechanism of skin contamination with radioactive materials.

The contaminative behavior of the different radioisotopes on the film surface was found to reflect their respective properties, and varied accordingly.     

Study and optimization of boronization in Alcator C-Mod using the Surface Science Station (S3)

A Surface Science Station (S³) on the Alcator C-Mod tokamak is used to study and optimize the location and rate of boron film deposition in situ during electron cyclotron (EC) discharge plasmas using 2.45 GHz radio-frequency (RF) heating and a mixture of helium and diborane (B₂D₆) gasses. The radial profile of boron deposition is measured with a pair of quartz microbalances (QMB) on S³, the faces of which can be rotated 360° including orientations parallel and perpendicular to the toroidal magnetic field B_T ～0.1 T. The plasma electron density is measured with a Langmuir probe, also on S³ in the vicinity of the QMBs, and typical values are ～1 × 10¹⁶ m^?3. A maximum boron deposition rate of 0.82 μg/cm²/min is obtained, which corresponds to 3.5 nm/min if the film density is that of solid boron. These deposition rates are sufficient for boron film applications between tokamak discharges. However the deposition does not peak at the EC resonance as previously assumed. Rather, deposition peaks near the upper hybrid (UH) resonance, ～5 cm outboard of the EC resonance. This has implications for RF absorption, with the RF waves being no longer damped on the electrons at the EC resonance. The previously inferred radial locations of critical erosion zones in Alcator C-Mod also need to be re-evaluated. The boron deposition profile versus major radius follows the ion flux/density profile, implying that the boron deposition is primarily ionic. The application of a vertical magnetic field (B_V ～0.01 T) was found to narrow the plasma density and boron deposition profiles near the UH resonance, thus better localizing the deposition. A Monte Carlo simulation is developed to model the boron deposition on the different QMB/tokamak surfaces. The model requires a relatively high boron ion gyroradius of ～5 mm, indicating a B⁺¹ ion temperature of ～2 eV, to match the deposition on QMB surfaces with different orientation to B_T. Additionally, the boron ion trajectories become de-magnetized at high neutral gas throughput (～0.5 Pa m³ s^?1) and pressure (～2 Pa) when the largest absolute deposition rates are measured, resulting in deposition patterns, which are independent of surface orientation to B_T in optimized conditions.     

X-ray impact induced desorption of gases from surfaces

Measurements of gases released from 302 stainless steel and gold surfaces before and after discharge cleaning were made in ultrahigh vacuum using X-rays with an energy distribution typical of a tungsten bremsstrahlung spectrum. Similar measurements were also made for Al₂O₃ surfaces which had not been discharge cleaned. For the non-discharge-cleaned surfaces of stainless steel, Al₂O₃, and gold the predominant gas species observed mass spectrometrically was CO₂. For some stainless steel and Al₂O₃ surfaces CO and O₂ were also readily observed. Mean quantum yields for CO, O₂ and CO₂ release from such stainless steel surfaces, for example, ranged from < 6 × 10^?5 to 9 × 10^?4 molecules per photons in the bremsstrahlung spectrum characteristic for 50 keV electron energy. After discharge cleaning a decrease in the mean quantum yields was observed for the stainless steel and gold surfaces.