Polycrystalline Tungsten Samples Research Articles

Influence of plasma impurities on the deuterium retention in tungsten exposed in the linear plasma generator PSI-2

The hydrogen retention in fusion reactors can be significantly influenced by the presence of plasma impurities. Earlier studies showed that helium can reduce the retention in tungsten wall materials. This paper gives the results of experiments on this topic in the linear plasma generator PSI-2. Exposures of polycrystalline tungsten samples to a deuterium plasma were performed at low temperatures (380K) under the variation of the impurity species (He, Ar) and concentration (0–5%). For the experiments with He, the total deuterium fluence was varied between 2⋅1024m−2 and 2⋅1026m−2. Subsequently, the surface morphology and deuterium retention were investigated. The results show a reduction of the deuterium retention by a factor of 3 for helium, and an increase by up to 30% for argon. A diffusion model for the helium case was developed, in which a shallow layer of porous helium nanobubble structures reduces the total deuterium content.

Microscopic evolution of pre-damaged and undamaged tungsten exposed to low-energy and high-flux helium ions

High-energy (260keV) He+ pre-damaged and undamaged polycrystalline tungsten samples were irradiated with low-energy (220eV) and high-flux (∼1021ions/m2s) He+ at a sample temperature of 873K to a fluence of 1.0×1025ions/m2. Microscopic evolution of these samples was carried out using non-destructive conductive atomic force microscopy and a nanohardness test. Analysis indicates that a large number of nanometer-sized protuberances of irradiated tungsten samples results from over-high internal pressure of nanometer-sized helium bubbles. Ordered and nanostructured helium bubbles with the same diameter and average spacing can be formed due to the self-trapping and self-organizing of helium atoms in the tungsten materials. In the case of pre-damaged, low-energy He+ irradiation results in a random distribution of nanostructured helium bubbles, indicating that high-energy He+ implantation results in serious irradiation damage of tungsten materials, acting as nuclei for helium bubbles.

Deuterium Retention in Tungsten with Different Grain Elongation Direction Irradiated by Plasma in APSEDAS

Deuterium retention after the low energy plasma irradiation was investigated using the polycrystalline tungsten samples of which grain elongation directions are parallel and perpendicular with respect to the surface. Fluence dependence of the retention measured by means of thermal desorption spectroscopy showed that it is 2-5 times larger for the sample with the perpendicular grain elongation. Thermal desorption of trapped deuterium has been modeled under the fast diffusion assumption with the defects trapping energy of about 2 eV. Possible mechanism of the deuterium retention and thermal desorption has been proposed.

Temperature dependent defects evolution and hardening of tungsten induced by 200 keV He-ions

Tungsten has been selected as one of the potential candidate materials to cover some parts of the divertor in the future International Thermonuclear Experimental Reactor (ITER). The accumulation of defects and He induced by neutron irradiation and their impact on the mechanical properties of tungsten are of very importance. In this work, the high pure polycrystalline tungsten samples were implanted by 200keV He+ with a fluence of 5×1016He+/cm2 at temperatures of room temperature (RT), 200, 400 and 800°C. Vacancy-type defects were detected in all implanted samples by means of positron annihilation spectroscopy. Vacancy-type defects produced by He implantation exist in the damaged layer and are decorated by He atoms. At higher implantation temperature, He–V complexes could grow larger through absorbing mobile vacancies. The nano-hardness values were measured by nano-indentation technique. It is observed that implantation hardening occurred for all the implanted samples. With increasing implantation temperature from 200°C to 800°C, the change of the average hardness values which are lower than the value at RT has a tendency of enhancement for the shallower layer and degradation for the deeper layer. The hardness variations are discussed to be the pinning effects of the defects with different density or size.

A Step-By-Step Analysis of the Polishing Process for Tungsten Specimens

Abstract The different stages of the polishing process of polycrystalline tungsten samples were investigated by scanning electron microscopy of both the sample surface and of cross-sections prepared with the help of a focused ion beam. It is shown that a distortion layer is present at the sample surface after mechanical fine grinding and even after polishing with diamond suspension, although the sample has a mirror-like finish. A sufficiently long chemo-mechanical polishing step using an alkaline colloidal silica suspension was able to remove this distortion layer. Although electropolishing produced an even smoother surface, the microstructure quality after chemo-mechanical polishing is comparable to that of an electropolished sample.

Deuterium Trapping in Rolled Polycrystalline Tungsten Exposed to Low Energy Plasma

Deuterium retention after low energy plasma exposure was investigated using the polycrystalline tungsten samples of which grain elongation directions are parallel and perpendicular with respect to the surface. The deuterium retention calculated from TDS spectra is 2-10 times larger in the sample with perpendicular grain elongation direction than in the parallel one for the irradiation temperatures of 450-750K. Removing of the grain elongation anisotropy by the recrystallization resulted in disappearance of the difference in the deuterium retention of the both kinds of samples. No bubbles and no blisters have been observed in the near surface layer of the samples. Plasma irradiation should produce high energy defects that are responsible for the deuterium retention. The difference in the retention of the both kinds of samples seems to be attributed to the different effective diffusion coefficients depending on the grain elongation direction. c

Statistical analysis of blister bursts during temperature-programmed desorption of deuterium-implanted polycrystalline tungsten

During temperature-programmed desorption (TPD) of stress-relieved polycrystalline tungsten samples exposed to a deuterium plasma, short, intense bursts of D2 were observed on the low-temperature flank of the main desorption peak. These bursts are attributed to the rupturing of blisters filled with high-pressure D2 gas. A statistical analysis of the size distribution and temporal correlation of the bursts is presented. The influence of different measurement intervals and TPD heating rates on the observed bursts is simulated based on these statistics and compared to the experimental results. The contribution of bursts to the total D inventory in the sample is also estimated.

The response of polycrystalline tungsten to 30 keV helium ion implantation at normal incidence and high temperatures

Polycrystalline tungsten samples were irradiated with 30keV helium ions in the Materials Irradiation Experiment (MITE-E). Samples were held at temperatures of 500–900°C while irradiated to fluences of ∼6×1016 to ∼4×1019He/cm2. Drastic changes to the surface morphology were observed at implant doses exceeding ∼1×1018He/cm2 and classified as blisters, pitting, or a highly directional surface structure dubbed “grass.” Morphological details of the grass structure depended on the crystal orientation of the grains at the sample surface. A sub-surface semi-porous layer was formed during irradiation. Post-implantation analysis revealed that this layer extended up to ∼1000nm below the surface in the highest fluence cases. Specimens irradiated to fluences in excess of ∼1×1018He/cm2 experienced statistically significant mass loss which increased with higher implant dose. Implications of this mass loss are discussed for the plasma facing components of both magnetic and inertial fusion reactors.

Retention and Surface Pore Formation in Helium Implanted Tungsten as a Fusion First Wall Material

Single- and polycrystalline tungsten samples were implanted with 30 keV3He ions to fluences of 5e16, 4e17 and 5e18 He/cm2 at temperatures ranging from ˜850 - 1000 °C. After implantation tungsten’s retention characteristics were studied using 3He(d,p)4He nuclear reaction analysis (NRA) and 3He(n,p)T neutron depth profiling (NDP). Morphological analyses included scanning electron microscopy (SEM), focused ion beam (FIB) milling, and X-ray diffraction on the single crystalline W samples (XRD).SEM analysis showed that the threshold forsurface pore formation occurs in both single-crystalline tungsten (SCW) and polycrystalline tungsten (PCW) between ˜5e16 - 4e17 He+/cm2. Both surface and sub-surface pore formation is observed to increase with higher implant fluences. Focused ion beam (FIB) milling revealed a sub-surface porous layer in both SCW and PCW, which increased in depth with implanted fluences. NRA measured the retained He fluence in SCW between 1.1e16 - 1.1e17 He/cm2 and in PCW between 1.3e17 - 1.5e17 He/cm2. NDP analysis measured the retained He fluence in SCW between 2.0e16 - 2.7e17 He/cm2 and in PCW between 4.1e16 - 3.2e17 He/cm2. Both of these analysis techniques reveal that the retained helium saturates in both single and polycrystalline W at ˜4e17 cm-2. The NDP analysis showed that the peak helium concentration shifted deeper into the specimens as the dose was increased, indicating a decrease in the effective density of the surface layer with an increased dose. Average retained helium concentrations were found to range from 0.7 - 8.6 at% in SCW and from 1.3 - 11.4 at% in PCW.

Helium behaviour and vacancy defect distribution in helium implanted tungsten

The distribution and the nature of 3He implantation-induced defects in polycrystalline tungsten samples were studied by Positron Annihilation Spectroscopy as a function of implantation fluence. The implanted helium profile was determined by Nuclear Reaction Analysis, and its evolution under different thermal annealings was investigated. Results show that vacancy-like defects are generated along the path of the ions and that their concentration varies directly as the implantation fluence. No helium desorption was observed under any thermal treatments. However, a change in 3He depth profile under specific annealing conditions suggests the formation of nanometric-size He bubbles.

Retention and surface blistering of helium irradiated tungsten as a first wall material

The first wall of an inertial fusion energy reactor may suffer from surface blistering and exfoliation due to helium ion irradiation and extreme temperatures. Tungsten is a candidate for the first wall material. A study of helium retention and surface blistering with regard to helium dose, temperature, pulsed implantation, and tungsten microstructure was conducted to better understand what may occur at the first wall of the reactor. Single crystal and polycrystalline tungsten samples were implanted with 1.3 MeV 3He in doses ranging from 10 19 m −2 to 10 22 m −2. Implanted samples were analyzed by 3He(d,p) 4He nuclear reaction analysis and 3He(n,p)T neutron depth profiling techniques. Surface blistering was observed for doses greater than 10 21 He/m 2. For He fluences of 5 × 10 20 He/m 2, similar retention levels in both microstructures resulted without blistering. Implantation and flash heating in cycles indicated that helium retention was mitigated with decreasing He dose per cycle.

Incident energy dependence of blistering at tungsten irradiated by low energy high flux deuterium plasma beams

Polycrystalline tungsten samples have been irradiated at near room temperature by high flux (1 × 10 22 D/m 2/s) deuterium plasma beams with incident ion energies ranging 7–98 eV/D. Surface blistering occurred at all energies as observed by means of scanning electron microscopy. At all energies, the blisters increased in their size and number with fluence within the corresponding low fluence ranges. The size increase tended to saturate at certain fluences within the experimental fluence ranges, which might be attributed to rupturing of blisters. The critical fluence for blistering Φ cr was found to increase with decreasing the incident energy. At energies <20 eV/D, Φ cr increased more rapidly. This energy dependence of Φ cr may be explained by a proposed model dealing with the oxide barrier to deuterium uptake into and release from the bulk W. The presence of oxide layers of some monolayers on the surfaces was verified by means of X-ray photoelectron spectroscopy analysis and secondary ion mass spectroscopy before and after irradiation to different fluences.

Helium retention and surface blistering characteristics of tungsten with regard to first wall conditions in an inertial fusion energy reactor

The first wall of an inertial fusion energy reactor may suffer from surface blistering and exfoliation due to helium ion fluxes and extreme temperatures. Tungsten is a candidate for the first wall material. A study of helium retention and surface blistering with regard to helium dose, temperature and tungsten microstructure was conducted to learn how the damaging effects of helium may be diminished. Single crystal and polycrystalline tungsten samples were implanted with 1.3MeV 3He in doses ranging from 1019/m2 to 1022/m2. Implanted samples were analyzed by 3He(d,p)4He nuclear reaction analysis and neutron depth profiling techniques. Surface blistering occurred for doses greater than 1021He/m2 and was analyzed by scanning electron microscopy. Repeated cycles of implantation and flash annealing indicated that helium retention was reduced with decreasing implant dose per cycle. A carbon foil energy degrader, currently in development, will allow a continuous spectrum of helium implantation energy matching the theoretical models of He ion fluxes within the IFE reactor.

Surface erosion of tungsten and the morphology of erosion products in experiments simulating plasma disruption

The surface erosion of different sorts of tungsten subjected to high-power pulsed plasma streams simulating plasma disruption is studied. With W-13I polycrystalline and (111) single-crystal tungsten samples used as examples, the size distributions for the erosion products collected at different angles to the target are compared. The typical drop erosion of the surface is observed. Fine drops either return to the surface or fly away in a direction parallel to the surface. Coarse drops leave the surface nearly at right angles to the surface. The single-crystal surface displays the absence of fine (<0.125 µm) drops typical of a polycrystalline tungsten surface. The erosion of the single-crystal samples is least among the tungsten sorts considered.

Grain Boundaries for Field Emission Microscopy

Heretofore the observation of grain boundaries with the field emission microscope has been limited by the accidental and infrequent appearance of suitable specimens. A process for preparing polycrystalline tungsten samples by bending a tungsten wire, etching a tip at the bend and heating is described. Both large and small angle grain boundaries result. As many as five tungsten grains may share a cross section of less than one micron in diameter. All the parameters required to describe the geometry of the boundaries may be obtained experimentally. Detailed observations on the dislocation structure of boundaries with angles as great as 6° are potentially possible. However, the field emission intensity from the region immediately adjoining and including the grain boundary must first be increased appreciably. Possible methods for achieving improved electron emission from the grain boundary region are discussed.