V. Efimov

Effect of nitrogen seeding into deuterium plasma on deuterium retention in tungsten

An interaction of deuterium plasma with seeding species with a material is, in particular, an issue for medium- and high-Z plasma-facing materials for fusion devices to reduce the power load in front of material surfaces. In this paper, we investigated the influence of seeding of nitrogen (N) into deuterium plasma on the accumulation of deuterium (D) in tungsten (W). Tungsten samples were exposed to pure and N-seeded deuterium plasmas generated by an electron-cyclotron resonance plasma source in the PLAQ (Plasma Quelle) experiment. D and N retention in each sample was subsequently analysed by nuclear reaction analysis for depth profiling up to 6 μm. It was found that the amount of N in W is (7–9)×1019 N m−2 in the temperature range 300–650 K and slightly decreases down to 6×1019 N m−2 at 800 K. This means that the nitrogen-containing layer formed upon exposure of W to N-seeded D plasma is thermally stable and does not decompose at least up to 800 K. It is shown that the seeding of nitrogen into D plasma does not prevent blister formation and even results in an increase of the size of blisters in some cases. Depth profile measurements show that there is an enhancement of D diffusion into the bulk and, consequently, an increase of D retention in W in the presence of N seeding in D plasma compared to pure D plasma. The influence of N seeding on D retention depends strongly on the applied bias and fluence in our plasma conditions. The mechanisms of deuterium retention in W in the presence of nitrogen seeding are discussed.

Investigation of a fine structure of deuterium thermal desorption spectra from tungsten

The results of deuterium thermal desorption from tungsten after irradiation by 10 keV ions are discussed. A special heating procedure allows features of the fine structure of the thermal-desorption spectra maxima to be revealed.

Surface modification and deuterium retention in reduced-activation steels under low-energy deuterium plasma exposure. Part I: undamaged steels

In this paper, reduced-activation ferritic/martensitic (RAFM) steels including Eurofer (9Cr) and oxide dispersion strengthening (ODS) steels by the addition of Y2O3 particles with different amounts of Cr, namely, (9-16)Cr were exposed to low energy deuterium (D) plasma (~20–200 eV per D) up to a fluence of 2.9 × 1025 D m−2 in the temperature range from 290 K to 700 K. The depth profile of D in steels was measured up to 8 µm depth by nuclear reaction analysis (NRA) and the total retained amount of D in those materials was determined by thermal desorption spectroscopy (TDS). It was found that the D retention in ODS steels is higher compared to Eurofer due to the much higher density of fine dispersoids and finer grain size. This work shows that in addition to the sintering temperature and time, the type, size and concentration of the doping particles have an enormous effect on the increase in the D retention. The D retention in undamaged ODS steels strongly depends on the Cr content: ODS with 12Cr has a minimum and the D retention in the case of ODS with (14-16)Cr is higher compared to (9-12)Cr. The replacing of Ti by Al in ODS-14Cr steels reduces the D retention. The formation of nano-structure surface roughness enriched in W or Ta due to combination of preferential sputtering of light elements and radiation-induced segregation was observed at incident D ion energy of 200 eV for both Eurofer and ODS steels. Both the surface roughness and the eroded layer enhance with increasing the temperature. The surface modifications result in a reduction of the D retention near the surface due to increasing the desorption flux and can reduce the overall D retention.

Surface modification and deuterium retention in reduced-activation steels under low-energy deuterium plasma exposure. Part II: steels pre-damaged with 20 MeV W ions and high heat flux

The reduced-activation ferritic/martensitic (RAFM) steels including Eurofer (9Cr) and oxide dispersion strengthened (ODS) steels by the addition of Y2O3 particles investigated in Part I were pre-damaged either with 20 MeV W ions at room temperature at IPP (Garching) or with high heat flux at FZJ (Juelich) and subsequently exposed to low energy (~20–200 eV per D) deuterium (D) plasma up to a fluence of 2.9 × 1025 D m−2 in the temperature range from 290 K to 700 K. The pre-irradiation with 20 MeV W ions at room temperature up to 1 displacement per atom (dpa) has no noticeable influence on the steel surface morphology before and after the D plasma exposure. The pre-irradiation with W ions leads to the same concentration of deuterium in all kinds of investigated steels, regardless of the presence of nanoparticles and Cr content. It was found that (i) both kinds of irradiation with W ions and high heat flux increase the D retention in steels compared to undamaged steels and (ii) the D retention in both pre-damaged and undamaged steels decreases with a formation of surface roughness under the irradiation of steels with deuterium ions with incident energy which exceeds the threshold of sputtering. The increase in the D retention in RAFM steels pre-damaged either with W ions (damage up to ~3 µm) or high heat flux (damage up to ~10 µm) diminishes with increasing the temperature. It is important to mention that the near surface modifications caused by either implantation of high energy ions or a high heat flux load, significantly affect the total D retention at low temperatures or low fluences but have a negligible impact on the total D retention at elevated temperatures and high fluences because, in these cases, the D retention is mainly determined by bulk diffusion.

Plasma-Facing Material Erosion Products Formed Under ITER-Like Transient Loads at QSPA-T Plasma Gun Facility

Abstract Edge-localized mode (ELM) simulation experiments were held on the quasi-stationary plasma accelerator QSPA-T to study the formation of plasma-facing material (PFM) erosion products. Parameters of the deuterium plasma heat loads in QSPA-T were close to those expected during transient events in ITER. A diagnostic system for measuring the deposition rate of the erosion products with resolution time of 0.02 ms (pulse duration 0.5 ms) was designed. It allowed defining the deposition rate dependence on time and property changes of the deposited film during the pulse. The average deposition rate in QSPA-T under exposures to ultra-short D plasmas was in the range of (0.1 to 100)×1019 at·cm2·s−1, which was much higher than that for stationary processes. It has been found that deuterium concentration in the deposited W films depends on substrate temperature and deposition rate approximately in the same way as for stationary processes. As the substrate temperature and deposition rate increased, the D/W atomic ratio in the W films decreased. For describing the evolution of the D/W ratio with the substrate temperature and the tungsten deposition rate, an empirical equation proposed by De Temmerman and Doerner (J. Nucl. Mater., 2009), but with alternative parameters, has been used.

Helium concentration measurement in tungsten fuzz-like nanostructures by means of thermal desorption spectroscopy

The concentration of helium in tungsten fuzz-like nanostructures has been measured by means of thermal desorption spectroscopy. Fuzz was formed on the W surface under intensive plasma irradiation at 1500 K. The helium content was measured first in the as-irradiated sample, and then in a similar sample with the fuzz mechanically scraped from the sample surface. The difference gave the He content in the fuzz, which was estimated to be He/ W = (13 ± 4)%.

Deuterium removal from radiation damage in tungsten by isotopic exchange with hydrogen atomic beam

The tungsten samples were pre-irradiated with self-ions to create radiation-induced defects and then exposed to the deuterium atomic beam. The deuterium removal was studied by isotopic exchange with atomic hydrogen beam. Modification of the deuterium depth profile in self-ion irradiated tungsten under isotopic exchange up to a depth of 6 μm was measured in- situ by nuclear reaction analysis. The total deuterium retention after isotopic exchange was measured by thermal desorption spectroscopy. It is shown that the efficiency of the deuterium removal increases with increasing of the hydrogen incident flux, incident energy and temperature of the tungsten sample.

Erosion products of plasma facing materials formed under ITER-like transient load and deuterium retention in them

Erosion of the plasma-facing materials in particular evaporation of the materials in a fusion reactor under intense transient events is one of the problems of the ITER. The current experimental data are insufficient to predict the properties of the erosion products, a significant part of which will be formed during transient events (edge-localized modes (ELMs) and disruptions). The paper concerns the experimental investigation of the graphite and tungsten erosion products deposited under pulsed plasma load at the QSPA-T: heat load on the target was 2.6 MJ/m2 with 0.5 ms pulse duration. The designed diagnostics for measuring the deposition rate made it possible to determine that the deposition of eroded material occurs during discharge, and the deposition rate is in the range (0.1–100) × 1019 at/(cm2 s), which is much higher than that for stationary processes. It is found that the relative atomic concentrations D/C and D/(W + C) in the erosion products deposited during the pulse process are on the same level as for the stationary processes. An exposure of erosion products to photonic energy densities typical of those expected at mitigated disruptions in the ITER (pulse duration of 0.5–1 ms, integral energy density of radiation of 0.1–0.5 MJ/m2) significantly decreases the concentration of trapped deuterium.

Deuterium accumulation in beryllium under irradiation with powerful pulsed plasma flows

Data on the thermal desorption of deuterium from beryllium irradiated with powerful pulsed plasma flows are presented. Two grades of beryllium, namely, TGP-56FW and S-65C, are investigated. Deuterium desorbs mainly in the temperature range 800–1400 K with a maximum at 1200 K. On average, the accumulation of deuterium in S-65C beryllium samples exceeds that in TGP-56FW beryllium samples by a factor of 3.

Deuterium retention after deuterium plasma implantation in tungsten pre-damaged by fast C+ ions

Thermal desorption of deuterium from W was investigated. Virgin samples and samples damaged by 10 MeV C 3+ ions were implanted from plasma in the LENTA facility at 370 K and 773 K. In comparison with the undamaged sample, deuterium retention in the damaged sample slightly increased in the case of deuterium implantation at RT, but decreased in the case of deuterium implantation at 773 K. At 773 K, deuterium was concluded to diffuse far behind the D ion range in the virgin sample, while C implantation region was concluded to be a barrier for D diffusion in the damaged sample.