Helium Retention Research Articles

Helium retention feature in the boron deposited layer on tungsten substrate by laser-induced breakdown spectroscopy and machine learning approach

ITER is designed for a burning plasma operation in which Tungsten (W) tiles are used as the first wall and diverter materials. Studying He dynamics in B-coated W wall is essential to understanding the effect of boronized plasma-facing components in fusion reactors, as these post-conditioning materials significantly influence helium retention and release. Plasma-wall interactions (PWI) are an important issue in ITER fusion reactors. PWI would lead to wall erosion and impurity redeposition. As a product of the D-T burning plasma, helium (He) ash would be retained or co-deposited on plasma-facing components (PFCs), affecting the stable operation of burning plasma. Laser-induced breakdown spectroscopy (LIBS) is proposed as a promising in-situ diagnostic approach for monitoring D/T and He retention and impurity deposition on PFCs of tokamak devices. In this study, the LIBS technique was used to investigate the helium retention feature in the boron (B) layer on tungsten substrate in 10−5 mbar. Five He-retention samples on the boron deposition layer on tungsten substrates were prepared by pulsed laser deposition (PLD) method at different ambient pressures in our lab. The investigations indicate that the atomic spectral line of helium (He-I 587.56 nm) was observed in the spectra of the first three laser shots. The depth profiles of He, B, and W in the boron-deposited layer on tungsten substrate were performed by LIBS to determine the co-deposition layer thickness. The concentration of He in the co-deposition layer samples measured by TDS is 7 × 1020 He/m2. The plasma parameters, such as plasma electron temperature and electron number density, were calculated to validate the local thermodynamic equilibrium. Machine learning (ML) algorithms are used to classify the co-deposits and substrates. The first three principal components (PC1, PC2 & PC3) of the unsupervised ML algorithm (PCA) give a classification accuracy of 91.7 %. The supervised ML algorithm neural network achieved training and testing accuracy of 100 % and 96.7 %, respectively.

Exploring helium retention from technical materials: Development and investigation

Materials used to study nuclear fusion can retain atmospheric helium unless pretreated before an experiment. Understanding helium outgassing is important for accurate diagnostics in experiments surrounding nuclear fusion. The presence of helium is often cited as the primary evidence that a nuclear reaction has occurred, so it is imperative that known sources of helium are mitigated prior to proceeding with novel nuclear experiments. It is also necessary to ensure hermeticity when transferring gas aliquots from an experiment to a mass spectrometer. In this article, we present studies of helium leak rates in systems used in novel nuclear experiments. We also present studies of helium retention in materials subjected to various heating profiles and atmospheric concentrations. Without pretreatment, 12-inch lengths of both 3/8” diameter tubes and 1/2″ diameter stainless-steel 316 tubing yielded an average areal outgassing amount of 0.64 pmol/cm2. If pretreatment is impractical, then the results may be scaled based on the tubing length necessary for constructing custom experimental equipment. It also may reabsorb 4He from the atmosphere in time. These studies also demonstrate that it is necessary to pretreat most materials prior to performing experiments where the presence of 4He is being used as an indicator for novel nuclear reactions.

Helium aggregation and surface morphology near grain boundaries in plasma-facing tungsten

We conduct molecular dynamics simulations of helium in tungsten to study the interaction of helium with grain boundaries. Model systems with grain boundary planes perpendicular to the surface and parallel to the surface are considered. The net attraction of mobile helium to the grain boundary results in a “depleted region” within approximately 3.5 nm of the grain boundary plane at low fluence, and once on the plane of the grain boundary, helium transport slows considerably. Helium retention is also strongly affected by the grain boundaries and their density: grain boundary planes approximately 6 nm beneath the plasma-facing surface and parallel to the surface tend to reduce the maximum bubble size due to the attraction of mobile clusters to the grain boundary plane, which lowers the concentration of helium near the surface (where it is being implanted); grain boundaries perpendicular to the surface tend to increase retention due to retention on the grain boundary plane. For grain boundaries parallel to the surface, the strong gettering effect of the grain boundaries on helium results in essentially no helium penetration through the grain boundary during the first 1.5 μs of plasma exposure at a flux of 1.6×1025 m−2s−1, corresponding to fluences on the order of 1020 m−2. Coarse-grained simulations capable of capturing the long-term dynamics of helium aggregation near grain boundaries would be required to determine whether these effects would have any measurable impact on phenomena, such as tungsten fuzz growth.

Laser-induced breakdown spectroscopy for helium detection in beryllium coatings

Laser Induced Breakdown Spectroscopy (LIBS) method is considered to be a promising tool for analyzing the retention of hydrogen isotopes (D and T) and helium (He) on the first walls and divertor regions of future fusion reactors. Helium will be produced in DT reactions but could also be used in the initial non-nuclear phases of DEMO concepts. The present study investigates the He detection by LIBS method in the Be coatings simulating the deposits on the divertor plasma-facing components of JET while the results are also relevant for He detection in the deposits of other wall materials. The study was carried out in a vacuum vessel filled with 2–40 mbar argon background gas. It was shown that 2.8 at. % of He was confidently detectable by LIBS at optimized measurement conditions and the estimated limit of detection at used experimental conditions is approximately 0.7 at. %. The intensity of the He emission line at 587.56 nm was the strongest at the center of the laser-induced plasma plume. The He line intensity increased with the pressure of Ar gas but the broadening of the He line and the increase of the background emission and noise set an upper limit to the Ar background pressure usable for He detection. The application of the calibration-free LIBS procedure resulted in the overestimation of the He/Be ratio by several orders of magnitude. The overestimation can be explained by the deviation of LIBS plasma from the local thermodynamic equilibrium, which is caused by the very high excitation energy of He atoms.

Diagenesis of fossil gar fish scales with implications for geochronology and paleoenvironmental applications

Fossil scales from gar fish are made of bioapatite and are common in the continental sedimentary record from the Cretaceous to the present, prompting interest in using these fossil scales as geochemical archives. We investigated the diagenetic characteristics of the two different components of gar scales: ganoine, which has similarities to tooth enamel, and bone. We examined microscopic textural characteristics, spatially resolved trace element patterns, and apparent gar scale ages using the (U-Th)/He and U-Th-Pb systems, in modern scales as well as fossil scales from Cretaceous to Paleogene age strata in the North American Williston and San Juan Basins. Structural and textural characteristics of modern and fossil gar scales suggest that ganoine is more resistant to diagenetic alteration than bone. Trace element concentrations in fossil bone are at least an order of magnitude higher than in ganoine and can be spatially heterogeneous, indicative of a complex and prolonged diagenetic history. Trace element concentrations in fossil ganoine often decrease systematically with distance from the ganoine surface, which may be consistent with a fossilization model in which trace element concentrations are controlled by an inwardly propagating recrystallization front. However, (U-Th)/He dates of ganoine are substantially younger than expected based on basin thermal histories and compared to conventional detrital apatite (U-Th)/He dates from the same localities. The ganoine (U-Th)/He dates are also older than a few Ma for only the lowest effective uranium concentrations. We interpret the young (U-Th)/He dates and the date-effective uranium pattern to be indicative of either protracted or late-stage incorporation of trace elements, which is inconsistent with a recrystallization front model alone. The young (U-Th)/He dates may also reflect microstructurally-controlled low helium retentivity in gar scale ganoine, although this does not explain the observed date- effective uranium systematics. Late-stage incorporation of trace elements is supported by the U-Th-Pb systematics of ganoine, which require addition of U and Th at a time significantly later than the depositional age. Late-stage, or protracted, diagenetic uptake of parent nuclides in gar scale ganoine contrasts with recent studies of bioapatite from conodont elements, which instead appear to exhibit late-stage parent nuclide loss. We conclude that the (U-Th)/He system in gar scale bioapatite is not currently useful as a chronometer and suggest that applications of other trace element proxies to gar scale bioapatite be approached with caution.

Laser temperature programmed desorption: A flexible technique to study ion-surface interaction.

Understanding the physical-chemical processes ruling the interaction of particles (atoms, molecules, and ions) with surfaces is fundamental in several research fields, such as heterogeneous catalysis, astrochemistry, and nuclear fusion. In particular, the interaction of hydrogen isotopes with plasma facing materials represents a high-priority research task in the fusion community. Such studies are essential to ensure the successful operation of experimental fusion reactors, such as the tokamak ITER. In this work, we present a surface science apparatus developed to study ion-surface interaction in fusion relevant systems. It combines laser-based techniques with contaminant-free ion/molecular beams, mass spectrometry, and surface science tools such as low-energy electron diffraction and Auger electron spectroscopy. It allows to cover a wide range of sample temperatures, from 130 to 2300K, by changing the heating rate of samples from 0.1 to 135 K/s and maintaining the linearity of the heating ramps, a powerful feature to gain insight on adsorption, absorption, and desorption mechanisms. Experimental calibration and performance are presented in detail. Moreover, to provide a factual overview of the experimental capabilities, we focus on two different applications: the protocol used to clean a W(110) single crystal sample and the development of laser temperature programmed desorption to study helium retention in tungsten.

Evolution of structures and internal stress of ZrC-SiC composite under He ion irradiation and post-annealing

Composite ceramics show great potential for utilization in nuclear systems. However, the component phases therein may exhibit large variations in structural response to the ion irradiation including degree of lattice expansion, ability to maintain the crystallinity, types and density of as-formed defects, etc. Such discrepancy may result in the evolution of internal stress and consequently more complicated interactions with respect to the irradiation behavior, which remains less understood. In this study, a ZrC–30 vol% SiC composite is chosen to study the irradiation and helium retention behavior under 540 keV He ion irradiation with a fluence of 1 × 1017/cm2 and subsequent annealing at 1000 °C and 1500 °C. Irradiation-induced volume dilation of ZrC lattice and especially from an amorphization process of SiC leads to a maximum compressive stress of ∼66 GPa at a certain irradiation depth, at which horizontal cracks are generated but only in ZrC grains. During high-temperature annealing, migration of defects, diffusion and coalesce of helium species, growth of He bubbles, recovery of crystal lattices and formation of dislocations are found to be closely correlated to the irradiation-induced gradient of helium retention concentration, lattice damage and internal stress along the irradiation depth along with the recrystallization process of amorphous SiC. The as-observed irradiation behavior and structural evolution during annealing are discussed by theoretical consideration of internal stress in the composite using the COMSOL Multiphysics package.

Surface modification and helium release of tungsten irradiated by 40 keV helium ions

The effects of grain boundaries and surface roughness on tungsten irradiated by 40 keV He ions with a flux of 1.6 × 1017 He+/(m2s) to a fluence of 6.6 × 1021 He m−2 at room temperature were investigated. It was found that rough surfaces can suppress the surface blistering on tungsten. This may be due to the grooves appeared on rough tungsten limiting crack growth, resulting in the release of helium. However, the influence of grain boundaries on denseness of blistering was not obvious except the difference in blistering cap. The number of blister with detached cap on large grain tungsten samples (LGW, with the grain size of ∼50 μm) is more than that on small grain tungsten samples (SGW, with the grain size of ∼5 μm). In addition, the effect of grain boundaries on helium retention in tungsten cannot be ignored. The total He release from SGW was 2.56 × 1021 He m−2, which was smaller than the value (4.10 × 1021 He m−2) of LGW.

Investigation of helium exhaust dynamics at the ASDEX Upgrade tokamak with full-tungsten wall

An efficient removal of He ash by active pumping in future fusion devices is necessary to avoid fuel dilution and not degrade the core confinement properties. Therefore, a deep understanding of the underlying physics mechanisms is mandatory. Helium exhaust has been experimentally investigated at the ASDEX Upgrade tokamak. This is an ideal test environment, thanks to the ITER-like divertor geometry, an extensive diagnostics coverage and the presence of plasma-facing components made of tungsten. The exhaust efficiency, characterized by the He compression in the divertor, was found to improve with increasing divertor neutral pressure but to degrade with detachment. A multi-reservoir particle balance model was developed to interpret the observed exhaust dynamics, accounting for plasma transport and wall retention. The limited performance of the pumping system and the efficient helium retention capability of the tungsten wall were identified to have the strongest impact on the exhaust dynamics.

Impact of Soret effect on hydrogen and helium retention in PFC tungsten under ELM-like conditions

In our previous work, we have demonstrated using nonequilibrium molecular-dynamics simulations that the fluxes of helium and self-interstitial atoms in the presence of a thermal gradient in tungsten are directed opposite to the heat flux, indicating that species transport is governed by a Soret effect, namely, thermal-gradient-driven diffusion, characterized by a negative heat of transport that drives species transport uphill, i.e. from the cooler to the hot regions of the tungsten sample. In this work, the findings of our thermal and species transport analysis have been implemented in our cluster-dynamics code, Xolotl, which has been used to compute temperature and species profiles over spatiotemporal scales representative of plasma-facing component (PFC) tungsten under typical reactor operating conditions, including extreme heat loads at the plasma-facing surface characteristic of plasma instabilities that induce edge localized modes (ELMs). We demonstrate that the steady-state species profiles, when properly accounting for the Soret effect, vary significantly from those where temperature-gradient-driven transport is not accounted for and discuss the implications of such a Soret effect on the response to plasma exposure of plasma-facing tungsten. Although our cluster-dynamics simulations do not yet include self-clustering of helium or hydrogen blister formation, our simulation results show that the Soret effect substantially reduces helium and hydrogenic species retention inside PFC tungsten.

Intense γ -Photon and High-Energy Electron Production by Neutron Irradiation: Effects of Nuclear Excitations on Reactor Materials

The effects of neutron irradiation on materials are often interpreted in terms of atomic recoils, initiated by neutron impacts and producing crystal lattice defects. In addition, there is a remarkable two-step process, strongly pronounced in the medium-weight and heavy elements. This process involves the generation of energetic {\gamma} photons in nonelastic collisions of neutrons with atomic nuclei, achieved via capture and inelastic reactions. Subsequently, high-energy electrons are excited through the scattering of {\gamma} photons by the atomic electrons. We derive and validate equations enabling a fast and robust evaluation of photon and electron fluxes produced by the neutrons in the bulk of materials. The two-step n-{\gamma}-e scattering creates a nonequilibrium dynamically fluctuating steady-state population of high-energy electrons, with the spectra of photon and electron energies extending well into the mega-electron-volt range. This stimulates vacancy diffusion through electron-triggered atomic recoils, primarily involving vacancy-impurity dissociation, even if thermal activation is ineffective. Tungsten converts the energy of fusion or fission neutrons into a flux of {\gamma} radiation at the conversion efficiency approaching 99%, with implications for structural materials, superconductors, and insulators, as well as phenomena like corrosion, and helium and hydrogen isotope retention.

Examination of Early-Stage Helium Retention and Release in Dispersion-Strengthened Tungsten Alloys

Tungsten is the material of choice for plasma-facing components in the divertor region of future nuclear fusion reactors. Exposure to low-energy helium ion irradiation results in microstructural changes as helium is trapped at defects in the tungsten matrix. High-temperature exposure results in the formation of helium bubbles in the subsurface. Dispersion-strengthened tungsten materials are tungsten-based materials with added transition metal carbides to alter the impurity distribution and grain structure. In this work, the thermal release of helium from dispersion-strengthened tungsten is investigated. After irradiation at 1073 K to a 1024 m−2 fluence, thermal desorption spectroscopy was performed to elucidate the helium trapping and desorption behavior. Post-desorption microscopy was performed to correlate the microstructural changes with helium release spectra. The amount of desorbed helium was highest in the 1.1 and 5 wt% alloys, and significantly lower in the 10 wt% alloys. Helium bubbles were observed in the pure tungsten and 1.1 wt% alloys within the tungsten grains. Correlating the composition with helium release spectra revealed the importance of tailoring grain size and oxide vacancy concentrations by varying the dispersoid content on the helium retention and release behavior. These first results of helium desorption from dispersion-strengthened tungsten indicate compositionally dependent retention and reveal the need to examine helium retention in advanced tungsten alloys under reactor-relevant exposure.

Deuterium and helium retention in W and W-Ta coatings irradiated with energetic ion beams

Different studies reveal the behaviour of Ta alloying to decrease deuterium retention in tungsten bulk materials aiming to be used for nuclear fusion applications. In this work, W and W-5 %Ta coatings were produced via magnetron sputtering techniques. The elemental composition of the samples and the spread of Ta in the W lattice was confirmed by GDOES, PIXE and by μ-PIXE, respectively. Sets of W and W-Ta coatings were simultaneously irradiated with a single 2H2+ or with sequential 4He+/2H2+ ion beam implantations using incident energies of 30 keV and ion fluences of 5 × 1017 ion/cm2. The irradiated samples were analysed by NRA/RBS and by ToF-ERDA to quantify the retained deuterium and helium amounts. The results reveal significantly lower contents of both isotopes in W-Ta.

Flux dependence of helium retention in clean W(1 1 0): Experimental evidence for He self-trapping

Helium (He) retention in tungsten (W) is a concern in fusion reactors since it could be detrimental to plasma facing components performance and influence the fusion fuel balance. He being not soluble in W, it tends to agglomerate on preexisting defects (vacancy, grain boundary), but it could in theory also self-trap (be immobilized on a non-preexisting vacancy) through the emission of a vacancy/self-interstitial W pair in the vicinity of a Hen interstitial cluster. In the present study, we prepared a pure single crystal W(110) sample with a clean surface in order to evidence the self-trapping of He in the W bulk at a sample temperature of 300 K and for a constant fluence of 2.0x10$^{21}$ He$^+$.m$^{-2}$. At a He$^+$ kinetic energy of 130 eV and a flux of 0.3x10$^{17}$ He$^+$.m$^{-2}$.s$^{-1}$, we only observed a small He desorption peak below 600 K. Rising the ion flux to 0.7x10$^{17}$ He$^+$.m$^{-2}$.s$^{-1}$, we observed the sudden appearance of two desorption peaks at 950 K and 1700 K. For the highest flux studied in this work, 5.0x10$^{17}$ He$^+$.m$^{-2}$.s$^{-1}$, an additional desorption peak at 1800 K and a desorption shoulder at 1900 K are observed. The temperature position of these He desorption peaks are consistent with the density functional theory literature and points to the occurrence of self-trapping once the 0.7x10$^{17}$ He$^+$.m$^{-2}$.s$^{-1}$ flux is attained at 300 K and to the possible realization of trap-mutation for the flux of 5.0x10$^{17}$ He$^+$.m$^{-2}$.s$^{-1}$. The present set of results should be used to constrain the development of He retention and He bubbles growth models based on ab initio quantities.

The influence of displacement damage on helium uptake and retention in tungsten

The influence of displacement damage on helium uptake and retention in tungsten

Effect of dislocation on helium irradiation defects in low-activation martensitic steel

Reduced-activation martensitic steel is a main candidate structural material for key components of advanced nuclear energy systems because of its good mechanical properties at room temperature, low neutron activation characteristics and satisfactory radiation resistance. In this work, we study the interaction between dislocations and helium-irradiation-induced defects in the steel and the effect of dislocations on the behavior of helium migration and desorption. The well-annealed samples are pre-deformed to 10% and 20% reductions in thickness by using cold rolling mill. After pre-deformation, samples are heat-treated to remove vacancy defects and retain dislocation defects. Then, the samples with reserved dislocations are irradiated with helium at room temperature (50 keV, 1×10<sup>17</sup> He/cm<sup>2</sup>). After irradiation, the samples are characterized by synchrotron radiation grazing incident X-Ray diffraction, positron annihilation Doppler broadening spectroscopy, and thermal desorption spectroscopy. The results show that dislocations hinder the diffusion of helium and helium-vacancy complexes, and reduce the accumulation of radiation damage. Such an effect becomes more significant with the increase of dislocation density. The BCC → FCC phase transition of low activation martensitic steel occurs at 1179 K. The increase of dislocation density will lead to the forward shift of helium desorption peak induced by phase transition. The retention of helium in the undeformed sample, 10% deformed sample and 20% deformed sample is 10.3%, 15.7% and 17.9%, respectively, indicating that high density dislocations promote the retention of helium.

First-principles study of He retention and clustering in Al-Ga alloy

Understanding the effect of alloying elements on the retention and clustering behavior of Helium (He) in Aluminum (Al) is a great help to study the radiation damage process of Al-based nuclear structure materials under He irradiation. Based on the first-principles calculations, we investigated the influence of Gallium (Ga) dopant on the formation of [Formula: see text] and [Formula: see text] clusters in Al and Al-0.9[Formula: see text]at% Ga (Al-Ga) alloy. We found that Ga dopant could influence the formation and growth of He clusters. In addition, the presence of Ga reduces the binding energy of both [Formula: see text] and [Formula: see text] clusters, resulting in He dissociation from the clusters more easily. Moreover, Ga could serve as a trapping center by reducing the charge density in its vicinity to induce He nucleation in Al-Ga alloy compared to that in pure Al. Furthermore, the binding energy of He to vacancy around Ga is weaker than that around Al, suggesting that Ga will decrease the trapping capability of vacancy to He. We thus propose that Ga plays a key role in He clustering behavior. Our results are significant to understand the effect of dopant on He bubbling behavior and the distribution of damages in Al-Ga alloy under He irradiation.

Characterization of helium retention in the inhomogeneous co-deposited layers using a long pulse laser induced ablation-quadrupole mass spectroscopy

• The 2D-resolved quantitative characterization of the He retention in the inhomogeneous co-deposited layers has been carried out using 750 microsecond pulse laser induced ablation-quadrupole mass spectrometry (LIA-QMS). • The concentration of He atoms in the co-deposited layer has been determined based on the measured He mass spectral intensity and the ablation mass of laser ablated crater. • LIA-QMS approach has a significantly potential for the 2D-resolved quantitative diagnosis for the He or D/T retention in the non-uniform co-deposited layers on PFMs in nuclear fusion devices. In this work, a spatially lateral-resolved study of the co-deposition of helium (He) and aluminum (Al) in non-uniform layers is carried out by a long pulse Laser-Induced Ablation-Quadrupole Mass Spectrometry (LIA-QMS) approach. He was co-deposited with Al on the silicon (Si) substrate by Pulse Laser Deposition (PLD) method at He ambient pressure of 4.0 Pa. The Al is a proxy of beryllium (Be) because the Be is toxic. The thickness of the He-Al co-deposited layers decreases from approximately 3.9 µm to 0.2 µm with increasing lateral distance measured using white light confocal 3-D profilometer. The areal number density of He-retention in the inhomogeneous co-deposited layers decreases from 5.8 × 10 21 to 7.5 × 10 19 He/m 2 along the lateral distance, which is consistent with the variation trend of the deposition layer thickness. The 2-D imaging of the He atomic areal density measured by LIA-QMS shows that the He-retention in the co-deposition layer is significantly inhomogeneous. The maximum and minimum of the He concentration in atom ratio in the co-deposited layer are about 4 % and 0.4 %, respectively. The results indicate that LIA-QMS diagnostic approach has a significantly potential for the 2D-resolved quantitative characterization of the He (or D/T) retention in the co-deposited layers on Plasma Facing Materials (PFMs).

First lithium experiments in HIDRA and evidence of helium retention during quasi-steady-state stellarator plasma operations

Recent experiments in Hybrid Illinois Device for Research and Applications (HIDRA) have had operational discharges between t discharge = 60 and 1000 s using electron cyclotron resonant heating (ECRH) of the plasma. This means that quasi-steady-state plasma discharges reach conditions to study long-pulse plasma material interactions (PMIs). The newly commissioned HIDRA-Material Analysis Test-stand PMI diagnostic is used to place a drop of lithium onto a heated tungsten surface, transfer the sample in-vacuo and expose it in a helium plasma. Helium is of interest as there is an open question to whether lithium will be able to remove helium ash in real fusion devices. The introduction of the W-Li sample in HIDRA resulted in evaporation of lithium into the helium plasma during a 600 s pulse and caused a reduction of over 90% in neutral pressure during the discharge. It was also observed that the plasma density and temperature increased by over 2.5 times. Using spectroscopy and a helium collisional radiative model, the peak temperature and density of the helium plasma can be monitored during the discharge. During lithium evaporation, as significant lithium ionization occurs, there is a 85% drop in the HIDRA vessel neutral pressure, despite a constant flow rate of He gas. This reduction in neutral pressure is supported by spectroscopy data with corresponding reductions in He I line intensities (587 nm, 667 nm, 706 nm, and 728 nm), as well as those of other impurities. At one point in the discharge a lithium plasma is created, as indicated by an increase in Li+ emission and a complete reduction in He+ emission, but the electron density jumps from n e = 3 × 1018 m−3 to over n e = 8 × 1018 m−3 while the core temperature stays relatively constant between T e = 16 eV and 20 eV. Once lithium has completely evaporated from the sample and the majority of the ionized lithium has diffused from the plasma to the vessel walls, pressure and spectroscopy data paired with He collisional radiative model calculations shows a re-establishment of a helium plasma in a low recycling regime. In this regime, the density drops down to n e = 2 × 1018 m−3 and the electron temperature increases from T e = 20 eV to over T e = 50 eV indicating an increase in helium heating efficiency. This is also indicated by the He+ emission re-establishing and having a higher intensity. In this paper we show the results from the first lithium campaign in HIDRA. In the presence of lithium, and in particular when lithium ions are present, the helium disappears from the plasma via an as of yet unknown complex relationship that needs to be further studied. The most likely explanation is that the lithium ions are distributed around the vessel and able to trap helium to the surface turning HIDRA into a large gettering surface. These results have potential implications on future plasma facing component design using liquid lithium for impurity and recycling control using limiters and divertors.

Helium and deuterium retention in Eurofer97 under sequential irradiation at low fluxes

The helium (He) retention and the influence of He-induced defects on the deuterium (D) retention and migration in Eurofer97 were investigated by in-situ and ex-situ thermal desorption spectroscopy technique. Samples were pre-irradiated with 3 keV He ions in the fluence range of 1019–1022 He/m2, then irradiation with 2 keV D3+ ions (667 eV/D) with a fluence of 1021 D/m2 was performed. Experiments were carried out at the sample temperature of 300 K. It was shown that the D retention in Eurofer97 increases and decreases with pre-irradiation with He fluences below and above 1021 He/m2, respectively. However, in all cases of He pre-irradiation, the D retention is higher than that in Eurofer97 without He pre-irradiation. The effect of the reduction of the D retention at He fluences above 1021 He/m2 can be connected with either the formation of open porosity due to interconnected bubbles or the strain field induced by high pressurized He bubbles that preventing D penetration into the bulk of a metal. Both effects reduce the D diffusion towards the bulk of a metal and, therefore, lead to a decrease in the D retention. Our data of in-situ and ex-situ experiments revealed that HemVn complexes in the temperature range of 600–800 K are unstable and transformed in stable HemVn complexes in the temperature range of 900–1100 K after air exposure. Due to a formation of oxide layer during a contact with the atmosphere, ex-situ TDS of D leads to a shift of the D release temperature in the high-temperature range compared to in-situ TDS in both cases with and without He pre-irradiation. Finally, the presence of He in a metal lattice will have the dominant influence on the D retention in the future fusion devices together with neutron irradiation and high temperature gradient. In the case of He saturation, the similar D retention was measured in different bcc metals and their alloys.