• Energy Research

Nuclear fusion plasma-facing materials (PFM) will suffer from irradiation, leading to significant changes in the material properties. This study investigates the impact of displacement damage on the deuterium retention near room temperature.ITER grade tungsten, Eurofer-97, and HiperFer 17Cr5 steel samples are irradiated with a tandem accelerator with ~3 MeV protons at currents of 100–600 nA on 250–550 µm spots at 320±10 K. In total 33 spots from 0 to 0.9 displacements per atom (DPA) at 0–4 µm depth are irradiated on 5 samples. After irradiation, the samples are exposed to D2 plasmas with a peak ion-flux of 2.1 × 1021 D/m²s for 4 h at <420 K in PSI-2. Lastly, D retention is measured via 3He nuclear reaction analysis with a spot size of 200 µm up to 4.5 µm depth.The long-term D retention in both W and steel increases with DPA with a saturation starting around 0.2 DPA. Retention in W increased by a factor 12 with up to 3.2 at.% D, while in steel increases up to 180 times with up to 0.08 at.% D were observed. The results highlight the importance of using steels also in PFMs. Compatibility of the results with heavy ion irradiations boosts the confidence in inter-comparability between different ion types, but also between ions and neutrons.

Total deuterium (D) retention from the bulk material of reduced-activation ferritic-martensitic (RAFM) steel Eurofer’97 (EU’97) and commercial ferritic-martensitic grade P92 was traced experimentally by means of thermal desorption spectroscopy (TDS) and linked to the role of krypton (Kr) and argon (Ar) seeding during high-fluence plasma exposure. The influence of impurity seeding on the steel microstructure was determined using scanning electron microscopy (SEM) and extended with focused ion beam (FIB) cross-sectioning. D capture at depths in the µm range was measured by nuclear reaction analysis (NRA). Plasma exposure of the steel samples occurred at 470 K with ion energy of 30–40 eV in the linear plasma device PSI-2 with up to 10% simultaneous impurity admixture in plasma. D inventory achieved values in the 1020 D/m2 range after plasma exposure with high fluences of up to 1 × 1026 D+/m2. In pure and mixed plasmas, the majority of D was trapped in the steel bulk. The Kr and Ar seeding of D plasma resulted in the population of multiple already existing D trapping sites. A similarity of D desorption spectra suggests that D trapping in P92 follows the same mechanism as in EU’97. Kr and Ar seeding mostly contributed to the surface sputtering of the steel samples, yielding less D retention due to the material loss. Keywords: Eurofer, RAFM, Ferritic-martensitic, Impurity seeding, Retention, Desorption

AbstractExperiments were performed in the linear plasma device PSI-2 in order to investigate the synergistic effects of combined steady-state He-plasma and thermal shock exposure. Tungsten produced according to the ITER material specifications by Plansee SE, Austria, was loaded sequentially and simultaneously by steady-state He plasma and transient thermal loads induced by a laser beam. All tungsten samples were exposed to helium plasma for 40min at a base temperature of ca. 850°C and a flux of ca. 2.8×1022m−2s−1. Before, during and after the plasma exposure 1000 thermal shock pulses with a pulse duration of 1ms and a power density 0.76GW/m² were applied on the samples. The thermal shock exposure before and after plasma exposure was done at room temperature in order to investigate helium induced surface effects also within cracks. The obtained results show that the combination of He plasma with transient thermal shock events results in a severe modification such as reduced height or agglomeration of the sub-surface He-bubbles and of the created nanostructures, i.e. W-fuzz.

AbstractIn this study the effect of impurities in deuterium plasma on the tungsten microstructure is investigated. W samples were exposed in the linear plasma generator PSI-2 at a sample temperature of 500K with an incident ion flux of about 1022m−2s−1, an incident ion fluence of 5×1025m−2 and an incident ion energy of 70eV. Samples were exposed to pure D+ plasma and with additional impurities of He (3%), Ar (7%), Ne (10%) or N (5%). After the PSI-2 exposure a part of each sample was additionally loaded with tritium to measure the tritium uptake using the imaging plate technique.The surface morphology was investigated using scanning electron microscope (SEM) combined with a focused ion beam (FIB) utilized for cross-sectioning and thin lamella preparation for the transmission electron microscope (TEM) analysis.Blistering with grain orientation dependence was observed on all exposed samples. Most pronounced blistering is reported for grains with orientation close to (111). The addition of Ar or Ne results in surface erosion with different yields depending on grain orientation. Highest erosion yield is observed for grains with orientation close to (100). Large blisters are present but show signatures of erosion. Less pronounced erosion is visible when adding N. The highest uptake of tritium was reported for the sample exposed to D+He plasma which corresponds to the largest – 18nm, near surface damage zone revealed by TEM. Lowest tritium accumulation was observed for samples exposed to D+Ar and D+Ne plasmas, which corresponds to the shallowest near surface damage zone, as confirmed by TEM.

Before decommissioning of the TEXTOR tokamak in 2013, the machine was conditioned with a comprehensive migration experiment where MoF6 and 15N2 were injected on the very last operation day. Thereafter, all plasma-facing components (PFCs) were available for extensive studies of both local and global migration of impurities – Mo, W, Inconel alloy constituents, 15 N, F – and fuel retention studies. Measurements were performed on 140 limiter tiles out of 864 throughout the whole machine to map global transport. One fifth of the introduced molybdenum could be found. Wherever possible, the findings are compared to results obtained previously in other machines. This review incorporates both published and unpublished results from this TEXTOR study and combines findings with analytical methods as well as modelling results from two codes, ERO and ASCOT. The main findings are: • Both local and global molybdenum transport can be explained by toroidal plasma flow and E⇀×B⇀ drift. The suggested transport scheme for molybdenum holds also for other analysed species, namely tungsten from previous experiments and medium-Z metals (Cr-Cu) introduced on various occasions.• Analytical interpretation of several deposition profile features is possible with basic geometrical and plasma physics considerations. These are deposition profiles on the collector probe, the lower part of the inner bumper limiter, the poloidal cross-section of the inner bumper limiter, and the poloidal limiter.• Any deposition pattern found in this TEXTOR study, including fuel retention, has neither poloidal nor toroidal symmetry, which is often assumed when determining deposition profiles on global scale.• Fuel retention is highly inhomogeneous due to local variation of plasma parameters – by auxiliary heating systems and impurity injection – and PFC temperature.• Local modelling with ERO yields good qualitative agreement but too high local deposition efficiency.• Global modelling with ASCOT shows that the radial electric field and source form have a high impact on global deposition patterns, while toroidal flow has little influence. Some of the experimental findings could be reproduced. Still, qualitative differences between simulated and experimental global deposition patterns remain. The review closes with lessons learnt during this extensive TEXTOR study which might be helpful for future scientific exploitation of other tokamaks to be decommissioned.

MoF6 injection from a localised source into plasma edge in the TEXTOR tokamak was the last experiment before the final shut-down of the TEXTOR machine. During decommissioning all plasma-facing components (PFCs) became available for surface studies. Detailed mapping of Mo deposition was performed in order to determine its migration on global scale. The concentration of Mo on PFC decays exponentially with distance from the source. The decay length is of the order of 0.1m on the main PFC and 1m on the receded components. Also the decay lengths modelled with the ERO code are between 0.15–1.3m, depending on the anomalous cross-field diffusion coefficient. The inner bumper limiter is found to be the major repository for Mo. Material balance measurements show that only up to 22% of the injected Mo was detected on all the PFCs thus indicating that a large fraction of injected Mo may have been pumped out before being deposited. Keywords: Global transport, Material migration, High-Z metals, TEXTOR, Plasma-facing components, Molybdenum