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Surface morphology and its evolution during the plasma irradiation is known to have a large influence on the erosion and resulting lifetime of plasma-facing components as well as tritium retention. For instance, surface roughness can affect physical sputtering, re-deposition, as well as angular distributions of the sputtered species. In this study the effect of surface roughness is implemented into the 3D Monte-Carlo code ERO2.0. First modelling results for molybdenum (Mo) irradiated with deuterium (D) in the conditions foreseen for the planned experiments at the linear plasma device PSI-2 are presented. Using the constructed examples of surfaces with various (regular and fractal) roughness types it is shown that the effective sputtering yield decreases for rough surfaces in comparison to smooth ones. The angular distribution of particles escaping from the rough surface collimates with the increase of the surface structure's aspect ratio. Moreover, the modelling predicts flattening of the surface during the plasma irradiation due to the preferable re-deposition in the “valleys” and sputtering of the peak tops.

Tungsten (W) is a preferred plasma-facing material for areas of high particle and power impact in present day and future fusion devices. The lifetime of W plasma-facing components (PFCs) under steady-state conditions is limited by erosion processes induced by energetic impinging particles such as impurities (carbon C, oxygen O, nitrogen N, etc.) and hydrogen isotopes above the threshold energy for Physical Sputtering (PS). We discovered for the first time, in addition to the bare physical sputtering process of W, a second W erosion mechanism at PFCs in TEXTOR (limiter surfaces) and ASDEX Upgrade (divertor target plates) during deuterium plasma bombardment. The tungsten deuteride molecule WD has been identified spectroscopically via the 6Π→6Σ+ transition in parallel to ordinary neutral W line emission of the WI transition (5d5(6S)6s 7S3 → 5d5(6S)6p 7P4) at λ=400.9nm used as measure to quantify the bare PS process or, more precisely, the gross erosion of W. We identified the underlying process for the molecular release as Chemically Assisted Physical Sputtering (CAPS) as observed also in the case of beryllium or lanthanum. Measurements in TEXTOR and ASDEX Upgrade showed a dependence of the WD band emission on the surface temperature - connected to the deuterium content in the near W surface - as well as on the flux and energy of impinging energetic particles. A quantification of the released WD is not yet possible due to lack of appropriate molecular data for the conversion of photons into particles, but the spatially resolved ratio of atomic (WI) and molecular photon flux (WD) indicates that the release takes place at the same time, but differently distributed along the target plates and limiters reflecting the variation in the described critical parameters (impinging ion flux, ion impact energy, and material temperature) along the surface. The plasma conditions in front of the interaction zone are in both cases fully ionising with an electron temperature Te > 10eV, thus, recombination into WD can be excluded. Keywords: Tungsten, TEXTOR, ASDEX Upgrade, Physical Sputtering, Divertor

Surface morphology development and sputtering behavior of Cr, as a test material, have been explored under He plasma exposure at a low incident ion energy of ∼80 eV in multiple linear plasma devices: PISCES-A, PSI-2, and NAGDIS-II. From comparison of the experiments in these devices, deposition of a small amount of heavier impurities (Mo in NAGDIS-II and Ta in PISCES-A) onto Cr is found to result in the formation of cone structures on the Cr surface due to preferential sputtering, resulting in a significant reduction (up to ∼10 times) in the sputtering yield of Cr due to line-of-sight redeposition onto the neighboring cones. The heavier impurities are thought to originate from a sample holding cap/cover, which can be sputtered by a trace amount of intrinsic impurities (C, O, etc) as well as by Cr ionized in the plasma. It can be concluded from the Cr experiments, as well as additional Be data collected in PISCES-B, that heavier impurity deposition plays a major role in the cone structure formation, while other mechanisms (e.g. surface irregularity and oxide) also exist. Keywords: Impurity deposition, Sputtering, Nanostructure

ABSTRACT: 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. Keywords: Global migration, Tokamak, high-Z material, Molybdenum, Modelling, ASCOT, ERO, Fuel retention, Deposition

In this paper the impact of steady state pure D plasma on WCrY smart alloys at ion energies of 120 and 220 eV is reported. For this purpose a comparison with simultaneously exposed pure W samples is drawn. Different analysis techniques employed for pre- and post-plasma sample analysis hint at a significant depletion of Cr and enrichment of W for lower ion energies. Preferential sputtering leads to enhanced volumetric loss at 220 eV. Analysis of redeposited material indicated local redeposition of Cr. Modelling the ion irradiation with SDTrimSP is used to further interpret experimental results. Depending on the sample temperature during plasma exposure and the magnitude of the ion flux, diffusion of Cr towards the surface is a determining factor for erosion of smart alloys for higher ion energies. Keywords: Smart alloys, Plasma-wall-interaction, Plasma ion irradiation, DEMO, W-based alloys

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

Tracer injection experiments in TEXTOR with MoF6 and WF6 lead to local deposition of about 6% for Mo and about 1% for W relative to the injected amount of Mo and W atoms. Modelling of these experiments has been done with ERO applying updated data for physical sputtering. The dissociation of the injected molecules has been treated in a simplified manner due to the lack of dissociation rate coefficients. However, with this it was possible to reproduce the observed radial penetration of Mo and W atoms into the plasma. The modelled local deposition efficiencies are about 50% for Mo and 60% for W assuming typical plasma parameters for the experimental conditions used. To reproduce the measured deposition efficiencies an enhancement factor for the erosion of deposited Mo and W has to be assumed (∼10 for Mo and ∼25 for W). Due to the rather low electron temperature Te of these plasma conditions (Te∼15eV at the location of injection), Mo and W are mostly sputtered by impurities whereas sputtering due to deuterium is negligible. A parameter study applying larger electron temperature leads to increased sputtering and thus to reduced local deposition efficiencies of about 30% for Mo and 5% for W. Though, even under these conditions enhanced erosion, albeit with reduced enhancement factors, is needed in the modelling to obtain the small measured deposition efficiencies. keywords: Erosion & Deposition, Sputtering, Impurity transport, TEXTOR, PACS: 52.40.Hf, 52.77.Dq, 52.65.Pp

An overview of diagnostic tools – test limiters and collector probes – used over the years for material migration studies in the TEXTOR tokamak is presented. Probe transfer systems are shown and their technical capabilities are described. This is accompanied by a brief presentation of selected results and conclusions from the research on material erosion – deposition processes including tests of candidate materials (e.g. W, Mo, carbon-based composites) for plasma-facing components in controlled fusion devices. The use of tracer techniques and methods for analysis of materials retrieved from the tokamak are summarized. The impact of research on the reactor wall technology is addressed.