[en] Rotating collectors (RC) were installed in JET during the period 2005-2007, each providing a time-resolved deposition pattern on the surface of a rotating silicon disc, which could be analysed once retrieved from the vessel. This paper reports results from the silicon disc removed from the RC located under the load-bearing septum replacement plate in JET in 2007. Nuclear reaction analysis results of the deposits on the disc have been correlated directly with the pattern of erosion and deposition observed by the quartz microbalance (QMB) located in an equivalent position. The thickest film in the time-resolved region (i.e. deposited in ∼60 pulses) was ∼250 nm, and the Be/C ratio was generally found to be 0.1 or lower, with two regions where the ratio rose to 0.2. The deposition observed with the QMB appears to be about a factor of four less.

[en] The fusion reactor ITER, which is under construction in France, will be a milestone on the way towards fusion energy, generating (as currently planned) for the first time conditions in which 10 times more fusion power output is expected than power input for plasma heating. Currently, the largest device is the Joint European Torus (JET), located in Great Britain. Since August 2011 it has been operated with a full metal first wall with beryllium (Be) limiters in the main chamber and tungsten (W, coatings and bulk metal) in the so-called divertor; JET with the ITER-like wall (JET-ILW). Plasma-material interactions constitute an important part of the research programme. The understanding of fuel retention and material migration belong to the top missions of JET-ILW. This work is focused on the determination of erosion, re-deposition/co-deposition of eroded species with fuel atoms and, on resultant retention of hydrogen isotopes on the JET-ILW wall components. Samples of the components retrieved from the torus after the three consecutive ILW campaigns (ILW-1, ILW-2 and ILW-3) were examined with accelerator-based ion beam analysis (IBA) techniques. This powerful set of methods is one of the crucial tools for surface/sub-surface studies of wall materials from controlled fusion devices.

[en] The First Mirror Test in Joint European Torus (JET) with the International Thermonuclear Experimental Reactor-like wall was performed with polycrystalline molybdenum mirrors. Two major types of experiments were done. Using a reciprocating probe system in the main chamber, a short-term exposure was made during a 0.3 h plasma operation in 71 discharges. The impact on reflectivity was negligible. In a long-term experiment lasting 19 h with 13 h of X-point plasma, 20 Mo mirrors were exposed, including four coated with a 1 μm-thick Rh layer. Optical performance of all mirrors exposed in the divertor was degraded by up to 80% because of beryllium, carbon and tungsten co-deposits on surfaces. Total reflectivity of most Mo mirrors facing plasma in the main chamber was only slightly affected in the spectral range 400–1600 nm, while the Rh-coated mirror lost its high original reflectivity by 30%, thus decreasing to the level typical of molybdenum surfaces. Specular reflectivity was decreased most strongly in the 250–400 nm UV range. Surface measurements with x-ray photoelectron spectroscopy and depth profiling with secondary ion mass spectrometry and heavy-ion elastic recoil detection analysis (ERDA) revealed that the very surface region on both types of mirrors had been modified by neutrals, resulting eventually in the composition change: Be, C, D at the level below 1 × 10¹⁶ cm⁻² mixed with traces of Ni, Fe in the layer 10–30 nm thick. On several exposed mirrors, the original matrix material (Mo) remained as the major constituent of the modified layer. The data obtained in two major phases of the JET operation with carbon and full metal walls are compared. The implications of these results for first mirrors and their maintenance in a reactor-class device are discussed. (paper)

[en] Nuclear fusion offers a potential future energy supply and is an area of worldwide research with this aim in mind. One such research effort is at the world’s largest operating tokamak device based in UK, known as JET – Joint European Torus. JET consists of a 6 m diameter torus (doughnut) shaped vacuum vessel in which high temperature hydrogen plasma is confined by magnetic field to facilitate fusion of hydrogen isotopes, deuterium (D) and tritium (T), to release high energy 14 MeV neutrons. The inside of the JET vacuum vessel is protected by wall tiles. In the extreme conditions the plasma interacts with the wall components and as a result components undergo erosion, material migration and deposition. This plasma wall interaction has an impact on operational lifetime of components and has the potential to generate dust which poses a safety risk in the event of accident scenarios. Plasma interaction with the wall materials also results in the retention of the hydrogen fuel, including the radionuclide tritium, which is regulated to ensure safe operation of fusion facilities. For these reasons an extensive programme to study materials removed from JET has been ongoing for more than three decades and have influenced decisions on upgrades to JET and the choice of wall materials for ITER, under construction in France. The use of accelerator-based techniques has played a major role in the materials analysis programme and results have advanced the understanding of erosion, material migration, deposition and fuel retention in tokomaks and provided benchmarking for modelling.

[en] Full text: Post mortem analysis of components removed from the JET ITER-like wall (JET-ILW) provides an overall picture of long term fuel inventory. Results from the first 2011–’12 (ILW-1) and second 2013–’14 (ILW-2) JET-ILWcampaigns are now available making a comprehensive overview possible. Overall plasma times for ILW-1 and ILW-2 are similar. ∼6 h limiter plasma and ∼14/∼13 h divertor plasma; however variation in strike point distribution influences overall material migration and fuel retention. Following ILW-1, the global long term fuel retention in JET-ILWis ∼0.2% of injected fuel (deuterium, D) — at least an order of magnitude lower than the carbon wall configuration. Of this ∼65% of the retained fuel is found in the divertor, with the remaining inventory located in the main chamber. Fuel retention at the inner divertor, 17 × 10²² D atoms, occurs predominantly by codeposition. The main deposition area is at the inner top horizontal surface where deposits > μm after ILW-2 are found. Overall the outer divertor surfaces remain a net erosion zone and have a lower fuel inventory, 3.9 × 10²² D atoms. Fuel retained on the bulk tungsten load bearing plate at the base of the divertor is ∼5 x 10²¹ D atoms following ILW-1. This contributes only a small inventory to the divertor and is consistent with the surface being a net erosion zone. In the remote divertor corners deposition occurs by line of sight transport of neutrals sputtered from the SP; this demonstrates the influence of SP location on remote material migration. Analysis also shows that deposition and fuel retention is higher in the outer corner than the inner corner. The total fuel inventory of plasma facing tiles in the main chamber contributes ∼30%, 8.7 x 10²² D atoms, to the vessel inventory. Recessed areas, whilst having a relatively low concentration of fuel, can contribute significantly due to the large areas involved. Since the recessed areas do not interact with plasma ions, the fuel retention is due to implantation of charge exchange neutrals. Overall recessed areas and gaps in tiles contribute > 30% of the main chamber inventory, 4.6 x 10²² D atoms, with the recessed inner wall making the largest contribution. (author)

[en] A complete global balance for material transport in JET requires knowledge of the net erosion in the main chamber, net deposition in the divertor and the amount of dust in the divertor region. Following the end of the first JET ITER-like wall campaign a set of tiles has been removed from the main chamber and the divertor. This paper describes the initial tile surface profiling results for evaluating the erosion in the main chamber and deposition in the divertor. Tile profiling was performed on upper dump plate tiles and inner wall guard limiters made of beryllium and on inner divertor tiles made of tungsten coated carbon (C)-fibre composites. Additionally, the mass of dust collected from the JET divertor is also reported. Present results are compared with JET-C campaign results with the all-carbon C wall. (paper)

[en] Accumulation of dust on the equipment for remotely handled (RH) operations was studied in JET with the ITER-like wall (JET-ILW) during the shutdown period following the third ILW campaign. The topic is connected to licensing procedures for ITER: the need to assess risks related to the external transfer of beryllium and radioactive matter with tritium and activation products. Ten adhesive carbon pads were placed in different locations on the robotic arm operated in-vessel for 672 h. Also air samplers were used during the RH operation and smear tests of the RH boom were performed to quantify specific contamination levels by beryllium and tritium. Dust morphology was determined by microscopy techniques. The areal density of dust varies at different sticker position on the boom. On some parts (e.g. ‘wrists’ of the robot) the density of particles exceeds 1000 per mm². Their morphology is very diverse but most collected objects originate from the construction material (aluminium) of the RH equipment itself. The accumulation of Be- and W-based particles is negligible. The study confirms earlier experimental evidence that Be-rich co-deposits (and also W coatings on CFC) adhere well to plasma-facing components and they are not easily mobilised. (topical issue article)

[en] Full text: Comprehensive and systematic surveys of dust particles were performed at JET with the ITER-like wall (JET-ILW) after two experimental campaigns, ∼19 h of plasma each. Thought the total amounts removed were small (around 1 g) the study of dust categories is crucial for ITER because these are unique data from a full metal-wall (beryllium and tungsten) machine. The identification of various categories of particles allows conclusions on mechanisms underlying their generation and mobilization. This work deals with dust collected with sticky pads from the divertor tiles and, with metal splashes on erosion-deposition probes in the divertor and the main chamber wall. The local sampling is essential for: i) finding a correlation between the type of dust and the deposition pattern, and ii) comparison of dust identified in a given location after consecutive campaigns The search has identified several forms. a) Flakes of Be-rich deposits (up to 800 μm) with embedded tiny metal particles: Ni, W. Irregular droplet-like W inclusions, up to 200 nm, are nearly uniformly distributed in the deposit with an exception of the bottom of the layer, i.e., film formed at the early commissioning phase without high power beam operation. There is also a significant content of nitrogen retained after plasma edge cooling. Films are crystalline, but the presence of amorphous regions cannot be fully excluded. b) Regular Be droplets, diameter 5-10 μm and - on probes - Be splashes with small bubbles thus indicating boiling of the droplet. c) Spherical W droplets, ∼100 μm diameter, which could be formed in the experiment on tungsten melting. d) Droplets of Inconel. e) Irregular debris or flakes up to 300 μm containing W, Mo-W. f) Ceramics containing boron nitride, zirconium oxide, alumina. In conclusion, the study clearly shows a correlation between the operation mode, material erosion, growth of codeposits and generation of dust. (author)

[en] The impact of edge localized modes (ELMs) carrying energies of up to 450 kJ on carbon erosion in the JET inner divertor is assessed by means of time resolved measurements using an in situ quartz microbalance diagnostic. The inner target erosion is strongly nonlinearly dependent on the ELM energy: a single 400 kJ ELM produces the same carbon erosion as ten 150 kJ events. The ELM-induced enhanced erosion is attributed to the presence of codeposited carbon-deuterium layers on the inner divertor target, which are thermally decomposed under the impact of ELMs

[en] Collection and ex situ studies of dust generated in controlled fusion devices during plasma operation are regularly carried out after experimental campaigns. Herewith results of the dust survey performed in JET after the second phase of operation with the metal ITER-like wall (2013–2014) are presented. For the first-time-ever particles deposited on silicon plates acting as dust collectors installed in the inner and outer divertor have been examined. The emphasis is on analysing metal particles (Be and W) with the aim to determine their composition, size and surface topography. The most important is the identification of beryllium dust in the form of droplets (both splashes and spherical particles), flakes of co-deposits and small fragments of Be tiles. Tungsten and nickel rich (from Inconel) particles are also identified. Nitrogen from plasma edge cooling has been detected in all types of particles. They are categorized and the origin of various constituents is discussed. (paper)