| 1. |
- Almqvist, Nils, et al.
(author)
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AFM and STM characterization of surfaces exposed to high flux deuterium plasma
- 1995
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In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 220-222:1-3, s. 917-921
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Journal article (peer-reviewed)abstract
- This paper reports the results of scanning tunneling (STM) and atomic force microscopy (AFM) studies of D+ irradiated graphite and graphite-silicon mixtures. The microscopes were used for studying surface topography and for measuring the surface roughness. The substrates were exposed at various temperatures (60 and 700°C) to different doses of deuterium ions in simulators of plasma - surface interactions and in the TEXTOR tokamak. Also nuclear reaction analysis (NRA) and Rutherford backscattering spectroscopy were applied for the qualitative and quantitative determination of surface composition. The initial stages of radiation damage, nanometer-sized bubbles/blisters, were found in plasma-eroded surfaces. These structures only appeared in the graphite phase on the multicomponent material. The microroughness of the surfaces was measured. We also used the AFM for probing the thickness of the plasma-modified layers. The results correlate with the presence of deuterium measured by NRA depth-profiling. Moreover, the AFM reveals the co-deposited layers formed on surfaces facing the tokamak plasma. The appearance of these layers is clearly correlated to the amount of co-deposited atoms.
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| 2. |
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| 3. |
- Coad, J. P., et al.
(author)
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Erosion/deposition in JET during the period 1999-2001
- 2003
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 313, s. 419-423
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Journal article (peer-reviewed)abstract
- Coated divertor and wall tiles exposed in JET for the 1999-2001 operations have been used to assess erosion/deposition. Deposited films of up to 90 mum thickness at the inner wall of the divertor tiles are, for the most part, enriched in beryllium and other metals, whilst carbon is probably chemically sputtered from these tiles and transported to shadowed regions of the inner divertor. However, from the composition at the surface of the tiles, it appears that the chemical erosion was 'switched off' by reducing the JET vessel wall temperature for the last part of the operations to 200 degreesC. Thick powdery deposits localised at the ion transport limit at each corner of the divertor may be due to physical sputtering. Erosion of the coatings is seen at the outer divertor wall, and on all the inner wall and outer limiter tiles.
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| 4. |
- Emmoth, Birger, et al.
(author)
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Particle collection at the plasma edge by a fast reciprocating probe at the TEXTOR tokamak
- 2003
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 313, s. 729-733
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Journal article (peer-reviewed)abstract
- A fast reciprocating probe system capable of transferring different types of heads has been constructed and implemented at the TEXTOR tokamak for diagnosing the plasma edge. It gives the possibility of using a particle collector technique to extend studies of material transport from the scrape-off layer to the near plasma edge. For the first time, the system was used for exposures of graphite samples (pure and coated with a-C:H or W) at positions both within and outside the last closed flux surface. Various surface analysis methods were applied to investigate the probe morphology and, by this, to determine radial deposition profiles of boron impurities and deuterium. The profiles for boron are remarkably flat whilst those for deuterium are characterised by a steep decay with the e-folding length of approximately 15 mm. On tungsten-coated samples almost no deuterium was found, most likely because of little carbon co-deposition, shallow implantation and low trapping coefficient of deuterons in the tungsten layer. Reconstruction of experimental results by means of a multifluid TECXY code helped to identify the contribution of impurity sources (limiters, wall) to the observed radial distribution of species.
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| 5. |
- Gueneau, C., et al.
(author)
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Thermodynamic assessment of the uranium-oxygen system
- 2002
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 304:03-feb, s. 161-175
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Journal article (peer-reviewed)abstract
- A thermodynamic assessment of the uranium-oxygen system is presented. A consistent set of experimental data is selected among the numerous data in the literature on the phase diagram and oxygen chemical potential. The thermodynamic properties of the phases are described using the compound energy model with ionic constituents for the solids and an ionic two-sublattice model for the liquid. For the uranium dioxide, the structure is described using three sublattices, one for the cations U3+, U4+ and U6+, one for the normal site of oxygen ions, and one for the interstitial oxygen ions. Vacancies are included in both oxygen sublattices. In this first approach, the homogeneity ranges of the U4O9-y, and U3O8-y compounds are not represented. A set of consistent model parameters that describes both the phase diagram and the oxygen chemical potential data in the whole composition range is thus obtained. The description of this basic binary system will be used to calculate higher order systems such as O-U-Zr and Fe-O-U which are important for simulating severe nuclear accidents.
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| 6. |
- Hermansson, P., et al.
(author)
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An effective method for calculation of diffusive flow in spherical grains
- 2002
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 304:2-3, s. 204-211
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Journal article (peer-reviewed)abstract
- The accuracy of a numerical fission gas release algorithm developed by Forsberg and Massih for solving the problem of diffusive flow to a spherical grain boundary is analysed. Estimates of numerical errors are derived for both steady-state and time varying conditions. We also present a method through which the accuracy of the algorithm can be improved or optimised for most applications. © 2002 Elsevier Science B.V. All rights reserved.
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| 7. |
- Hirai, T., et al.
(author)
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Performance and erosion of a tungsten brush limiter exposed at the TEXTOR tokamak
- 2003
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 313, s. 67-71
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Journal article (peer-reviewed)abstract
- To examine the performance of a castellated structure under plasma loading, a hemispherical solid tungsten brush limiter was exposed to the plasma in the TEXTOR-94 tokamak. Due to the thermally isolated column of W segments, IR camera showed a non-uniform temperature distribution. The maximum incident power density was calculated to be about 35-40 MW/m(2). Concerning impurity generation, the structure did not show any particular effects. During plasma exposure, only some minor cracks developed in one of the columns, however, the crack propagation was interrupted by a groove. It can be concluded that the W brush limiter had comparable performance and superior mechanical behaviour compared to a solid W limiter. To study erosion and long-range transport of W atoms, a graphite limiter was exposed simultaneously with the brush limiter. As a result, the deposited W atoms via long-range transportation were estimated to be 10(15) cm(-2) shot(-1) at 46.5 cm from the plasma centre of TEXTOR.
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| 8. |
- Huber, A., et al.
(author)
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Comparison of impurity production, recycling and power deposition on carbon and tungsten limiters in TEXTOR-94
- 2001
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 290, s. 276-280
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Journal article (peer-reviewed)abstract
- Impurity production, hydrogen recycling and power deposition on carbon and tungsten limiters have been investigated in TEXTOR-94 using a C-W twin test limiter. Considerable differences have been observed on W and C surfaces, which can be explained by the different particle and energy reflection coefficients of hydrogen on these surfaces. The measurements show in addition that the majority of the carbon release is from recycled carbon and that only a small part (below 10%) is due to net-erosion from the bulk carbon material. The heat deposition on C and W sides differs under the same plasma conditions significantly and is typically about 30% larger on the cal bon surface. The behaviour of the impurity production: recycling and power deposition for various discharge conditions is presented.
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| 9. |
- Ihde, J., et al.
(author)
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Wall conditioning by microwave generated plasmas in a toroidal magnetic field
- 2001
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 290, s. 1180-1184
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Journal article (peer-reviewed)abstract
- The suitability of microwave generated plasmas for the purpose of wall coating in purr toroidal magnetic fields is investigated in a special test bench. We report on the results of layer deposition using methane plasmas as a systematic case study for future boron or silicon deposition by other gases (B2H6, SiH4). The produced coatings can be characterized as polymer-like soft a-C:D-films with a high DIG-ratio due to the low energy of particles hitting the wall. Neutral hydrocarbon radicals could be identified to play the major role for film deposition. On the other hand, strong re-erosion induced by deuterium ions is observed in regions with plasma-wall contact. The spatial homogeneity and the characteristics of produced coatings are presented and observations are correlated with measured plasma parameters. The use of pulsed plasmas for wall conditioning is compared with steady-state discharges,
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| 10. |
- Laux, M., et al.
(author)
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Arcing at B4C-covered limiters exposed to a SOL-plasma
- 2003
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In: Journal of Nuclear Materials. - 0022-3115 .- 1873-4820. ; 313, s. 62-66
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Journal article (peer-reviewed)abstract
- Plasma sprayed B4C-layers considered as wall coatings for the W7X stellarator have been studied during and after exposure to TEXTOR and after arcing experiments in vacuum. Arcing through the B4C layer occurred favoured by high power fluxes and not restricted to less stable phases. But this arcing implies an especially noisy scrape-off layer (SOL). Instead of moving retrograde in the external magnetic field, the arc spot on the B4C-layer sticks to the same location for its whole lifetime. Consequently, the arc erodes the entire B4C layer, finally burning down to the Cu substrate. In the neighbourhood of craters the surface contains Cu originating from those craters. This material, hauled to the surface by the arc, is subject to subsequent erosion, transport, and redeposition by the SOL-plasma. The behaviour of arcs on B4C is Most probably caused by the peculiar temperature dependences of the electrical and heat conductivity of B4C.
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