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The silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composite system for fusion applications has seen a continual evolution from development a fundamental understanding of the material system and its behavior in a hostile irradiation environment to the current effort which is directed at a broad-based program of technology maturation program. In essence, over the past few decades this material system has steadily moved from a laboratory curiosity to an engineering material, both for fusion structural applications and other high performance application such as aerospace. This paper outlines the recent international scientific and technological achievements towards the development of SiC/SiC composite material technologies for fusion application and discusses future research directions. It also reviews the materials system in the larger context of progress to maturity as an engineering material for both the larger nuclear community and broader engineering applications.

In fusion devices, in the region next to high temperature plasma, material could be eroded from plasma-facing materials in one location and is transported to other, sometimes remote, locations throughout the device. These plasma-created materials are so-called mixed materials. Previous studies on JET samples have shown that compositional changes in the depth profiles of the mixed-material surfaces can be correlated to the operational history of the machine. SEM and detailed EDX analysis of CFC Be and T contaminated samples coming from the JET divertor (shutdown 2007) have been carried out in the SCKCEN hot cell facilities. The main aim of these measurements is to have a clear image of the erosion/deposition pattern in those samples for each element. The processes for material migration and the morphology of the deposits are discussed. XPS/AES measurements will also be carried out to understand the possible compound processes, mainly focussed on Be 2 C formations.

Insulator materials required for ITER and beyond must operate in a significant radiation field, extending well beyond the first wall. As a result, these materials will be subjected not only to neutron and gamma irradiation, but also to particle bombardment, due mainly to ionization of the residual gas and acceleration of the resulting ions by local electric fields. A systematic study was carried out on the main insulating candidate materials for ITER (Al2O3, SiO2, BeO, and AlN), in order to assess this potential surface degradation issue, and clarify possible mechanisms. Severe surface optical and electrical degradation has previously been reported for these materials bombarded with H+, D+, and He+ ions, at different energies, temperatures, and dose rates, as well as for electron irradiation. In all cases, dramatic degradation has been found and related to loss of oxygen (nitrogen) from the implanted/irradiated zone due to preferential radiolytic anion sputtering. In the work reported here, a model to explain the surface electrical degradation under ion bombardment will be presented. A mathematical expression for the evolution of the surface conductivity with dose of an insulator or extrinsic semiconductor, in which the temperature effect is taken into account is given. This model allows one to obtain the diffusion activation energy of the anion in an interstitial position under an electronic excitation environment. Experimental data for both silica and alumina bombarded with He ions at different temperatures have been fitted. In both cases, the fits obtained from theoretical predictions are in very good agreement with the experimental observations.

Reliable plasma diagnostic systems are key elements for an efficient and safe operation of future fusion reactors. These systems use particular components, such as ceramic insulators, dielectric and optical windows, optical fibres and complete sensor assemblies. These materials, in addition to neutron and gamma radiation, will be subjected to bombardment by low energy ions and neutral particles. Alumina (Al 2 O 3 ) is one of the insulating candidate materials to be used in diagnostic systems for ITER, where it will play important roles as electrical insulation and in optical components. Possible material damage has been examined by implanting He into sapphire at different temperatures to simulate ion bombardment. The electrical conductivity in the implanted region increases by more than nine orders of magnitude. Such severe surface electrical degradation is due to the loss of oxygen from the implanted surface. The loss of oxygen also reduces the material band gap in the surface region and as a consequence the optical transmission is severely reduced. Implantation temperature plays an important role, where one observes that although electrical degradation is higher for higher temperature implantation, optical degradation is lower. The electrical conductivity in the implanted region increases by more than nine orders of magnitude. Such severe surface electrical degradation is due to the loss of oxygen from the implanted surface. The loss of oxygen also reduces the material band gap in the surface region and as a consequence the optical transmission is severely reduced. Implantation temperature plays an important role, where one observes that although electrical degradation is higher for higher temperature implantation, optical degradation is lower.

The fusion plasma, with a typical temperature of 10 keV, has to be brought into contact with a physical wall in order to remove the helium produced and drain the excess energy in the fusion plasma. The fusion plasma is far too hot to be brought into direct contact with a physical wall. It would degrade the wall and the debris from the wall would extinguish the plasma. Therefore, schemes are developed to cool down the plasma locally before it impacts on a physical surface. The resulting plasma wall-interaction in ITER is facing several challenges including surface erosion, material redeposition and tritium retention. In JET facility both beryllium and carbon are used together as plasma facing materials, and the resultant surfaces show considerable mixing between the two elements. In this work a first XPS characterisation has been performed on the surface of several PFC (plasma facing components) samples obtained from various JET tiles from the Mk II GB divertor in JET. The substrates of the tiles are CFC, and the deposited films have various D/C ratios depending on the position in the tokamak. Ion beam analysis shows that the D/C ratio is ∼1 in the shadowed areas and Be/C ∼1–2 in the plasma-exposed regions of the inner divertor. These values could give rise to the formation of beryllium carbides, and indications of a carbide component are found in the XPS spectra. The processes for material migration and the morphology of the deposits are discussed.

Two types of oxides, silica and sapphire have been irradiated with 1.8 MeV electron and 54 keV He' in order to study surface electrical and optical degradation processes. It has been found that both materials suffer severe surface degradation as a consequence of radiation induced oxygen removal for either 1.8 MeV electron irradiation or 54 keV He' implantation. Degradation is higher in the case of alpha particle bombardment, but the results strongly suggest that the fundamental processes taking place are basically the same. Ionizing radiation rather than knock-on displacements appears to be the origin of the severe oxygen removal from the irradiated surface. The possibility of such surface degradation in insulators for ITER and future fusion reactors needs to be fully assessed.

Insulators will be used in ITER diagnostic systems where they will play an important role for optical components, as well as in neutral beam injector system. In addition to neutron and gamma radiation, these materials will be subjected to bombardment by low energy ions and neutral particles. Previous works carried out in silica have shown a dramatic surface electrical and optical degradation with a marked temperature dependence when the material is subjected to proton or alpha particle bombardment, due to loss of oxygen from the surface caused by preferential sputtering by energetic ions. Therefore, it was necessary to extend the study to the main candidate ceramic insulators for ITER (alumina, BeO and AlN), in order to evaluate possible issues for both diagnostic and heating and current driven systems. A clear optical and electrical degradation have been found in all the materials studied after implantation, in general this appears to be an issue common to all the insulator materials studied. This degradation is related with oxygen loss from the implanted zone for alumina and BeO, as observed previously for silica, and nitrogen loss for AlN. For silica implanted at higher energies data show surface degradation in all cases again due to loss of oxygen with a markedly different behaviour for each implanted ion.

KS-4V quartz glass (SiO2) is a candidate material to be used in ITER diagnostic systems where it will play an important role for optical components and possibly as electrical insulator. In addition to neutron and gamma radiation, the material will be subjected to bombardment by low energy ions and neutral particles. Previous work has shown severe electrical and optical degradation with a marked temperature dependence when the material is subjected to proton or alpha particle bombardment, due to loss of oxygen from the surface caused by preferential sputtering by energetic ions. In this work, a systematic study of ion bombardment induced surface degradation as a function of the ion mass, energy and dose rate has been carried out. A clear dependence on ion mass has been observed, with degradation occurring much more rapidly for heavier ions. Also the surface electrical degradation depends strongly on the implanted particle energy, degradation is higher for higher energy. No dependence on dose rate was observed.

Ceramic insulators and windows in ITER will be subjected to bombardment by energetic hydrogen isotopes and helium as a consequence of ionization of the residual gas by gamma radiation and acceleration of the ions by local electric fields. Most of the energy carried by these particles will be deposited at or very near the surface giving rise to possible electrical and optical degradation. In this work, radiation induced electrical conductivity for air, helium, and hydrogen for pressures from 1000 to 10 −3 mbar, at 3 Gy/s, and applied voltages up to 1500 V (10 5 V/m) has been measured. The ion currents at the low pressure ((0.2–2) × 10 −6 A/(Gy s −1 m 3 )) are much higher than those expected from both theoretical extrapolations and experimental measurements near to atmospheric pressure (10 −8 A/(Gy s −1 m 3 )). If such high ion currents bombard the insulator surfaces, severe degradation will occur.

CVD diamond is the prime candidate material for use as windows in the ITER ECRH system, and can also be used as transmission component in ECE diagnostics. Hence electrical conductivity, radioluminescence, and optical absorption are important properties for the use of this material. In case of use for ECE diagnostics, the main concerns are absorption and radioluminescence. To address these issues measurements of surface electrical degradation, optical absorption, and radiolumniscence have been made. Intense radioluminescence bands related to nitrogen aggregates and carbon vacancies are observed within the bulk of CVD diamond during electron irradiation. Severe surface degradation occurs in CVD diamond when subjected to ion bombardment, increasing the surface electrical conductivity by many orders of magnitude and drastically reducing the optical transmission.

The long-term objective of the European Fusion Development Agreement (EFDA) fusion materials programme is to develop structural and armor materials in combination with the necessary production and fabrication technologies for reactor concepts beyond the International Thermonuclear Experimental Reactor. The programmatic roadmap is structured into four engineering research lines which comprise fabrication process development, structural material development, armor material optimization, and irradiation performance testing, which are complemented by a fundamental research programme on "Materials Science and Modeling." This paper presents the current research status of the EFDA experimental and testing investigations, and gives a detailed overview of the latest results on materials research, fabrication, joining, high heat flux testing, plasticity studies, modeling, and validation experiments. (C) 2013 Elsevier B.V. All rights reserved.