Near future fusion devices (ITER and DEMO) will have a strong need of systems to allow visual inspection of the first wall, control of its temperature and even some diagnostics where light from/to the plasma are involved.
The conditions will be much more problematic for the materials used as mirrors, windows, fibres and lenses due to the increasing levels of neutron radiation that they will be subjected to. Other challenging conditions are added in some cases, like some vacuum windows. They must be a tritium barrier, sustain pressure differences and sometimes have a broadband wavelength transmission even after neutron irradiation.
Therefore, since 2014, the EUROFusion Materials Program has included functional materials as an important branch. Its main scope are issues of optical and dielectric materials for DEMO applications. Evidently, R&D on these materials is essential for optical diagnostics, safety and control. Examples are visual inspection, spectroscopic studies and first wall temperature monitoring. Discussion concerning the use of transmission windows in DEMO diagnostics is still an open question and the answer will depend on the results obtained in new materials and tests after their irradiation at higher neutron doses (DEMO relevant). Previous studies were mainly focused on ITER conditions and, besides, new materials have been available in recent years and need evaluation.
This work presents an updated overview of the work programme concerning optical materials for mirrors, windows and lenses done by several European research units. Field with little information have been the radiation effects on infrared (IR) materials and new high potential materials not yet considered for fusion application, e.g. aluminium oxynitride (ALON). Despite being the best transparent polycrystalline material in terms of mechanical properties, its post-irradiation behaviour was not available.
Another important issue is the possibility of using optical coatings to improve optical transmission (%T) for which radiation hardness has to be tested. Results of samples with and without coatings are also compared to distinguish between bulk and coating damage.
Finally, when possible, windows from several suppliers were compared to highlight potential effects due to varying impurities or fabrication processes. Contrary to the results observed in gamma or low neutron fluence irradiations, our results shows that, after some threshold, the optical damage is very similar, regardless of the manufacturer. This has been a very important result for future DEMO design.
• Irradiation campaigns.
After a material selection and extensive pre-irradiation characterization, two neutron irradiation campaigns were performed. The conditions are summarized in Table 1. The goal was to obtain data at much higher fluence than in previous ones and target moderate temperatures, as could be expected in DEMO.
Table 1. Fluence and temperature of performed irradiations
Batch....Fluence (n/m2 )....Temperature (C)
• Results on Mirrors
Concerning mirrors, they are one of the most sensible elements, as they are subjected to plasma erosion, co-deposition and neutron/ions damage. It is also very difficult, if not impossible, to really reproduce the real operation conditions. Therefore, the studies have been focused on ion irradiation, using self-ion irradiation for damage production and H and He bombardment for bubble formation studies. The chosen material presently is polycrystalline Molybdenum in as-grown as well as highly deformed state (nanograined). Using this method, the influence of a large quantity of grain boundaries on the defect/bubbles production just started to give results. The W deposition (to mimic W deposition from first wall) is also under study to check its possible effect on reflectivity. One of the main results so far is that the change of reflectivity has been much more related to He ions irradiation than to self-ion irradiation. The cause is the He bubble formation on the top layer. Double irradiation (Mo + He) gives the same results than He irradiation only.
• Refractive optics
In the case of transmission optics, irradiated materials include several grades of MgAl2O4 spinel, amorphous silica, sapphire and YAG (some types with optical coating) plus ZnS and CVD diamond. Most of them could be measured almost directly after irradiation while some need more time due to increased activation issues.
In the EUROfusion Workpackage Materials, there is also some part devoted to new material development; e.g. for optical windows, C12A7 (12CaO·7Al2O3) was studied because of its possible radiation resilience, although finding a negative behaviour. Nano-grained transparent spinel production by SPS is in progress, in order to check its post-irradiation behaviour in the future.
CVD diamond was also checked to obtain information of its optical behaviour, as it exhibits very good physical properties in as-grown state. Unfortunately, as it happens with thermal conductivity, the optical transmission is rapidly dropping with neutron radiation, being totally black after only 0.1 dpa.
The general observed trend can be summarized as follows: finding a material for the UV region is still the most complicated endeavour, because %T is heavily reduced in all the materials studied and presently no solution seems feasible. In contrast, excellent materials exist for the IR region with negligible observed radiation damage for several of them, like sapphire (no damage up to 0.4 dpa). For the visible range we have found some excellent resilience to neutron/gamma radiation in some amorphous silica windows. For all other observed window materials, the damage goes from moderate to high. An example of damage observed in Sapphire is shown in figure 1.
• Modelling activities:
Past experimental evidences indicate that point defect production and aggregation kinetics (temperature dependent) are responsible for optical and dielectric properties degradation. This stage is mainly controlled by the F centre migration energy. Radiation defect migration and interaction simulations in oxides are therefore an excellent approach. This would allow us to obtain information concerning its radiation stability and the role of impurities. This has been a task since 2014 together with comparison to present and previous experimental data.
Annealing of defects in many oxides subjected to different radiation (electrons, ions, neutrons) and doses have already revealed a common pattern: Defect annealing parameters (e.g. migration energies) differ considerably for electron- and neutron/heavy ion- irradiated materials. The effective diffusion energy of defects (thermally activated process) is very dependent on the magnitude of the lattice damage/disorder. Similar effect is known in chemical kinetics and semiconductor physics as Meyer-Neldel rule. Future developments will add information helping us to understand the intrinsic behaviour of radiation damage and hopefully will aid to mitigate it.
• General remarks
- Results obtained so far have increased significantly our knowledge on functional/optical materials behaviour at higher neutron fluence, with further data on irradiated functional materials coming soon.
- Several solutions for applications in the visible and IR-range up to 0.4 dpa have already been found
- Interaction with the working group on diagnostics for DEMO is crucial, concerning the specifications of materials, design reviews, level of radiation and recent developments for control and safety systems.
|Affiliation||Laboratorio Nacional de fusión – CIEMAT. Avda. Complutense 40, 28040, Madrid. Spain|
|Country or International Organization||Spain|