A. Moroño

Radiation enhanced degradation of aluminium mirrors for remote handling and diagnostics applications: Effect of humidity

Abstract High quality mirrors will be required for different diagnostic and remote handling systems in International Thermonuclear Experimental Reactor (ITER). The possibility of radiation enhanced surface degradation has been assessed. Unprotected Duralumin discs and high quality SiO overcoated aluminium on Pyrex mirrors have been irradiated with 60 Co gamma rays at 12 Gy/s up to 50 MGy. Irradiations from 303 to 523 K have been performed in air and nitrogen. It has been observed that humidity is a key factor in the degradation process and produces severe corrosion on the aluminium mirror surface even with SiO overcoating. SEM and XPS analysis of chemical species formed on the surface indicate that the degradation produces aluminium hydroxide compounds, and the corrosion corresponds to a filiform mechanism.

Effect of pressure on the radiation induced electrical conductivity of NBI insulator gases

Abstract A study of the effect of pressure on the radiation induced electrical conductivity (RIC) of candidate international thermonuclear experimental reactor (ITER) neutral beam injectors (NBI) insulation gases has been carried out. Large volume, pressure and voltage values similar to those for the ITER system are not readily achievable experimentally. To overcome this problem different experiments have been performed in a Van de Graaff electron accelerator at lower values and the results compared with a theoretical model. Excellent agreement is found between theoretical and experimental results allowing extrapolation to the higher voltage, gas volume and particularly higher gas pressure required in the ITER design. The results show that the use of high gas pressure to increase the voltage breakdown threshold and reduce size may also result in enhanced leakage current losses.

Radiation enhanced dielectric breakdown in insulating gases for NBI systems

During the operation of future machines such as ITER, insulating gases in the NBI systems will be subjected to a radiation field of the order of 1 Gy/s and hence the electric field threshold for dielectric breakdown could be modified. Radiation enhanced dielectric breakdown has been studied for N 2 , CO 2 , N 2 /CO 2 mixture, SF 6 , and dry air at atmospheric pressure and 20 °C. Dry air, N 2 , CO 2, and N 2 /CO 2 mixture all exhibit moderate radiation enhanced breakdown, with a reduction in the limiting voltage between 5 and 18% at 20 Gy/s. In contrast SF 6 does not. Following breakdown N 2 and CO 2 show no quenching of the breakdown current until the applied voltage is reduced to almost zero. Both dry air and SF 6 are good in terms of quenching the breakdown.

Surface Electrical Degradation of Helium Implanted Sapphire

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 (Al2O3) 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.

A dedicated system for in situ testing of gamma ray induced optical absorption and emission in optical materials

Serious radiation damage due to the high energy neutron/gamma fluxes is expected for optical materials such as scintillators, windows, and lenses which will be part of the plasma diagnostics in future fusion devices. Radiation induced absorption represents a major concern for these components for which experimental validation under as near as possible reactor conditions becomes essential. A new experimental system has been developed at the CIEMAT Nayade 60Co gamma irradiation facility for in situ radiation induced optical absorption measurements, covering a spectral range between 370 and 730 nm. This setup consists in a rotating sample holder which allows one to collect incident light (reference signal) and transmitted light through the material to be tested as a function of irradiation dose. This is an advanced and robust system which overcomes the important experimental difficulties that radiation involves providing a valuable testing capability for transmission components and scintillators under realistic fusion conditions. A detailed description of the experimental arrangement, together with preliminary tests carried out for system validation is given in this paper.