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.