Radiation effects on optical and electrical properties of diamond

CVD diamond is the prime candidate material for use as windows in the ITER ECRH system, and could also be used as transmission component in ECE diagnostics. Hence dielectric behaviour, electrical conductivity, radioluminescence, and optical absorption are important properties for the use of this material. Particularly important for ECRH use are the dielectric properties and thermal conductivity, however electrical conductivity is also an important factor. During operation in ITER and future fusion reactors the window will be subjected to neutron and gamma radiation and also to bombardment by energetic ions produced by sputtering and residual gas ionization. Such bombardment could degrade the surface electrical resistivity of the material, thus increasing the microwave absorption and may provoke window failure due to thermal strain. In the case of use for ECE diagnostics the main issues are absorption and radioluminescence. Radioluminescence is produced by excitation of defects which are in the material before irradiation or defects generated by the radiation itself. Hence radioluminescence in addition to being a problem, may serve as a way to monitor production of defects during irradiation. CVD diamond was electron irradiated in the beam line of a 2 MeV Van de Graaff accelerator, and radioluminescence from 200 to 800 nm was measured at a dose rate of 700 Gy/s at room temperature. In order to assess possible surface electrical degradation a CVD diamond sample was implanted with 54 keV He+ at 50 oC up to a dose of 1017 ions/cm2. Two gold electrodes placed on the implanted surface permitted the surface electrical conductivity to be measured during implantation. After implantation the electrical conductivity was measured as a function of temperature from 20 to 450 oC. Intense radioluminescence bands associated with nitrogen aggregates and carbon vacancies are observed from the onset of irradiation. Radioluminescence intensity decreases with irradiation dose probably due to radiation damage induced quenching. In the case of the helium implanted sample an enormous increase in electrical conductivity occurs after implantation (surface resistance lower than 1000 ohms at 50 oC was measured). The implanted area became completely black indicating a high susceptibility to amorphization or graphitization for diamond when subjected to particle bombardment. (author)