Neutron radiation effects on the dielectric loss tangent of ceramic insulators

Full text of publication follows: The radiation induced changes occurring in the physical properties of ceramic materials has a significant effect in the design of fusion reactor components. With ceramic materials used in plasma heating and control systems, diagnostic windows, and voltage feed-through, their satisfactory performance under harsh environments is essential in the operation of the reactor. Dielectric properties, specifically the loss tangent (tan δ are critical to the performance of radio frequency heating systems. Defined as the ratio of the imaginary to the real part of the dielectric permittivity, tan δ is directly proportional to the power absorbed by the dielectric from a transmitted electromagnetic wave. The value of tan δ for a given class of ceramics is highly dependent on temperature, frequency, material impurities and radiation. In ion cyclotron heating (ICH) systems proposed for the international thermonuclear experimental reactor (ITER), it is required that insulators have a tan δ value below 10-3 to avoid overheating. There are only a limited number of experimental studies on loss tangent degradation associated with neutron irradiation, particularly at conditions relevant for operation of ICH systems in ITER. Neutron irradiation experiments were conducted at the High Flux Isotope Reactor at ORNL on single crystal sapphire and spinel along with polycrystalline alumina, beryllia, silicon nitride and aluminum nitride. Different commercially available grades of alumina and aluminum nitride investigated. Samples were irradiated at 65 deg. C at a neutron flux of 1x1015 n/cm2s (E>0.1 MeV) to displacement damage levels of 0.001, 0.01 and 0.1 dpa (1018 to 1020 n/cm2). Post- irradiation testing of the materials was conducted at room temperature and 100 MHz. Specific grades of alumina and sapphire were found to be suitable (tan δ ∼1x10-4 for conditions up to 0.1 dpa) for ICH applications in ITER up to the radiation damage levels tested, assuming active cooling. Beryllia was relatively insensitive to neutron irradiation, and considering that BeO has also exhibited relatively good resistance to thermal conductivity degradation in some of our other recent studies, beryllia remains a very attractive radiation-resistant dielectric for ICH applications. Covalently bonded aluminum nitride and silicon nitride showed unacceptably high values of tan δ which may preclude their use in the ICH antenna design. (authors)