Electrical and optical characteristics of dielectrics for fusion use under irradiation

Abstract

High values of electrical conductivity reported in several papers and ascribed to radiation-induced conductivity (RIC, with the magnitude of ionizing efficiency K exceeding 10 −10 s/Gy Ω m) and radiation-induced electrical degradation in applied electric field (RIED) can not be explained within the framework of a reasonable physical model of bulk conductivity. Owing to charge carrier recombination on traps the RIC is not so high as to restrict the application of dielectrics in fusion reactors. The phenomenon of `bulk RIED' is probably caused by surface contamination of dielectric materials during irradiation testing. More important problems concerning electric charging of dielectrics under irradiation and subsequent dielectric break-down are discussed. The radiation-induced dielectric break-down has been shown to occur in alumina without applying the electric field. The electric charging as well as features of RIC and RIL in dielectrics are attributed to the common mechanism of non-equilibrium charge carrier relaxation, namely, recombination on electron and hole traps. The presented results on RIL in Al 2O 3 based materials indicate local charge accumulation in the bulk of the materials caused by charge partitioning under irradiation. Electric charging and break-down are probably caused by charge partitioning in dielectrics under irradiation rather than primary particle charge accumulation.