E.R. Hodgson

Radiation-enhanced electrical breakdown in fusion insulators

Abstract Radiation-enhanced electrical breakdown is observed in Al 2 O 3 under conditions simulating fusion reactor devices. Ionization and displacement damage play key roles. Similarity with colloid production and radiation-enhanced impurity aggregation is noted.

Irradiation effects in ceramics for fusion reactor applications

Abstract A review is given on the status of R&D of ceramics for fusion reactor applications. Emphasis is placed on the engineering design activity of fusion reactor, the International Thermonuclear Experimental Reactor, Engineering Design Activities (ITER-EDA) as well as related research of electrical, dielectric and mechanical properties, and fundamental aspects of defects. The paper is based mainly on the presentations and discussion, including a keynote lecture and several short presentations by invited panelists, at the discussion session of `Ceramic Insulators for Fusion Energy Applications at ICFRM-8, on 30th October 1997 in Sendai, Japan. This report illustrates how the ITER-EDA and other related international activities have functioned not only to coordinate, but also to promote international collaborations and to mobilize the limited research resources effectively. A database which is necessary for reliable design of diagnostic components in ITER is included in the review.

General radiation problems for insulating materials in future fusion devices

It is envisaged that ITER will come into operation early in the next century. This next step machine will present problems due to the radiation damage effects in the numerous insulator components. The effects on the electrical conductivity, optical and mechanical properties are discussed in the light of recent advances and results. Material successes for the ECRH windows are presented, together with reasons for concern in the NBI system due to enhanced gas conductivity.

KU1 quartz glass for remote handling and LIDAR diagnostic optical transmission systems

Abstract KU1 quartz glass, a radiation-resistant material, is being considered for use in different fusion applications such as remote handling optical transmission components and diagnostic windows. Gamma (60Co) and 1.8 MeV electron irradiations have been carried out to study the ionizing radiation-induced optical absorption in this material. The results show that while the optical absorption induced by irradiation in the KU1 silica is acceptable for the remote handling optical transmission components, this is not the case with diagnostic window applications, where background absorption may pose a problem for LIDAR. However, this disadvantage may be minimised by pre-irradiation at about 200°C.

The paramount importance of dose rate in RIED effect experiments

Abstract Electron irradiation experiments on radiation-induced electrical degradation (RIED) of aluminium oxide, have highlighted the importance of dose rate in the degradation process, together with the role of irradation temperature and sample type. From these results it has been possible to account for the wide range of results obtained for degradation using proton and neutron irradiations, and to suggest why in certain cases degradation is not observed.

An initial model for the RIED effect

Abstract A simple model based on electron acceleration in the conduction band giving rise to an increased F + oxygen vacancy lifetime provides an explanation for several radiation induced electrical degradation (RIED) associated observations in Al 2 O 3 . The increased F + radioluminescence noted during RIED is a direct consequence of the lifetime increase. The model predicts the observed electric field threshold for RIED, and an increase in the field threshold with increasing impurity content. RIED for RF electric fields is also explained. In addition the lifetime increase provides an explanation for the enhanced oxygen vacancy aggregation including colloid and gamma alumina production observed under RIED conditions.

The magnetic diagnostic set for ITER

This paper presents the multiple set of requirements for the ITER magnetic diagnostic systems and the current status of the various R&D activities performed by the EU partners.

Dose rate dependence of the radiation-induced electrical conductivity in MgO

Abstract The radiation-induced electrical conductivity (RIC), has been measured for single crystal MgO at dose rates between 102 and 107 Gy h−1 and temperatures from 14 to 450°C. A correlation is observed between the RIC dose rate dependence, and the thermal stability of well defined electron and hole traps, in agreement with the model of Klaffky et al. The dose rate dependence factor is observed not to be constant, but to depend on dose rate. The possible effects of displacement damage and radiation induced impurity diffusion are noted.

Radiation enhanced electrical breakdown in fusion insulators from dc to 126 MHz

Serious electrical degradation occurs in Al2O3 when subjected to concurrent electron irradiation and an applied electric field. The process occurs from dc to at least 126 MHz and a voltage threshold has been observed for the effect.

Enhanced oxygen vacancy aggregation and colloid production in Al2O3

Abstract Optical luminescence and absorption measurements on Al2O3 (sapphire) electron irradiated at temperatures between 200 and 270°C with and without an applied electric field have been used to identify an oxygen vacancy aggregation process leading to the formation of aluminium colloids. This process, which is only observed when an electric field is applied during irradiation, occurs within the volume and is considered as a possible precursor for RIED. The results help to clarify the observed similarity between RIED and colloid production, and help to provide an explanation for the observation of gamma alumina in sapphire which has suffered RIED.