Comprehensive nuclear materials 4 22 radiation effects on the physical properties of dielectric insulators for fusion reactors

Comprehensive nuclear materials 4 22 radiation effects on the physical properties of dielectric insulators for fusion reactors Comprehensive nuclear materials 4 22 radiation effects on the physical properties of dielectric insulators for fusion reactors Comprehensive nuclear materials 4 22 radiation effects on the physical properties of dielectric insulators for fusion reactors Comprehensive nuclear materials 4 22 radiation effects on the physical properties of dielectric insulators for fusion reactors Comprehensive nuclear materials 4 22 radiation effects on the physical properties of dielectric insulators for fusion reactors

4.22 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors E R Hodgson Euratom/CIEMAT Fusion Association, Madrid, Spain T Shikama Tohoku University, Sendai, Japan ß 2012 Elsevier Ltd All rights reserved 4.22.1 4.22.2 4.22.3 4.22.4 4.22.5 4.22.6 4.22.7 4.22.8 References Introduction Fusion-Relevant Radiation Damage in Insulating Materials Simulation Experiments Degradation of Insulator Electrical Resistance Degradation of Insulator AC/RF Dielectric Properties Degradation of Insulator Thermal Conductivity Degradation of Optical Properties Concluding Remarks Abbreviations AC/RF BA CDA CIEMAT CVD DC DEMO ECRH EDA EVEDA FIRE H&CD HFIR HFR ICRH IEA IFMIF IMR IR ITER JET Alternating current/radio frequency Broader approach Conceptual design activity Centro de Investigaciones Energe´ticas, Medioambientales, y Tecnolo´gicas Chemical vapor deposition Direct current Demonstration Electron cyclotron resonant heating Engineering design activity Engineering Validation and Engineering Design Activities Fusion ignition research experiment Heating and current drive High Flux Isotope Reactor (Oak Ridge, USA) High Flux Reactor (Petten, Holland) Ion cyclotron resonant heating International Energy Agency International Fusion Materials Irradiation Facility Institute for Materials Research Infrared International Thermonuclear Experimental Reactor (Cadarache, France) Joint European Torus (Culham, UK) KfK KU1, KS-4V LAM LAMPF LH LIDAR MACOR MI NBI ORNL OSIRIS PIE RAFM RIA RIC RIED RIEMF RF RL SCCG TEM UV 702 703 705 706 712 715 717 720 721 Kernforschungszentrum Karlsruhe (Germany) Russian radiation-resistant quartz glasses Low-activation materials Los Alamos Meson Physics Facility (USA) Lower hybrid Light Detection and Ranging Machinable Glass Ceramic (Corning Incorporated) Mineral insulation/insulated Neutral beam injector Oak Ridge National Laboratory (USA) From the Greek for Us-yri ‘the king’ (Reactor at Saclay, France) Postirradiation examination Reduced activation ferritic martensitic Radiation-induced absorption Radiation-induced conductivity Radiation-induced electrical degradation Radiation-induced electromotive force Russian Federation Radioluminescence Subcritical crack growth Transmission electron microscope Ultraviolet 701 702 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors 4.22.1 Introduction It is envisaged that early in the twenty-first century ITER (International Thermonuclear Experimental Reactor) will come into operation, and it is expected that this intermediate ‘technology’ machine will help to bridge the gap between the present-day large ‘physics’ machines and the precommercial DEMO power reactor, thus paving the way for commercial fusion reactors to become available by the end of the century Although this ‘next-step’ device will undoubtedly help to solve many of the problems, which still remain in the field of plasma confinement, it will also present additional operational and experimental difficulties not found in present-day machines These problems are related to the expected radiation damage effects as a result of the intense radiation field from the ‘burning’ plasma This ignited plasma will give rise to high-energy neutron and gamma fluxes, penetrating well beyond the first wall, from which one foresees a serious materials problem that has to be solved In the initial physics phase of operation of such a machine, it is the radiation flux, which will be of concern, whereas in the later technology phase, both flux and fluence will play important roles as fluence (dose)-dependent radiation damage builds up in the materials For structural metallic materials, radiation damage in ITER is expected to be severe, although tolerable, only near to the first wall However, the problem facing the numerous insulating components is far more serious because of the necessity to maintain not only the mechanical, but also the far more sensitive physical properties intact An additional concern arises from the need to carry out inspection, maintenance, and repair remotely because of the neutron-induced activation of the machine This ‘remote handling’ activity will employ machinery, which requires the use of numerous standard components ranging from simple wires, connectors, and motors, to optical components such as windows, lenses, and fibers, as well as electronic devices such as cameras and various sophisticated sensors All these components use insulating materials It is clear, therefore, that we face a situation in which insulating materials will be required to operate under a radiation field, in a number of key systems from plasma heating and current drive (H&CD), to diagnostics, as well as remote handling maintenance systems All these systems directly affect not only the operation, but also the safety, control, and long-term reliability of the machine Even for ITER, the performance of some potential insulating materials appears marginal In the long term, beyond ITER, the solution of the materials problem will determine the viability of fusion power The radiation field will modify to some degree all of the important material physical and mechanical properties Some of the induced changes will be flux dependent, while others will be modified by the total fluence Clearly, the former flux-dependent processes will be of concern from the onset of operation of future next-step devices The fluence-dependent effects on the other hand are the important parameters affecting the component or material lifetime The properties of concern which need to be considered for the many applications include electrical resistance, dielectric loss, optical absorption, and emission, as well as thermal and mechanical properties Numerous papers have been published discussing both general, and more recently, specific aspects of radiation damage in insulating materials for fusion applications, and those most relevant to the present chapter are included.1–26 In recent years, because of the acute lack of data for insulators and the recognition of their high sensitivity to radiation, most work has concentrated on the immediate needs for ITER A comprehensive ceramics irradiation program was established to investigate radiation effects on a wide range of materials for essentially all components foreseen for H&CD and diagnostics in ITER, and to find solutions for the problems which have been identified A large number of relevant components and candidate materials have been, and are being, studied systematically at gradually increasing radiation dose rates and doses, under increasingly realistic conditions A considerable volume of the work discussed here was carried out within the ITER framework during the CDA, EDA, and EDA extension (Conceptual and Engineering Design Activities 1992–2002) as specific tasks assigned to the various Home Teams (T26/28 and T246; EU, JA, RF, US; T252/445 and T492; EU, JA, RF).27,28 Since these last ITER tasks, no new coordinated tasks related to insulators have been formulated However, despite the lack of an official framework in which to develop and assign further common tasks following the end of the ITER-EDA extension, collaborative work has continued between the EU, JA, RF, and US Home Teams on both basic and applied aspects of radiation damage in insulator materials This has resulted in considerable progress being made on the understanding of the pertinent effects of radiation on in-vessel components and materials in particular for diagnostic applications Problems which have been addressed and for which irradiation testing Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors has been performed include comparison of absorption and luminescence for different optical fibers and window materials, RIEMF (radiation-induced electromotive force) and related effects for MI (mineral insulated) cables and coils, alternative bolometers to the reference JET type gold on mica, hot filament pressure gauges, and electric field effects in aluminas One must however remember that ITER is only an intermediate ‘technology’ machine on the road to a precommercial power reactor Such power reactors will face the same radiation flux problems as anticipated in ITER, but the fluence problems will be far more severe It is also important to note that the radiation flux and fluence levels will be different from one type of device to another depending on the design (e.g., in ITER and the Fusion Ignition Research Experiment (FIRE)26), and also on the specific location within that device Because of the general uncertainty in defining radiation levels, most radiation effects studies have taken this into account by providing where possible data as a function of dose rate (flux), dose (fluence), and irradiation temperature Although the task ahead is difficult, important advances are being made not only in the identification of potential problems and operational limitations, but also in the understanding of the relevant radiation effects, as well as materials selection and design accommodation to enable the materials limitations to be tolerated Following a brief introduction to the problem of radiation damage in both metals and insulators, the important aspect of simulating the operating environment for the component or material under examination will be presented, with reference to present experimental procedures The chapter will then concentrate on the problems facing the use of insulators, with the radiation effects on the main physical properties being discussed, concentrating in particular on the dielectric properties 4.22.2 Fusion-Relevant Radiation Damage in Insulating Materials The study of intense radiation effects in metals has been closely associated with the development of nuclear fission reactors, and as a result at the beginning of the 1980s when the urgent need to consider radiation damage aspects of materials to be employed in future fusion reactors was fully realized, a considerable amount of knowledge and expertise already existed for metallic materials.29 This was not the case 703 for the insulating materials, mainly because of the fact that the required use of insulators in fissiontype reactors is in general limited to low radiation regions, well protected from the reactor core However, despite the late start and the reduced number of specialists working in related fields at the time, together with the complexity of the mechanisms involved in radiation damage processes in insulators, considerable progress has been made not only in assessing the possible problem areas, but also in finding viable solutions Several general reviews give a good introduction to the specific problem of radiation damage in insulators.30–36 The materials employed in the next-step fusion machine will be subjected to fluxes of neutrons and gammas originating in the ignited plasma The radiation intensity will depend not only on the distance from the plasma, but also in a complex way on the actual position within the machine because of the radiation streaming along the numerous penetrations required for cooling systems, blanket structures, heating systems, and diagnostic and inspection channels, as well as the radiation coming from the water in the outgoing cooling channels due to the 16O(n, p)16N nuclear reaction However one-, two-, and even three-dimensional models are now available, which enable the neutron and gamma fluxes to be calculated with confidence at most, if not all, machine positions.37–40 Radiation damage is generally divided into two components: displacement damage and ionization effects In a fusion environment, displacement damage, which affects both metals and insulators, will result from the direct knock-on of atoms/ions from their lattice sites by the neutrons, giving rise to vacancies and interstitials Those primary knock-on atoms (PKAs) with sufficient energy may go on to produce further displacements, so-called cascades The numerous point defects thus produced may either recombine, in which case no net damage results, or they may stabilize and even aggregate producing more stable extended defects These secondary processes which determine the fate of the vacancies and interstitials are governed by their mobilities These mobilities are highly temperature dependent, and in the case of insulators even depend on the ionizing radiation level (radiation-enhanced diffusion) Displacement damage is measured in ‘dpa’ (displacements per atom) where dpa is equivalent to displacing all the atoms once from their lattice sites At the first wall of ITER, the primary displacement dose rate will be of the order of 10À6 dpa sÀ1 704 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors In contrast, ionizing radiation although absorbed by both metals and insulators, in general, only produces heating in metals However, certain aspects of radiation damage in metals, such as radiation-enhanced corrosion and grain boundary modification are related to ionization The effects of ionization on insulators are in comparison quite marked because of the excitation of electrons from the valence to the conduction band giving rise to charge transfer effects Ionizing radiation is measured in absorbed dose Gy (Gray) where Gy ¼ J kgÀ1 At the first wall of ITER, the dose rate will be of the order of 104 Gy sÀ1 The response of insulators to both displacement and ionizing radiation is far more complex than in the case of metals Apart from a few specific cases (diamond for example), insulating materials are polyatomic in nature This leads to the following: (i) We have in general two or more sublattices which may not tolerate mixing (ii) This gives rise to more types of defects than can exist in metals (iii) Because of the electrically insulating nature, the defects may have different charge states, and hence different mobilities (iv) The displacement rates and thresholds, as well as the mobilities, may be different on each sublattice (v) We may have interaction between the defects on different sublattices (vi) Defects can be produced in some cases by purely electronic processes (radiolysis); however, in the insulating materials of interest for fusion, this is generally not the case As a consequence of these factors, while radiation damage affects all materials, the insulators are far more sensitive to radiation damage than metals While stainless steel, for example, can withstand several dpa and GGy with no problem, some properties of insulating materials can be noticeably modified by as little as 10À5 dpa or a few kGy Because of this, the present ongoing programs of radiation testing for diagnostics are concentrating mainly on the insulating components of the systems The results of these radiation damage processes are flux- and fluencedependent changes in the physical and mechanical properties of the materials, which may be particularly severe for the insulators The properties of concern which suffer modification are the electrical and thermal conductivity, dielectric loss and permittivity, optical properties, and to a lesser extent the mechanical strength and volume The effects of such changes are that the insulators may suffer Joule heating because of the increased electrical conductivity or lower thermal conductivity, and absorption in windows and fibers can increase from the microwave to the optical region and they emit strong luminescence (radioluminescence, RL); in addition, the materials may become more brittle and may suffer swelling Clearly, some materials are more radiation resistant than others The organic insulators, which are widely used in multiple applications in general, degrade under purely ionizing radiation and are not suitable for use at temperatures above about 200  C; as a result their use will be limited to superconducting magnet insulation and remote handling applications during reactor shutdown Inorganic insulators of the alkali halide class have been widely studied and are used as optical windows; however, they are susceptible to radiolysis (displacement damage induced by electronic excitation) and in general become opaque at low radiation fluences Of the numerous insulating materials, it is the refractory oxides and nitrides, which in general show the highest radiation resistance, and of these the ones which have received specific attention within the fusion program include MgO, Al2O3, MgAl2O4, BeO, AlN, and Si3N4 In addition, different forms of SiO2 and materials such as diamond and silicon have been examined for various window and optical transmission applications One other aspect of radiation damage that should be mentioned is nuclear transmutation The highenergy neutrons will produce nuclear reactions in all the materials giving rise to transmutation products.1 These will build up with time and represent impurities in the materials, which may modify their properties The physical properties of insulators are particularly sensitive to impurities Furthermore, some of these transmutation products may be radioactive and give rise to the need for remote handling and hot cell manipulation in the case of component removal, repair, or replacement For the structural materials, in the present concepts mainly steel alloys, considerable work has been carried out on the development of so-called low or reduced activation materials (LAM, RAFM – reduced activation ferritic/ martensitic) for possible use in DEMO and future commercial fusion reactors.41–45 This work with the aim of reducing the amount of nuclear waste has studied not only the substitution of radiological problem alloying elements such as Mo and Nb in steels, but also the viability of other materials such as vanadium and SiC/SiC composites In the case of the insulating materials, no equivalent study or Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors development has been carried out, in part because of the small fraction of the total material volume represented by the insulators, and also because the important physical properties of these materials are expected to be degraded before the transmutation products become of concern Certainly, for a next-step machine such as ITER, transmutation products, with the possible exception of hydrogen and helium, are not expected to present a serious problem 4.22.3 Simulation Experiments Within the fusion community, there is an acute awareness of the necessity to construct a suitable irradiation testing facility for materials, which will enable both testing and development of materials for future fusion reactor devices with a fusion-like neutron spectrum Within this context, both conceptual and engineering design activities were undertaken during the 1990s within the IEA framework with the view of providing such a facility, the IFMIF (International Fusion Materials Irradiation Facility).46–50 This work has been recently renewed under the EU-Japan Broader Approach (BA) activities with the EVEDA (Engineering Validation and Engineering Design Activities) tasks.51,52 However, at the present time no entirely suitable irradiation testing facility exists, and as a consequence experiments have been performed in nuclear fission reactors and particle accelerators, as well as g- and X-ray sources, in an attempt to simulate the real operating conditions of the insulating materials and components The experiments required must simulate the neutron and g radiation field, that is, the displacement and ionization damage rates, the radiation environment, that is, vacuum and temperature, and also the operating conditions such as applied voltage, or mechanical stress As will be seen, for the insulator physical properties, it is furthermore essential that in situ testing is carried out to determine whether or not the required physical properties of the material or component are maintained during irradiation Examples of this include the electrical conductivity, which can increase many orders of magnitude due to the ionizing radiation, or optical windows, which may emit intense RL Experimental nuclear fission reactors clearly have the advantage of producing a radiation field consisting of both neutrons and g-rays, although in most cases the actual neutron energy spectrum and the dpa to ionization and He ratios are not those which will 705 be experienced in a fusion reactor.50 However, it is worthwhile noting that to date experimental fission reactors have mainly been used for irradiations in the metals programs where the emphasis is on the neutron flux and little consideration is given to the g field As a result, the irradiation channels have in general been designed and installed with this criterion However, it should be possible to select positions within the reactors which, together with suitable neutron absorber materials and neutron to g converters, provide acceptable radiation fields The main difficulties with in-reactor experiments come from the inaccessibility of the radiation volume and are concerned with the problem of carrying out in situ measurements and achieving the correct irradiation environment While considerable success has been attained in the in situ measurement requirement, with parameters such as electrical conductivity, optical absorption and emission, and even radiofrequency dielectric loss being determined, the problem of irradiating in vacuum still remains, with most experiments being performed in a controlled He environment Irradiation in a controlled atmosphere such as He causes an immediate problem for in situ electrical and dielectric measurements because of the radiation-enhanced electrical conductivity of the gas,53 and even in the case of irradiation in vacuum at about 10À3 mbar spurious leakage currents will occur.54 Furthermore, many in-reactor experiments rely on nuclear heating to reach the required temperature, and hence have difficulty maintaining a controlled temperature, in part because of the changes in the reactor power, and also because of the problem of calculating the final sample or component temperature These aspects will be further discussed later One additional difficulty comes from the nuclear activation of the sample or component, which generally means that postirradiation examination (PIE) has either to be carried out in a hot cell or postponed until the material can be safely handled Particle accelerators, on the other hand, are ideal for carrying out in situ experiments in high vacuum and at well-controlled temperatures because of the easy access and the very localized radiation field High levels of displacement damage and ionization can be achieved with little or no nuclear activation It is however in the nonnuclear aspect of the radiation field where their disadvantage is evident, and great care has to be taken to ensure that appropriate displacement rates are deduced to enable reliable comparison with the expected fusion damage A further serious disadvantage is due to the limited irradiation 706 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors volume and particle penetration depth This in general means that only small thin material samples or components can be tested The present-day situation of materials and component radiation testing for fusion applications takes full advantage not only of fission reactors and particle accelerators, but also 60Co g irradiation facilities and even X-ray sources The use of such widely different radiation sources can be justified as long as the influence of the type of radiation on the physical parameter of interest is known This, in certain cases, is true for radiation-induced electrical conductivity and RL for example, where for low total fluences it is the ionizing component of the radiation field which is important In situ measurements can now be made during irradiation of the important electrical, dielectric, and optical properties In addition other aspects such as mechanical strength and tritium diffusion are being assessed during irradiation Undoubtedly, successful modeling could be of help to address this diverse use of irradiation sources; however, general modeling for the insulators has hardly got off the ground because of the difficulties associated with describing radiation effects in polyatomic bandstructured materials As a result, in contrast to the extended activity for metallic structural materials, to date there has been no coordinated activity for the insulators, with only specific models for aspects such as electrical and thermal conductivity being developed 4.22.4 Degradation of Insulator Electrical Resistance Electrical resistance, more generally discussed in terms of the electrical conductivity (the inverse of the resistance), is an important basic parameter for numerous systems and components including the NBI (neutral beam injector) heating system, ICRH (ion cyclotron resonant heating) windows and supports, magnetic coils, feedthroughs and standoffs, MI cables, and wire insulation Any reduction in the electrical resistance of the insulator material in these components may give rise to problems such as increased Joule heating, signal loss, or impedance change The main candidate material for these applications is Al2O3 and is also the one which has been most extensively studied, both in the polycrystalline alumina form and as single crystal sapphire To a lesser extent, MgO, BeO, MgAl2O4, AlN, and SiO2 have also been studied At the present time, three types of electrical degradation in a radiation environment are recognized and have been investigated; these are radiation-induced conductivity (RIC), radiation-induced electrical degradation (RIED), and surface degradation Of these types of degradation, RIC was the first to be addressed in a fusion context, as this enhancement of the electrical conductivity is flux dependent and hence a possible cause for concern from the onset of operation of any fusion device Fortunately, RIC had been studied for many years, and a sound theoretical understanding already existed.55–59 The ionizing component of the radiation field causes an increase in the electrical conductivity because of the excitation of electrons from the valence to the conduction band and their subsequent trapping in levels within the band gap near to the conduction band from where they are thermally excited once again into the conduction band Figure shows schematically RIC as a function of irradiation time and ionizing dose rate (flux) The increase in saturation depends not only on the dose rate as indicated, but also in a complex way on the temperature and sample impurity content, as may be seen in Figure for MgO:Fe.60 Nevertheless, such behavior, including the initial step, is well predicted by theory.57 However, at the dose rates of interest for fusion applications, in the range of approximately Gy sÀ1 to >100 Gy sÀ1, saturation is reached within minutes to seconds, and it is this saturation level which is usually the value of interest The RIC process can lead to increases in the electrical conductivity of many orders of magnitude For example, a standard high-purity alumina has a room temperature conductivity of generally less than 10À16 S mÀ1, which increases to approximately 10À10 S mÀ1 for an ionizing dose rate of only Gy sÀ1.61 The first experiments carried out within a fusion application context, that is, refractory oxide materials, high-dose rates, and temperatures, gave an insight into the effects of dose rate, temperature, and material impurity, and established the well-known relationship at saturation, between the total electrical conductivity measured during irradiation and the ionizing dose rate: stotal ẳ s0 ỵ KRd where s0 is the conductivity in the absence of radiation, R is the dose rate, and K and d are constants.59,61–63 Although d % 1, the detailed studies found temperature, dose, and dose rate dependence in this parameter, with extreme values in certain cases ranging between 0.5 and 1.5, and in addition a temperature dependence was observed for K At the present time, extensive RIC data are available for materials irradiated with X-rays, g-rays, electrons, protons, positive ions, and fission and Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors 707 Schematic RIC 2.5 Increasing dose rate RIC (a.u.) 1.5 0.5 0 60 40 Irradiation time (a.u.) 20 80 100 Figure Schematic RIC Saturation is reached more rapidly at higher dose rate For fusion applications, it is generally the saturation level that is of interest MgO:Fe 0.1 Gy s–1 14 ЊC, 180 ppm 172 ЊC, 180 ppm 14 ЊC, 650 ppm 136 ЊC, 650 ppm RIC 1.0 n n(t = ¥) RIC (pA) 0.6 0.8 (e = 0,h = 0) (100,0.1) (10,0.1) (10,1) (6,0.2) (3,1) 0.4 0.2 0 gG t N T 50 100 150 Time 200 Irradiation time (min) Figure RIC for single crystal MgO, doped with 180 and 650 ppm Fe g irradiation at 0.1 Gy sÀ1 for different temperatures (14, 136, and 172  C).60 Theoretical predictions are shown inset Reproduced from Huntley, D J.; Andrews, J R Can J Phys 1968, 46, 147 14 MeV neutrons Many of the additional results, although in some cases limited to one temperature, and/or one dose rate, add confirmation to the earlier extended studies, but more importantly show that RIC is essentially a function of the ionization, independent of the irradiating particle or source With very few exceptions, all the data taken together over a range of dose rates from RIC ðpolycrystalÞ and RIC ðpureÞ > RIC ðimpureÞ However, the indication on the impurity dependence needs to be qualified, as certain impurities introduce levels near to the conduction band, and increase the RIC.59,60 This would imply therefore that the vast majority of the impurities in the materials act as recombination centers for the electrons and holes, thereby reducing the free charge carrier lifetimes, and not introduce electron levels near to the conduction band The reduction of the electron lifetime in the conduction band has important consequences for the RIED effect in different materials, as discussed below From all the data available, at the present time one can safely say that RIC is sufficiently ‘well understood’ to allow this type of electrical degradation to be accommodated by the design, and that materials exist which give rise to electrical conductivities 10À6 S mÀ1 for ionizing dose rates of up to >103 Gy sÀ1 One only expects possible problems or influence near the first wall Unfortunately, this is precisely the region where magnetic coil diagnostics that can tolerate only very low leakage conductivity will be employed It is important to remember that RIC is a flux-dependent effect and will be present from the onset of operation of the next-step machines Hence, devices which are sensitive to impedance changes, which will occur for example in MI cables, must take RIC into account Furthermore, as RIC is strongly affected by impurity content, the buildup of transmutation products will modify the RIC with irradiation time (fluence), although this is not expected to be of serious concern for ITER In contrast to RIC, RIED is a more serious problem because it has been observed under certain conditions to permanently increase, that is, degrade, the electrical conductivity with irradiation dose Figure shows a schematic RIED-type degradation The initial increase in the conductivity corresponds to the RIC as described above Following a certain irradiation time, or accumulated dose, the conductivity again begins to increase as s0 degrades In Al2O3 for which most work has been performed, RIED is observed as a permanent increase or degradation of the electrical conductivity (s0) when a small electric field (%100 kV mÀ1) is applied during irradiation at moderate temperatures (%450  C) At considerably higher temperatures and voltages, but without an irradiation field,67 or for irradiations performed without an applied electric field,68 no degradation occurs Even at the present time, this type of degradation is still not fully understood; nor is there general agreement as to whether RIED is a real degradation in the volume Following the first report of RIED effect in electron-irradiated sapphire (Al2O3) and MgO,8 numerous experiments were carried out to assess its possible relevance to fusion insulator applications These addressed the effect of the applied electric RIED influence RIC + RIED (a.u.) 1.5 RIC dominates Permanent degradation 0.5 0 20 40 709 60 80 100 Irradiation time (a.u.) Figure Schematic RIED Initially, during irradiation RIC dominates, but with irradiation time (dose) the measured conductivity increases because of permanent degradation 710 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors field, DC or AC/RF69 and voltage threshold,70 the irradiation temperature,71,72 and the ionizing dose rate,73 as well as observations that in addition to electrons, RIED occurred with protons (Figure 674), as,75 and neutrons,76–78 and the observation of RIED effects in other materials, for example, MgAl2O4.74 In addition, further experiments were performed in which RIED-like effects were also observed in sapphire that was electron irradiated in air,79 for thin Al2O3 films,80 and MgO insulated cable.81 In contrast, some experiments did not observe any RIED effect, with some reporting enhanced surface conductivity or even cracking of the material.82–88 This led to suggestions that the RIED degradation is not a real volume effect, but is caused by surface contamination.82,86 Because of the potential importance of electrical degradation and the uncertainty, extensive discussions on RIED were held at several IEA Workshops,89,90 including the experimental techniques employed in the irradiations to separate and identify volume degradation from surface effects It was pointed out at an early stage of the discussions that important factors such as dose rate, and in particular material-type differences, and irradiation temperature, all of which could cause RIED not to be observed were not being taken into account.73 For example, under identical conditions RIED was observed in Vitox alumina but not in Wesgo AL995 alumina,75 strongly suggesting a material (possibly impurity and/or grain size) dependence, and further observations showed that the low purity, large grain size Wesgo AL995 material was highly susceptible to surface degradation when irradiated in high vacuum.91 The in-reactor RIED experiment in HFIR at ORNL also threw light on the complex RIED problem.92,93 Initial results indicated no significant increase in electrical conductivity for 12 different samples However, moderate to substantial electrical degradation was later reported for some of the sapphire and alumina samples, so material type is an important parameter.94 One of the major difficulties for in-reactor experiments is the determination of s0, the conductivity in the absence of radiation, and its temperature behavior The use of nuclear heating and the residual reactor radiation level mean that changes in this parameter with temperature and its corresponding activation energy are not generally measured, although these are the main indicators for the onset of degradation; hence, RIED only becomes measurable when the material conductivity in the absence of radiation is larger than the RIC; that is, s0 ! KRd Furthermore, some experiments were performed at temperatures either near room temperature85 or above 600  C,95 considerably outside the expected effective temperature range for RIED of approximately 400–500  C In an attempt to clarify the situation, work was performed to identify possible basic causes of RIED These experiments detected specific volume effects in Al2O3 that are observed only for irradiations carried out with an applied electric field A marked Log10 displacements per atom -4.0 -3.5 -3.0 -4.5 -2.5 Log10 electrical conductivity (W-1 m-1) Vitox Al2O3 500 ЊC -5 -10 400 ЊC -15 8.5 9.0 9.5 10.0 Log10 ionization dose (Gy) Figure RIED observed in alumina during 18 MeV proton irradiation, with an applied field of 0.5 MV mÀ1 Reproduced with permission from Pells, G P J Nucl Mater 1991, 184, 177 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors enhancement of the well-characterized Fỵ-center (oxygen vacancy with one trapped electron) was observed,71 and TEM identified large regions of g-alumina within the bulk of RIED degraded Al2O3.96 The increase in Fỵ-center production gave rise to enhanced oxygen vacancy mobility, and led to vacancy aggregation and aluminum colloid formation, as may be seen in Figure 7.97 This clarified the observed close similarity between the RIED effect and colloid production in the alkali halides,68 and helped to explain the formation of g-alumina and associated bulk electrical and mechanical degradation.96 The combined work led to a RIED model being formulated, which successfully explained the role of the electric field (both DC and AC/RF), the ionization, and the anion (oxygen) vacancies.98 The model predicted a threshold electric field for degradation depending on the impurity/defect concentration which, as mentioned above in the discussion of RIC, reduces the free electron lifetime This helps to explain the negative RIED results for Wesgo AL995 alumina where the applied experimental field was below the predicted value of >0.6 MV mÀ1.75,87 It also highlighted the importance of the ionization, in agreement with earlier conclusions.73,84 Additional support for the model, and RIED as a volume effect, came with the TEM identification of aluminum colloids, as well as previously observed g-alumina, in Al2O3 irradiated with an electric field applied.99 At that time, an alternative model based on charge buildup and breakdown was also developed, but was not extended to explain many of the important observations.100 During the intense activities related to RIED during the 1990s, two important factors emerged, one concerned with surface electrical degradation, and the other related to the importance of the experimental irradiation environment For insulating components in future fusion devices, surface electrical degradation may prove to be more serious than the RIC and RIED volume effects At that time, two types of surface degradation were reported, a contamination caused by poor vacuum, sputtering, or evaporation,83,88 and a real surface degradation related to radiation-enhanced surface vacuum reduction and possibly impurity segregation.101,102 Both forms are affected by the irradiation environment and ionizing radiation However, the real surface degradation effect is strongly material dependent, and occurs in vacuum but not in air or helium.102 This stresses the extreme importance of a representative irradiation environment for material testing Most insulating materials required for fusion applications in ITER and beyond must indeed operate in high vacuum, and in consequence accelerator experiments to study electrical conductivity have been performed in vacuum, whereas to date, with few exceptions,76–78,103,104 in-reactor experiments for technical reasons have been performed in helium Another significant aspect of in-reactor experiments performed in helium is the radiation-induced leakage current in the gas,53 which makes it difficult to Optical absorption (OD cm-1) Colloid band 310 ЊC 290 ЊC 270 ЊC 711 Energy (eV) Figure Aluminum colloid band in sapphire irradiated with 1.8 MeV electrons at different temperatures with an electric field of 0.2 MV mÀ1 applied Reproduced from Moron˜o, A.; Hodgson, E R J Nucl Mater 1997, 250, 156 712 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors determine volume conductivity.81,104 One should also mention that severe electrical surface degradation has recently been observed when oxide insulator materials are bombarded with keV H and He ions.105 The mechanism giving rise to such surface degradation is believed to be the loss of oxygen from the vacuum insulator surface region due to preferential radiolytic sputtering Similarly, in future fusion devices such as ITER ceramic insulators and windows may also degrade, as they will be bombarded by energetic H isotope and He ions because of ionization of the residual gas by g radiation and acceleration by local electric fields.54 At the present time, the role of the irradiation environment in electrical degradation clearly requires further study Additional difficulties experienced in performing in-reactor experiments include temperature control and also component testing.104,106–108 It is also important to note that several in-reactor experiments have suffered from electrical breakdowns related to the difficulty of maintaining high voltages in a radiation field, precisely what is required for some H&CD and diagnostics systems in a next-step device Whether or not these are due to RIED, temperature excursions, He gas breakdown, or problems with the MI cables, terminations, and feedthroughs remains unexplained 4.22.5 Degradation of Insulator AC/RF Dielectric Properties As with the DC electrical properties, it soon became apparent, even before ITER CDA, that data for radiation effects on the AC/RF dielectric properties (dielectric loss and permittivity) of suitable insulating materials for fusion applications were almost nonexistent Such materials will be needed for both H&CD and diagnostic applications, where they will be required to maintain their dielectric properties from kHz to GHz under a radiation field in high vacuum Initial work concentrated on the characterization of candidate materials (Al2O3, MgAl2O4, BeO, AlN, and Si3N4), and also PIE of neutron- and protonirradiated materials.109–114 In general, changes in permittivity were observed to be small ( 5%) and considered to be acceptable for fusion applications However, results for dielectric loss (loss tangent measurements) showed orders of magnitude variation for similar materials (%10À5–10À2 for different forms of alumina at 100 MHz) even before irradiation To address this problem, a standard material (MACOR) was distributed and measured by the main laboratories involved (EU, JA, US) to check the different measuring systems used However, the results showed good agreement,115 and the large variation in reported loss tangent values was later shown to be real, in part because of the effect of the different impurity contents of the materials.116,117 This may be clearly seen in Figure 8, where loss tangent data for different aluminas over a wide frequency range are given, showing marked absorption band structures due to polarizable defects (impurities).116 During the early postirradiation loss tangent measurements, there was an indication of recovery, suggesting that loss during irradiation could be significantly higher.65,109–111 This implied that the already difficult measurements should be made in situ during irradiation In a simple way, dielectric loss can be considered as being due to two contributions: Loss a DC conductivityị=Frequency ỵ Polarization term Clearly, both terms can be modified by the radiation RIC and RIED will increase the DC conductivity and give rise to dose rate (flux) and dose (fluence) effects, although the contribution will decrease with frequency The polarization term depends on the defects in the material, which exist as, or can form, dipoles through charge transfer processes due to ionization (impurities, vacancies), and produces the absorption band structure observed in the loss as a function of frequency (Figure 8) This term also gives rise to both flux and fluence effects Furthermore, defects which are modified by radiation-induced charge transfer processes, for example, levels in the band gap occupied by electrons from the conduction band, are unstable and decay after irradiation This process is responsible for the slow decrease in electrical conductivity observed at the end of RIC experiments, and will similarly cause a slow decrease in the polarization term Hence, the initial observations of recovery in dielectric loss are to be expected, and the effort required to make measurements during irradiation fully justified Following the earlier measurements made during X-ray and proton irradiation,65,109,118 work concentrated on the needs for ICRH at about 100 MHz with the first measurements being made during pulsed neutron irradiation (Figure 9).119,120 These pulsed neutron experiments with ionizing dose rates >104 Gy sÀ1 found increases in loss of only about a factor Such a small increase is not compatible with the PIE results, which indicated that the order of Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors ICRH LH 713 ECRH Loss tangent 10-2 10-4 10-6 102 105 108 1011 Frequency (Hz) Figure Loss tangent versus frequency for different aluminas and sapphire (lowest loss) Reproduced from Molla´, J.; Heidinger, R.; Ibarra, A J Nucl Mater 1994, 212–215, 1029, with permission from Molla Loss tangent (´10-3) AIN Sapphire (´10) 0.1 0.14 0.18 0.22 0.26 0.3 Time (s) Figure The first in-reactor loss measurements at 100 MHz during a narrow neutron pulse (14 ms FWHM), showing the slow recovery for AlN Reproduced from Stoller, R E.; Goulding, R H.; Zinkle, S J J Nucl Mater 1992, 191–194, 602, with permission from Zinkle magnitude increases during irradiation This discrepancy may be related to the pulsed nature of the irradiation; although the peak dose rate was high, the integrated dose is only about 500 Gy per pulse, far too low for RIC to reach saturation.59–63 However, recent results indicate that for low dose (fluence), that is, at the beginning of operation, the influence of the DC conductivity term (RIC) is small for frequencies above about MHz even for dose rates >1 kGy sÀ1.121 Furthermore, in these pulsed experiments, the dpa per 714 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors pulse (%10À7 dpa) is too small to affect either the DC conductivity (RIC) or the polarizable defects, even though this term at these dose rates becomes important even down to 100 kHz Candidate RF heating systems for ITER (IC, ion cyclotron; LH, lower hybrid; EC, electron cyclotron) operating at about 100 MHz, GHz, and 200 GHz will require insulators (feedthroughs, standoffs, windows) to operate with large electric fields in a radiation field In general, the in situ experiments employed low-voltage RF, and the question then arises as to whether RIED could possibly affect the dielectric loss.120 At a time of intense RIED activity, two quite different theoretical models were presented in an attempt to explain why the application of a relatively small electric field during irradiation can substantially modify the damage production process and lead to volume electrical degradation.98,100 The earlier model was based on charge buildup and breakdown, that is, a DC mechanism, but failed to explain many of the results observed during RIED experiments.100 The later model however explained the role of the ionization taking into account the production of highly unstable Fỵ-centers,122 the electric field threshold, as well as g-alumina and colloid production, but more importantly predicted that RIED could occur for applied fields at frequencies >100 GHz.98 This was in agreement with early observations of RIED from DC to >100 MHz, and indications for RIED at frequencies above GHz.69 Dielectric loss measurements at 15 GHz, made during electron irradiation at kGy sÀ1, and postirradiation from kHz to 15 GHz, for sapphire, alumina, BeO, and MgAl2O4, show very varied results.123,124 Sapphire, the purest alumina grade, and BeO showed no prompt increase in loss, nor with a dose up to 50 MGy However, the 999 and 997 alumina grades showed significant prompt and dose-dependent increases in loss, consistent with a modification in the polarization term Furthermore, these in situ measurements show postirradiation recovery similar to the early reports for proton- and neutron-irradiated materials.65,109–111 In addition, sapphire samples, which had been preirradiated to MGy, 10À6 dpa at 450  C with a DC electric field (210 kV mÀ1) to produce RIED showed a significant increase in the loss (2 increase), and also in the prompt dielectric loss (%5 increase) Similar increases have only been observed for sapphire neutron irradiated, without an electric field applied, to >10À3 dpa.9 In this context, one should also mention recent work concerned with RF ion sources for NBI systems, where in situ measurements of dielectric loss during and following electron irradiation of alumina (Deranox 999) to 110 MGy with a MHz RF voltage (0.8 MV mÀ1) applied indicate a permanent increase in loss for irradiation at 240  C, but not at 120  C, as expected from previous RIED studies.125 While various alumina and BeO grades were available with adequate initial properties (dielectric loss, thermal conductivity, and mechanical strength) before irradiation for NBI, IC, and even LH applications, and with potential to withstand the expected ITER radiation levels, this was not the case for ECRH windows Sapphire or high-purity alumina, the initial ECRH window reference materials with low dielectric loss in the MHz to GHz range,116,126–128 exhibit increasing loss with increasing frequency reaching !10À4 (loss tangent) by 100 GHz Hence, to transmit the megawatts of RF power that will be required,9 these materials would have to be employed at cryogenic temperatures, and furthermore with a very low neutron tolerance level, 1020 n mÀ2.128 However, in recent years, considerable progress has been made with CVD diamond, a material with the required combination of low dielectric loss, high thermal conductivity, and mechanical strength.19,25,129–134 In this context, initial work began to examine both high-purity silicon and diamond homopolar crystalline materials which as a result of their decreasing loss with increasing frequency offered the possibility for operation at frequencies above 150 GHz with loss tangents 10À5, at room temperature.129 These two materials required development in completely opposite directions The initial high-resistivity silicon had very low loss but extreme radiation sensitivity Because of its perfection, electrons excited into the conduction band by purely ionizing radiation had very long lifetimes (no defect recombination sites) leading to high dielectric loss through the high electrical conductivity In contrast, the CVD diamond, initially almost black in color, had high loss because of the numerous defects in the material giving rise to polarization losses, but was almost insensitive to ionizing radiation because of the extremely short lifetime of the conduction band electrons Although the radiation sensitivity of silicon could be notably reduced by electron irradiation and also by Au doping because of the introduction of recombination defects, the main limitation for silicon comes from its small 1.1 eV band gap This allows electrons to be readily thermally excited into the conduction band at temperatures only slightly above room temperature, Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors Window grade CVD diamond 145 GHz Dicl loss tangent (10-4) 0.9 715 Unirr 10–21 n m−2 10–22 n m−2 0.6 0.3 0.0 150 200 250 300 Temperature (K) Figure 10 Diamond dielectric loss at 145 GHz while being unirradiated, and neutron irradiated at 320 K (pool temperature) to 1021 and 1022 n mÀ2 Reproduced with permission from Thumm, M.; Arnold, A.; Heidinger, R.; Rohde, M.; Schwab, R.; Spoerl, R Fusion Eng Des 2001, 53, 517 which rapidly increases the dielectric loss.135–138 In the case of CVD diamond, the progress has been remarkable, available samples going from black and irregular in shape to almost transparent mm thick 100 mm diameter disks, with room temperature loss %1  10À5 at 145 GHz, comparable with sapphire at 77 K, and furthermore increasing only to about  10À5 by 450  C.130,132 Loss measurements during electron and X-ray irradiation at 18 and 40 GHz, respectively of the developed CVD diamond, show almost negligible contributions of conductivity (RIC) and polarizable defects, and successful high-power transmission tests have now been carried out.132,133 As may be seen in Figure 10, PIE loss tangent measurements of neutron-irradiated ‘window grade’ CVD diamond indicate that even by 1022 n mÀ2 (10À3 dpa), the room temperature loss only increases to  10À5 at 145 GHz (6  10À5 at 190 GHz).134 During the intense activity to find suitable materials for the high-power IC, LH, and EC heating applications, work was also being carried out on materials for diagnostic systems In particular, KU1 quartz glass provided by the Russian Federation within the ITER-EDA task sharing agreement was shown to be highly radiation resistant with respect to its optical properties for use in both diagnostic and remote handling applications, and became the main reference material not only for optical windows, but also fibers.26,139,140 In view of this, the material was also examined for possible use in DC and RF applications Both RIC and RIED, together with dielectric loss and permittivity, have been determined for as-received, as well as electron and neutron irradiated material A large number of different experimental setups were employed to obtain the dielectric spectrum of KU1 over a very wide frequency range (10 mHz to 145 GHz), and where possible, values were obtained during electron irradiation In addition, data have been obtained for samples neutron irradiated to 10À4 dpa The results indicate that for low radiation doses the electrical and dielectric properties are only slightly degraded, and in particular the use of KU1 for electron cyclotron emission (ECE) windows and low-loss DC applications is feasible.134,141 4.22.6 Degradation of Insulator Thermal Conductivity Work began at an early stage to assess the thermomechanical properties of candidate insulating materials for fusion applications In an attempt to determine the best combination of mechanical, thermophysical, and dielectric properties for the demanding H&CD applications, Al2O3 (both alumina and sapphire), AlN, Si3N4, BeO, and MgAl2O4 in numerous different grades were examined ‘as-received’ and following irradiation.142–149 At room temperature, the unirradiated thermal conductivity of a typical alumina is of the order of 30 W mÀ1 KÀ1, and that of BeO about 280 W mÀ1 KÀ1 These values are sufficiently high 716 Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors for IC and LH heating systems to ensure adequate cooling in most cases; however, the thermal conductivity in ceramics is reduced because of increased phonon scattering, by the presence of point defects and to a lesser extent by extended defects or aggregates Hence, one expects a reduction in thermal conductivity on irradiation, together with a notable influence of the irradiation temperature, that is, irradiation above temperatures at which the radiationinduced defects become mobile and can either recombine or aggregate should lead to a lower degradation of the thermal conductivity, while lowtemperature irradiation should have a marked effect because of the increased point defect stability The expected general behavior was confirmed by the early data (Figure 11), and indicated that a maximum reduction to about one-third of the room temperature thermal conductivity value could be expected.142–145 This will occur for a neutron fluence value (dpa), which strongly depends on the irradiation temperature For near room temperature irradiation (300 K), reduction to the lower saturation level was observed by about 1023 n mÀ2 (0.01 dpa), whereas at 600 K this lower saturation level was only reached following a fluence of above 1024 n mÀ2 Within reasonable margins, these values applied for Al2O3, AlN, and MgAl2O4 Similar PIE results were obtained at a later date for reactor irradiations at different temperatures of a wide range of ceramic materials.150 Because of the importance of point defects in the reduction of thermal conductivity, it is reasonable to expect that postirradiation measurements may underestimate the effect due to possible postirradiation annealing An attempt to measure thermal conductivity in situ during reactor irradiation, although unable to quantify the degradation, did highlight a very rapid decrease in thermal conductivity by 1022 n mÀ2 (0.001 dpa) at the startup of irradiation, followed by saturation.151 Finally, one should mention the specific case of sapphire and CVD diamond, the original and the present reference materials for ECRH For sapphire, the need for low-temperature (