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

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.

4.22.2 Fusion-Relevant Radiation Damage in Insulating Materials 703

4.22.5 Degradation of Insulator AC/RF Dielectric Properties 712

CDA Conceptual design activity

CIEMAT Centro de Investigaciones Energe´ticas,

Medioambientales, y Tecnolo´gicas

CVD Chemical vapor deposition

ECRH Electron cyclotron resonant heating

EDA Engineering design activity

EVEDA Engineering Validation and Engineering

Design Activities

FIRE Fusion ignition research experiment

H&CD Heating and current drive

HFIR High Flux Isotope Reactor

(Oak Ridge, USA)

HFR High Flux Reactor (Petten, Holland)

ICRH Ion cyclotron resonant heating

IEA International Energy Agency

IFMIF International Fusion Materials Irradiation

Facility

IMR Institute for Materials Research

ITER International Thermonuclear

Experimental Reactor (Cadarache,

France)

JET Joint European Torus (Culham, UK)

KfK Kernforschungszentrum Karlsruhe

(Germany) KU1,

(Reactor at Saclay, France) PIE Postirradiation examination RAFM Reduced activation ferritic martensitic RIA Radiation-induced absorption RIC Radiation-induced conductivity RIED Radiation-induced electrical

degradation RIEMF Radiation-induced electromotive force

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

experi-mental difficulties not found in present-day machines

These problems are related to the expected

radia-tion damage effects as a result of the intense radiaradia-tion

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

opera-tion of such a machine, it is the radiaopera-tion 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

How-ever, the problem facing the numerous insulating

components is far more serious because of the

neces-sity to maintain not only the mechanical, but also the

far more sensitive physical properties intact An

addi-tional 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

machin-ery, 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,

there-fore, 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

sys-tems 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 materialsproblem will determine the viability of fusion power.The radiation field will modify to some degree all

of the important material physical and mechanicalproperties Some of the induced changes will be fluxdependent, while others will be modified by the totalfluence Clearly, the former flux-dependent pro-cesses will be of concern from the onset of operation

of future next-step devices The fluence-dependenteffects on the other hand are the important para-meters affecting the component or material lifetime.The properties of concern which need to be consid-ered for the many applications include electricalresistance, dielectric loss, optical absorption, andemission, as well as thermal and mechanical proper-ties Numerous papers have been published discuss-ing both general, and more recently, specific aspects

of radiation damage in insulating materials for fusionapplications, and those most relevant to the presentchapter are included.1–26

In recent years, because of the acute lack of datafor insulators and the recognition of their high sensi-tivity to radiation, most work has concentrated on theimmediate needs for ITER A comprehensive cera-mics irradiation program was established to investi-gate radiation effects on a wide range of materials foressentially all components foreseen for H&CD anddiagnostics in ITER, and to find solutions for theproblems which have been identified A large number

of relevant components and candidate materials havebeen, and are being, studied systematically at gradu-ally increasing radiation dose rates and doses, underincreasingly realistic conditions A considerable vol-ume of the work discussed here was carried outwithin the ITER framework during the CDA, EDA,and EDA extension (Conceptual and EngineeringDesign 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).27,28Since these last ITER tasks, no new coordinatedtasks related to insulators have been formulated.However, despite the lack of an official framework

in which to develop and assign further common tasksfollowing the end of the ITER-EDA extension, col-laborative work has continued between the EU, JA,

RF, and US Home Teams on both basic and appliedaspects of radiation damage in insulator materials.This has resulted in considerable progress beingmade on the understanding of the pertinent effects

of radiation on in-vessel components and materials inparticular for diagnostic applications Problems whichhave been addressed and for which irradiation testing

has been performed include comparison of absorption

and luminescence for different optical fibers and

win-dow materials, RIEMF (radiation-induced

electro-motive 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

antici-pated 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

spe-cific location within that device Because of the

gen-eral 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

tempera-ture Although the task ahead is difficult, important

advances are being made not only in the

identifica-tion of potential problems and operaidentifica-tional

limita-tions, but also in the understanding of the relevant

radiation effects, as well as materials selection and

design accommodation to enable the materials

lim-itations 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

envi-ronment for the component or material under

exam-ination will be presented, with reference to present

experimental procedures The chapter will then

con-centrate on the problems facing the use of insulators,

with the radiation effects on the main physical

prop-erties 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

begin-ning 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

consid-erable amount of knowledge and expertise already

existed for metallic materials.29This was not the case

for the insulating materials, mainly because of thefact that the required use of insulators in fission-type reactors is in general limited to low radiationregions, well protected from the reactor core How-ever, despite the late start and the reduced number

of specialists working in related fields at the time,together with the complexity of the mechanismsinvolved in radiation damage processes in insulators,considerable progress has been made not only inassessing the possible problem areas, but also infinding viable solutions Several general reviewsgive a good introduction to the specific problem ofradiation damage in insulators.30–36

The materials employed in the next-step fusionmachine will be subjected to fluxes of neutronsand gammas originating in the ignited plasma Theradiation intensity will depend not only on the dis-tance from the plasma, but also in a complex way

on the actual position within the machine because

of the radiation streaming along the numerous trations required for cooling systems, blanket struc-tures, heating systems, and diagnostic and inspectionchannels, as well as the radiation coming from thewater in the outgoing cooling channels due tothe 16O(n, p)16N nuclear reaction However one-,two-, and even three-dimensional models are nowavailable, which enable the neutron and gammafluxes to be calculated with confidence at most,

pene-if not all, machine positions.37–40Radiation damage is generally divided into twocomponents: displacement damage and ionizationeffects In a fusion environment, displacement dam-age, which affects both metals and insulators, willresult from the direct knock-on of atoms/ions fromtheir lattice sites by the neutrons, giving rise tovacancies and interstitials Those primary knock-onatoms (PKAs) with sufficient energy may go on toproduce further displacements, so-called cascades.The numerous point defects thus produced mayeither recombine, in which case no net damageresults, or they may stabilize and even aggregateproducing more stable extended defects These sec-ondary processes which determine the fate of thevacancies and interstitials are governed by theirmobilities These mobilities are highly temperaturedependent, and in the case of insulators even depend

on the ionizing radiation level (radiation-enhanceddiffusion) Displacement damage is measured in ‘dpa’(displacements per atom) where 1 dpa is equivalent todisplacing all the atoms once from their lattice sites

At the first wall of ITER, the primary displacementdose rate will be of the order of 10
6dpa s
1

In contrast, ionizing radiation although absorbed by

both metals and insulators, in general, only produces

heating in metals However, certain aspects of

radia-tion damage in metals, such as radiaradia-tion-enhanced

corrosion and grain boundary modification are related

to ionization The effects of ionization on insulators

are in comparison quite marked because of the

exci-tation 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 1 Gy¼ 1 J kg
1 At the first wall of ITER, the

dose rate will be of the order of 104Gy s
1

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

sev-eral dpa and GGy with no problem, some properties

of insulating materials can be noticeably modified by

as little as 10
5dpa or a few kGy Because of this, the

present ongoing programs of radiation testing for

diagnostics are concentrating mainly on the

insulat-ing components of the systems The results of these

radiation damage processes are flux- and

fluence-dependent 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

mechan-ical 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 inwindows and fibers can increase from the microwave

to the optical region and they emit strong cence (radioluminescence, RL); in addition, thematerials may become more brittle and may sufferswelling Clearly, some materials are more radiationresistant than others The organic insulators, whichare widely used in multiple applications in general,degrade under purely ionizing radiation and are notsuitable for use at temperatures above about 200C;

lumines-as a result their use will be limited to superconductingmagnet insulation and remote handling applicationsduring reactor shutdown Inorganic insulators of thealkali halide class have been widely studied and areused as optical windows; however, they are suscepti-ble to radiolysis (displacement damage induced

by electronic excitation) and in general becomeopaque at low radiation fluences Of the numerousinsulating materials, it is the refractory oxides andnitrides, which in general show the highest radia-tion resistance, and of these the ones which havereceived specific attention within the fusion programinclude MgO, Al2O3, MgAl2O4, BeO, AlN, and

Si3N4 In addition, different forms of SiO2 andmaterials such as diamond and silicon have beenexamined for various window and optical transmis-sion applications

One other aspect of radiation damage that should

be mentioned is nuclear transmutation The energy neutrons will produce nuclear reactions inall the materials giving rise to transmutation pro-ducts.1These will build up with time and representimpurities in the materials, which may modify theirproperties The physical properties of insulatorsare particularly sensitive to impurities Furthermore,some of these transmutation products may be radio-active and give rise to the need for remote handlingand hot cell manipulation in the case of componentremoval, repair, or replacement For the structuralmaterials, in the present concepts mainly steel alloys,considerable work has been carried out on the devel-opment of so-called low or reduced activation mate-rials (LAM, RAFM – reduced activation ferritic/martensitic) for possible use in DEMO and futurecommercial fusion reactors.41–45This work with theaim of reducing the amount of nuclear waste hasstudied not only the substitution of radiological prob-lem alloying elements such as Mo and Nb in steels,but also the viability of other materials such as vana-dium and SiC/SiC composites In the case ofthe insulating materials, no equivalent study or

high-development has been carried out, in part because of

the small fraction of the total material volume

repre-sented by the insulators, and also because the

impor-tant 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

possi-ble 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

concep-tual and engineering design activities were

under-taken during the 1990s within the IEA framework

with the view of providing such a facility, the

IFMIF (International Fusion Materials Irradiation

Facility).46–50This work has been recently renewed

under the EU-Japan Broader Approach (BA)

activ-ities 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

experi-ments have been performed in nuclear fission

reac-tors and particle accelerareac-tors, as well as g- and X-ray

sources, in an attempt to simulate the real operating

conditions of the insulating materials and

compo-nents The experiments required must simulate the

neutron and g radiation field, that is, the

displace-ment and ionization damage rates, the radiation

envi-ronment, 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

consist-ing 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

be experienced in a fusion reactor.50 However, it isworthwhile noting that to date experimental fissionreactors have mainly been used for irradiations inthe metals programs where the emphasis is on theneutron flux and little consideration is given to the

g field As a result, the irradiation channels have

in general been designed and installed with thiscriterion However, it should be possible to selectpositions within the reactors which, together withsuitable neutron absorber materials and neutron

to g converters, provide acceptable radiation fields.The main difficulties with in-reactor experimentscome from the inaccessibility of the radiation volumeand are concerned with the problem of carrying out

in situ measurements and achieving the correct diation environment While considerable success hasbeen attained in the in situ measurement require-ment, with parameters such as electrical conductivity,optical absorption and emission, and even radiofre-quency dielectric loss being determined, the problem

irra-of irradiating in vacuum still remains, with mostexperiments being performed in a controlled Heenvironment Irradiation in a controlled atmospheresuch as He causes an immediate problem for in situelectrical and dielectric measurements because of theradiation-enhanced electrical conductivity of the gas,53and even in the case of irradiation in vacuum at about

10
3mbar spurious leakage currents will occur.54Furthermore, many in-reactor experiments rely onnuclear heating to reach the required temperature,and hence have difficulty maintaining a controlledtemperature, in part because of the changes in thereactor power, and also because of the problem ofcalculating the final sample or component tempera-ture These aspects will be further discussed later.One additional difficulty comes from the nuclearactivation of the sample or component, which gener-ally means that postirradiation examination (PIE) haseither to be carried out in a hot cell or postponeduntil the material can be safely handled

Particle accelerators, on the other hand, are idealfor carrying outin situ experiments in high vacuumand at well-controlled temperatures because of theeasy access and the very localized radiation field.High levels of displacement damage and ionizationcan be achieved with little or no nuclear activation

It is however in the nonnuclear aspect of the radiationfield where their disadvantage is evident, and greatcare has to be taken to ensure that appropriate dis-placement rates are deduced to enable reliable com-parison with the expected fusion damage A furtherserious disadvantage is due to the limited irradiation

volume and particle penetration depth This in

gen-eral means that only small thin material samples or

components can be tested

The present-day situation of materials and

com-ponent radiation testing for fusion applications takes

full advantage not only of fission reactors and particle

accelerators, but also60Co g irradiation facilities and

even X-ray sources The use of such widely different

radiation sources can be justified as long as the

influ-ence of the type of radiation on the physical

parame-ter of inparame-terest 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,

dielec-tric, and optical properties In addition other aspects

such as mechanical strength and tritium diffusion are

being assessed during irradiation Undoubtedly,

suc-cessful 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

band-structured 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

sup-ports, 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 Al2O3and is also the one which has

been most extensively studied, both in the

polycrys-talline 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 gated; these are radiation-induced conductivity (RIC),radiation-induced electrical degradation (RIED), andsurface degradation

investi-Of these types of degradation, RIC was the first

to be addressed in a fusion context, as this ment of the electrical conductivity is flux dependentand hence a possible cause for concern from theonset of operation of any fusion device Fortunately,RIC had been studied for many years, and asound theoretical understanding already existed.55–59The ionizing component of the radiation field causes

enhance-an increase in the electrical conductivity because

of the excitation of electrons from the valence to theconduction band and their subsequent trapping inlevels within the band gap near to the conductionband from where they are thermally excited onceagain into the conduction band.Figure 1shows sche-matically RIC as a function of irradiation time andionizing dose rate (flux) The increase in saturationdepends not only on the dose rate as indicated, butalso in a complex way on the temperature and sampleimpurity content, as may be seen in Figure 2 forMgO:Fe.60Nevertheless, such behavior, including theinitial step, is well predicted by theory.57 However,

at the dose rates of interest for fusion applications, inthe range of approximately 1 Gy s
1to>100 Gy s
1,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 ofmagnitude For example, a standard high-purityalumina has a room temperature conductivity ofgenerally less than 10
16S m
1, which increases toapproximately 10
10S m
1for an ionizing dose rate

of only 1 Gy s
1.61The first experiments carried outwithin a fusion application context, that is, refractoryoxide materials, high-dose rates, and temperatures,gave an insight into the effects of dose rate, tem-perature, and material impurity, and established thewell-known relationship at saturation, between thetotal electrical conductivity measured during irradia-tion and the ionizing dose rate:stotal¼ s0þ KRdwhere

s0is 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 ture, dose, and dose rate dependence in this parameter,with extreme values in certain cases ranging between0.5 and 1.5, and in addition a temperature dependencewas observed for K At the present time, extensive RICdata are available for materials irradiated with X-rays,g-rays, electrons, protons, positive ions, and fission and

tempera-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 ofdose rates from<1 Gy s
1to about 104Gy s
1show

d  1, as may be seen in Figure 3, and lie within

a narrow band with the spread in conductivity values

at any given dose rate being about two orders ofmagnitude13; see also, for example, Noda et al.,66

Irradiation time (a.u.)

136⬚C, 650 ppm 172⬚C, 180 ppm

0 0.2 0.4

n ¥) 0.60.8 1.0

2

N T

(6,0.2) (10,0.1) (10,1)

Figure 2 RIC for single crystal MgO, doped with 180 and 650 ppm Fe g irradiation at 0.1 Gy s
1for different temperatures (14, 136, and 172C).60Theoretical predictions are shown inset Reproduced from Huntley, D J.; Andrews, J R Can.

J Phys 1968, 46, 147.

where 14 MeV neutron results are given together with

a small selection of other RIC data For all the RIC

data available, because of the different experimental

conditions, it is difficult to draw any conclusions as to

the reason for the spread in RIC values at any given

dose rate However, data obtained from electron

irradiations of different aluminas and other materialsunder identical conditions of dose rate and temper-ature give an indication that the RIC is inverselyproportional to the sample impurity content.19 Fromthese results (Figure 4), two general conclusions/indications may be drawn:

10 -6

1.8 MeV e - 450⬚C 700 Gy s –1 10 -10 dpa s–1

Al2O3MgO MgAl2O4

Figure 4 RIC for different single and polycrystalline materials measured during 1.8 MeV electron irradiation at 700 Gy s
1,

450C, plotted as a function of the estimated total impurity content The line is of slope
1 Reproduced from

Hodgson, E R J Nucl Mater 1998, 258–263, 226.

Hodgson and Clement 60

Figure 3 Representative data for RIC as a function of dose rate for different oxide materials Irradiation with electrons, protons, and neutrons Reproduced from Shikama, T.; Pells, G P J Nucl Mater 1994, 212–215, 80.

RICðsingle crystalÞ > RIC ðpolycrystalÞ and

RICðpureÞ > RIC ðimpureÞ

However, the indication on the impurity dependence

needs to be qualified, as certain impurities

intro-duce levels near to the conduction band, and increase

the RIC.59,60This 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

life-times, and do not introduce electron levels near to

the conduction band The reduction of the electron

lifetime in the conduction band has important

con-sequences 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

degrada-tion to be accommodated by the design, and that

materials exist which give rise to electrical

conduc-tivities 10
6S m
1 for ionizing dose rates of up

to>103

Gy s
1 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 isstrongly affected by impurity content, the buildup oftransmutation products will modify the RIC with irra-diation time (fluence), although this is not expected to

be of serious concern for ITER

In contrast to RIC, RIED is a more serious lem because it has been observed under certain con-ditions to permanently increase, that is, degrade, theelectrical conductivity with irradiation dose.Figure 5shows a schematic RIED-type degradation The ini-tial increase in the conductivity corresponds to theRIC as described above Following a certain irradia-tion time, or accumulated dose, the conductivityagain begins to increase as s0 degrades In Al2O3

prob-for which most work has been perprob-formed, RIED isobserved as a permanent increase or degradation

of the electrical conductivity (s0) when a smallelectric field (100 kV m
1) is applied during irradi-ation at moderate temperatures (450C) At con-

siderably higher temperatures and voltages, butwithout an irradiation field,67or for irradiations per-formed without an applied electric field,68no degra-dation occurs Even at the present time, this type ofdegradation is still not fully understood; nor is theregeneral agreement as to whether RIED is a realdegradation in the volume

Following the first report of RIED effect inelectron-irradiated sapphire (Al2O3) and MgO,8numerous experiments were carried out to assessits possible relevance to fusion insulator applications.These addressed the effect of the applied electric

Figure 5 Schematic RIED Initially, during irradiation RIC dominates, but with irradiation time (dose) the measured conductivity increases because of permanent degradation.

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,76–78and 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,79for thin

Al2O3films,80and MgO insulated cable.81In 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

contamina-tion.82,86Because of the potential importance of

elec-trical degradation and the uncertainty, extensive

discussions on RIED were held at several IEA

Workshops,89,90 including the experimental

techni-ques 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

partic-ular material-type differences, and irradiation

temper-ature, 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 grainsize Wesgo AL995 material was highly susceptible tosurface degradation when irradiated in high vacuum.91The in-reactor RIED experiment in HFIR at ORNLalso threw light on the complex RIED problem.92,93Initial results indicated no significant increase in elec-trical conductivity for 12 different samples However,moderate to substantial electrical degradation was laterreported for some of the sapphire and alumina samples,

so material type is an important parameter.94One ofthe major difficulties for in-reactor experiments is thedetermination of s0, the conductivity in the absence

of radiation, and its temperature behavior The use ofnuclear heating and the residual reactor radiation levelmean that changes in this parameter with temperatureand its corresponding activation energy are not gener-ally measured, although these are the main indicatorsfor the onset of degradation; hence, RIED onlybecomes measurable when the material conductivity

in the absence of radiation is larger than the RIC; that

is, s0 KRd Furthermore, some experiments wereperformed at temperatures either near room tempera-ture85 or above 600C,95 considerably outside theexpected effective temperature range for RIED ofapproximately 400–500C

In an attempt to clarify the situation, work wasperformed to identify possible basic causes of RIED.These experiments detected specific volume effects

in Al2O3 that are observed only for irradiationscarried out with an applied electric field A marked

Log10 ionization dose (Gy)

enhancement of the well-characterized Fþ-center

(oxygen vacancy with one trapped electron) was

observed,71 and TEM identified large regions of

g-alumina within the bulk of RIED degraded

Al2O3.96The increase in Fþ-center production gave

rise to enhanced oxygen vacancy mobility, and led

to vacancy aggregation and aluminum colloid

for-mation, as may be seen inFigure 7.97This 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

degrada-tion.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

con-centration which, as mentioned above in the

discus-sion 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 m
1.75,87

It also highlighted the importance of the ionization,

in agreement with earlier conclusions.73,84Additional

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 importantobservations.100

During the intense activities related to RIEDduring the 1990s, two important factors emerged,one concerned with surface electrical degradation,and the other related to the importance of the exper-imental irradiation environment For insulating com-ponents in future fusion devices, surface electricaldegradation may prove to be more serious than theRIC and RIED volume effects At that time, twotypes of surface degradation were reported, a con-tamination caused by poor vacuum, sputtering,

or evaporation,83,88 and a real surface degradationrelated to radiation-enhanced surface vacuum reduc-tion and possibly impurity segregation.101,102 Bothforms are affected by the irradiation environmentand ionizing radiation However, the real surfacedegradation effect is strongly material dependent,and occurs in vacuum but not in air or helium.102This stresses the extreme importance of a represen-tative irradiation environment for material testing.Most insulating materials required for fusion applica-tions in ITER and beyond must indeed operate

in high vacuum, and in consequence acceleratorexperiments to study electrical conductivity havebeen performed in vacuum, whereas to date, withfew exceptions,76–78,103,104 in-reactor experimentsfor technical reasons have been performed in helium.Another significant aspect of in-reactor experimentsperformed in helium is the radiation-induced leakagecurrent in the gas,53 which makes it difficult to

Figure 7 Aluminum colloid band in sapphire irradiated with 1.8 MeV electrons at different temperatures with

an electric field of 0.2 MV m
1applied Reproduced from Moron˜o, A.; Hodgson, E R J Nucl Mater 1997, 250, 156.

determine volume conductivity.81,104One 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

degrada-tion 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

win-dows 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.54At 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.104,106–108It is also

impor-tant 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

radi-ation effects on the AC/RF dielectric properties

(dielectric loss and permittivity) of suitable insulating

materials for fusion applications were almost

nonex-istent 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

proton-irradiated 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

mea-surements) showed orders of magnitude variation for

similar materials (10
5–10
2 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 ferent measuring systems used However, the resultsshowed good agreement,115and the large variation inreported loss tangent values was later shown to bereal, in part because of the effect of the differentimpurity contents of the materials.116,117 This may

dif-be clearly seen inFigure 8, where loss tangent datafor different aluminas over a wide frequency rangeare given, showing marked absorption band struc-tures due to polarizable defects (impurities).116During the early postirradiation loss tangentmeasurements, there was an indication of recovery,suggesting that loss during irradiation could be signif-icantly higher.65,109–111This implied that the alreadydifficult measurements should be madein situ duringirradiation In a simple way, dielectric loss can beconsidered as being due to two contributions:Loss aðDC conductivityÞ=Frequency

þ Polarization termClearly, both terms can be modified by the radiation.RIC and RIED will increase the DC conductivityand give rise to dose rate (flux) and dose (fluence)effects, although the contribution will decrease withfrequency The polarization term depends on thedefects in the material, which exist as, or can form,dipoles through charge transfer processes due toionization (impurities, vacancies), and produces theabsorption band structure observed in the loss as afunction of frequency (Figure 8) This term alsogives rise to both flux and fluence effects Furthermore,defects which are modified by radiation-inducedcharge transfer processes, for example, levels in theband gap occupied by electrons from the conductionband, are unstable and decay after irradiation Thisprocess is responsible for the slow decrease in electricalconductivity observed at the end of RIC experiments,and will similarly cause a slow decrease in the polari-zation term Hence, the initial observations of recovery

in dielectric loss are to be expected, and the effortrequired to make measurements during irradiationfully justified

Following the earlier measurements made duringX-ray and proton irradiation,65,109,118work concen-trated on the needs for ICRH at about 100 MHzwith the first measurements being made duringpulsed neutron irradiation (Figure 9).119,120 Thesepulsed neutron experiments with ionizing dose rates

>104

Gy s
1 found increases in loss of only about afactor 4 Such a small increase is not compatible withthe PIE results, which indicated that the order of