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 1 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.


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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 1 Gy ¼ 1 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 1 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 2 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 1 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 1 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 ¼ s0 þ 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.)

2

1.5

1

0.5

0

0

60
40
Irradiation time (a.u.)

20

80


100

Figure 1 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
4
14 ЊC, 180 ppm
172 ЊC, 180 ppm
14 ЊC, 650 ppm
136 ЊC, 650 ppm

RIC

2
1.0

n
n(t = ¥)

RIC (pA)

3

0.6

0.8

(e = 0,h = 0)

(100,0.1)
(10,0.1)

1

(10,1)

(6,0.2)
(3,1)

0.4
0.2
0

0

0

1

2
gG
t
N
T

0

50


100

150

3

4

Time

200

Irradiation time (min)
Figure 2 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 <1 Gy sÀ1 to about 104 Gy sÀ1 show
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 of
magnitude13; see also, for example, Noda et al.,66



708

Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

10-2
Superconducting
magnet

Electrical conductivity (W-m)-1

10-4

First wall

10-6

3

10-8
1

2
10-10

10-12

Al2O3


Van Lint et al.64
Klaffky et al.59
Pells et al.65

MgO

Hodgson and Clement60

4
MgAl2O4

10-14 -4
10

10-2

100

Pells et al.65

102
104
Dose rate (Gy s–1)

106

108

1010


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.

1.8 MeV e- 450 ЊC 700 Gy s–1 10-10 dpa s–1
Al2O3
MgO
MgAl2O4
BeO

RIC (S m−1)

10-6

10-7

10-8
100

1000
Impurity content (ppm)

104

Figure 4 RIC for different single and polycrystalline materials measured during 1.8 MeV electron irradiation at 700 Gy sÀ1,
450  C, plotted as a function of the estimated total impurity content. The line is of slope À1. Reproduced from
Hodgson, E. R. J. Nucl. Mater. 1998, 258–263, 226.

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 materials
under identical conditions of dose rate and temperature give an indication that the RIC is inversely
proportional to the sample impurity content.19 From
these results (Figure 4), two general conclusions/
indications may be drawn:


Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

RIC ðsingle crystalÞ > 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 do 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 5
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


2
RIED influence

RIC + RIED (a.u.)

1.5
RIC dominates
1

Permanent
degradation

0.5

0

0

20

40

709

60

80

100


Irradiation time (a.u.)
Figure 5 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)

0
Vitox Al2O3

500 ЊC

-5

-10
400 ЊC
-15

8.5

9.0


9.5

10.0

Log10 ionization dose (Gy)
Figure 6 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 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
Al2O3.96 The increase in Fþ-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
2
310 ЊC
290 ЊC

1

0

270 ЊC

3

4

711

5

6

Energy (eV)
Figure 7 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 ðDC conductivityÞ=Frequency
þ 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 4. 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 8 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.

7

Loss tangent (´10-3)

6
5
4
AIN
3
2
1
Sapphire (´10)
0
0.1

0.14

0.18

0.22

0.26


0.3

Time (s)
Figure 9 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 1 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, 5 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 Fþ-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 1 GHz.69
Dielectric loss measurements at 15 GHz, made during
electron irradiation at 2 kGy sÀ1, and postirradiation
from 1 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 7 MGy, 10À6 dpa at
450  C with a DC electric field (210 kV mÀ1) to produce RIED showed a significant increase in the loss
(2Â increase), and also in the prompt dielectric
loss (%5Â 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 1 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 2 mm thick
100 mm diameter disks, with room temperature
loss %1 Â 10À5 at 145 GHz, comparable with sapphire
at 77 K, and furthermore increasing only to about
5 Â 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 5 Â 10À5
at 145 GHz (6 Â 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 (<100 K) operation
to minimize dielectric loss also provided a gain in
thermal conductivity (200 W mÀ1 KÀ1 at 100 K, c.f.
about 30 W mÀ1 KÀ1 at room temperature). However,
in addition to the dielectric loss showing a very low

neutron tolerance ( 1020 n mÀ2) at this low temperature,128 the high thermal conductivity was reduced
by over two orders of magnitude also by 1020 n mÀ2
(10À5 dpa), because of the enhanced point defect
stability.147,152 In the case of CVD diamond, the
increase in the room temperature dielectric loss
was still tolerable up to 1022 n mÀ2 (10À3 dpa).134
Unfortunately, although the extremely high thermal
conductivity at room temperature (%1800 W mÀ1 KÀ1)
already began to degrade by 1020 n mÀ2 (10À5 dpa), it
was at the tolerance limit by 1021 n mÀ2 (Figure 12).134
Almost identical results were reported after electron
irradiation to 3 Â 10À6 dpa where the thermal conductivity was reduced by about 9%, confirming the
importance of point defects.153

0.35
lµT -1

Thermal conductivity (W cm-1 K)

0.30

Nonirradiated
KfK® cyclotron

lµT -0.9

LAMPF
0.25

Petten-HFR

lµT -0.7

OSIRIS

0.20

0.15
lµT -0.4

0.10

0.05
0

0

200

400

600

800

1000

1200

1400


1600

1800

Temperature (K)

Figure 11 Effect of neutron and a particle irradiations at different temperatures and doses on alumina (AL23: Degussit)
thermal conductivity (KfK 700 K 0.001 dpa, LAMPF 600 K 0.5 dpa, Petten 473 K 0.4 dpa, OSIRIS 823 K 5 dpa).
Reproduced from Rohde, M.; Schulz, B. J. Nucl. Mater. 1990, 173, 289, with permission from Rohde.


Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

717

2000

Thermal conductivity (W mK-1)

1800
1600
1400
1200
1000
800
600
400
200

Refinement grade

Refinement grade
Scale-up grade
Window grade
Torus window disc

0
Unirradiated

1020
1021
Fluence (n m-2)

1022

Figure 12 Thermal conductivity reduction for different diamond grades as a function of neutron dose. Reproduced
from Heidinger, R.; Rohde, M.; Spo¨rl, R. Fusion Eng. Des. 2001, 56–57, 471, with permission from Heidinger.

4.22.7 Degradation of Optical
Properties
Within the fusion program, another area of concern is
related to the effects of radiation on the optical properties of the dielectric materials to be used as transmission components (windows, lenses, and optical
fibers) for the UV, visible, and near-IR wavelength
diagnostic systems needed for control and safety, as
well as maintenance (remote handling).21,26,140,154,155
Radiation-induced optical absorption (RIA) and light
emission or RL impose severe limitations on the use
of any optical material within an intense radiation
field. For remote handling applications, the optical
components will be expected to maintain their transmission properties under high levels of ionizing radiation ( 1 Gy sÀ1) during hundreds of hours. For such
applications, RIA imposes the main limitation, but

can be tolerated. However, in the case of diagnostic
applications, in addition to a higher level of ionizing
radiation (tens to hundreds Gy sÀ1), the materials
will also be subjected to atomic displacements !10À9
dpa sÀ1. It soon became clear that both RIA and
RL would impose severe limitations on the main
candidate materials (sapphire and silica). Of these
two materials, sapphire is by far the most resistant
to ionizing radiation. Although ionizing radiation can
cause an increase in optical absorption because of
trace impurities and vacancy defects present in the
material, it is in general the displacement damage
mechanism which induces absorption at first in the
UV region as a result of oxygen vacancy-related

defects.30,33,156–158 This fluence (dose) effect reduces
the transmission in the UV region to essentially zero
for doses above about 10À4 dpa, and more slowly in
the visible as the tails of the absorption bands begin to
overlap into this region. Although sapphire shows
more radiation resistance than SiO2 in terms of optical absorption, the material was found to be unsuitable for many diagnostic applications because of its
intense RL, as will be seen below.
As with RIC, RL is ionizing flux (dose rate)
dependent and hence will be a problem from the
onset of operation of future fusion devices. Furthermore, to assess RL clearly requires in situ measurements during irradiation. While many studies had
been carried out on luminescence phenomena in
SiO2 and sapphire, the problem was only addressed
in a quantitative way because of fusion application
requirements.159–164 Sapphire was quickly excluded
from high-dose rate applications when it was shown

that the photon emission for a typical diagnostic
window dose rate would be comparable with the
photon emission from the plasma.159 In contrast,
certain grades of silica show virtually no RL in the
UV-visible region, the emission being limited almost
to the Cherenkov background. Quantitative luminescence data comparing UV grade sapphire and two
types of silica, both of which show low RL, are given
in Figure 13, indicating that suitable materials do
exist in which the RL can be reduced to a minimum,
although there are limited data on RL as a function
of fluence.162–164 In particular, the KU1 and KS-4V
quartz glass materials, provided by the Russian


718

Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

1011 photons/(s.Å.sr.cm3)

1000

Sapphire

100

10

1


Anhydroguide

KU1
0.1
2000

3000

4000

6000
5000
Wavelength (Å)

7000

8000

Figure 13 Quantified RL emission for sapphire and two silica grades during 1.8 MeV electron irradiation at 700 Gy sÀ1,
15  C. Reproduced from Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1998, 258–263, 1889.

Federation for the ITER diagnostics radiation testing
program, have proved to be highly resistant to RL
and RIA because of ionizing radiation and displacement damage, and are now reference materials.26,165–170
For ionizing radiation doses up to at least 100 MGy
and for temperatures at or above about 100  C, very
little absorption is induced in the KU1 material over
the whole visible range; one must keep in mind
however that with irradiation displacement dose
the optical absorption related to oxygen vacancies

in SiO2, as in all oxide materials, eventually renders
them opaque in the UV and visible range.171–175
In an analogous way to the ECRH transmission
windows, mention should be made of windows
required for high-power laser transmission, that is,
the LIDAR (light detection and ranging) system.
This demanding diagnostic system being considered
for ITER will require very high-quality transmission
windows for the high-power laser pulses at about 500
and 1000 nm. It is estimated that transmission losses of
the order of 5% may cause problems with the window
integrity because of laser damage. However, such
small decreases in the transmission corresponding to
an optical density increase of only 0.02 are extremely
difficult to measure by standard PIE of irradiated
optical materials. Such measurements have to be performed in situ. In situ measurement is also required in
order to determine possible radiation-enhanced
absorption which can easily reach such small values.
The possibility of radiation-enhanced dielectric
breakdown due to the intense laser pulse and the

ionizing radiation has also to be considered. However,
such a determination requires an elaborate in situ
experiment. Work on laser-induced damage in KU1
and KS-4V has confirmed the limited influence of
RIA and RIC on the damage threshold for highpower laser transmission.176 However, metallic deposition due to sputtering or evaporation can seriously
reduce the damage threshold even for a few nanometer thickness, as may be seen in Figure 14. The effect
is strongly material dependent, and furthermore selfcleaning with subthreshold laser pulses is not effective
for all deposited materials.177,178
Although in general RL is considered to be a

problem for diagnostic systems in future devices,
it may be employed as a detector/converter for
X-ray, UV, and particle emission from the plasma.
The intense RL from Al2O3:Cr, for example, has
been used for many years in ceramic fluorescent
screens for accelerator beam alignment,179 and is
now being developed with improved radiation resistance and rapid decay times for fusion applications,
along with other alternative luminescent materials
(Figure 15).180–182 Furthermore, RL is a potentially
powerful tool capable of monitoring material modification during irradiation, but has been largely
neglected within the fusion materials activities, in
part because of the difficulty in interpreting the
resulting emission spectra. However, the technique is
now being successfully employed to study insulating
materials such as aluminas and silicas, as well as
breeding ceramics for fusion applications.183,184


Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

719

1.2

Damage probability

1

0.8


0.6

KU1 +
steel layer

0.4

KS-4V +
steel layer
KU1
KS-4V
polished

0.2

0
1014

1015
1016
Laser power density (W m-2)

1017

Figure 14 Effect of a thin sputtered steel layer on the laser induced damage for KU1 and KS-4V quartz glasses.
Reproduced from Martin, P.; Moron˜o, A.; Hodgson, E.R. J. Nucl. Mater. 2004, 329–333, 1442.

1.4

Relative luminescent intensity (a.u.)


14-MeV neutron irradiation
1.2

SrAl2O4:Eu2+, Dy3+
Sr4Al14O25:Eu2+, Dy3+

1
0.8

0.6
0.4
0.2
0
300

350

400

550
450
500
Wavelength (nm)

600

650

700


Figure 15 14 MeV neutron-induced luminescence in doped aluminates. Reproduced from Toh, K.; Shikama, T.;
Katsui, H.; et al. J. Nucl. Mater. 2009, 386–388, 1027.

Finally, in connection with optical transmission
components, one should note the flexibility and simplification in diagnostic design that the use of optical
fibers would allow. However, this is not straightforward; although RIA and RL are problems for optical
window and lens components, in the case of optical
fibers the situation is far worse because of the length
of the optical path. Furthermore, because of the

manufacturing techniques, fibers with characteristics
as good as those observed for the KU1 quartz glass for
example have not been obtained. This has prompted
an extensive collaborative research program to find
the most suitable types of radiation-resistant fiber.
Several different optical fibers have been examined
to assess RIA and light emission, the viability of
high-temperature operation and annealing, jacketing


720

Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

material, and the influence of hydrogen loading.
In addition, parallel work is being carried out on the
possibility of photobleaching using high-intensity
lasers to recover transmission, ‘holey’ fibers for
improved transmission and radiation resistance, and

fibers with extended blue – UV transmission.26,185–190
Irradiations have been carried out to total doses
above 10 MGy and 1022 n mÀ2, and temperatures
from about 30 to 300  C. The most promising fibers
are the hydrogen loaded KU1 and KS-4V, where
above 400 nm they show the lowest RIA, as may
be seen in Figure 16.139 Although the KU1 is
the slightly better material up to about 700 nm, the
intrinsic OH band and its harmonics notably affect
transmission above 800 nm, so for a fiber required to
transmit in the visible and IR regions, the hydrogen
loaded KS-4V may be a better choice. For silica
materials up to about 10 MGy, the main radiation
damage mechanisms involve electron and holetrapping; hence, the wide differences observed in
induced absorption of the fibers tested are due to
variations in intrinsic trapping centers (defects and
impurities). In general, these trapping centers are
thermally unstable, hence the effective thermal
annealing for irradiation at higher temperature, or
postirradiation thermal annealing. For higher doses,
displacement damage leading to extensive structural
damage begins to dominate, but by this time the
fibers are of little use for diagnostic applications.
Limited work is underway to examine the possibility

Radiation-induced absorption (dB m-2)

16

of in situ photobleaching of the radiation-induced

damage using high-intensity UV lasers, the potential
of so-called ‘holey’ fibers (fibers containing an array
of vacuum, air, or liquid filled holes) to improve
radiation resistance, as well as fibers to extend transmission into the blue – UV region.

4.22.8 Concluding Remarks
Since the end of the 1970s, the fusion materials community has been providing the necessary insulator
research and database for H&CD, and diagnostic
systems for a next-step burning plasma device
(ITER). As described above, the continuous research
has identified and highlighted the limitations and
potential problems related to electrical, dielectric,
and thermal conductivity degradation, window and
fiber absorption, and luminescence, while at the same
time providing possible solutions for the ECRH windows with the development of diamond for example,
identifying safer operating conditions for the insulators, assessing optical materials for low RL, and characterizing dielectric properties over a wide range of
frequencies (IC to EC). In addition to channeling the
necessary expertise, numerous unique experiments
and installations have been developed to study candidate materials under relevant conditions, in particular during irradiation. All this has produced data of
direct relevance to both ITER and future fusion

5.2 ´ 1012 n cm-2, 25 MGy, 130 ЊC

14

KU1
12

F1-doped


10

KS4V

8
6
4
2

KU-H2G

0

450

500

550

600 650 700
Wavelength (nm)

750

800

850

900


Figure 16 In-reactor radiation-induced optical absorption (RIA) for four types of fiber. The hydrogen loaded KU1
(KU-H2G) shows the best performance. Reproduced from Brichard, B.; Fernandez Fernandez, A.; Ooms, H.;
et al. J. Nucl. Mater. 2004, 329–333, 1456.


Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

devices. As a result of the in-reactor experience,
research was then able to concentrate on the required
irradiation testing and screening of prototype components, urgently required for ITER. These have
included bolometers, Hall probes, MI cables, coated
mirrors, pressure gauges, as well as the optical fibers
discussed above.26,108,139,191–193
The irradiation testing required for ceramic
insulator materials is far more complex than that
required for structural materials. For almost all of
the properties of interest, in situ testing is mandatory.
The difficulties associated with in situ testing
together with the high cost of reactor irradiations
have meant that g sources and particle accelerators
have been used to full advantage, where the behavior
with irradiation temperature and dose rate can be
more easily assessed. The need for irradiation in
vacuum is an added difficulty; one must remember
that most in-reactor irradiations are performed in He.
However, despite the difficulties, several experimental systems have been developed to enable testing in
both static and dynamic (pumped) vacuum, thereby
fully simulating the expected environment. In addition, the hostile, inaccessible, and noisy environment
of experimental fission reactors makes the measurement of often small but important effects difficult.
However, over the years considerable in-reactor testing expertise has been gained and much needed

experiments performed.
One must remember that ITER is only a ‘next
step’; the final goal is to provide a safe and reliable
fusion reactor within a reasonable time. Undoubtedly,
beyond ITER the use of insulators will be severely
restricted to those essential to operation and maintenance, but they will be of paramount importance to
the success of fusion power. Hence, future ceramic
insulator research activity, while keeping in mind
the short-term ‘urgent’ ITER needs, must address
the expected fluence degradation effects on all the
material properties and enable viable solutions to be
available in time. The problems to be addressed
are related to long-term degradation of the required
properties because of aggregation and segregation of
defects and impurities. For high fluence, not only
H and He, but also other transmutation elements
will begin to play a role in the modification of the
material physical properties. Although not discussed
here, mechanical property degradation has been
studied since the beginning of the fusion materials
activity. During this early work, considerable attention was paid to the mechanical properties of refractory oxides and nitrides, where PIE indicated that

721

significant degradation of the mechanical strength
would only occur for radiation damage levels of
the order of 1 dpa or above.148 However, evidence
was found for two types of radiation-enhanced
degradation of the mechanical strength, ‘enhanced’
implying degradation of damage levels <<1 dpa.

One is related to RIED, where the formation of
small regions of g-alumina within the a-alumina
matrix caused materials to become more fragile.75,96
The other type is related to subcritical crack growth
(SCCG), where for certain aluminas the ‘time to
fracture’ was markedly shortened for tests performed
during g-irradiation.194 Clearly, these aspects of radiation damage related to synergistic effects of radiation plus an electric field and mechanical stress
should be reexamined to be able to predict their
long-term importance, as also should be RIED itself
which still remains an unresolved potential longterm problem. Finally, one should note that to date
none of the in-reactor experiments has been performed without encountering some unexpected difficulty, related to, for example, feedthroughs, MI
cables, electrical contacts, applied voltages, gas leakage currents, poor vacuum, and temperature control.
These are precisely the types of problem that must be
avoided in future fusion devices, and a renewed effort
will be required to overcome them. (See also Chapter
1.02, Fundamental Point Defect Properties in
Ceramics).

References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.

12.
13.
14.
15.
16.

Rovner, L. H.; Hopkins, G. R. Nucl. Technol. 1976, 29, 274.
Phillips, D. C. AERE Harwell Report R-8923, 1978.
Clinard, F. W., Jr. J. Nucl. Mater. 1979, 85–86, 393.
Clinard, F. W., Jr.; Hurley, G. F. J. Nucl. Mater. 1981,
103–104, 705.
Pells, G. P. J. Nucl. Mater. 1984, 122–123, 1338.
Clinard, F. W., Jr. Cryst. Latt. Def. Amorph. Mater. 1987,
14, 241.
Pells, G. P. J. Nucl. Mater. 1988, 155–157, 67.
Hodgson, E. R. Cryst. Latt. Def. Amorph. Mater. 1989,
18, 169.
Heidinger, R. J. Nucl. Mater. 1991, 179–181, 64.
Zinkle, S. J.; Hodgson, E. R. J. Nucl. Mater. 1992,
191–194, 58.
Clinard, F. W., Jr.; Farnum, E. H.; Griscom, D. L.; et al.
J. Nucl. Mater. 1992, 191–194, 1399.
Kinoshita, C. J. Nucl. Mater. 1992, 191–194, 67.
Shikama, T.; Pells, G. P. J. Nucl. Mater. 1994,
212–215, 80.
Hobbs, L. W.; Clinard, F. W., Jr.; Zinkle, S. J.; Ewing, R. C.
J. Nucl. Mater. 1994, 216, 219.
Kinoshita, C.; Zinkle, S. J. J. Nucl. Mater. 1996,
233–237, 100.
Zinkle, S. J. Mat. Res. Soc. Symp. Proc. 1997, 439,

667–678.


722
17.
18.

19.
20.
21.
22.
23.
24.
25.
26.
27.
28.

29.
30.

31.

32.
33.
34.

35.
36.


37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.

Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors
Zinkle, S. J.; Kinoshita, C. J. Nucl. Mater. 1997, 251,
200–217.
Hodgson, E. R. In Diagnostics for Experimental
Thermonuclear Fusion Reactors; Stott, P. E., Gorini, G.,
Prandoni, P., Sindoni, E., Eds.; Plenum Press: New York,
1998; Vol. 2.
Hodgson, E. R. J. Nucl. Mater. 1998, 258–263, 226.
Shikama, T.; Yasuda, K.; Yamamoto, S.; Kinoshita, C.;
Zinkle, S. J.; Hodgson, E. R. J. Nucl. Mater. 1999, 271, 560.
Yamamoto, S.; Shikama, T.; Belyakov, V.; et al. J. Nucl.
Mater. 2000, 283–287, 60.
Shikama, T.; Zinkle, S. J. Nucl. Mater. 2002, 307–311,
1073.
Hodgson, E. R. Nucl. Instrum. Meth. Phys. Res. B 2002,
191, 744.
Decreton, M.; Shikama, T.; Hodgson, E. R. J. Nucl.
Mater. 2004, 329–333, 125.

Ibarra, A.; Hodgson, E. R. Nucl. Instrum. Meth. Phys.
Res. B 2004, 218, 29.
Vayakis, G.; Hodgson, E. R.; Voitsenya, V.; Walker, C. I.
Fusion Sci. Technol. 2008, 53, 699.
Yamamoto, S. ITER Document G 55 DDD 27 97–12–05
W0.1.
Yamamoto, S.; et al. In Diagnostics for Experimental
Thermonuclear Fusion Reactors; Stott, P. E., Gorini, G.,
Prandoni, P., Sindoni, E., Eds.; Plenum Press: New York,
1998; Vol. 2.
Schiller, P.; Ehrlich, K.; Nihoul, J. J. Nucl. Mater. 1991,
179–181, 13.
Sonder, E.; Sibley, W. A. In Point Defects in Solids;
Crawford, J. H., Slifkin, L. M., Eds.; Plenum Press:
New York, 1972; Vol. 1.
Clinard, F. W.; Hobbs, L. W. In Physics of Radiation
Effects in Crystals; Johnson, R. A.; Orlov, A. N., Eds.;
North Holland: Amsterdam, 1986; p 387.
Hughes, A. E. Rad. Eff. 1986, 97, 1.
Agullo-Lopez, F.; Catlow, C. R. A.; Townsend, P. D. Point
Defects in Materials; Academic Press: London, 1988.
Itoh, N. Ed. Defect Processes Induced by Electronic
Excitation in Insulators; Directions in Condensed Matter
Physics; World Scientific: Singapore, 1989; Vol. 5.
Stoneham, A. M. Nucl. Instrum. Meth. Phys. Res. 1994,
A91, 1.
Itoh, N.; Stoneham, A. M. Materials Modification by
Electronic Excitation; Cambridge University Press:
Cambridge, 2001.
Santoro, R. T.; Iida, H.; Khripunov, V. ITER Report IDoMS

No. NAG-47-8-1-97, 1997.
Khripunov, V.; Santoro, R. T. ITER Report IDoMS No.
NAG-50, 1997.
Nightingale, M.; Taylor, N. ITER NBI Design Task N53 TD
11 FE D322, 1996.
Costley, A. E.; Campbell, D. J.; Kasai, S.; Young, K. E.;
Zaveriaev, V. Fusion Eng. Des. 2001, 55, 331.
Harries, D. R.; Butterworth, G. J.; Hishinuma, A.;
Wiffen, F. W. J. Nucl. Mater. 1992, 191–194, 92.
Kohyama, A.; Hishinuma, A.; Gelles, D. S.; Klueh, R. L.;
Dietz, W.; Ehrlich, K. J. Nucl. Mater. 1996, 233–237, 138.
van der Schaaf, B.; Gelles, D. S.; Jitsukawa, S.; et al.
J. Nucl. Mater. 2000, 283–287, 52.
Baluc, N.; Gelles, D. S.; Jitsukawa, S.; et al. J. Nucl.
Mater. 2007, 367–370, 33.
Nishitani, T.; Tanigawa, H.; Jitsukawa, S.; et al. J. Nucl.
Mater. 2009, 386–388, 405.
Kondo, T.; Doran, D. G.; Ehrlich, K.; Wiffen, F. W. J. Nucl.
Mater. 1992, 191–194, 100.
Kondo, T.; Shannon, T. E.; Ehrlich, K. J. Nucl. Mater.
1996, 233–237, 82.

48.
49.
50.
51.
52.
53.
54.
55.

56.
57.
58.
59.
60.
61.
62.
63.
64.
65.

66.
67.
68.
69.
70.
71.
72.
73.
74.
75.

76.
77.
78.
79.
80.
81.
82.
83.

84.

85.

Noda, K.; Ehrlich, K.; Jitsukawa, S.; Mo¨slang, A.;
Zinkle, S. J. J. Nucl. Mater. 1998, 258–263, 97.
Mo¨slang, A.; Antonnucci, C.; Daum, E.; et al. J. Nucl.
Mater. 1998, 258–263, 427.
Ehrlich, K.; Bloom, E. E.; Kondo, T. J. Nucl. Mater. 2000,
283–287, 79.
Garin, P. J. Nucl. Mater. 2009, 386–388, 944.
Garin, P.; Sugimoto, M. Fusion Eng. Des. 2009,
84, 259.
Hodgson, E. R.; Moron˜o, A. J. Nucl. Mater. 2002,
307–311, 1660.
Hodgson, E. R.; Moron˜o, A.; Gonzalez, S. M. J. Nucl.
Mater. 2009, 386–388, 999.
Rose, A. Phys. Rev. 1955, 97, 322.
Marshall, J. F.; Pomerantz, M. A.; Shatas, R. A. Phys.
Rev. 1957, 106, 432.
Huntley, D. J.; Andrews, J. R. Can. J. Phys. 1968,
46, 147.
Hughes, R. C. Phys. Rev. 1979, B19, 5318.
Klaffky, R. W.; Rose, B. H.; Goland, A. N.; Dienes, G. J.
Phys. Rev. 1980, B21, 3610.
Hodgson, E. R.; Clement, S. Rad. Eff. 1986, 97, 251.
Pells, G. P. Rad. Eff. 1986, 97, 39.
Pells, G. P.; Hill, G. J. J. Nucl. Mater. 1986, 141–143, 375.
Hodgson, E. R.; Clement, S. J. Nucl. Mater. 1988,
155–157, 357.

van Lint, V. A. J.; Harrity, J. W.; Flanagan, T. M. IEEE
Trans. Nucl. Sci. 1968, NS-15(6), 194.
Pells, G. P.; Buckley, S. N.; Agnew, P.; Foreman, A. J. E.;
Murphy, M. J.; Staunton-Lambert, S. A. B. Radiation
Effects in Electrically Insulating Ceramics; Harwell AERE
Report 13222, 1988.
Noda, K.; Nakazawa, T.; Oyama, Y.; Yamaki, D.;
Ikedo, Y. J. Nucl. Mater. 1996, 233–237, 1289.
Weeks, R. A.; Sonder, E.; Narayan, J. Am. Ceram. Soc.
Bull. 1978, 57, 316.
Hodgson, E. R. J. Nucl. Mater. 1991, 179–181, 383.
Hodgson, E. R. J. Nucl. Mater. 1992, 191–194, 552.
Hodgson, E. R. Nucl. Instrum. Meth. 1992, B65, 298.
Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1994,
212–215, 1119.
Pells, G. P.; Sowden, B. C. J. Nucl. Mater. 1995, 223,
174.
Hodgson, E. R. J. Nucl. Mater. 1994, 212–215, 1123.
Pells, G. P. J. Nucl. Mater. 1991, 184, 177.
Mo¨slang, A.; Daum, E.; Lindau, R. In Proceedings of the
18th Symposium on Fusion Technology, Karlsruhe,
August 1994; Fusion Technol. 1994; p 1313.
Shikama, T.; Narui, M.; Endo, Y.; Sagawa, T.; Kayano, H.
J. Nucl. Mater. 1992, 191–194, 575.
Shikama, T.; Narui, M.; Endo, Y.; Ochiai, A.; Kayano, H.
J. Nucl. Mater. 1992, 191–194, 544.
Shikama, T.; Narui, M.; Kayano, H.; Sagawa, T. J. Nucl.
Mater. 1994, 212–215, 1133.
Zong, X. F.; Shen, C. F.; Liu, S.; et al. Phys. Rev. B 1994,
49, 15514.

Hunn, J. D.; Stoller, R. E.; Zinkle, S. J. J. Nucl. Mater.
1995, 219, 169.
Farnum, E. H.; Shikama, T.; Narui, M.; Sagawa, T.;
Scarborough, K. J. Nucl. Mater. 1996, 228, 117.
Kesternich, W.; Scheuermann, F.; Zinkle, S. J. J. Nucl.
Mater. 1993, 206, 68.
Jung, P.; Zhu, Z.; Klein, H. J. Nucl. Mater. 1993, 206, 72.
Farnum, E. H.; Clinard, F. W., Jr.; Sommer, W. F.;
Kennedy, J. C.; Shikama, T. J. Nucl. Mater. 1994,
212–215, 1128.
Snead, L. L.; White, D. P.; Zinkle, S. J. J. Nucl. Mater.
1994, 212–215, 1107.


Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors
86.
87.
88.
89.
90.

91.
92.
93.
94.
95.

96.
97.
98.

99.

100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
110.
111.
112.
113.
114.
115.
116.
117.
118.
119.
120.

Kesternich, W.; Scheuermann, F.; Zinkle, S. J. Nucl.
Mater. 1995, 219, 190.
Snead, L. L.; White, D. P.; Zinkle, S. J. J. Nucl. Mater.
1995, 226, 58.
Kesternich, W. J. Nucl. Mater. 1998, 253, 167.
Chen, Y.; Clinard, F. W.; Evans, B. D.; et al. J. Nucl.

Mater. 1994, 217, 32.
Zinkle, S. J.; Hodgson, E. R.; Shikama, T. IEA Workshop
on Radiation Effects in Ceramic Insulators, Cincinnati, May
1997 (ORNL/M-6068, DOE/ER-0313/22).
Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1996,
233–237, 1299.
Shikama, T.; Zinkle, S. J.; Shiiyama, K.; Snead, L. L.;
Farnum, E. H. J. Nucl. Mater. 1998, 258–263, 1867.
Shiiyama, K.; Howlader, M. M. R.; Zinkle, S. J.; et al.
J. Nucl. Mater. 1998, 258–263, 1848.
Shikama, T.; Zinkle, S. J. J. Nucl. Mater. 1998, 258–263,
1861.
Chernov, V. M.; Khorasanov, G. L.; Plaksin, O. A.;
Stepanov, V. A.; Stepanov, P. A.; Belyakov, V. A. J. Nucl.
Mater. 1998, 253, 175.
Pells, G. P.; Hodgson, E. R. J. Nucl. Mater. 1995, 226, 286.
Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1997, 250, 156.
Hodgson, E. R.; Moron˜o, A. J. Nucl. Mater. 2000,
283–287, 880.
Yasuda, K.; Tanaka, K.; Shimada, M.; Yamamoto, T.;
Matsumura, S.; Kinoshita, C. J. Nucl. Mater. 2004,
329–333, 1451.
Zong, X. F.; Shen, C. F.; Liu, S.; et al. Phys. Rev. B 1996,
54, 139.
Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1996,
233–237, 1299.
Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1998,
258–263, 1798.
Narui, M.; Shikama, T.; Endo, Y.; Sagawa, T.; Kayano, H.
J. Nucl. Mater. 1992, 191–194, 592.

Ooms, H.; Hodgson, E. R.; Decre´ton, M.; et al. Fusion
Eng. Des. 2007, 82, 2531.
Gonza´lez, S. M.; Moron˜o, A.; Hodgson, E. R. Fusion Eng.
Des. 2007, 82, 2567.
Narui, M.; Sagawa, T.; Endo, Y.; et al. J. Nucl. Mater.
1994, 212–215, 1645.
Narui, M.; Sagawa, T.; Shikama, T. J. Nucl. Mater. 1998,
258–263, 372.
Gusarov, A.; Vermeeren, L.; Brichard, B.; et al. Fusion
Eng. Des. 2005, 75–79, 819.
Pells, G. P.; Hill, G. J. J. Nucl. Mater. 1986, 141–143, 375.
Frost, H. M.; Clinard, F. W., Jr., J. Nucl. Mater. 1988,
155–157, 315.
Buckley, S. N.; Agnew, P. J. Nucl. Mater. 1988,
155–157, 361.
Heidinger, R.; Ko¨niger, F. J. Nucl. Mater. 1988,
155–157, 344.
Heidinger, R. J. Nucl. Mater. 1990, 173, 243.
Dienst, W.; Fett, T.; Heidinger, R.; Ro¨hrig, H. D.;
Schulz, B. J. Nucl. Mater. 1990, 174, 102.
Pells, G. P.; Heidinger, R.; Ibarra-Sanchez, A.; Ohno, H.;
Goulding, R. H. J. Nucl. Mater. 1992, 191–194, 535.
Molla´, J.; Heidinger, R.; Ibarra, A. J. Nucl. Mater. 1994,
212–215, 1029.
Vila, R.; Gonzalez, M.; Molla´, J.; Ibarra, A. J. Nucl. Mater.
1998, 253, 141.
Farnum, E. H.; Kennedy, J. C.; Clinard, F. W.; Frost, H. M.
J. Nucl. Mater. 1992, 191–194, 548.
Stoller, R. E.; Goulding, R. H.; Zinkle, S. J. J. Nucl. Mater.
1992, 191–194, 602.

Goulding, R. H.; Zinkle, S. J.; Rasmussen, D. A.;
Stoller, R. E. J. Appl. Phys. 1996, 79, 2920.

121.
122.
123.
124.
125.
126.
127.
128.
129.

130.
131.
132.

133.
134.
135.
136.
137.
138.
139.
140.
141.
142.
143.
144.
145.

146.
147.
148.
149.
150.
151.
152.

153.

154.
155.

723

Vila, R.; Hodgson, E. R. J. Nucl. Mater. 2000,
283–287, 903.
Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1997,
249, 128.
Molla´, J.; Ibarra, A.; Hodgson, E. R. J. Nucl. Mater. 1994,
212–215, 1113.
Molla´, J.; Ibarra, A.; Hodgson, E. R. J. Nucl. Mater. 1995,
219, 182.
Vila, R.; Hodgson, E. R. J. Nucl. Mater. 2011;
doi:10.1016/j.jnucmat.2011.01.080.
Heidinger, R.; Hofmann, A.; Nickel, H. U.; Norajitra, P.
Fusion Eng. Des. 1991, 18, 337.
Ibarra, A.; Heidinger, R.; Molla, J. J. Nucl. Mater. 1992,
191–194, 530.
Heidinger, R. J. Nucl. Mater. 1994, 212–215, 1101.

Heidinger, R. In Proceedings of 19th International
Conference on Infrared and Millimeter Waves, Sendai,
Japan, 1994.
Braz, O.; Kasugai, A.; Sakamoto, K.; et al. Int. J. Infrared
Millim. Waves 1997, 18, 1495.
Ibarra, A.; Gonzalez, M.; Vila, R.; Molla, J. Diamond Relat.
Mater. 1997, 6, 856.
Thumm, M.; Arnold, A.; Heidinger, R.; Rohde, M.;
Schwab, R.; Spoerl, R. Fusion Eng. Des. 2001,
53, 517.
Imai, T.; Kobayashi, N.; Temkin, R.; Thumm, M.; Tran, M. Q.;
Alikaev, V. Fusion Eng. Des. 2001, 55, 281.
Heidinger, R.; Rohde, M.; Spo¨rl, R. Fusion Eng. Des.
2001, 56–57, 471.
Molla´, J.; Ibarra, A.; Heidinger, R.; Hodgson, E. R. J. Nucl.
Mater. 1995, 218, 108.
Vila, R.; Ibarra, A.; Hodgson, E. R. J. Nucl. Mater. 1996,
233–237, 1340.
Molla, J.; Vila, R.; Heidinger, R.; Ibarra, A. J. Nucl. Mater.
1998, 258–263, 1884.
Heidinger, R.; Ibarra, A.; Molla, J. J. Nucl. Mater. 1998,
258–263, 1822.
Brichard, B.; Fernandez Fernandez, A.; Ooms, H.; et al.
J. Nucl. Mater. 2004, 329–333, 1456.
Donne, A. J. H.; et al. Nucl. Fusion 2007, 47, S337.
Vila, R.; Molla´, J.; Heidinger, R.; Moron˜o, A.; Hodgson, E. R.
J. Nucl. Mater. 2002, 307–311, 1273.
Schulz, B. J. Nucl. Mater. 1988, 155–157, 348.
Dienst, W. J. Nucl. Mater. 1990, 174, 102.
Rohde, M.; Schulz, B. J. Nucl. Mater. 1990, 173, 289.

Dienst, W.; Fett, T.; Heidinger, R.; Ro¨hrig, H. D.;
Schulz, B. J. Nucl. Mater. 1990, 174, 102.
Dienst, W. J. Nucl. Mater. 1992, 191–194, 555.
Burghartz, St.; Schulz, B. J. Nucl. Mater. 1994, 212–215,
1065.
Dienst, W. J. Nucl. Mater. 1994, 211, 186.
Hazelton, C.; Rice, J.; Snead, L. L.; Zinkle, S. J. J. Nucl.
Mater. 1998, 253, 190.
Snead, L. L.; Zinkle, S. J.; White, D. P. J. Nucl. Mater.
2005, 340, 187.
Snead, L. L.; Yamada, R.; Noda, K.; et al. J. Nucl. Mater.
2000, 283–287, 545.
Schulz, B.; Haase, G. T246 Final Report Ceramics for
Heating and Current Drive, and Diagnostic Systems;
Hodgson, E. R., Ed.; EUR-CIEMAT 92; European Fusion
Technology Programme, 1998.
Heidinger, R.; Danilov, I.; Meier, A.; Rohde, M. Final
Report D13: Irradiation Effects in Ceramics for Heating
and Current Drive, and Diagnostic Systems; EFDA Task
TW4-TPDC-IRRCER; European Fusion Technology
Programme, 2006.
Costley, A. E. Rev. Sci. Instrum. 1995, 66, 296.
Ramsey, A. T. Rev. Sci. Instrum. 1995, 66, 871.


724

Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors

156. Arnold, G. W.; Compton, W. D. Phys. Rev. Lett. 1960, 4, 66.

157. Lee, K. H.; Crawford, J. H. Phys. Rev. 1977, B15, 4065.
158. Evans, B. D.; Stapelbroek, M. Solid State Commun.
1980, 33, 765.
159. Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1995, 224, 216.
160. Cooke, D. W.; Bennett, B. L.; Farnum, E. H.; Thomas, D. E.;
Portis, A. M. J. Nucl. Mater. 1998, 255, 180.
161. Sato, F.; Oyama, Y.; Iida, T.; Maekawa, F.; Ikeda, Y.
J. Nucl. Mater. 1998, 258–263, 1897.
162. Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 1998,
258–263, 1889.
163. Gorshkov, A.; Orlinski, D.; Sannikov, V.; et al. J. Nucl.
Mater. 1999, 273, 271.
164. Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater. 2007,
367–370, 1048.
165. Orlinski, D. V.; Vukolov, K. Y. Plasma Device Oper. 1999,
17, 195.
166. Garcia-Matos, M.; Moron˜o, A.; Hodgson, E. R. J. Nucl.
Mater. 2000, 283–287, 890.
167. Sugie, T.; Nishitani, T.; Kasai, S.; Kaneko, J.;
Yamamoto, S. J. Nucl. Mater. 2002, 307–311, 1264.
168. Vukolov, K. Yu.; Levin, B. A. Fusion Eng. Des. 2003,
66–68, 861.
169. Vukolov, K. Yu. Fusion Eng. Des. 2009, 84, 1961.
170. Leo´n, M.; Martı´n, P.; Vila, R.; Molla, J.; Ibarra, A. Fusion
Eng. Des. 2009, 84, 1174.
171. Griscom, D. L. J. Non-Cryst. Solids 1985, 73, 51.
172. Friebele, E. J.; Griscom, D. L.; Marrone, M. J.
J. Non-Cryst. Solids 1985, 71, 133.
173. Griscom, D. L.; Ceram, J. Soc. Jpn 1991, 99, 923.
174. Orlinski, D. V.; Altovsky, I. V.; Bazilevskaya, T. A.; et al.

J. Nucl. Mater. 1994, 212–215, 1059.
175. Gritayna, V. T.; Bazilevskaya, T. A.; Voitsenya, V. S.;
Orlinski, D. V.; Tarabrin, Yu. A. J. Nucl. Mater. 1996,
233–237, 1310.
176. Martin, P.; Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater.
2000, 283–287, 894.
177. Martin, P.; Moron˜o, A.; Hodgson, E. R. J. Nucl. Mater.
2004, 329–333, 1442.

178.
179.
180.
181.

182.
183.
184.
185.
186.

187.
188.
189.
190.
191.
192.
193.
194.

Gorbunov, A. V.; Klassen, N. V.; Orlinski, D. V.;

Vukolov, K. Yu. Fusion Eng. Des. 2005, 74, 815.
Johnson, C. D. CERN/PS/90–42 (AR), 1990.
MacCarthy, K. J.; Baciero, A.; Zurro, B.; et al. J. Appl.
Phys. 2002, 92, 6541.
Toh, K.; Shikama, T.; Nagata, S.; Tsuchiya, B.;
Yamauchi, M.; Nishitani, T. J. Nucl. Mater. 2007,
367–370, 1128.
Toh, K.; Shikama, T.; Katsui, H.; et al. J. Nucl. Mater.
2009, 386–388, 1027.
Moron˜o, A.; Martı´n, P.; Gusarov, A.; Hodgson, E. R.
J. Nucl. Mater. 2009, 386, 1030.
Nagata, S.; Katsui, H.; Tsuchiya, B.; et al. J. Nucl. Mater.
2009, 386, 1045.
Cooke, D. W.; Bennett, B. L.; Farnum, E. H. J. Nucl.
Mater. 1996, 232, 214.
Deparis, O.; Me´gret, P.; Decre´ton, M.; Blondel, M.;
Golant, K. M.; Tomashuk, A. L. In Diagnostics for
Experimental Thermonuclear Fusion Reactors; Stott,
P. E., Gorini, G., Prandoni, P., Sindoni, E., Eds.; Plenum
Press: New York, 1998; Vol. 2, p 291.
Kakuta, T.; Sakasai, K.; Shikama, T.; Narui, M.;
Sagawa, T. J. Nucl. Mater. 1998, 258–263, 1893.
Kakuta, T.; Shikama, T.; Nishitani, T.; et al. J. Nucl. Mater.
2002, 307–311, 1277.
Toh, K.; Shikama, T.; Nagata, S.; et al. J. Nucl. Mater.
2004, 329–333, 1495.
Sporea, D.; Sporea, A.; Constantinescu, B. Fusion Eng.
Des. 2005, 74, 763.
Nishitani, T.; Shikama, T.; Reichle, R.; et al. Fusion Eng.
Des. 2002, 63–64, 437.

Shikama, T.; Nishitani, T.; Kakuta, T.; et al. Nucl. Fusion
2003, 43, 517.
Orlovskiy, I. I.; Vukolov, K. Yu. Fusion. Eng. Des. 2005,
74, 865.
Pells, G. P.; Boothby, R. M. J. Nucl. Mater. 1998,
256, 25.