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 Gys-1 to >100 Gy s~ , 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 10~16Sm~ which increases to approximately 10~ 0Sm-1 for an ionizing dose rate of only 1 Gys~161 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 = s0 + KRR 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

14MeV 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 Gys-1 to about 104Gys-1 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 a/.,66

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:

RIC (single crystal) > RIC (polycrystal) and

RIC (pure) > RIC (impure)

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 <10~6Sm_1 for ionizing dose rates of up to >103Gys~ . 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 m j 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

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 contamina — tion.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 ofthe 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 > KR. Furthermore, some experiments were performed at temperatures either near room tempera — ture85 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

enhancement of the well-characterized F+-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 F+-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 degrada- tion.96 The combined work led to a RIED model being formulated, which successfully explained the role of the electric field (both DC and AC/RF), the ionization, and the anion (oxygen) vacancies.98 The model predicted a threshold electric field for degradation depending on the impurity/defect 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 m-1.75,87 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.1

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 ofthe 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, 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

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.