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 proton- irradiated materials.109-114 In general, changes in permittivity were observed to be small (<5%) and considered to be acceptable for fusion applications. However, results for dielectric loss (loss tangent mea­surements) showed orders of magnitude variation for similar materials («10~5-10~2 for different forms of alumina at 100 MHz) even before irradiation. To address this problem, a standard material (MACOR) was distributed and measured by the main laboratories involved (EU, JA, US) to check the dif­ferent measuring systems used. However, the results showed good agreement,115 and the large variation in reported loss tangent values was later shown to be real, in part because of the effect of the different impurity contents of the materials.116,117 This may be clearly seen in Figure 8, where loss tangent data for different aluminas over a wide frequency range are given, showing marked absorption band struc­tures due to polarizable defects (impurities).116

During the early postirradiation loss tangent measurements, there was an indication of recovery, suggesting that loss during irradiation could be signif­icantly higher.65,109-111 This implied that the already difficult measurements should be made in situ during irradiation. In a simple way, dielectric loss can be considered as being due to two contributions:

Loss a (DC conductivity)/Frequency + 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 ofrecovery 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 >104Gys-1 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

Time (s)

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 s~ . Furthermore, in these pulsed experiments, the dpa per

pulse («10-7 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 F+-centers,1 2 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 2kGys-1, 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, 10-6dpa at 450 °C with a DC electric field (210 kVm-1) to pro­duce RIED showed a significant increase in the loss (2 x increase), and also in the prompt dielectric loss («5x increase). Similar increases have only been observed for sapphire neutron irradiated, with­out an electric field applied, to >10-3dpa.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 (Dera — nox 999) to 110 MGy with a 1 MHz RF voltage (0.8 MV m-1) 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 >10-4 (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, <1020nm-2.12 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 <10- , 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,

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 x 10~5 at 145 GHz, comparable with sapphire at 77 K, and furthermore increasing only to about 5 x 10~5 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.13 ,

As may be seen in Figure 10, PIE loss tangent mea­surements of neutron-irradiated ‘window grade’ CVD diamond indicate that even by 1022 n m~2 (10~3 dpa), the room temperature loss only increases to 5 x 10- at 145 GHz (6 x 10-5 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 10~4dpa. The results indicate that for low radiation doses the electrical and dielectric properties are only slightly degraded, and in particu­lar the use of KU 1 for electron cyclotron emission (ECE) windows and low-loss DC applications is feasible.134,141