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 s~ ) 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 Gys~ ), the materials will also be subjected to atomic displacements >10~ dpas-1. 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 10~4dpa, 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

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 high — power 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).1 0-1 2 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

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
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 10MGy and 1022nm~ , 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 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.