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 m-1 K~ , and that of BeO about 280Wm~1K~1. These values are sufficiently high
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 radiation- induced defects become mobile and can either recombine or aggregate should lead to a lower degra­dation of the thermal conductivity, while low — temperature 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 m — (0.01 dpa), whereas at 600 K this lower saturation level was only reached following a fluence of above 1024nm-2. 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 <1022nm — (0.001 dpa) at the startup of irradiation, followed by

151

saturation.

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 (200Wm-1K-1 at 100 K, c. f. about 30 W m-1 K-1 at room temperature). However, in addition to the dielectric loss showing a very low neutron tolerance (<1020nm- ) at this low temper­ature,128 the high thermal conductivity was reduced by over two orders of magnitude also by 1020nm — (10-5dpa), 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 1022nm-2 (10-3 dpa).134 Unfortunately, although the extremely high thermal conductivity at room temperature («1800 W m-1 K-1) already began to degrade by 1020nm-2 (10-5dpa), it was at the tolerance limit by 1021 nm-2 (Figure 12).134 Almost identical results were reported after electron irradiation to 3 x 10-6dpa where the thermal con­ductivity was reduced by about 9%, confirming the importance of point defects.153