R. Heidinger

Investigations on ceramic materials for fusion technology

Abstract Thermophysical and mechanical properties were measured on SiC as an irradiation-resistant First Wall (FW) protection material and on various low loss microwave window materials. An evaluation shows that SiC may not to be excluded from use for FW protection, but only moderate plasma disruption impacts could be tolerated. For the HF window development several grades of alumina, A1N, and spinel were extensively tested for their thermal and mechanical properties and with respect to HF absorption at ≈ 145 GHz. Alumina and particularly sapphire are superior in dielectric properties, but if thermal stresses are taken into account, the best grades of A1N are preferable. This could be demonstrated by parametric lifetime calculations for windows of a given design. The tolerable HF power densities are, however, still unsatisfactory. Irradiations deteriorate both thermal conductivity and dielectric loss.

Alumina ceramics for heating systems

Abstract Data on the dielectric properties of several high purity (> 99.7%) commercial alumina grades and sapphire are presented in wide frequency (1 kHz-145 GHz) and temperature (100 K–500 K) ranges. Loss tangent values show differences over more than three orders of magnitude. For fusion applications and in the frequency range of ICRH and LH, the loss tangent effect is consistent with relaxation processes and it is found that values as low as 10 −5 are available. On the other side, in the frequency range of ECRH the loss tangent can be dominated either by relaxation processes or by the interaction with intrinsic lattice vibrations.

Design parameters of ceramic insulator materials for fusion reactors

Abstract The parameters relevant to the use of ceramic insulator materials in resistive components for direct currents (dc) and in transmission components for electromagnetic (em) waves are discussed. In resistive components, radiation-induced conductivity and radiation-enhanced electrothermal breakdown are of major concern. The limits for em power transmission are considered for the different plasma heating concepts. Special attention is given to the dielectric loss tangent. In addition, the mechanical and thermophysical properties relevant to thermal crack formation are reviewed. For the particularly critical gyrotron window, the technical analysis is exemplified and the motivation of cryogenic cooling is presented.

Dielectric measurements on sapphire for electron cyclotron wave systems

Three different grades of sapphire have been investigated in the unirradiated state at 25–300 K and 145 GHz. The initial T2 dependence of dielectric loss was found to become steeper below 150 K, reaching exponents around 4 at 70 K. In the HEMEX grade, the loss starts to level off to constant values below about 100 K. A specimen subjected to the RIED effect under electron irradiation showed a similar behaviour but reaching higher constant loss levels. Fast neutron irradiations were performed in the fluence ranges of 1020–300 K were observed in HEMEX grades irradiated up significant property changes in the dielectric properties at 70–300 K were observed in HEMEX grades irradiated up to 1021 n/m2. The use of the present data base for the assessment of tolerable fluence levels for a cryogenically-cooled torus window in electron cyclotron wave systems is discussed.

Mechanical strength of neutron-irradiated window materials

Highly specialised window materials considered for transmission lines in plasma heating and diagnostic systems in nuclear fusion reactors were studied in terms of mechanical strength properties and potential radiation effects introduced by neutron irradiation up to 10 21 n/m 2 (E > 0:1 MeV). Small disks of CVD diamond cut from model windows for high power transmission and from rods of a special fused silica grade (KU1) with radiation-hard optical properties were tested together with disks of commercially available high quality silica grades. Based on a biaxial mechanical test method, the influence of specimen machining could be kept under special control. The results obtained for CVD diamond clearly indicate that median strength values of 400 MPa and high Weibull moduli of 20 can be maintained with structural damage introduced at 10 � 4 dpa. For high quality silica grades, median strength levels of 300 MPa were reached in the test geometries applied. However, they tend to be very sensitive to changes in the surface quality. Median values of about 120 MPa and Weibull moduli of 10 can be taken for conservative strength evaluations for spontaneous failure.

Electron-irradiated silicon: An optimized material for gyrotron windows

Abstract The special potential of high resistivity (HR) silicon for electron cyclotron wave transmission components for nuclear fusion is related to the dominant contribution of free charge carriers to the mm-wave absorption. It is demonstrated by dielectric property measurements that electron irradiation can practically overcome the problems of special concern for applications, such as the sensitivity of dielectric loss (tan δ) to ionizing radiation and the complex temperature dependence of tan δ. Also the loss levels are lowered at room temperature. The improvements are correlated with the total dose of electron irradiations and are of long term stability. It is inferred that new structural defects are formed, which act on the charge carriers as centers of trapping, recombination or scattering. As a consequence, tan δ values are measured at 145 GHz that fall below those of sapphire in the range of 120–350 K.

Radiation effects on dielectric losses of Au-doped silicon

Effects of electron and neutron irradiation on dielectric properties of Au-doped silicon are examined as a function of the frequency between 1 kHz and 150 GHz. The studies compare the Au-doped Si with a high resisitivity (HR) pure Si in the as-received state and after electron irradiation. The obtained data for both materials show that electron irradiation and neutron irradiation do not cause degradation of the dielectric loss behaviour, but even improve it. This beneficial effect already observed earlier in pure silicon is also observed in Au-doped silicon. Loss data obtained in-beam under electron irradiation are also reported.

Dielectric properties of alumina/zirconia composites at millimeter wavelengths

Alumina‐zirconia composites with ZrO2 contents up to 20 wt % and negligible porosity were investigated at millimeter (mm) wavelengths to determine the changes appearing in the dielectric properties of pure alumina ceramics when unstabilized or partially stabilized ZrO2 is added to improve the mechanical strength. It is demonstrated that it is essential to distinguish between the contributions of the monoclinic and the tetragonal phase of zirconia (m‐ZrO2, t‐ZrO2). Permittivity is raised with increasing content of either phase; the effective permittivity can be assessed by the rule of mixtures (Maxwell–Garnett formulation of the generalized Clausius–Mossotti relation) using permittivity values of 10 for Al2O3, 14–21 for m‐ZrO2, and 40–45 for t‐ZrO2. The permittivity data show only a small variation in the investigated range of 9–145 GHz. For the dielectric loss, there is evidence of a predominant contribution of m‐ZrO2; in addition, the marked increase in loss with frequency becomes sharper. The t‐ZrO2, which is responsible for strengthening, does not show any significant influence on losses. It is therefore concluded that ZrO2 strengthening of alumina is feasible without affecting mm‐wave losses at room temperature as long as the presence of m‐ZrO2 is avoided.

New potentials for high mechanical strength grades of polycrystalline alumina for EC waves windows

A fine grained high purity alumina grade and a composite of alumina and 5% zircona have been characterized as possible candidates to be used in rf windows. Both dielectric and mechanical properties are measured. The room temperature permittivity is around 9.84 and 10.28, respectively without significant frequency dependence. Its temperature dependence shows a T 2 law for both types of materials. The room temperature loss tangent goes from 2 to 15×10 −5 and from 6 to 39×10 −5 respectively in the frequency range between 17 to 145 GHz. The temperature dependence shows a strong decrease at low temperatures for the fine grained alumina and almost no temperature dependence for the alumina/zircona composite. The obtained bending strength data are 370 ± 50 and 584 ± 66 MPa, respectively. These results indicate that the material grades are attractive for use in cryogenically cooled and in backup rf window designs.

Pre- and post-irradiation studies on mm-wave losses in reference window materials for electron cyclotron wave systems

Dielectric materials for transmission windows in electron cyclotron (EC) wave systems are studied for the radiation effects on their dielectric properties (permittivity, loss tangent). Based on comparative neutron irradiation experiments performed at cryogenic and reactor pool temperatures, it is shown here that fast neutron fluences up to at least 2-4 x 10 20 n/m 2 (E > 0.1 MeV) do not critically affect the mm-wave losses in HEMEX grade sapphire, the reference material for the cryogenically cooled high power EC window. Fast neutron (10 20 n/m 2 ) and electron (5 x 10 6 dpa) irradiations on specially developed CVD diamond grades for conventionally cooled windows demonstrate acceptable mm-wave loss levels, whereas loss at lower frequencies can be significantly affected. In-beam measurements under X-ray irradiation show no additional loss terms in sapphire and CVD diamond, at least up to 0.45 Gy/s. Dielectric data obtained for synthetic quartz indicate that the observed changes due to neutron damage are not severe for fast neutron fluences at least up to 10 22 n/m 2 .