Measurements of the Dielectric Properties of Irradiated Low-Loss by Microwave Reflectometry and Micro-Calorimetry
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MEASUREMENTS OF THE DIELECTRIC PROPERTIES OF IRRADIATED LOW-LOSS BY MICROWAVE REFLECTOMETRY AND MICRO-CALORIMETRY S.N. BUCKLEY AND P. AGNEW AEA Technology, Harwell, United Kingdom ABSTRACT An rf reflectometer has been used to measure dielectric losses in alumina ceramics during irradiation by5 3 MeV protons. The equipment was able to detect tan 6 changes of 5x10" , but calibration by micro-calorimetry revealed large systematic losses, up to 2.5x10-'. Irradiation at 300 K resulted in a progressive increase in dielectric loss but no prompt increments. INTRODUCTION Radiofrequency heating is an important technique for raising and modeling the temperature profiles in nuclear fusion plasmas. In one system, Ion Cyclotron Resonance Heating which operates at 107-10 Hz, the energy is transmitted into the torus down coaxial lines which have to be sealed by windows to prevent the magnetrons becoming contaminated by tritium. The ideal electrical location of windows is close to the plasma chamber, but it is feared that nuclear radiation might then enhance dielectric losses sufficiently to trigger runaway dielectric heating and a likely heal of window fracture. The problem for design engineers and scientists is that in-reactor electrical experiments are not feasible because reactor environments are inimical to the apparatus required to measure dielectric properties. Post-irradiation measurements on the other hand could be inappropriate since the radiation effects might be transitory, e.g. due to temporarily excited point defects in the crystal lattice. Ion irradiation obviates the pernicious background radiation present in reactors, but specimens have to be thinner than the ion range in the solid, some 50 .unfor 3 MeV protons; higher energies would induce nuclear ractions. Conventional Q meters cannot be used for such small specimens and the only viable alternative at these frequencies is the less accurate technique, measurement of the reflectivity of electromagnetic waves by air/dielectric/metal interface. High frequency dielectric properties are always difficult to determine with precision even in the absence of irradiation. In theory one just has to assess how the dielectric perturbes or modifies an electric field. Complications however arise because at high frequencies electric fields are additionally perturbed by measurement probes and are hard to measure or calculate, electrical energy leaks out of the experimental system, and energy is dissipated via ohmic losses in conductors as well as through dielectric losses in specimens. Thus, although the experimentalist's lot has recently been eased by the availability of high stability rf sources and powerful desk top computers, systematic errors remain. Absolute numerical magnitudes of permittivity and loss cannot be trusted unless they have been validated by some independent procedure. The present paper describes a reflectometer system used to measure the complex permittivity of low-loss ceramic materials while they were actually being irradiated, and a micro-calorimeter develop
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