Precision Measurement of Low Loss Window Materials
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G2.8.1
Precision Measurement of Low Loss Window Materials
ABSTRACT A precision measurement technique for measuring complex dielectric permittivity in the 118 – 178 GHz frequency range is described. The combination of a high-quality Fabry-Perot resonator, excited by a free-running BWO (backward wave oscillator), combined with the processing capability of a sensitive receiver based on the Tektronix 2782 spectrum analyzer, using the WM782 (F-G) harmonic mixer, yields a very sensitive measurement technique. The development of techniques for measuring extremely low losses in dielectric materials has been driven primarily by the requirements of the fusion energy research program. The heating of a plasma to the temperatures required for fusion is expected to require the delivery of megawatts of RF power through vacuum windows into the plasma vessel. A material with very low loss as well as excellent thermal and mechanical properties must be utilized to avoid a catastrophic window failure. The relevant loss mechanisms and measured results for several candidate materials are discussed I. INTRODUCTION Plasma fusion experiments require external-heating systems based on the absorption of electromagnetic waves by the plasma particles. Electron Cyclotron Waves (ECW) systems operating in the millimeter wave (mm-wave) spectral region are often utilized in fusion devices to provide a well-localized energy deposition for building-up and controlling burning plasma. Gyrotron tubes most commonly generate the mm-wave power for this purpose. The growing use of such systems in plasma experiments requires gyrotron tubes capable of producing CW power of 1 MW or more in the frequency range of 110-200 GHz [1]. In these tubes, the radio frequency window forms a critical component, as it must not only provide a vacuum barrier but also a tritium barrier between the plasma chamber and its surroundings. The requirement of high-power, cw operation places extreme demands on the materials properties of the dielectric window material. If the critical balance between the heat generation rates (due to the dielectric absorption in the window materials) and the heat extraction rates (based on thermal conduction in the window materials) is not mastered, the window component may fail catastrophically by thermal run-away or thermal crack formation [2]. As regards cooling liquid, only fusion-reactor compatible cooling liquid can be used. Special attention must be given to the plasma containment window as radiation damage, possibly induced by the presence of neutrons and γ-rays, may degrade the window performance. The important parameters for high-power window development are the dielectric properties of the material, i.e., the loss tangent factor (tanδ), and the permittivity, (ε ′), as well as the thermal and mechanical properties. Accurate knowledge of the dielectric parameters is particularly crucial for basic window design. The permittivity must be known to design for minimum power reflection by the window, either by proper choice
G2.8.2
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