8 research outputs found
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Diffusion-controlled processes in microwave-fired oxide ceramics
Processing oxide-based ceramics using microwave heating leads to a number of unexpected results, which can only be interpreted in terms of enhanced diffusion. Enhanced sintering has been observed in alumina and zirconia. Accelerated grain growth in dense, hot-pressed alumina has been demonstrated. Increased diffusion coefficients have been observed for diffusion of oxygen in sapphire. As yet, a satisfactory theory to account for these phenomena has not been developed. This paper reviews the experimental work conducted at the Oak Ridge National Laboratory during the past four years on the processing of oxides in both 2.45 and 28 GHz microwave furnaces. 18 refs., 10 figs
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RFTF ECH microwave system
A Radio-Frequency Test Facility (RFTF) has recently been constructed at Oak Ridge National Laboratory (ORNL) for development and testing of Ion Cyclotron Heating (ICH) antennas under realistic fusion reactor plasma edge conditions. High-power ICH antennas must be immersed in the plasma for proper coupling of rf power and therefore are subject to particle bombardment and heat flux. In RFTF, plasma is generated and heated by electron cyclotron resonance heating (ECRH) with 28-GHz microwave power from a gyrotron tube. The plasma is confined in a simple magnetic mirror formed by two superconducting coils surrounding a box-shaped vacuum vessel. Using 50 kW of microwave power, a plasma with density of 5 x 10/sup 11/ cm/sup 3/ and temperature of 8 eV is obtained, a fairly good fusion research edge plasma. This presentation covers the microwave generation and transmission system plus some of the electron cyclotron heated (ECH) results on RFTF
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Pure mode swept frequency TE/sub 02/ measurements in 6. 35 cm circular waveguide
A mode transducer was developed which generates high purity (>99%) TE/sub 02/ mode in small diameter (1.64 cm) circular waveguide. The device works from 52 to 62 GHz which allows relatively wideband, accurate swept frequency measurements of insertion and return loss in this mode. TE/sub 01/ to TE/sub 02/ insertion loss for the device is approximately 0.2 dB and the Te/sub 01/ mode rejection from the TE/sub 02/ port is 15 dB. A 2-degree linear taper is used to connect to 6.35 cm diameter waveguide. Mode purity of the TE/sub 02/ mode after this taper is >98%. Some typical measurements of waveguide components as well as a description of the device will be presented
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Studies on the scale-up of the microwave-assisted nitridation and sintering of reaction-bonded silicon nitride
Studies using laboratory test samples have shown that microwave heating produces sintered reaction-bonded silicon nitride materials with improved properties. The final challenge for processing this material by microwave heating is the development of a technology for processing larger batch-size quantities of these materials. Initial microwave scale-up experiments were performed using powder compacts of a bucket tappet geometry. In experiments using microwave-transparent boron nitride sample crucibles, temperature gradients within some crucibles led to larger variations in the sample densities than were obtained with the conventionally processed samples. The use of a microwave-suscepter type crucible made of silicon carbide and boron nitride resulted in an improved temperature uniformity and in density variations comparable to those obtained for the control groups
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Initial results of a high-power microwave sintering experiment at ORNL
Experiments have recently begun at Oak Ridge National Laboratory to develop microwave sintering techniques suitable for large ceramic parts. Microwave sintering offers the advantages of faster heating rates, more uniform heating, and greater energy efficiency than conventional sintering techniques. We are using 28-GHz, 200-kW cw gyrotrons as the heating source. An untuned cavity is used as the applicator to eliminate geometry sensitivity in coupling efficiency
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Microwave processing of silicon carbide. CRADA final report
A Cooperative Research and Development Agreement (CRADA) between Lockheed Martin Energy Systems, Inc. (LMES) and Dow Chemical Company was initiated on May 3, 1993. (Lockheed Martin Energy Research Inc. (LMER) replaced LMES). The completion date for the Agreement was December 1996. The purpose of this project is to develop microwave processing techniques to produce superior silicon carbide. Sintered silicon carbide is an attractive material for use in high-stress, high-temperature, high-wear, or highly corrosive applications. However, use in these applications has been hampered by a lack of consistency in strength, density, and other physical properties. It is proposed that the enhanced sintering that has been achieved using microwaves in oxide and halide systems be applied to the sintering of these materials to produce a more highly controlled density and microstructure. This will, in turn, increase the strength and Weibull modulus of the sintered body. The use of microwave energy to anneal for a moderate temperature (1,400--1,600 C) anneal in a high vacuum (< 10{sup {minus}4} Torr) results in an improvement in the sintered density and density distribution. These changes in turn result in improved properties of the sintered compacts. Further, scale up of the process has resulted in the routine production of 3 kg components in excess of 4 cm in thickness
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A mode-transforming polarization-rotatable launcher for the ATF (Advanced Toroidal Facility) fusion experiment
The Advanced Toroidal Facility (ATF) fusion energy experiment at the Oak Ridge National Laboratory (ORNL) requires high-power microwaves for startup and plasma heating. Power from a gyrotron oscillator tube at 53.2 GHz will be used to ionize and heat the plasma by the electron cyclotron heating (ECH) process. The confining magnetic field of the device is either 0.95 or 1.9T. The gyrotron tube generates 200 kW in the TE/sub 02/ mode, which is transported in an overmoded 6.35-cm-diam circular waveguide to the ATF vacuum vessel. The launcher consists of a mode-converting Vlasov section, which converts the nonpolarized TE/sub 02/ wave into a linearly polarized narrow beam. The beam reflects off a tiled spherical reflector grating and is focused at the center of the plasma. The polarization can be rotated to optimize the absorption efficiency by rotating the grating in the spherical reflector. Overall system efficiency is kept high by making the twist reflector large enough to catch the Vlasov converter sidelobe power, which is partially due to mode conversion in the waveguide system. The launcher design and laboratory measurements are discussed. 3 refs., 3 figs
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Gyrotron: an ECH system component
The gyrotron, or electron-cyclotron maser, in the form of a gyromonotron, is being developed as a source of millimeter wave energy for fusion plasma heating. The characteristics of this high power, high efficiency electron tube are described in terms of the requirements for the beam power supply system, the mechanical support system, the cooling system, the focusing and tuning magnets, and the waveguide system. Requirements of power level and transmission efficiency dictate the use of oversize waveguide. The implications, both to the user and to the interaction mechanisms in the gyrotron, of the use of oversize waveguide are treated. The effects of variations of various operating parameters upon the gyrotron's power output and stability are also discussed. Data from gyrotron development and system operation are used where appropriate