Dr. Stephen C. Bates，Thoughtventions Unlimited LLC
40 Nutmeg Lane Glastonbury, CT 06033
The Problems.Windows that transmit high power microwave (μ W) and radio frequency (RF) energy are a necessary enabling technology for plasma heating for magnetic confinement nuclear fusion devices. Window technology is marginal for microwave tubes currently being developed, whereas the level of generated microwave power that is technically feasible is continually increasing as is the frequency being used. When the transmitted power exceeds the capability of the windows (higher frequencies also usually increase losses), multiple feeds or complex windows must be used, which is very expensive for both the microwave transmission system and for the fusion vessel. Microwave transmission windows must be as thin as possible to minimize absorbed power, yet they must withstand pressure against vacuum. Other limitations on window materials are compatibility with high vacuum systems, resistance to radiation damage, and tolerance of thermal stress. The high strength and low dielectric constants of ceramics are attractive for this application, but the statistical failure of ceramics severely limits their design use. At the beginning of this program major advances in materials and/or design engineering were needed to provide large-aperture, low-loss windows for microwave transmission.
The Innovations. Specially processed and mounted sapphire windows has been shown to provide a large improvement in microwave/RF power transmission capabilities compared with current technology. By processing the sapphire surface for strengthening, using stress-minimizing designs, and minimizing thermal stresses, sapphire can be a mechanically equivalent replacement for high strength steel. In the past, sapphire has been the best material for a microwave window because of its high strength, low absorbed power and good tolerance of radiation damage. Properly mounted, strengthened sapphire windows have been experimentally demonstrated by Thoughtventions Unlimited (TvU) to have a design strength that is more than 10 times larger than the current window specifications using sapphire. Further improvements that have been investigated include a surface cooled grid window concept that has been shown to have very promising power level capabilities while simultaneously being highly reliable and economical to manufacture. A quasi-optical resonant ring device has been developed at Oak Ridge National Laboratory that permits high power testing of the windows using low power, less expensive microwave drivers. This device allows testing of windows at power levels of 1.5 MW or greater using 200 kW gyrotron power sources, with gains of 25 or greater at higher frequency.
Ultrathin sapphire microwave windows have been developed and tested in this Phase II program. Very high fracture strength (failure at very high pressures: 5-6 atm) has been experimentally demonstrated for very thin (0.5 mm) but large (100 mm diameter) windows. Polish strengthening of these disks provides part of the large performance improvement relative to the current state of the art, and stress minimization resulting from special mounting procedures provides the rest.
Polish strengthening of windows was demonstrated, and polish strengthened disks were used to fabrication a window fixture. Polish strengthened disks were purchased commercially, and polishing and inspection techniques were developed at TvU. Although reliable polish strengthening had not been obtained at the writing of this report tests indicate that continuing Phase 3 work will achieve this result.
Modeling was developed to predict the stress, strain, and deflection behavior of very thin sapphire disks. The modeling was successfully used to predict disk strain and deflection behavior. The modeling was then used to explain the widely scattered failure pressure data for extensive window breaking tests. Strengthening, mounting, and membrane effects on disk failure in response to pressure loading were explained, allowing an ultimate high power microwave window to be designed - 100 mm in diameter and 0.1 mm thick.
A resonant ring microwave power amplification device was constructed and successfully tested at low power at ORNL. This device will allow high power testing of window assemblies using only modest input power. High power testing will be performed through continued testing. A water-cooled grid window was developed, fabricated and demonstrated to be microwave transparent. The grid window was also experimentally demonstrated to tolerate 1 kW of deposited power at TvU, making it an appropriate candidate as a microwave window.
A microwave double window fixture was fabricated to demonstrate thin window power transmission capabilities and perform basic testing of the windows. The window design and the window fixture were evaluated to definitively establish the feasibility of the sapphire strengthening and mounting techniques. The fixture is ready for high power testing when it becomes available at ORNL.
The accomplishments of Phase II work are summarized as follows:
Demonstrated more than an order of magnitude increase in window design strength compared with standard sapphire window technology.
Developed polishing procedures for strengthening sapphire.
Pressure tested sapphire windows to failure to validate modeling and demonstrate strengthening.
Demonstrated sapphire disk membrane stress behavior and consequent maximum stress reduction.
Demonstrated stress reduction in sapphire windows by modifying the support structure of the window.
Developed modeling to predict stress, strain and deflection for all windows tested.
Failure tested numerous windows in different mounting configurations.
Designed, fabricated, and tested a high power microwave window fixture.
Demonstrated microwave window fixture fabrication techniques.
Developed a microwave resonant ring to test windows under high microwave power loads using a moderate-power source.
Tested strengthened sapphire window fixtures for system function and microwave response.
Developed alternate concepts for novel microwave windows, concentrating on a grid-cooled single window design.
Developed electromagnetic, thermal and mechanical models for the grid-cooled window.
Fabricated and tested the grid cooled window concept experimentally for cooling and microwave response.