The diameter uptaper from 38.3 mm to 63.5 mm was then fabricated to match the diameter of the resonant ring and windows to be tested. A square root profile was designed which provides the greatest mode purity for the shortest length. This section was drawn and fabricated. CW cooling capability was added for the high power tests.
Other gyrotron facility setup tasks completed include the fabrication of a frame to support the TE01-TM11 and TM11 to HE11 converters and a second frame to support the resonant ring and test window. High vacuum was established in the waveguide and the resonant frequency of the ring was tuned to match gyrotron frequency. The ring resonant frequency must also be controlled to overcome thermal drift, and optical arc detectors must be installed. A cooled miter bend coupler has been shown to be practical for cw operation; this coupler was also fabricated.
The goal of creating a resonant ring apparatus that can test microwave components at high power using available lower power drivers was also achieved.The resonant ring concept feasibility was demonstrated through both modeling and experiments, indicating that large power gains are practical and can be used for high power component testing with currently available microwave sources. Furthermore, the ring is expected to perform even better at the higher frequencies of future microwave systems. High power operation of the ring was close at this writing, delayed by competing programs at ORNL.
Window fixture tasks. Work was done to modify the existing double disk window fixture is nearing completion. The two waveguide sections have been corrugated and machined to accept a short section that has a weld lip to be welded to the sapphire disk braze assemblies. The weld sections will be brazed into the waveguide halves when a final gap dimension can be determined. The window reflection vs. Double-disk gap has been modeled using a wave impedance code. Due to the window deflection that occurs with vacuum loading and from FC75 coolant pressure, it is expected that final tuning of the match frequency will be required by adjusting the gap with shims. A nominal gap of either 2 or 4 mm is a low reflection spacing. The 4 mm central gap will be chosen so that the edge gap with the deflection of two disks subtracted will be greater that ~ 2mm to allow for sufficient FC-75 flow.
Using all of the information and developments of the previous four Tasks a final prototype window fixture design was developed, and a final prototype fabricated. This fixture has a clear aperture diameter of 63.5 mm to be consistent with the microwave ring apparatus at ORNL that was used for microwave testing. Quite a few microwave window fixture designs were considered and are discussed in Task 2. Strengthened sapphire was used as the microwave window material. The thickness of the window was determined by the available thicknesses made by Union Carbide. The window is a factor of 3 thinner than standard windows of this diameter and thus suitable as megawatt power level window, since current windows are used up to 500 kW.
The strengthened sapphire windows were built into a standard fixture that was modified for the new windows. The sapphire windows were first face-brazed onto flat stainless steel rings. Similar stainless steel rings were welded into the stainless steel fixture. The two stainless steel rings were then e-beam edge welded together to complete the fixture. This is a simpler and more robust manufacturing technique than being used currently. In standard double window microwave fixtures, the windows are brazed onto a copper cup with a relatively low temperature braze after the copper cup has been high temperature brazed into the stainless steel fixture. The soft copper allows the braze joint to be formed with low stresses. The copper cup is structurally weak, however, so that the window assembly must be supported against pressure loads by a backing ring. The present design needs no backing ring, since the window rests against the bulk waveguide. The standard fixture also has a relatively low bakeout temperature because of the double braze technique.
A schematic of the final Phase 2 fixture design is shown in Fig. 20. Modeling allowed reassessment of the cantilevering approach and suitable adjustment of the design for both simplicity and the greatest stress reduction. This design also incorporates the program advances made in window brazing. In other respects the fixture design is unchanged; face cooling and mounting to the microwave duct.
Figure 20. Prototype high power window fixture design.
Although the Phase 2 prototype window uses Union Carbide sapphire and a 63.5 mm aperture, a larger optimized prototype window has been designed, using identical fabrication techniques. The large deflection modeling of Task 1 was used to design a nominal sapphire window for a 102 mm diameter (multi) megawatt waveguide. The specifications for this window are:
Double window
FC-75 cooled at 1 atm pressure (design based on 100% pressure safety factor)
100 mm diameter aperture
0.1 mm thick
600°C Bakeout
These specifications represent significant advances both in the thinness of the window and the temperature of the bakeout. The thickness of the windows is specified using large deflection modeling, taking advantage of the physical effects of both the higher design strength of the sapphire from strengthening and the lowering of peak stress in the window through membrane effects during deflection of the very thin window. Membrane forces allow stress to be distributed throughout the thickness of the window, rather than concentrating them at the surface away from the pressure as in flexing disks. The membrane regime becomes more important as the disk becomes thinner; the disk thickness has been reduced both by reducing the pressure loading from the standard 4 atm, and by increasing the failure stress through polish strengthening. The higher bakeout temperature has been achieved by developing a higher temperature brazing process.
Although advances in polishing have been made in the program, the 100 micron thickness windows that are optimal can not be made at this time, and will be part of a Phase III effort if commercialization is possible.
When the windows become extremely thin their larger deflection changes the interaction with the microwaves passing through them. For a 1 atm pressure load a 65 µ m thick window with a 31.8 mm radius will have a central deflection of 1 mm, whereas for the a 0.43 mm thick disk of the same radius, the central displacement is approximately 0.5 mm. For a double window fixture with pressurized coolant between the windows and vacuum outside the windows, both windows will bow outward, so that the spacing between the windows will increase by a maximum of twice the central deflection. The increase in total spacing will depend on the width of the cooling channel.
Testing of the final window fixture took place in stages as the fixture was assembled. The testing consisted of:
1) Pressure testing.
2) Heat testing.
3) Leak testing/bakeout.
4) Low power microwave testing (ORNL)
5) High power testing (ORNL).
Pressure testing consisted first of vacuum leak checking of the brazed windows at 3E Labs where they were brazed, followed by deflection testing under pressure at TvU. The windows were vacuum tested again at ORNL after they were welded into the fixture, and pressure tested (FC-75) during cooled operation at high power.
Heating and heat flux testing was performed on an isolated sapphire window and is described in Task 1. The windows are so thin that temperature gradients cannot be created across the thickness of the window, and thermal stresses resulting from face cooling are eliminated. An extremely high bakeout temperature was demonstrated by rebrazing a window at 900°C without harming the window or the window seal. Furthermore, by brazing the window into a ring, brazing stresses could be separated from the rest of the fixture fabrication process.
Low power microwave testing was performed at ORNL without problems, as expected from a clear-aperture sapphire window.
The window will be tested in the quasi-optical high power resonant ring system at ORNL to progressively greater power levels up to at least 1.5 MW. Fixture behavior will be monitored with an IR camera and by examining the microwave characteristics of the ring, which by its operation will be able to detect losses in its components. Unfortunately commercial concentration on diamond windows has caused the effort in this program to focus on the resonant ring itself as the program development with the greatest potential impact on future microwave component progress.
The goal of this program was to demonstrate the feasibility of making ultrathin sapphire microwave windows. The goal of making ultrathin windows has been shown to be conclusively achieved through pressure failure experiments where sapphire windows a factor of 3-5 thinner than current practice have been shown to support the same pressure that must be supported by current windows. There is no question that these windows will support Megawatt microwave power levels, since microwave absorption is known to be a linear function of material thickness, and power absorption is the current limit on microwave windows. The remaining question of whether the thinner sapphire windows could be assembled into a fixture was answered positively by successfully fabricating a standard double window fixture using the ultrathin windows. Commercialization has been frustrated by the development of competing diamond window technology.
A number of contributing results and developments have also been achieved during the Phase II effort. The contributing results include:
Development of polish strengthening techniques.
Development of modeling that allows accurate prediction of peak stress of a circular window under pressure loading.
Failure testing to demonstrate sapphire strengthening and validate modeling.
Development of a simplified double-window fixture design.
Development of thin-window brazing techniques.
Pressure testing of the fixture.
Temperature testing of the fixture.
Low power microwave testing.
High power microwave testing.
The final window fixture used 0.43 mm thick sapphire in a microwave fixture that had an aperture of 63.5 mm. The fixture was pressure tested to 1 atm and will be tested under high power microwaves to power levels of greater than 1 MW if possible.
A number of ancillary important developments have also taken place. A resonant ring microwave power amplifier was demonstrated and the facility was used to test strengthened sapphire window fixtures for system function and microwave response. The novel quasi-optical resonant ring device was both modeled and tested, providing large power gains. It will allow testing of the windows developed in this program at power levels of over 1 MW using existing 200 kW gyrotron power sources at ORNL. This was the only way that the new microwave windows could have been tested, since few megawatt microwave facilities are available at this time. Microwave power tube companies have their own in-house development efforts (in this case using diamond windows) and are not willing to test unknown systems. DOE fusion facilities are under such heavy use that they cannot spare the time and effort required to test new systems.
It has been found that the new thin sapphire windows are immune to the standard failure mode of microwave windows - thermal stress fracture. The most common cause for microwave window failure is the development of a hot spot on the window, which boils the coolant and further reduces cooling, increasing local thermal stress until the window fails. The thin windows are thin enough so that they have no significant thermal gradients through their thickness and are thus not subject to large thermal stresses as a result of hot spots that may occur.
Additional novel microwave window concepts have been developed through a low-level, secondary effort. A tube-grid cooled single sapphire window was built and tested. It was shown to be transparent to microwaves of the proper frequency, and to tolerate a 1 kW heat load without effect. This window shows long term promise for use in a variety of high power, single frequency microwave systems. Another promising concept is the use of He gas cooling to practically and dramatically reduce the microwave absorption in sapphire windows.
Acknowledgements: The financial support of DOE under Grant No. DE-FG02-95ER-86038 is gratefully acknowledged, as well as the support of the project manager, T.V. George, without whose backing this work could not have been performed.
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