Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
  • H01P1/00—Auxiliary devices
  • H01P1/08—Dielectric windows

Definitions

• the invention relates to vacuum-tight microwave windows, used in principle at the output of a power electronic tube to transmit microwave electromagnetic energy between the interior of the tube (under high vacuum) and the exterior (in atmospheric atmosphere by example).
• the tube can in particular be an amplifier such as a traveling wave tube (TOP in French, TWT in English for "traveling wave tube"), or a klystron for example. It can also be an oscillator (magnetron, etc.). Typically, it is desired to send the amplified energy inside the tube to a waveguide which contains air.
• the microwave window ensures the free passage, at least in a given frequency band, of the electromagnetic energy towards the waveguide while maintaining the sealing of the vacuum inside the tube.
• the windows comprise a flat disc of insulating dielectric, through which electromagnetic energy passes.
• this disc is made of alumina or another ceramic having not only very good dielectric properties but also good thermal conductivity and good resistance to high temperatures and strong temperature gradients. Indeed, for high power tubes and operating with high electric fields, the passage of energy generates losses in the dielectric, therefore significant heating.
• the tubes concerned here can provide powers of several tens of kilowatts.
• the dielectric disc can typically have dimensions of ten centimeters in diameter for a thickness of 1 millimeter to a few millimeters.
• the dielectric disc is brazed over its entire periphery against the interior surface of a metallic cylindrical skirt (generally made of copper) which surrounds it.
• the cylindrical skirt 10 is itself surrounded by a holding block 20, for example stainless steel, which serves as a support for fixing the dielectric disc 30 and its skirt 10 between the tube power and a waveguide.
• the dielectric disc 30 is brazed inside the skirt.
• the holding block 20 can be used for radiator and transition times between the tube and the waveguide. It has a lower part 22 constituting the start of the waveguide, with a peripheral flange 24 serving to fix the waveguide to the block 20.
• the upper part 26 of the block 20 is cylindrical and it surrounds the cylindrical skirt 10. It is intended to be welded or brazed around an outlet opening of the power tube (not shown). The bottom and the top of the skirt are brazed inside the holding block 20.
• solders between the dielectric disc and the skirt, as well as the solders between the skirt and the retaining block participate in maintaining the vacuum tightness.
• the thermal stresses in operation can be very high given the high power dissipation which occurs in the dielectric disc.
• the power dissipation is generally maximum towards the center and lower at the edges.
• the thermal conduction qualities of the ceramic (especially alumina) allow the heat to be removed radially towards the edges; the copper skirt and the stainless steel retaining block serve as a radiator.
• the thermal stresses are very high because of the temperature difference between different areas of the ceramic. They are aggravated by the fact that the distribution of the power dissipation in the window is not necessarily completely radial.
• the stresses can cause ruptures of the ceramic, or of the copper, or of the various solders which ensure the vacuum tightness.
• the defects which may result from these thermal stresses are unacceptable for the tube as soon as they cause a loss of vacuum tightness. This is why one is limited in the power level likely to pass through the window during the use of the tube.
• An object of the invention is to produce a microwave window having higher power handling capacities than in the prior art, while retaining the advantages of existing windows.
• a microwave window comprising a dielectric disc, which is characterized by the fact that it comprises a prestressing ring surrounding the periphery of the disc and exerting at rest, over the entire periphery of the disc, a directed radial compression stress. toward the center of the disc.
• a metal skirt brazed around the disc as in the prior art, the preload ring surrounding the metal skirt around the periphery of the dielectric disc.
• the prestressing ring is constituted by an annular portion of a holding block used to fix the window on the outlet of a microwave tube, this annular portion pressing locally on an outer surface of the skirt all around the periphery. of the dielectric disc.
• the retaining block is made of a much more rigid material than the metal skirt.
• the holding block can be made of stainless steel, the skirt being of copper and the disc of alumina.
• the thickness of the compression preload ring can be approximately 2 or 3 millimeters.
• the prestressing ring may consist of an additional thickness (in total approximately 3 millimeters) of metal.
• the temperature rise of the dielectric disc does not change compared to the prior art, but the breaking strength is considerably improved.
• the prestressing is preferably of the order of several tens of bars, or more. It is exercised at rest, when the tube is cold; in operation, the window heats up the more the power transmitted through the window is high.
• the preload ring expands and relieves the internal thermal stresses that appear in the window due to the large temperature gradients in the window; but, since the ring exerts cold a radial compression stress, its hot expansion does not exert an unacceptable stress on the solder between the ring and the window.
• the invention also relates to a method for producing a microwave window.
• the method consists in placing a dielectric disc inside a metallic skirt, with a brazing material, and with a small clearance between the periphery of the disc and the skirt, in placing the whole of the disc and the skirt in the inside of a retaining block comprising a prestressing ring surrounding the skirt over the entire periphery of the disc, a slight clearance being provided between the ring and the skirt, to place a brazing hoop around this ring with a slight clearance between the hoop and the ring, the material of the hoop having a lower coefficient of expansion than that of the ring, to bring the hoop, ring, skirt and disc assembly to a temperature sufficient to ensure the brazing of the disc in the skirt, and to allow to cool, cooling ensuring a prestress in radial compression of the ring on the skirt and on the disc.
• the brazing band is then removed.
• top and bottom of the skirt are brazed to the holding block.
• a brazing bead is therefore placed in the appropriate places before passing the assembly through the brazing oven.
• a lubricating substance preferably graphite, is interposed between the strain ring and the metal skirt, preventing sintering between the ring and the skirt in the event of a rise in temperature of the ring, for example during the cycles of steaming of the microwave tube 5 comprising the window.
• These steaming cycles are intended to ensure the degassing of the different parts of the tube while a high vacuum is made inside the tube. Sintering between the ring and the skirt, due to pressure, friction and temperature, would lead to a stiffening of the ring / skirt connection and to risks of subsequent leakage.
• FIG. 2 shows a window according to the invention
• FIG. 3 shows the brazing operation necessary for the realization of the window according to the invention.
• the holding block is made of metal which is more rigid than the metal skirt 5 and ensures the rigidity of the entire window.
• the prestressing ring according to the invention is in this case an integral part of the retaining block 20 which surrounds the skirt 10. This ring is designated by the reference 40. It is constituted for simplicity by a local reinforcement of the thickness of the holding block. It could also be independent of the block.
• the holding block has a cylindrical central part generally surrounding the metal skirt 10, and the ring is formed by a localized zone of this cylindrical part, this zone having a greater thickness and being situated around the periphery of the dielectric disc. 30.
• the prestressing ring 40 may have a radial thickness of approximately 3 millimeters over a height of a few millimeters.
• the holding block is made of stainless steel with a thickness generally about 1 millimeter around the skirt 10, and it locally includes an extra thickness (2 millimeters thick in addition all around the disc 30)
• the inside diameter of the retaining block, in its cylindrical central part, is greater than the outside diameter of the metal skirt, except at the location of the prestressing ring 40; at this point, the inner diameter of the ring is equal to that of the metal skirt, the ring permanently exerting a compressive stress on the skirt 10 and, through the skirt, on the periphery of the dielectric disc 30.
• the inside diameter of the central cylindrical part of the holding block 20 is approximately 1 millimeter greater than the outside diameter of the skirt 10.
• the metal skirt is preferably welded or brazed to the retaining block by its upper part on the one hand, by its lower part on the other hand, the dielectric disc being located in a central part of the skirt 10.
• the upper part 26 of the holding block is shaped so that it can be welded tightly on the power tube, around an electromagnetic energy outlet opening. The tube and its outlet opening are not shown.
• the lower part of the block 20 is shaped so that it can be fixed (but not necessarily sealed) to an element for using the output power of the tube, for example a waveguide not shown.
• a lower fixing flange 24 can be provided for this purpose.
• the lower part 22 of the holding block 20 can be machined internally to form a waveguide of dimensions corresponding to the waveguide to which it is to be fixed.
• the prestressing ring 40 exerts a strong compression on the periphery of the skirt and the dielectric disc and there is preferably interposed between the ring and the skirt a thin layer of a lubricating substance, preferably a thin layer of graphite, to avoid any joining by sintering in the event of a rise in temperature.
• a lubricating substance preferably a thin layer of graphite
• the disc is mounted in the skirt and mounting the skirt in the holding block, then the assembly is passed through a brazing furnace in the presence of a brazing band tightened around the holding block at the preload ring.
• Figure 3 shows this operation.
• the brazing hoop is a circular ring 50 whose cold internal diameter is equal, with a slight clearance, to the cold external diameter of the retaining block at the location of the prestressing ring 40.
• the hoop 50 is in one rigid material with a lower coefficient of expansion than that of the retaining block (and the skirt).
• the dielectric disc also has a lower coefficient of expansion than that of the holding block and the skirt.
• the skirt is made of copper
• the holding block is made of stainless steel
• the hoop is made of molybdenum, or tungsten, or even alumina.
• the coefficient of expansion of molybdenum is about 3 times lower than that stainless steel and copper.
• the coefficient of expansion of alumina is also much lower than that of the holding block (approximately 3 times).
• the dielectric disc is placed in the skirt with a peripheral brazing bead 60.
• a clearance exists between the disc and the skirt.
• the molten solder will penetrate by wetting in the gap between the disc and the skirt at the periphery of the disc.
• the cold clearance is such that thereafter, when hot, during the brazing operation, the clearance becomes very small (of the order of 0.1 millimeter, however allowing the brazing material to penetrate into the gap between disc and skirt.
• the skirt is placed in the holding block with a slight cold clearance between the block and the skirt at the location of the prestressing ring 40.
• the interior of the ring 40 has been previously coated with a lubrication substance mentioned above (graphite in principle) Brazing beads 62 and 64 are placed at the top and bottom of the skirt 10.
• the molybdenum hoop 50 is slid cold around the holding block containing the metallic skirt.
• the hoop could also be heated before being placed around the cold holding block. It is placed at the level of the prestressing ring 40.
• the clearance between the hoop and the holding block is low during the introduction (normally cold) of the fret turn of the block. This clearance becomes zero during the brazing operation.
• the assembly is heated to the temperature necessary for the brazing operation.
• the solder cords melt.
• the disc, the skirt, the holding block, and the hoop expand. However, the hoop 50 expands much less than the holding block (and than the skirt). It therefore partially prevents expansion of the preload ring 40.
• the preload ring 40 contracts, the holding block 20 contracts, as does the skirt 10 at the dielectric disc (the stainless steel ring is made of much more rigid material than the skirt).
• the ring 40 exerts compression on the dielectric disc 30 which contracts less.
• This compression is permanent: it is present at rest, but it remains in operation. It is radial. It depends on the play between the parts, the materials used, the diameters of the elements, and the thickness of the prestressing ring. It can reach 100 bars. It is therefore a strong compression preload.
• the hoop 50 is removed after cooling. In the presence of this permanent prestressing, the overall structure will be much more resistant to breakage when the window is subjected to thermal stresses, either in steaming or in operation.
• the output window according to the invention can be used in particular at the output of TOPs, or of klystrons.

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• Waveguide Connection Structure (AREA)
• Ceramic Products (AREA)

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• 2001
  • 2001-02-23 FR FR0102512A patent/FR2821487B1/fr not_active Expired - Fee Related
• 2002
  • 2002-02-22 WO PCT/FR2002/000666 patent/WO2002069434A1/fr active IP Right Grant
  • 2002-02-22 US US10/470,001 patent/US7049909B2/en not_active Expired - Lifetime
  • 2002-02-22 EP EP02704882A patent/EP1364425B1/de not_active Expired - Lifetime
  • 2002-02-22 JP JP2002568451A patent/JP2004521543A/ja active Pending

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