Abstract:
A window for a waveguide for a beam of electromagnetic radiation. The window includes a layer of a material capable of allowing electromagnetic radiation to pass therethrough and a support on each side of the layer. Each support defines a passage for electromagnetic radiation. Further, the layer is provided with at least one recess formed in the periphery thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a window, for example a window for a waveguide for a beam of electromagnetic radiation. 
     2. Discussion of the Background 
     Electromagnetic radiation, and more particularly microwave radiation, is generated in gyrotrons and other high power microwave sources. A beam of the radiation is generated and this passes along a waveguide to an exit port. The waveguide comprises a passage, generally of circular cross-section, having a window extending across it. The beam of electromagnetic radiation passes through the window. The window is used for the protection of the electromagnetic sources or detectors from environmental factors. The window may be made of a variety of materials such as sapphire, diamond and the like. Such windows are generally planar, although curved profiles have been suggested. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a window for a waveguide for a beam of electromagnetic radiation, comprises a layer of a material capable of allowing electromagnetic radiation to pass therethrough and a support on each side of the layer, each support defining a passage for electromagnetic radiation, and the layer being provided with at least one recess formed in the periphery thereof. 
     Preferably a plurality of recesses are formed in the periphery of the layer, those recesses preferably being evenly spaced around the periphery. The number of recesses will vary according to the nature of the layer and will typically exceed six. 
     The depth of the recess or recesses into the layer will vary according to the nature of the material from which the layer is made, the size and shape of the layer and other such factors. For example, where the layer is disc-shaped, the depth of the recess or recesses may be chosen to reduce the peak level of hoop stresses in the layer. 
     In one form of the invention, each support is located inside the periphery of the layer and the or each recess has a depth equal to or less the distance of the supports from the periphery. 
     It is preferred that each recess is free of sharp corners, e.g. has a rounded end which extends into the layer. 
     Each support may take the form of a ring having a flange which bears against a surface of the layer. The flanges will generally extend outwards and away from the passage defined within the supports. To minimise expansion of the flanges, a retaining ring may be provided around the outwardly extending flanges. The retaining ring may be made of a material such as molybdenum. 
     The supports will generally be bonded to a surface of the layer by means of a braze or diffusion bond. The layer may take any suitable shape such as rectangular, but will generally be disc-shaped. 
     The material from which the layer is made will typically be sapphire, diamond, germanium, zinc selenide, silicon, doped silicon, silicon nitride, aluminium nitride or boron nitride. The window is preferably made of diamond. Such diamond is preferably produced using chemical vapour deposition (CVD). 
     The invention extends to a layer for use in a window as described above, such layer being of a material capable of allowing electromagnetic radiation to pass therethrough and being provided with at least one recess formed in the periphery thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an embodiment of a window of the invention, and 
     FIG. 2 is a section along the line  2 — 2  of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the invention will now be described with reference to FIGS. 1 and 2. Referring to the drawings, there is shown a window comprising disc-shaped layer  10 , preferably made of CVD diamond, having opposite major surfaces  12 ,  14  and a peripheral edge  16 . The layer  10  is mounted between opposed supports or cuffs  18 ,  20 . Each cuff comprises a ring section  18   a ,  20   a  and an outwardly extending flange  18   b ,  20   b . The flange  18   b  bears against the surface  12 , whilst the flange  20   b  bears against the surface  14 . The flanges  18   b ,  20   b , are bonded to the surfaces  12 ,  14  respectively, of the CVD diamond layer  10 , by means of a diffusion bond. The bonding material will be chosen to suit the application. Examples of suitable materials are aluminium and gold and alloys thereof. 
     A passage  22  is defined within the cuff  18  and a passage  24  is defined within the cuff  20 . The cuff  18  is in alignment with the cuff  20  so that the passages  22  and  24  have no discernible discontinuity. 
     Retaining rings  26  and  28  are provided around the outwardly extending flanges  18   b ,  20   b . The retaining ring preferably has a coefficient of expansion which is comparable to that of the layer  10  and lower than that of the cuffs  18 ,  20 . An example of a suitable material is molybdenum. 
     The cuffs  18 ,  20  will typically be made of a metal such as Inconel. The attachment of a CVD diamond layer to a metal cuff presents several problems. First, a compatible braze or diffusion bond must be chosen having adequate strength and high temperature creep resistance to withstand bakeout. Secondly, the metal cuffs must be chosen having a linear thermal expansion match to the diamond window. While primarily for the high temperature bonding process, this requirement also affects stresses encountered in subsequent vacuum bakeout treatments which can cause cumulative work hardening effects in some metal joints leading ultimately to fracture. Diamond has one of the lowest thermal expansion coefficients of any material and any metal cuff will expand considerably more than diamond over the 0-1000° C. temperature range causing mismatch stresses. Finally, the thermal variations from centre to edge in the CVD deposition process can lead to hoop stresses across the layer  10 . The magnitude of hoop stresses can sometimes be sufficient to promote radial cracks propagating in from the edge of the disc. It is important to contain these cracks if the disc is to be considered for vacuum window applications. Even if the cracks are not present initially, the presence of the stresses can lead to cracking later during the lifetime of the window. 
     A plurality of recesses or slots  30  are provided around the periphery  16  of the disc-shaped layer  10 . Each slot  30  extends into the layer  10  from the periphery  16  and each slot has a rounded end or base  32 . The slots, it has been found, give rise to the following advantages: 
     1. The slots facilitate the use of alignment pins and fixtures, which are simpler and lower in cost, directly against the cuffs  18 ,  20  to ensure that accurate concentric alignment of the two cuffs and their bonding to the layer is achieved. A typically error of 0.3 nm in this regard can be reduced to better than 0.05 mm. 
     2. The use of slots reduces the peak level of hoop stresses that exist in the layer  10  particularly in the region between the cuffs  18 ,  20  and the edge  16 . This in turn reduces the likelihood of uncontrolled radial cracks extending in from the edge  16 . The number of slots required preferably exceeds six and each slot should have a depth equivalent to the region between the cuffs  18 ,  20  and the edge  16 . 
     3. The thermal mismatch stresses between the cuffs  18 ,  20  and the layer  10  are reduced since the slotted layer is effectively more compliant in the region of the bonding. Twisting forces which may exist in the outer periphery of the layer  10  are reduced. The likelihood of bond failure during repeat thermal cycling is also reduced. 
     4. The slots can be used to modify the liquid cooling around the edge of the window in order to derive an advantage in the heat transfer. 
     The window described above may be mounted in a waveguide using methods and techniques known in the art.