Abstract:
The present invention refers to a method for assembling an electron exit window of an electron beam generating device, comprising the steps of: arranging a foil support plate on a housing of the electron beam generating device, bonding a window foil to a frame along at least one continuous bonding line, thus creating an exit window sub-assembly, and attaching the exit window sub-assembly onto the housing. The invention also relates to an electron exit window assembly.

Description:
THE FIELD OF THE INVENTION 
     The present invention refers to a method for assembling an electron exit window and an electron exit window assembly. 
     PRIOR ART 
     Electron beam generating devices may be used in sterilization of items, such as for example in sterilization of food packages or medical equipment, or they may be used in curing of e.g. ink. Generally, these devices comprise an electron exit window formed by a foil and a foil support plate. The foil support plate, which is preferably made of copper, has a plurality of apertures through which the electrons will exit from the electron beam generating device during operation. The foil may have a thickness of around 6-10 μm and may be made of titanium. Due to the thinness most of the electrons are able to pass through it. 
     The present invention primarily relates to electron beam generating devices used for irradiation of webs of material, i.e., electron beam generating devices having relatively large electron exit windows. 
     The method or process being used today for producing electron beam devices of the above type will be described in the following, referring to  FIG. 1  and  FIG. 2 . 
     The electron beam device  100  comprises two parts; a tube body  102  housing and protecting the assembly  103  generating and shaping the electron beam, and a flange  104  carrying components relating to the output of the electron beam, such as the window foil  106  and the foil support plate  108  preventing the window foil  106  from collapsing as vacuum is established inside the device  100 . Further, during operation of the electron beam device the foil  106  is subject to excessive heat. Thereby, the foil support plate  108  also serves the important purpose of conducting heat generated in the foil  106  during use away from the foil of the device. By keeping the foil temperature moderate a sufficiently long lifetime of the foil  106  may be obtained. 
     In the production the foil support plate  108 , being of copper, is bonded to the flange  104 , which is separate from the tube body  102  at this stage. The flange  104  is generally made of stainless steel. The window foil  106  is then bonded onto the foil support plate  108  along a line extending along the perimeter of the foil support plate  108 , and excess window foil  106  is trimmed off. The foil  106  may subsequently be coated, in order to improve its properties regarding for instance heat transfer. The coating is made on the side of the foil  106  facing the outside of the electron beam generating device  100 . The flange  104  is subsequently attached to the tube body  102  to form a sealed housing. 
     SUMMARY OF THE INVENTION 
     The inventors of the present invention have discovered that this prior solution is not optimal when the electron beam device is used in for example oxygen containing atmospheres. Under these circumstances the accelerated electrons will generate ozone, which is a highly corrosive substance. The ozone may corrode the copper foil support, which may in turn compromise the seal of the housing and the function of the electron beam generating device. In addition, in a packaging machine producing food packages, hydrogen peroxide is often used to sterilize the machine parts before production of packages starts. Thus, the copper foil support may come into contact with hydrogen peroxide as well. Hydrogen peroxide is also highly corrosive for the copper foil support. 
     The most sensitive location is the copper volume at the bonding line with the foil  106 . Here, the corrosion only needs to work underneath the bonding line, which is only a few tenths of a millimeter, in order to result in the unfortunate result described above. 
     The present invention aims at solving this problem by providing a method for assembling an electron exit window of an electron beam generating device, comprising the steps of: arranging a foil support plate on a housing of the electron beam generating device, bonding a window foil to a frame along at least one continuous bonding line, thus creating an exit window sub-assembly, and attaching the exit window sub-assembly onto the housing. 
     There are several advantages with the inventive method, one being that the attachment of the foil to the corrosion-proof frame, which in turn is bonded into a flange of the housing, will provide a seal, which will protect the copper foil support plate from being subjected to corrosive substances, which may cause corrosion and failing sealability. 
     A further advantage with this assembly method is that the foil may be coated on the inside, i.e. the side which will be facing the inside of the electron beam generating device. From a wear perspective it is an advantage to have the coating on the inside where it is protected from for example plasma. Further, if a coating is to be placed on the foil, only the sub-assembly comprising the foil and the frame need to be placed in the coating machine. 
     An additional advantage is related to component cost and the vulnerability of the foil. Assembling an electron beam device is a complex manufacturing method with several critical steps, one being for example the bonding of the foil to the frame and one being the optional coating of the foil. Further, the tube body and the flange are being costly components relative to the foil. By attaching the foil to a frame, and by attaching said frame to the rest of the electron beam generating device late in the manufacturing process, money may be saved if any of the steps related to the foil fails. 
     Preferred embodiments of the method are defined by the dependent claims. 
     The invention also comprises an electron exit window assembly of an electron beam generating device comprising a foil support plate and a window foil, wherein said foil support plate is arranged on a housing of the electron beam generating device, said window foil is bonded to a frame along at least one continuous bonding line, forming an exit window sub-assembly, and said exit window sub-assembly is attached to the housing. 
     The advantages discussed in relation to the method similarly apply also for the electron exit window assembly. 
     Preferred embodiments of the electron exit window assembly are defined by the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, an exemplary embodiment of the invention will be described in greater detail, with reference to the enclosed drawings, in which: 
         FIG. 1  is a schematic cross sectional isometric view of an electron beam device according to prior art, 
         FIG. 2  is a schematic partial cross section of the device of  FIG. 1 , shown as an exploded view, 
         FIG. 3  is a schematic partial cross section of a device according to a first embodiment of the invention, for comparison with the cross section of  FIG. 2 , as shown as an exploded view, 
         FIG. 4  is a schematic partial cross section similar to  FIG. 3 , but shown in an assembled state, and 
         FIG. 5  is a partial cross section of a foil provided with a coating according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1 and 2 , comprising a prior art solution, have already been described. In an exemplary embodiment of the present invention, as shown in  FIGS. 3 and 4 , a foil support plate  208  is arranged on the housing of an electron beam generating device. The foil support plate  208  is preferably made of copper and is bonded to a flange  204  of the housing. One possible bonding technique is brazing. The foil support plate  208  is bonded to an edge  210  of an opening  212  in the flange  204 . 
     In a separate step, or in the same manufacturing step, said flange  204  is plasma welded to the tube body  202  forming said housing of the electron beam generating device. In another embodiment, not shown, the tube body  202  and the flange  204  is made in one piece. 
     In a separate step the window foil  206  is bonded onto a frame  214  to form an exit window sub-assembly  216 . The word “frame” should here be interpreted as an element having a central hole configuration. The foil  206  is preferably made of titanium and said frame  214  is preferably made of stainless steel. Possible bonding techniques may be for example laser welding, electron beam welding, brazing, ultrasonic welding, diffusion bonding and gluing. In the exemplary embodiment the foil  206  is diffusion bonded onto the frame  214  along a continuous bonding line  218  partly shown in  FIG. 4 . The bonding line  218  is continuous to be able to maintain vacuum inside the electron beam device. The word “continuous” is used to define that the line is endless or closed. Further, it should be defined that the bonding line  218  extends along the hole configuration of the frame  214  but within the perimeter of the frame  214 . Preferably, the bonding line  218  extends at a distance from the perimeter of the frame  214 . Furthermore, at least one bonding line  218  is made. Thus, two or more bonding lines may be made. For example, an inner and an outer bonding line may be made on the frame  214 , and the two lines may, for instance, be concentric with each other. 
     At this stage the foil  206  may optionally be coated and in the coating process only the exit window assembly  216  needs to be processed. According to this embodiment the foil  206  may be coated on both sides, but preferably on the inside, i.e. the side of the foil  206  which will face the inside of the electron beam generating device once assembled. 
     In  FIG. 5  the foil  206  is shown with a coating denoted  206 C. 
     The coating  206 C serves the purpose of increasing thermal conductivity in order to increase the lifetime of the foil  206 . 
     As mentioned an advantage with the present invention is the possibility of being able to provide the coating  206 C to the inside of the foil  206 . Plasma, which is being built up outside of the electron exit window during operation of the electron beam device, can wear down the coating of an electron exit window. However, on the inside of the foil  206  the coating  206 C will be protected from the effects of plasma. Hence, there is an opportunity, with the present invention, to choose among several heat conductive coating materials, for example DLC (Diamond-like-carbon), copper, aluminium, graphite, silver and gold. 
     Subsequently, the frame  214 , and thereby the exit window sub-assembly  216  may be attached to the flange  204  portion of the housing. 
     The step of attaching the exit window sub-assembly  216  to the housing is made in such a way that it forms a protection for the foil support plate  208  ensuring that the foil support plate  208  is not exposed to the environment outside of the electron beam generating device. As may be seen from  FIG. 3  and  FIG. 4  the frame  214  is arranged in an indentation  220  partly formed by the flange  204  of the housing and partly formed by the foil support plate  208 . In an alternative embodiment, not shown, the indentation for receiving the frame may be formed in the housing only. 
     The frame  214  of the exit window sub-assembly  216  is preferably welded onto the housing. 
     It can be seen from  FIG. 4  that after attachment of the exit window sub-assembly  216  onto the housing no portion of the foil support plate  208  is exposed to the outside atmosphere, and thereby corrosion of the copper foil support plate  208  is prevented. 
     Further to the advantages, the exit window sub-assembly  216  may be tested separately, such that the hermetic sealing in the bond, along bonding line  218 , between the foil  206  and the frame  214  is confirmed before the exit window sub-assembly  216  is welded to the flange  204 . Should the seal be defective the exit window sub-assembly  216  may simply be discarded without affecting the cost or production time to any great extent. The frame  214  has in this exemplary embodiment a thickness of 3 mm, and is made of stainless steel. 
     With the inventive solution there is an additional advantage in that the window foil  206  will be exposed to few processing steps. It should be understood that, with regard to the thinness of the foil  206 , any processing work on or near the foil  206  may jeopardize the integrity of the foil  206 .