Abstract:
A cathode-housing suspension of an electron beam device having a tubular body of elongate shape with an exit window extending in the longitudinal direction and a connector end in one end of the tubular body is disclosed. The electron beam device further comprises a cathode housing having an elongate shape and comprising a free end and an attachment end remote to the free end, and the attachment end comprises an outwardly extending flange provided with threaded openings for set screws and non-threaded openings for attachment bolts, for attaching the attachment end to a corresponding socket of the tubular body, wherein a mechanism configured to bias the attachment end away from the socket are arranged in the tubular body.

Description:
TECHNICAL BACKGROUND 
     The present invention relates to an electron beam device (EBD), and in particular to an EBD with improved properties in relation to a cathode housing thereof. 
     A typical EBD comprises a vacuum tight body inside which a cathode housing is arranged. The cathode housing comprises a filament which is heated by a current in order for electrons to be produced. The thus produced electrons are accelerated by means of a high-voltage potential and exits through an exit window of the body, typically a thin window foil supported by a support grid. Electron beam devices may be used for several purposes, such as curing of paint or adhesives, or sterilisation of volumes or surfaces. Depending on the application properties such as acceleration voltage, beam profile, shape of the EBD will vary. The teachings of the present invention may advantageously be applied to EBD:s used for sterilization of a web of packaging material, since it may significantly improve the performance of EBD:s being designed for that purpose. It is to be understood, however that it may be applied to other EBD:s having a similar construction for which similar advantages may be obtained. 
     Within the field of sterilization of a web of packaging material, performance factors such as stability, durability and longevity are key issues, once the quality of the sterilization is ensured. All components mentioned and still more may be optimized in order for the EBD to produce the desired beam shape under any given circumstances. 
     The present invention relates to the context of elongate electron beam devices used for treatment of larger surface, such as webs of packaging material used for production of packaging containers. More specifically the present invention relates to improvements of such EBD:s, in terms of ensuring adequate quality while simplifying assembly of the EBD. 
     SUMMARY 
     According to the present invention there is provided a cathode-housing suspension of an electron beam device having a tubular body of elongate shape with an exit window extending in the longitudinal direction and a connector end in one end of the tubular body. The electron beam device further comprises a cathode housing having an elongate shape and comprising a free end and an attachment end remote to the free end, and the attachment end comprises an outwardly extending flange provided with threaded openings for set screws and non-threaded openings for attachment bolts, for attaching the attachment end to a corresponding socket of the tubular body, wherein means configured to bias the attachment end away from the socket are arranged in the tubular body. 
     One advantage of a suspension in accordance with the present embodiment is that it facilitates aligned mounting of cathode housing. In particular it enables perfect positioning of the cathode housing in relation to the tube body without the need of machining constructional details with overly small tolerances. This in turn enables a simplified production of components, and a faster assembly of the device. The resulting suspension will be flexible in regard of its assembly, yet rigid in its assembled state. Even small deviations in the position of the cathode housing may have considerable impact on the performance of the electron-beam device. It may for instance affect the beam profile, and alterations in the beam profile may in turn affect the longevity of the device. 
     In one or more embodiments the connector end may comprise concentrically arranged cylindrical connector elements separated by annular ceramic spacers, wherein the ceramic spacers are arranged in a staggered fashion, such that adjacent spacers are shifted in the longitudinal direction in relation to each other, which results in a number of advantages listed in the detailed description. In further embodiments where every other ceramic spacer is aligned in the longitudinal direction the stability of the suspension is even further increased. 
     In one or more embodiments the biasing means comprises a plate spring arranged between the attachment end and the socket. The plate spring will provide a reliable biasing means, which may easily be localized in the adequate position, e.g. by having openings through which the screws and bolts may extend. 
     In a number of embodiments the number of attachment bolts may be three, which provides a simplistic yet fully flexible solution, and in order to maximize symmetry the attachment bolts may be distributed evenly on the flange. The set screws may, irrespective of the number of attachment bolts, be located between adjacent attachment bolts. In some embodiments the set screws may be arranged equidistant from adjacent attachment bolts (in between two attachment bolts), and in other embodiments the set screws may be arranged closer to one attachment bolt. A reason for the latter may be that a lateral distance between opposing forces should be minimized (in the first example they are maximized), which may be desired depending on the dimensions of the flange, etc. 
     In one or more embodiments the connector end is delimited by a cylinder segment welded to the tubular body, wherein the socket is concentrically suspended in a ceramic isolator disc brazed to the inner perimeter of the cylinder segment. This construction enables for all tuning of the cathode-housing suspension to be performed outside of the constraints of the EBD, which would still be possible if desired. 
     In order to decrease the risk of generation of excessive electric field strength and the generation of sparks the socket may have a curved surface on a side remote to the cathode housing. 
     The present invention also relates to a method for suspending a cathode housing in a connector end of an electron beam device comprising the steps of
         arranging biasing means between an attachment end of the cathode housing and a corresponding socket of the connector end of the electron beam device   compressing the biasing means partially by means of attachment bolts extending from the cathode housing and being engaged in threaded openings of the socket, or vice versa,   adjusting the attachment bolts until the cathode housing has the desired inclination,   fixating the position of the cathode housing by means of tightening set screws.       

     The method may also comprise the step of welding the connector end to one end of a tubular body of an EBD. In one or more embodiments the method may also comprise the step of evacuating the tubular body and sealing it, for generation of a sealed electron beam device, where a vacuum pump is not required to maintain the adequate degree of vacuum in the EBD. 
    
    
     
       DETAILED DESCRIPTION 
         FIG. 1  is a schematic side view of an electron beam device which may comprise a suspension in accordance with one embodiment of the present invention. 
         FIG. 2  is a schematic cross section of the electron beam device of  FIG. 1 . 
         FIG. 3  is a schematic exploded view from the side of a suspension in accordance with one embodiment of the present invention. 
         FIG. 4  is an exploded view similar to  FIG. 3 , yet in perspective and in some more detail. 
         FIG. 5  is a perspective view of an electron beam device similar to  FIG. 1  yet in some more detail. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a side view of an electron beam device according to a first embodiment of the present invention. The purpose of the drawing is simply to illustrate the basic components of an electron beam device, and it should be emphasized that the purpose is not to provide a true constructional drawing or in any other way limit the present invention. 
     The electron beam device  100  of  FIG. 1  comprises a tube body  102  having an exit window arrangement  104 . The exit window arrangement  104  in turn comprises subassemblies not relevant for the present invention, yet having the properties of providing an outlet window for electrons while preserving vacuum inside the tube body  102 . Components inside the vacuum tube  102  are illustrated by the phantom lines. A cathode housing  106  extends from a connection end  108  of the tube body  102 . In this context it should be mentioned that the connection end  108  may be removable or rigidly attached to the rest of the tube body  102 . In the present embodiment an outer cylinder segment of the connection end  108  is welded to the tube body  102 , which is showed in more detail referring to  FIGS. 3-5 . 
     Within the constraints of the cathode housing  106  a filament  110  is arranged. A control grid  112  (not shown in  FIG. 1 ) may also be arranged as part of the cathode housing  106 , which control grid  112  is used for better control of the electron emission. The application of a separate and variable electrical potential to the control grid  112  makes it possible to use it for active shaping of the generated electron beam. In its simplest use a negative potential may be used in order to block the electrons from leaving the cathode housing. For these purposes the control grid  112  may be electrically connected to a separate power supply (not shown). 
     In use, an electron beam is generated by heating the filament, using a current, and by accelerating the electron towards the exit window  104  by means of a high-voltage potential. 
     An attachment end  114  of the cathode housing  106  comprises an outwardly extending flange  116 . The flange  116  connects to a socket  118  of the connection end  108  by means of screws, and this suspension will be explained in more detail referring to  FIG. 3 . First,  FIG. 2  is an end view further illustrating the shape of the EBD of  FIG. 1 . Again, the purpose is simply to illustrate the present invention, not to limit the invention in any unreasonable way, the skilled person will realize, upon reading the present application, that there are several applications for the present invention as defined by the claims. The socket may be made of stainless steel. 
       FIG. 3  is an exploded side view of a suspension in accordance with an embodiment of the present invention. Reference numerals already introduced in reference to  FIGS. 1 and 2  will be reused for like components. A plate spring  120  is sandwiched between the flange  116  of the attachment end  114  and the socket  118 . The purpose of the plate spring  120  is to bias the cathode housing  106  in the direction of its free end, away from the socket  118 . The flange  116  attaches to the socket  118  by means of three attachment bolts  122  (see also  FIG. 4 ) extending through through-holes of the flange  116  and engaging with threaded holes in the socket  118 . The attachment bolts  122  are preferably equidistantly distributed around the circumference of the flange  116 . In the illustrated embodiment the attachment bolts  122  extend through bores of the plate spring  120 , which bores also serve the purpose of localizing the plate spring  120  correctly. In the flange  116  and between adjacent attachment bolts  122 , set screws  124  are arranged (see also  FIG. 4 ). The set screws  124  are arranged in threaded bores of the flange  116  and extend through corresponding bores of the plate spring  120 . Unlike the attachment bolts  122  the set screws do generally not extend into the socket  118 . 
     During assembly the cathode housing  106  is attached to the socket  118  using the attachment bolts  122 . The attachment bolts  122  are tightened such that the plate spring  120  is only partially compressed. At this point dial indicators or various other techniques may be used to verify the position of the cathode housing  106 . If the position has to be adjusted, one or more of the attachment bolts  122  are adjusted, and since the plate spring  120  will bias the flange  116  towards the head of the bolt  122  each such adjustment will result in an alteration of the position of the cathode housing  106 . Once the position (inclination) of the cathode housing  106  adequate the set screws  124  are tightened. As the set screws  124  are tightened they will force the flange  116  towards the head of the attachment bolts  122 . Thereby they will fixate the flange  116  in the adequate position. 
     The socket  118  will have to carry the mechanical load comprising the weight of the cathode housing  106 , and also provide a seal able to sustain the vacuum inside the tube  102  under varying temperatures. Concentrically arranged cylindrical connectors  126  are arranged in the socket  118 . The cylindrical connectors  126  are electrically separated using annular isolators  128 , preferably ceramic isolators being brazed to adjacent connectors  126 . The annular isolators  128  are arranged in a staggered configuration, where every other isolator  128  is shifted in the longitudinal direction. This configuration enables the socket  118  to absorb the load generated by the weight of the cathode housing  106  as well as effects emanating from temperature variations, such as expansion of the material. The major part of the load is however absorbed by an outermost ring of the connector, and the main purpose of the staggered configuration is to avoid excessive stress during brazing of the isolators  128 , during which process the temperature may reach about 900° C., which generally is far higher than the temperature prevailing during operation of the device. The material used for the connectors may be FeNiCo, having a thermal expansion coefficient between that of the ceramic and the stainless steel. 
     The remote end of the socket  118  (in relation to the cathode housing  106 ) is brazed to a larger ceramic disc  130  along an inner perimeter thereof. The outer perimeter of the ceramic disc  130  is effectively brazed to an inner diameter of a cylinder segment  132 , which in turn is welded to, and forms a part of the tube body  102 , which has been discussed earlier. The main purpose of the ceramic disc  130  is to provide electrical insulation between the connection unit  126  and the tube body  102 , while also transfer and bear the load from the cathode housing  106 . The cylinder segment  132  may be formed from a material having a coefficient of thermal expansion between that of the tube body and of the ceramic material, suggestively FeNiCo. This will reduce the stress induced by temperature variations within the electron-beam device. The ceramic disc  130  may be formed from Al 2 O 3  as the major constituent. 
       FIG. 4  corresponds to  FIG. 3 , yet it illustrates a few more details of the suspension and surrounding components in a less schematic way. The mushroom shape of the socket  118  with a curved surface on the side remote to the cathode housing  106 , which is visible in the drawing is preferential since it reduces excessive electric field concentrations and the generation of sparks, which would hamper the operation of the EBD. The curved surface may have rotational symmetry around a central longitudinal axis of the EBD, as is visible in the same drawing. 
     Any electron beam device described in the present application may be a sealed electron beam device, where the vacuum inside the electron beam device inside the EBD is maintained without the continuous use of vacuum pumps. In one assembly step vacuum is generated inside the EBD, after which the opening through which vacuum is drawn, is sealed off permanently.