Abstract:
A direct-heated flat emitter for generating a homogenous electron beam, particularly for x-ray tubes, has two terminal lugs for the heating current supply formed at the edge of the perimeter of the emission surface and the emission surface is subdivided into interconnects by slits. The slits have a width no less than 10 μm and no greater than 1% of the length of a diagonal of the smallest rectangle which can circumscribe the emission surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a direct-heated flat emitter for creating an electron beam, particularly for x-ray tubes, with two terminal lugs formed at the edge of the perimeter for the heat supply. 
     2. Description of the Prior Art 
     For mammography there are x-ray tubes with rectangular surface emitters consisting of tungsten sheet that is about 50 μm thick. These flat emitters are provided with mutually parallel slits proceeding in alternating fashion from mutually opposite sides, so that interconnects are formed which produce a serpentine current path that enables a direct heating of the flat emitter. 
     Flat emitters are also known from French Patent 58 949, French Patent 978 627, British Specification 10 11 398, German OS 37 17 974 and German PS 39 01 337. 
     Such emitters share the problem that an electron beam having an optimally homogenous electron distribution over its cross-section can only be generated if the slits are very narrow. The slits cannot be made arbitrarily narrow, however, there is the danger of shorts between neighboring interconnects. Besides this, there is the danger of voltage arcing between neighboring interconnects. Both lead to a shortening of the lifetime of the flat emitter or even to its premature failure. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a flat emitter of the above type having a structure which allows a longer lifetime for the emitter to be achieved. 
     This object is inventively achieved in a flat emitter with a slitted emission surface wherein the slits have a width of no less than 10 μm and no greater than 1% of the length of a diagonal of the smallest rectangle which can circumscribe the emission surface. 
     Shorts and voltage arcing between neighboring interconnects are thus precluded, which is a precondition for a longer lifetime. It is also guaranteed that the cuts do not have a width which causes undesirable non-homogeneities to arise in the electron beam emanating from the emission surface. 
     The above object also is inventively achieved in a flat emitter having a round i.e. a substantially annular emission surface wherein the slits proceed such that each right-proceeding interconnect curve is followed by a left-proceeding interconnect, and each left-proceeding interconnect curve is followed by a right-proceeding interconnect. 
     It is assured in this way that, unlike in the case of a flat emitter described in British Specification 1 011 398 which is fashioned as double-threaded spiral, for example, the potential difference between immediately neighboring interconnects decreases. This leads to a low danger of voltage arcing, thereby preconditioning a longer lifetime. 
     If the interconnects have different electrical resistances, local &#34;hot spots&#34; or &#34;cold spots&#34; and thus a correspondingly different electron emission depending on resistance value can occur. To avoid this, in a version of the invention path exhibited by the slit at least partially conforms to the peripheral shape of the flat detector, and preferably such that the interconnects thus created have substantially the same electrical resistance over the entire emission surface. 
     In contrast to the known arrangements, there is a different current flow in the interconnects of the emitter formed by the slits because of the arrangement of the two terminal lugs formed at the outer edge (which should preferably be diametrically opposed) so that a uniform heating of all regions of the emission surface of the emitter and thus a very homogenous electron beam and guaranteed. This is true particularly for the central region of the emitter, which is not a point of origin for any of the electrical terminals. (The term &#34;point of origin&#34; as used herein means an originating or connection location (i.e. a &#34;root&#34;) of the terminal lug relative to the emission surface, which will not literally be a single point.) 
     The flat emitter is preferably annular with two opposing concentric curved slit pairs connected at one end to each other and to the point of origin of one of the terminal lugs by straight slits, these lugs being arranged diametrically to each other and offset 90° relative to the connecting line of the midpoints of the curved slits. Due to these curved slits and the few short straight connecting slits, a very good division of a round emission surface can be achieved, so that there are equal conductor widths and thus equal electrical resistances at practically all points. This in turn results in a uniform temperature of the entire emission surface and thus the generation of a homogenous electron beam. 
     The curved slits of each pair should preferably span different angles, with the center of the emission surface coinciding with the apex of these angles. It has further proven appropriate for the radii of the inner curved slits to be substantially 1/5 of the radius of the emission surface, and the radii of the outer curved slits are substantially 3/5 of the emission surface radius. 
     To prevent significant gradients of the emitter temperature in the regions of the respective points of origin of the terminal lugs due to the necessarily occurring heat runoff into the mounting rods which provide the current supply and thus to avoid non-homogeneities of the electron beam in this region, in a further embodiment of the invention each region of the point of origin of a terminal lug has a width which is modified relative to the interconnects of the emission surfaces and which balances the thermal conduction losses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic section through the cathode of an electron beam tube with a direct-heated flat emitter in accordance with the invention arranged inside a Wehnelt cylinder. 
     FIG. 2 is a plan view of the flat emitter in accordance with the invention before the bending of the terminal lugs to form the mounting legs. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The cathode schematically depicted in FIG. 1 has a Wehnelt cylinder 1 with a central bore 2 in which an annular direct-heated flat emitter 3 is arranged. The emitter 3 has a terminal lug 4 formed thereon, these being welded onto the current supply rods 5 and serving for mechanical mounting of the flat emitter 3 in addition to the current supply. The current supply rods 5 are led from an insulating part 7 to the exterior via tubes 6 and connected at the exterior with electrical terminal wires 8 in known fashion (not depicted in detail). In order to obtain an optimally homogenous electron beam, the annular surface of the emitter 3 is divided by two curved slit pairs 9, 10 and 9&#39;, 10&#39; (the two curved slits of each pair span different midpoint angles). The slits of the pairs are disposed concentrically relative to the midpoint of the emitter 3. The ends of the respective outer curved slits 9, 9&#39; and of the inner curved slits 10&#39;, 10 of the other pair which ends reside on the same side with respect to the diametrically opposed terminal lugs 4 are connected to each other by straight slits 11, 11&#39; and to the point of origin of one of the terminal lugs 4 by other straight slits 12, 12&#39;. This results in a configuration with the desired uniform width of the interconnects formed by the slits, with a uniform resistance and consequently a uniform temperature on the basis of the current flow through the direct-heated emitter 3. 
     Narrowed width regions indicated in dashed lines at the point of origin of each of the terminal lugs 4. The narrowed width regions can extend a greater or lesser distance beyond the length of the terminal lugs 4, this distance being selected after experimentation such that a compensation of the thermal conduction losses into the rods 5 (current supply) is achieved by an optimized width of the terminal lugs 4. 
     As can be seen in FIG. 2, the slits 9 to 12 and 9&#39; to 12&#39; are such that, proceeding from the left terminal lug 4 in FIG. 2, a right-proceeding interconnect curve R1 is followed by a left-proceeding interconnect L 1 , followed by a central region 6, and another left-proceeding interconnect curve L 2  is followed by another right-proceeding interconnect curve R2. 
     It is thus guaranteed that the potential difference between neighboring interconnects--and thus the danger of voltage arcing--is low. Increased potential differences relative to the terminal lugs 4 arise in the region of the slits 12 and 12&#39;, for which reason the slits 12 and 12&#39; are fashioned with correspondingly larger widths than the remaining slits (not depicted). 
     The innermost left interconnect curve L1 can be connected to the innermost left interconnect curve L2 either directly in the central region C or with the insertion of a short linear interconnect, or even with the insertion of a short interconnect that is bent to the right at which a short interconnect that is bent to the right is attached. Whatever connection is used in the central region C, as shown in FIG. 2 it will be a generally right-proceeding connection (according to the above nomenclature), so that the right-left alternation is preserved. 
     The remaining slits 9 to 11, and 9&#39; to 11&#39;, each have a width that is at least equal to 10 μm and at the most equal to 1% of the length of the diagonal of the smallest rectangle which can circumscribe the emission surface, which is drawn in FIG. 2 with the diagonal in dashed fashion. 
     In this way, in the interest of a longer lifetime, the occurrence of shorts and voltage arcing between neighboring interconnects is precluded. The emanation of a non-homogenous electron beam from the emission surface due to an excessively large width of the slit is simultaneously precluded. 
     Due to the small width the slits 9 to 11, and 9&#39; to 11&#39; are depicted in FIG. 1 as simple lines. 
     In the exemplary embodiment, as a result of the annular shape of the flat emitter 3 the smallest circumscribable rectangle for the emission surface is a square. In contrast, in the case of an elliptical flat emitter, for example, the smallest circumscribable rectangle for the emission surface would be a rectangle whose larger lateral length would correspond to the length of the major of the ellipse, and whose smaller lateral length would correspond to the minor axis of the ellipse. 
     The invention is not limited to the exemplary embodiment. It is thus also possible to inventively fashion flat emitters with an outer contour that deviates from the annular outer contour provided in the exemplary embodiment. 
     Emitters considered flat emitters in the framework of the invention are those wherein the electrons emanate from a preferably flat, but possibly bent region which, unlike in wire filaments, is fashioned in planar fashion, namely as emission surface. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of our contribution to the art.