Abstract:
A cathode structure for an electron gun of a cathode-ray tube having a reduced size and providing a fast cathode start-up time and excellent thermal efficiency. The cathode comprises a closed chamber consisting of a cap supporting an emitting part of the cathode and a dish-shaped skirt having an internal surface which is concave so as to reflect, by radiation, the thermal energy stored in the skirt walls, towards that region of the cap supporting the emitting part. The cathode structure also comprises filament supply leads which pass through the side walls of the skirt and connect to the filament heater contained in the closed chamber.

Description:
[0001]    The invention relates to a cathode for an electron gun of a cathode-ray tube, the structure of which is improved with a view to reducing the length of the cathode and improving the thermal efficiency of the cathode.  
         BACKGROUND OF THE INVENTION  
         [0002]    A cathode for an electron gun of a cathode-ray tube in general comprises a cap on which a material intended to emit an electron beam is placed, a cylindrical-shaped cathode skirt forming, with the cap, a unitary assembly, a heater filament inserted in the skirt, the filament comprising in general a spiral part placed close to the cap and the leads for connection to the power supply circuit, this connection being made via the skirt orifice situated opposite the cap; the filament leads are welded to rigid yokes secured to the structure of the gun through electrically non-conducting parts made, for example, of glass. The cathode itself is held in place in the lower part of the gun using a sleeve secured, for example by welding, to the end of the skirt opposite the cap. Such a structure is, for example, described in U.S. Pat. No. 4,403,169.  
           [0003]    In a gun of this type, part of the power provided by the filament to take the emitting part to its operating temperature, is lost by radiation at the rear opening of the cathode skirt. Furthermore, a lot of power is lost through the cathode support means in the gun, such as the sleeve mentioned above. In order to improve the thermal efficiency of the cathode, U.S. Pat. No. 5,013,965 describes the use a cylindrical cathode skirt having the particular feature of being closed at the end opposite the cap. This configuration improves the thermal efficiency of known cathodes by heat conduction from the skirt to the emitting part, but in an insufficient manner and in particular it does not especially speed up the cathode startup time, an important characteristic for rapidly obtaining an image on a television screen.  
           [0004]    A cathode according to the prior art, as illustrated in FIG. 1, comprises an emitting part  1  in the form of a layer of emitting material for an oxide cathode or of a pellet impregnated with emitting materials in the case of a so-called impregnated cathode. The emitting material is supported by a cathode cap  2 , placed at one of the ends of a cylindrical skirt  3 ; the skirt  3  lies in a direction Z perpendicular to the emitting surface of  1 ; the skirt  3  is closed at its end  4  opposite the cap, so as to form a closed cylinder in which the cathode filament  5  is enclosed; the filament leads pass through the cathode skirt via orifices  9  made in the end  4  opposite the cap. The cathode is supported in the structure of the gun by a sleeve  7  connected in a conventional manner to the other parts of the electron gun.  
           [0005]    However, this type of structure has two major drawbacks:  
           [0006]    its length along the main axis Z is large and contributes to extending the length of the electron gun which incorporates it and consequently, the depth of the tube fitted with such a gun; and  
           [0007]    the thermal efficiency of such a cathode is not optimized. A lot of energy is lost in the skirt and in the means for connecting the skirt to the other parts of the gun.  
         SUMMARY OF THE INVENTION  
         [0008]    The cathode for an electron gun of a cathode-ray tube according to the invention comprises emitting materials to generate an electron beam; a metal cap on which the emitting part is placed; a spiral-shaped heater filament placed under the cap and terminated by connection leads, a skirt surrounding the spiral part of the filament and forming, with the cap, a closed space.  
           [0009]    One improvement lies in that the internal surface of the rear part of the skirt opposite the cap reflects heat energy from the filament to the emitting part thereby providing fast cathode start-up time and excellent thermal efficiency. In one embodiment, the internal surface of the skirt is preferably thereby efficiently reflecting, by radiation, the thermal energy stored in the skirt walls, towards that region of the cap supporting the emitting parts.  
           [0010]    Another separate or included improvement lies in that the connection leads pass through the side walls of the skirt, preferably closest to the junction between the cap and the skirt. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention will now be described in greater detail, with relation to the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a sectional view of a cathode according to the prior art;  
         [0013]    [0013]FIG. 2 is a sectional view of a first embodiment of the invention;  
         [0014]    [0014]FIG. 3 illustrates a second embodiment of the invention;  
         [0015]    [0015]FIG. 4 is a top view of a cathode according to the invention;  
         [0016]    [0016]FIG. 5 is a perspective view of the emitting part and of the coiled filament of a cathode according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The cathode according to the invention, shown in a first embodiment in FIG. 2, has a length along the Z axis which is shorter in comparison with the prior art. FIG. 2 illustrates one embodiment in the case of an impregnated cathode, but can be applied in the same way to an oxide cathode.  
         [0018]    The cathode comprises a metal cap  11  supporting a pellet  10  of porous materials impregnated with emitting materials. A metal skirt  14  is attached to the cap. Unlike the prior art, the two connection leads  15  of the filament  13  pass, not through the bottom of the skirt  14 , but through its side wall via orifices or notches  19 . These orifices or notches are preferably placed close to the cap  11 , or else at the end of the skirt  14  closest to the cap. The cathode, consisting of its emitting part  10 , the cap  11 , its skirt  14  and the filament  13 , is held in place using an electrically insulating support  17  made, for example, of sintered glass, the mechanical connection between the support  17  and the cathode/filament assembly (emitting part  10 , cap  11 , filament  13 , and skirt  14 ) is made by a plurality of arms  12 , welded to metal pads  16  included in the insulating support  17 . The number of these arms may, for example, be three, placed at 120° to each other; they preferably lie in a plane substantially parallel to the support surface  17  and to the surface of the emitting part  10  of the cathode in order to reduce the axial length of the cathode. As illustrated in section in FIG. 2, one of the arms  12  acts as an electrical connection to bring the cathode to an ad hoc potential using a connector  18  connected to a power source. The filament  13  has a coating which ensures electrical insulation between, on the one hand, the conducting core of the filament electrically connected to the leads  15  and, on the other hand, the emitting part  10 , cap  11 , arms  12 , skirt  14 , electrically conducting pads  16  and connector  18  are electrically connected to each other. For example, this coating lies over the entire part of the filament contained in the space defined by the cap  11  and the skirt  14 , and also lies beyond the orifices  19 .  
         [0019]    In a second embodiment of the invention illustrated in FIG. 3, FIG. 4 and FIG. 5, the two connection leads  15  of the filament are used as cathode support arms, the leads being connected, for example by welding, to pads  16  included in the insulating support  17 . In this case, the two filament leads are able to pass through the side part of the skirt  14 , at 120° to each other, in a plane parallel to the surface of the emitting part  10 . Mechanical stability is ensured by at least one arm  12 , in this case placed at 120° to the two leads  15 . The cathode is assembled, for example, as follows:  
         [0020]    the filament  13 , with its two leads lying at 120° to each other, is placed under the cap  11 , to which an arm  12  has been previously welded;  
         [0021]    the skirt  14 , having on its periphery three notches  19  at 120° to each other, is attached to the cap  11  so that the leads  15  and the arm  12  pass through the notches  19 . The dimensions of the notches are matched to the dimensions of the leads  15  and of the arm  12  so as to achieve a close fit in order to prevent any subsequent movement of these components during operation;  
         [0022]    the skirt  14  is secured, for example by welding, to the cap  11 ; and the leads  15  and the arm  12  are secured, for example by welding, to metal pads  16  included in the insulating support  17 .  
         [0023]    The unitary module thus produced can be inserted alone or in threes, in the lower part of an electron gun for a monochrome or colour cathode-ray tube. In one advantageous embodiment, the first electrode of the gun comprises means into which the cathode module is inserted in order to keep the emitting part of the cathode facing the orifice of the grid and at a good distance from it.  
         [0024]    The invention thus allows especially the axial length of the cathode to be reduced, but also allows the thermal efficiency of the latter to be increased.  
         [0025]    In the cathode structure according to the invention, it is henceforth possible to close the lower part, opposite the cap  11 , so that it acts as a heat reflector by reflecting the radiant heat energy directly onto the emitting part of the cathode. The concave shape of the internal surface of the lower part of the skirt is designed to carry out this function of reflecting energy onto the cap  11  area supporting the emitting part  10 ; thus the internal surface of the closed lower part of the skirt can have any concave shape designed to carry out this function; preferably, it may have a conical or frustoconical shape which is easy to produce industrially, the apex angle of the cone being chosen so that a part of the heat which is not directly picked up by the emitting part  10  is sent by radiative reflection to the cap area supporting the emitting part  10 .  
         [0026]    In order to improve the heat exchange between the filament and the emitting part, the shape of the filament  13  is adapted so as to match the internal surface of the skirt; thus, the head  20  of the filament has a size in a plane parallel to the plane of the emitting surface of the cathode which is smaller than at its base located closest to the emitting part. The filament may, for example, be spiralled on a cone, so as to increase the surface area of the filament directly facing the surface of the cap  11  located under the emitting part and to decrease its mean distance from the surface.  
         [0027]    In the case of an impregnated cathode, it is still possible to improve the heat exchange by placing at least part of a turn of the filament  13  around the side wall  21  of the pellet, as illustrated in FIG. 2 and FIG. 3.  
         [0028]    In general, it is possible to improve the heat exchange between the filament  13  and the emitting part  10  by adapting the geometric shapes of components emitting part  10 , cap  11 , filament  13  and skirt  14  so as to favour transfer by heat radiation:  
         [0029]    from filament  13  to emitting part  10 , by increasing the surface areas of filament  13  and emitting part  10  directly in line with each other (more exactly, directly in line through component  11 ), and by decreasing the space between the surfaces; and  
         [0030]    from filament  13  to skirt  14 , by increasing the surface areas of filament  13  and skirt  14  directly in line with each other, and by decreasing the space between the surfaces.  
         [0031]    Moreover, the use of leads  15  as cathode support makes it possible, compared with a structure according to the state of the art, to decrease thermal losses, on the one hand by radiation and on the other hand by conduction, in the cathode support components such as the sleeve  7  of FIG. 1.  
         [0032]    Apart from the fact that the cathode structure according to the invention makes it possible both to reduce the size of the cathode and to improve its heat efficiency, the fact of having a filament closer to the emitting part with a lower part of the skirt in the form of a heat reflector makes it possible in addition to improve it by decreasing the cathode startup time thereof, the startup time corresponding to the time elapsing between applying the supply voltage to the filament and obtaining the electron current emitted by the cathode.