Process for producing a directly heated cathode
FIELD OF THE INVENTION 
     The present invention pertains to a process for producing a directly heated cathode for a cathode ray tube, especially for X-ray tubes and more particularly to a cathode coil consisting of tungsten fastened in a load bearing manner to cathode braces made of high melting metal by means of heat supplied in a punctiform manner. 
     BACKGROUND OF THE INVENTION 
     It has been known that cathode coils made of tungsten wire are used in X-ray tubes. Such tungsten coils are held on metal straps or metal leads, which consist of, e.g., molybdenum. Heating coils are fastened to the leads generally by welding, e.g., argon arc welding or microplasma welding. The cathode coils may be damaged in the area of fastening during such welding processes. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is a primary object of the present invention to provide an improved mode of fastening for a cathode coil on metal straps consisting of a difficult-to-melt metal. 
     According to the invention, a process is provided for producing a directly heated cathode for cathode ray tubes, particularly for X-ray tubes. The cathode coil is provided consisting of tungsten, and is fastened in a load-bearing manner to a support means of cathode braces formed of a high-melting metal by supplying heat in a punctiform manner. The coil has characteristics that promote heating of and consequent emission of electrons for x-ray production when an electric current is passed through the coil. The fastening is provided by employing platinum as the brazing filler. 
     The cathode coil is preferably fastened to the cathode braces made of molybdenum. The brazing may be performed by means of laser energy supplied in a punctiform manner. The brazing heat may also be provided by a welding arc ignited at the brazed joint. Preferably, the brazing is carried out under a protective gas atmosphere, particularly a protective gas stream such as argon. 
     One essential advantage of the present invention is considered to be the fact that damage, especially cross-sectional damage to the tungsten coil, is avoided. The brazing described is particularly suitable for fastening cathode coils made of tungsten to holding straps made of molybdenum. Thus, it is particularly suitable for producing cathodes for X-ray tubes, because it withstands even high temperatures. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     The only figure is a schematic representation showing the produced directly heated cathode according to the invention. 
    
    
     The figure shows a cathode coil 1 made of tungsten for an X-ray tube. At its two ends 3 it is fastened to a support means of holding straps 2, which preferably consist of molybdenum. These materials possess sufficient strength even at elevated temperatures. 
     According to the present invention, the ends 3 of the tungsten coil are fastened to the straps or braces 2 by brazing, using platinum as the brazing filler. 
     The heat needed for brazing is preferably supplied in the same manner as is usually done in the case of welding. Consequently, it is supplied in a directed, punctiform manner to the fastening site. 
     According to a preferred heating method, the heat is supplied via a laser beam of appropriate energy density directed toward the brazed joint or the brazing filler. The brazed joint is scavenged with a protective gas, e.g., argon. This may be provided by directing an argon protective gas stream at the brazing site. 
     According to another heating method, an arc is ignited at the brazed joint. A method such as used for argon arc welding is preferably used. 
     The methods described guarantee holding and contacting of the tungsten coil that is stable even at high temperatures, without the risk of damage to the coils. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.