US2770752A - Insulated shielding structure for gaseous discharge tube - Google Patents

Description

N 5 D. v. EDWARDS ET AL 2,770,752

INSULATED SHIELDING STRUCTURE FOR GASEOUS DISCHARGE TUBE Filed Jan. 26, 1952 JNVENTORS DVEdwarda And WR-Kruqcr. BY I M MM.

5 4 E H 2 l 1 a o I 8 L w i. 6 3 7 a 1 R R H 7 2 MM 8 u w 9.. N Z k .1 5 4 1 l 9 THE/R ATTORNEY.

United States Patent O INSULATED SHIELDING STRUCTURE FOR GASEOUS DISCHARGE TUBE Donald V. Edwards, Montclair, and William P. Kruger,

Morristown, N. J., assignors to Electrons, Incorporated,

This invention relates to high voltage controllable gaseous discharge tubes, and more particularly to an insulating structure of low potential metal shields for the leads of high voltage anodes of gaseous discharge tubes.

In certain types of gaseous discharge devices, such as a high voltage controllable rectifier tube, it is desirable to provide complete shielding of certain surfaces at anode potential by a metal shield at cathode or other appropriate low potential. The probability of effective ionization and an arc discharge through the gaseous medium between the shield and the surfaces at anode potential to be shielded, may be avoided by application of the principle of short or mean free path spacing; but it is difficult to maintain such close spacing at the surface of the anode lead adjacent its seal in the tube envelope, and still have adequate electrical insulation between the parts. If the shield is merely extended to the envelope wall around the seal for the anode lead, the electrical insulation or isolation between such closely spaced electrodes obtainable at the envelope wall is not adequate for the high anode voltages desired. Under such conditions, a high voltage on the anode lead is likely to result in a short circuit to the closely spaced shield, due either to a rupture of the intervening insulation afforded by the envelope wall, or a breakdown in the form of a surface arc of obscure origin but apparently due to the eifects of field emission from points of localized high potential associated with the film existing on the inner surface of glass envelopes. On the other hand, in order that the shielding of the anode lead may be effective, all gaps or spaces between the shield and the anode lead adjacent its seal must be closed in some way to avoid the long ionizing paths that would produce an unwanted arc discharge.

With these and other considerations in mind, the principal object of this invention is to provide a shielding structure for the anode leads of high voltage gaseous discharge tubes which will afford both short path shielding and adequate electrical insulation of the shield, including points on the surface of the anode lead adjacent its seal in the tube envelope, so that the desired complete shielding for high voltage anodes may be provided.

Generally speaking, and without attempting to define the nature and scope of the invention, it is proposed to employ a partially conductive element of insulating material adjacent the anode seal, preferably an extension of the glass wall of the tube envelope, for forming the joint or connection between the closely spaced end portion of the metal shield and the anode lead, said element having the appropriate dimensions to afford the desired electrical insulation, but avoiding efi'ective ionization in the intervening space due to its contour and the potential distribution caused by its partially conductive nature. Considered more specifically, the joint between theupper end portion of the metal shield and the anode lead adjacent its seal in the tube envelope is formed by a conical wall of partially conductive glass in conductive contact with the :closely spaced surfacesiof the shield and anode at points sufliciently separated to afford the desired insula- 2,770,752 Patented Nov. 13,

2 tion against a short-circuit, said glass wall providing a distribution of potential by its conductivity to avoid effective ionization and an arc discharge between the shield and anode lead in the region of such joint.

Various other objects, characteristic features, attributes and advantages of the invention will be in part apparent, and in part pointed out as the description progresses.

Although the structure of this invention may take various specific forms, and also may be used with various types of tubes, it is convenient in describing the nature of the invention and its characteristic features to refer to a tangible physical embodiment of the invention as applied to a simple typical tube structure, such as illustrated in the accompanying drawings.

In these drawings,

Fig. 1 is a longitudinal section through a simple form of hot cathode grid control rectifier tube, and illustrates the application of the anode shielding structure of this invention to such a tube, the parts being illustrated for the purpose of facilitating an understanding of the general structural organization of the tube, rather than to show the particular construction and arrangement of parts to be used in practice.

Fig. 2 is a transverse section through the tube of Fig. 1 on the section line indicated by 2--2.

Fig. 3 is a fragmentary view illustrating to a larger scale the upper part of the tube of Fig. l and those parts involving the insulating and shielding structure of this invention; and i Fig. 4 is an explanatory diagram representative of the distribution of potentials for a given anode voltage, and the general nature of the equi-potential linesof the electric field characteristic of the structure of this invention.

The present invention relates more particularly to shielding structures for the anode leads of gaseous discharge tubes, where complete anode shielding is desirable to avoid unwanted conduction through the tube at the higher levels of anodevoltage. In a grid control rectifier tube, for example, complete shielding of the anode and its supporting lead, except for an area opposite the control grid, is desirable to provide higher peak forward and inverse voltage ratings for the tube; and for the purpose of explaining this invention, a simple and conventional structure for a tube of this type has been assumed.

The tube structure shown comprises in general a cathode K heated to an emissive temperature from an external source of current and enclosed in a heat shield HS, a control grid G, and an anode A, these tube elements being suitably supported in an evacuated glass envelope E containing an ionizable medium in the form of a vapor or gas at the appropriate pressure for the structure and voltage rating of the tube, all in the manner conformingwith recognized principles and practices in the art.

is assumed to be of the filamentary type comprising a spirally wound coil 5 of wire or strips of nickel as a core material welded at its ends to supporting rods 6 which pass through tubular insulators 7 in the bottom end piece 8 of the cylindrical heat shield HS, and which are sealed in a circular mounting stem 10 to which the body of the envelope E is fused as indicated at 11. A midpoint of the cathode is connected by a welded piece 12 to the, bottom piece 8 of the heat shield HS. The insulators 7 are held in place by a relatively tight fit in the bottom 8 of the heat shield, supplemented by projections indicated at 13.0n the supporting rods 6 below these insulators, such projections, being formed by deformation of the rods or added welded pieces. The cathode K is provided with an emissive coating, preferably of the barium nickelate type formed and treated in the manner disclosed in the patent to D. V. Edwards et al., No. 1,985,855, December 25, 1934p The heat shield HS is formed in the usual way by a cylindrical body welded to the peripheral flanges of the bottom end piece 8 and a top header 16 having therein the usual discharge opening indicated at 17. This heat shield HS is supported by a plurality of legs 18 welded tothe bottom 8 ofthe heat shield and anchored in the Circular mounting stem 10. i i I i The control grid G in the arrangement illustrated comprises a'ring 20 to which the ends of grid bars 21 are welded at the appropriate spaced intervals. This grid ring 20 is supported by a plurality of rods 22 which are welded at their bent upper ends to a peripheral flange of this ring, and which extend through tubular insulators 23 "fitted in holes in the top and bottom ends 16 and 8 ofthe heat shield HS. These insulators 23 are made of steatite or like heat resistant insulated material, and are formed with recessed ends in accordance with the disclosure of the patent to E. K. Smith, No. 2,456,540, December 14, 1948. As shown, these tubular insulators 23 are retained in position by projections formed on the supporting rods 22 indicated at 24, and by the bent upper ends of these rods. The lower ends of the supporting rods 22 are connected together by a cross member 25, which in turn is'welded to the upper end of a grid supporting lead 26 sealed in th circular mounting stem 10. Although the two grid supporting rods 22 are illustrated in the conventional illustration, three such rods are preferably used in practice to constitute a more rigid threepoint support for the grid, and the same is true of the heat shield supporting legs 18.

The anode A in the structur shown is assumed to be a flat disc of a suitable metal, such as tantalum or nickel, which may have a peripheral flange and radial corrugations (not shown) to afford the desired stiffness and avoid undue warping at the high temperatures the anode assumes. in fabrication of the tube and in operation. The anode A is welded to an anode supporting rod 30 sealed in the upper end of the tube envelope E. In the type of seal illustrated, the anode supporting lead 30 is secured, such as by silver solder brazing, to the underside of an inverted cup 31 of a metal, such as the alloy commonly known as Kovar, which is capable of forming a gastight seal with glass. The periphery of this metal cup 31 is sealed to the glassof the envelope E in the ordinary manner; and the appropriate external lead 32 for the anode is brazed to the outside of the cup 31. The anode supporting rod 30 is to be surrounded by a closely spaced shield as later described; and in order to have dimensions suitable for such an arrangement, a metal sleeve 34 of the appropriate diameter is used around the anode supporting rod 30 in the structure illustrated. This sleeve 34 fits at its top closely inside the flange of the cup, and is formed with a flange at its lower end to closely fit the upper surface of the anode A, and be welded thereto if desired. It may b considered that the anode lead as a whole comprises this sleeve 34 together with the rod 30. In this connection, the particular structure of the seal for the anode supporting lead is not material to the in vention; and where the size of the anode lead permits, it may be directly sealed in the glass envelope E without a glass-to-metal seal of the type indicated.

The present invention relates to structures for providing complete shielding of an anode and its supporting lead for gaseous discharge tubes, and more particularly to the joint or connection between such an anode shield and the anode supporting lead adjacent its seal in the tube envelope. In the particular shielding arrangement illustrated, the anode shield S comprises a body 35 of imperforate sheet metal shaped to have the close spacing later discussed to the top or back surface of the anode A,-to its periphery, and also to the periphery of the grid G and the heat shield HS. An integral end portion or tubular extension 36 of the shield S is similarlyspaced closely around the anode lead 30 up to a point adjacent the seal of the anode lead, where the joint or connection of this invention is employed. In the particular structure shown, the glass envelope E is formed with a tapered or conical portion 40, conveniently termed a neck, which is in electrical conductive contact at its ends with the outer surface of the anode lead sleeve 34 and the tubular extension 36 of the shield S, in a manner readily understood from the drawings. In order to assure an effective conductive connection between the glass neck 40 and the metal surfaces of the anode lead sleeve 34 and the tubular portion 36 of th shield S, ring coatings of a suitable metal, such as platinum, are preferably formed by. a suitable metal sputtering, painting, or other glass surface treatment process on the inner surface of this glass neck, as indicated at 41 (see Fig. 3), in a position to contact with the engaging metal surfaces.

The cross section of the glass neck 40, and the distance of separation between the points of conductive contact with the anode lead sleeve 34 and the shield tubular portion 36 are chosen to afford the necessary dielectric strength and insulating qualities to stand high differences of potential without a conductive breakdown or short circuit. Also, in accordance with the principles of this invention as later discussed, this glass neck should be partially conductive electrically, that is, should con duct some small amount of current to a degree to afford the desired potential distribution, but not permit excessive or objectionable current at the anode voltages used. In general, insulating materials as they are commonly known conduct a very slight amount of current. In this connection, glass of the compositions commonly used for tube envelopes have as a rule sufficient electrical conductivity at the temperature of normal operation in tubes of this type for the desired results; but if necessary glass of a special composition may be employed, or even some form of a surface film may be deliberately applied toobtain the degree of conductivity desired.

After the tube construction as shown and described is assembled, it is subjected to a schedule of degassing, exhaust, activation of the cathode, and introduction of a gaseous medium, such as a vapor of mercury or other metal, or a gas such as argon, xenon, hydrogen or the like at the pressure appropriate to the particular type of tube, in accordance with well known practice, whereupon the tube is sealed off in the usual manner at the exhaust tubulation indicated at 45. Among other things, during this process of fabrication, the metal coatings 41 on the glass neck 40 of the envelope E become effectively fused to the. glass and metal surfaces to afford the desired conductive contact between these parts.

The general purpose of an anode shield, such as the shield S illustrated, is to avoid effective ionization and an arc discharge as between the anode and cathode, which otherwise limits the inverse voltage the tube can stand without conducting in the wrong direction, and the positive voltage that may be applied to the anode without loss of grid control. Such peak inverse and forward voltage ratings for a grid control rectifier may be raised by anode shielding to a degree dependent upon the effectiveness of such shielding. Since breakdown and cumulative ionization of the gaseous medium in the tube may occur along any ionizing path, no matter how restricted in cross section, if the differences of potential are high enough as between surfaces at anode potential and the cathode or the shield itself, it is important to have complete shielding of all surfaces of anode potential, except for the area opposite the control grid. Various anode shielding arrangements for gaseous discharge tubes have been proposed, but all of the structure with which we are familiar fail to afford the complete shielding effect required for satisfactory operation at a high level of anode voltage. 'In this connection, any spot on a surface at. anode potential, even a spot on the anode lead close to its seal in the, tube envelope, may bea source of an electric field to cause unwanted ionization alonga path to the cathode or. to the shield, In other words, all points on thesurfface of the anode lead, even those along the surface where the lead engages the tube envelope, should be shielded against long path ionization with respect to a cathode and the shield itself, otherwise the intended anode shielding is inadequate for high Voltages.

In connection with the matter of shielding surfaces at anode potential, it is a familiar phenomenon in gaseous discharges, sometimes termed the short or mean free path principle, that the space or distance between electrode surfaces may be made small enough, with due regard to the pressure, kind of gas and other factors, to a degree where the effective ionization and initiation of an arc discharge of substantial current between such electrodes requires extremely high difierences of potential. The particular spacing effective to avoid the unwanted arc discharge is dependent upon various complex factors, and is difficult to specify for the varying conditions. From one point of view, it may be said that ionization requires electron collision with gas molecules, and is not likely to occur if the distance between the electrode surfaces corresponds with the electron mean free path for the existing gas pressure, on account of the small probability of ionizing collisions; and for this reason the desired spacing is sometimes termed mean free path spacing. However, ionization to a degree sufiicient to cause a sustained arc discharge of substantial current involves other factors than the probability of electron collisions with gas molecules, such as the electron velocity, rate of ion dissipation, and the like, so that effective results may be obtained for practical operating conditions with an electrode spacing somewhat greater than the theoretical electron mean free path for the existing conditions. For the purpose of this case, it is convenient to refer to the desired spacing here involved as between the anode shield S and surfaces of anode potential, which serves to avoid cumulative ionization and an arc discharge of substantial current, as short path spacing, which is in the order of the electron mean free path, but is not limited to the theoretical electron mean free path for the existing set of conditions. By way of explanation, and without limiting the invention, a spacing in the order of 2 mm. is representative of the appropriate short path spacing for typical pressure and voltage conditions in grid control rectifier tubes, such as illustrated; and the anode shield S is designed to provide a spacing of this order of magnitude.

The anode shield S is conveniently supported with the necessary rigidity in this space relationship by a plurality of relatively stiff supporting rods 46 having their bent ends welded to the shield and their upper ends anchored in glass envelope E. Three such supporting rods 46 are preferably employed to constitute a three-point support, although only two are shown in the somewhat diagrammatic illustration of the tube structure. One of these supporting rods 46 preferably extends through a gas tight seal in the tube envelope E to provide an external connection permitting the shield S to be readily connected to the heat shield HS or a cathode lead and maintained at the appropriate potential.

According to the theoretical concepts concerning the complex process of ionization to be avoided in connection with anode shielding, the unwanted cumulative ionization may occur if the electric field between the electrode surfaces in question causes electron movement over distances "and with velocities suflicient to produce enough ionizing collisions with the gas molecules. The electric field between extensive electrode surfaces is essentially uniform with equi-potential lines in a general parallel relationship, and the electron movement under the influence of such electric field along paths perpendicular to its equi-potential lines is essentially straight toward an electrode surface for maximum distances of travel no longer than the spacing between the electrode surfaces, so that very high electrode voltages are required to produce enough ionizing collisions when a short path spacing for the concentration of gas molecules is employed. If, however, the electric field is not uniform with parallel equi-potential lines, as in the case of the boundary of an electrode surface, then the paths of electron movement may be much longer than I the electrode spacing, and long enough to produce enough ionizing collisions to initiate an arc discharge at moderate electrode voltages. Thus, while a short path spacing is efiective for the lower part of the anode lead and the anode shield S, any gap at the upper boundary of the shield will result in a non-uniform electric field, and the curvatures in its equi-potential lines tend to cause electron movement along long ionizing paths and an arc discharge through such gap to the outer surface of the shield, thereby limiting the anode voltage that may be employed without an arc short-circuit to the shield. If the shield is extended with the same short path spacing up to the wall of the tube envelope, with the idea of avoiding any gap or space for long path ionization, then the anode voltage is applied across closely spaced points at the wall of the tube envelope, and the anode voltage usable is limited by the breakdown strength of insulation that can be provided in such a short distance. The lower boundary edge of the anode shield S may be readily disposed opposite a surface, such as the heat shield HS in the structure illustrated, which i at substantially the same potential as the shield, so that different conditions exist and the problem of gap closing and insulation is not involved. In short, the region adjacent the seal of an anode lead in the tube envelope presents a dilficult problem of space closure to avoid long path ionization, and also adequate insulation between the parts if complete shielding for high voltage anodes is to be obtained.

Considering the structure of this invention, and re ferring to Fig. 3 and the explanatory diagram of Fig. 4, a partially conductive insulating element in the formof the glass neck it? spans the gap between the upper boundary edge of the tubular extension 36 of the shield S and the sleeve 34 at anode potential. This closes up all gaps or openings for long path ionization to the cathode or outer surface of the shield S. The small current conducted by this glass neck 40, indicated by an arrow in Fig. 4 for a positive anode voltage, produces a voltage drop, so to speak, lengthwise of the glass neck to establish surface potentials at points such as indicated in Fig. 4, which result in an electric field in this region having essentially parallel equi-potential lines, such as indicated at 48. In other words, the potential distribution on the surface of the glass neck 40 dueto its conductive nature, results in substantially parallel equipotential lines in the region of this neck, and the same effect upon ionization in this region as the uniform electric field between electrode surfaces having a short path spacing. Stated another way, the joint or connection between the shield S and the anode leadadjacent its seal in the tube envelope, maintains the same effect of short path spacing in this region applicable to the rest of the shield, while at the same time aifording adequate electrical insulation to avoid a short-circuit between any part at anode potential and the shield. In this connection, the taper for the conical glass neck may be readily selected to alford a length of insulating material between the points of conductive contact with the shield and anode lead suitable for the anode voltages involved.

Although the partially conductive insulating element characteristic of this invention is conveniently formed by a neck of the glass envelope itself, as in the tube structure illustrated, it will be evident that such insulating element may be a part separate from the envelope, as in the case of a metal envelope tube, and may be made of insulating materials other than glass which have the appropriate properties of dielectric strength at the temperatures involved, the desired degree of partial electrical conductivity, and the like. Also, while a sleeve 34 is used around the anode supporting lead 30 in the construction illustrated to afford more convenient dimensions for the portioning of the parts.

tubular extension 36 of the shield S for its short path spacing to the anode lead as a Whole, it is obvious that such additional sleeve may be omitted by proper pro- Further, the anode shielding structure of this invention is not limited to the particular type of tube assumed for illustration and description in this case; and it should be understood that various modifications, adaptations and additions may be made in the general type of tube structure, and in the specific arrangement and construction of parts shown and described without departing from the invention.

What we claim is:

1. A gaseous discharge tube comprising, an evacuated envelope containing a gaseous medium, an anode having a lead sealed in said envelope, a shield of imperforate metal shaped to have a short path spacing from said anode and from its lead, and a joint between the boundary edges of said shield and said anode lead, said joint including an element of material of very slight electrical conductivity in contact with the shield and the anode lead, the shortest path through said material between said shield and said lead being greater than said short path spacing, thereby affording a dimension of said material capable of withstanding without break-down relatively high diiferences of potential, said element being eifective to establish surface potentials influencing the electric field in the region of said joint, whereby the effect of said short path spacing is extended into the region of said joint.

2. A gaseous discharge tube comprising, an anode having a lead sealed in the tube envelope, a meta-l shield having a short path spacing to said anode and its lead, and an essentially rigid connecting joint between the boundary edges of said shield and said anode lead, said joint including an elongated tubular element of material being only very slightly conductive in response to the maximum voltage occurring between said anode and said shield and being in contact with said shield and said anode lead at spaced points, the shortest path through said material between said shield and said lead being greater than said short path spacing.

3. A controllable gaseous discharge tube comprising, an evacuated envelope containing a gaseous medium, an anode having a lead sealed in said envelope, a control grid, anode shielding means including a metal shield shaped to have short path spacing to the periphery of said grid and all surfaces of said anode and its lead except for an area opposite said grid, an imperforate solid element of material being only very slightly conductive for the maximum voltages occurring between said anode and said shield and being connected between the boundary edges of said shield and said anode lead, the minimum distance between said shield and said anode lead through said element being greater than said short path spacing, said element thereby affording a dimension of said material capable of withstanding without breakdown the maximum voltage appearing between said shield and said anode lead, said element providing due to its slight electrical conductivity a distribution of electric field potentials to avoid effective ionization in the adjacent region of the gaseous medium at anode to shield difierences of potential essentially as high as said s'hort path spacing permits.

4. A gaseous discharge tube comprising, an evacuated envelope containing a gaseous medium, an anode having a lead sealed in said envelope, and an electrically conductive structure completely shielding all surfaces of said anode and its lead except an area to receive the electron current conducted by the tube, said shielding structure including a tubular portion around the anode lead in a short path spacing relationship thereto, and an essentially rigid joint between the boundary ends of said tubular portion and said anode lead formed of a conieally shaped element of material being capable of conducting current to only a slight degree in response to the maximum potential difference occurring between said anode and said shield, the minimum path length through said element between said boundary ends of said tubular portion and said anode lead being substantially greater than said short path spacing.

5. The combination according to claim 1 wherein said element is formed of glass.

6. The combination according to claim 1 wherein said element has on its surface a metallic film in the areas of contact with the anode lead and shield.

7. A gaseous discharge tube comprising, an anode having a lead sealed in an envelope containing an ioniz able medium, an imperforate shield of electrically conductive material closely spaced in the order of the electron mean-free path for the ionizable medium to all points on the surface of said anode and its lead except for a discharge receiving area, said shield including a tubular portion around the anode lead adjacent its seal, and a joint of high voltage break-down characteristics between the boundary edges of said tubular portion of the shield and said anode lead, said joint comprising an elongated conical element of glass and afiording a minimum distance path between said boundtry edges of said shield and said anode lead being much greater than said close spacing, said element of glass being only very slightly conductive at the operating anode voltages to provide surface potentials modifying the electric field in said boundary region, whereby the break-down volt-age for the ionizable medium in the region of said joint is not materially less than that aifordcd by said close spacing.

References Cited in the file of this patent UNITED STATES PATENTS 2,070,816 Von Wedel Feb. 16, 1937 2,084,725 Dallenback June 22, 1937 2,106,847 Knickamp Feb. 1, 1938 2,504,706 Lempert Apr. 18, 1950

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