US2568552A - High-voltage electronic rectifier tube of the incandescent filament type - Google Patents

Description

p 1951 B. K. M. MAeNussdN 68,552

HIGH-VOLTAGE ELECTRONIC RECTIFIER TUBE O F THE INCANDESCENT FILAMENT TYPE Filed April 26, 1949 2 Sheets-Sheet 1 23 i I 5-2 1 1M Y i: lo 5::3 I I Q II 1 1| H 1 INVENTOR 5m fiflfla hussm ATTORNEYS l 18, 1951 B K. M. MAGNUSSON 2,568,552

HIGH-VOLTAGE ELECTRONIC RECTIFIER TUBE OF THE INCANDESCENT FILAMENT TYPE Filed April 26, 1949 2 Sheets-Sheet 2 INVENTOR ATTORNEYS Patented Sept. 18, 1951 HIGH-VOLTAGE ELECTRONIC RECTIFIER TUBE OF THE INCANDESCENT FlLA- MENT TYPE Bror K. M. Magnusson, Stamford, com, assignor to Machlett Laboratories Incorporated, Springdale, Conn, a corporation of Connecticut Application April 26, 1949, Serial No. 89,705

12 Claims. 1

This invention relates to electronic tubes of the rectifier type and is concerned more particularly with a rectifier tube which has elements of novel configuration and relative arrangement and, as a result, is superior electrically to prior rectiflers. In the new tube, the stresses in the cathode filament resulting from electrostatic forces acting thereon are minimized and, in addition, the possibility of bombardment of the filaments by energetic ions is reduced. It is, accordingly, possible to use thoriated tungsten wire as the filament material in the new tube, even though the tube is to operate at the extremely high voltages, such as 80,000 volts or more, commonly employed in X-ray apparatus.

In a rectifier tube for operation at extremely high voltages, it is desirable to construct and mount the filament in such manner that the required quantity of rectifier current will pass from the filament to the anode without a high space charge being built up in the interelectrode region. The reason is that, with a high space charge, the voltage drop between the electrodes on the conducting half cycle becomes high and, under such conditions, positive ion bombardment of the filament is bound to occur with resultant damage thereto, if the energy of the bombarding particles is high. The voltage drop between filament and anode is a function of the electron current density and of the spacing between the filament and the anode and the spacing is limited, in that it should be of such magnitude that the potential gradients developing on inverse voltage will not be high enough to cause the anode to emit an appreciable quantity of electrons by cold emission. It is, accordingly, advantageous to operate with as low an electron current density at the filament surface as possible and this requires the use of a filament of a total length which is large in relation to the curent fiowmg therethrough. In addition, the filament should be of such form that the stresses resulting from electrostatic forces acting thereon are minimized. The provision of such a filament presents obvious difiiculties.

Thoriated tungsten filaments have heretofore been regularly used in low voltage rectifier tubes because, for given emission currents, they consume much less power for heating than is required by filaments of pure tungsten. It has not, however, been considered feasible up to the present to employ thoriated tungsten filaments in high voltage rectifiers and particularly in tubes which are to operate at the high voltages commonly employed in X-ray apparatus. One reason against using thoriated tungsten filaments in high voltage tubes is that, if the surface of such a filament is bombarded by positive ions of appreciable energy, the emission efllciency of the filament is impaired, probably because the monatomic layer of thorium is sputtered away. As some positive ions will always be present in such a tube because of the lack of a perfect vacuum. bombardment of the filament is inevitable, but it can be tolerated if the energy of the bombarding ions is not too great. Another reason why thoriated tungsten filaments have not been employed in high voltage tubes is that they are liable to be broken by the mechanical forces imposed on them by the electrostatic forces to which they are subjected.

The present invention is directed to the provision of a high voltage rectifier tube, in which the elements are of novel construction and in which thoriated tungsten may be employed as a 20 filament material without the disadvantages attendant upon the use of such material in prior tubes to operate at comparable voltages. In the new tube, a sufficient total-length of filament may be readily employed and arranged in such manner that the required amount of rectifier current will pass without the building up of a high space from the anode surface at a substantially uniform distance, which is the smallest permissible in view of the potential gradients on inversevoltage, and the forward voltage drop required to utilize emission from all parts of the filament is thus kept at 5 p a minimum. The filament is so constructed that the electrostatic forces acting thereon are minimized and the filament is not subjected at any point to a stress with a bending moment. In addition, the prevention of the development of a high space charge reduces the possibility of bombardment of the filament with energetic ions, so that, when a thoriated tungsten filament is employed, deactivation by bombardment is inhibited.

The construction of thetiibe in such manner as to operate with a low space charge, as described, is advantageous, regardless of the filament material psed, because the operating efliciency of the tube is increased and the power loss in the high voltage circuit, of which the tube is a part, is reduced. Such power loss must be dissipated by the anode and a reduction in the loss reduces the temperature at which the anode operates and which might otherwise limit the operatload capacity of the tube.

In addition to the features above mentioned,

the new tube includes novel shielding means,

which prevent bombardment of'tlie glass envelope by electrons cold-emitted from the anode during inverse voltage and pulled with high energy toward the cathode structure. The new shielding means intercepts some of the electrons which would otherwise pass between the filament support wires and, after having been focused thereby, would continue across the tube and strike the glass wall with consequent overheating of the glass at the point of bombardment or liberation of gas, secondary electrons, or both.

For a better understanding of the invention, reference may be made to the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional view of a tifier tube embodying the invention; a

Fig. 2 is a fragmentary sectional view on the line 2-2 of Pig. 1;

Figs. 3 and 4 are sectional views on the lines 3-3 and 4-8, respectively, of Fig. 2;

Fig. 5 is a view partly in longitudinal section and partly in elevation of a modified electrode combination that may be used in the new tube;

Fig. 6 is a sectional view on the line 8-8 of Fig. 5;

Fig. '1 is a view partly in side elevation and a partly in longitudinal section showing another electrode combination; and

Figs. 8, 9, and 10 are sectional views on the lines 8-8, 9-8, and lO-IO, respectively, of Fig. '7.

The rectifier tube illustrated comprises an evacuated envelope [8 ordinarily of glass, which is re-entrant at one end II and formed at its otherend with a neck l2. At the re-entrant end, the envelope is formed with a tubulation l3, by which its interior may be exhausted, and at the inner end of the re-entrant portion, there is a press I4, through which are sealed parallel lead-in wires l8. The wires are connected to parallel stem wires 18 of the cathode structure, which extend inwardly from the press. Each stem wire is bent to provide a lateral portion l1 and an end portion I lying parallel to the main length of the wire. The lateral portions I! of two wires converge slightly so that the end portions lie closer together than the main portions of the wires. A supporting wire I! is embedded in the press between the wires I 8 and has a lateral portion 20 extending in the opposite direction from the portion "of the wires l8 and terminating in a loop 2|.

The filament 22 of the tube is made, in two parts of a single length of wire, which has its ends attached to the terminal portions ll of stem wires l8 and passes through the loop 2| on wire 28, so that each part of the filament is anchored at one terminal l8 of a stem wire and at loop 2|. 'Each filament part is substantially semi-circular in form and lies in a plane and the planes, in which the two parts of the filament lie, intersect at a substantial angle which, in the construction illustrated in Fig. 2, is an angle of approximately 90.

A cup-shaped shield 23 is mounted on the supporting wire I! and has openings, through which the stem wires I8 extend. The shield is concave toward the press l4 and projects well past the end of the press and the inmost end of the reentrant portion ll of the envelope. A second shield 24 of cylindrical form is mounted on the shield 23 to extend outwardly therefrom and enclose the wires l8 and I! to a point close to their first bends.

The anode of the tube comprises a stem 28 sealed through the end of the neck of the envelope and lying substantially in coaxial alignment with the wires l8 and I. A head 28 is attached to the inner end of the stem 28 and the head is concave toward the filament. In the construction shown, the head is an approximately hemispherical shell and it is of such size and position that its rolled rim 2l overlaps the end of the cylindrical shield 24. The portions of the wires l8 and I9 exposed beyond the end of shield 24, accordingly. lie within the concavity of the head as do also the two portions of the filament. The concave surface of the head of the anode and the two parts of the filament are substantially concentric and each part of the filament thus lies substantially uniformly spaced throughout its length from the concave surface of the head.

The filament is preferably of thoriated tungsten, which is quite brittle, but, since the parts of the filament are approximately concentric with the anode surface, the filament is not subjected at any point to a stress with a bending moment. Also, the uniform spacing of the parts of the filament from the anode makes it possible to keep the space charge at a minimum in all parts of the interelectrode region. With the filament arrangement described, the total length of the filament may be such that the required amount of electron current will pass from the filament to the anode without the building up of a high space charge with corresponding high forward voltage. As a result, the energy of the ions bombarding the filament is relatively low and they have little eifect on the emitting surface. The two parts of the filament are spaced angularly from one another to such an extent that their mutual shielding effect will not cause distortion of the electrostatic field sufiicient to produce any considerable unbalance of stresses on the two parts of the filament.

In the construction illustrated, each part of the filament is substantially semi-circular in form and lies in a plane, and the head of the anode is approximately hemispherical. It is to be understood that a single filament may be used, in which event the filament lies in a plane preferably through the axis of the anode and is opposed to the face of the anode, that is, the convexity of the filament extends toward the concavity of the anode face. It is not necessary that the .filament be semi-circular and the anode hemispherical and, for some purposes, the filament may be less than a semi-circle in length and the anode less than hemispherical in form. Also, the filament and the surface of the anode, which it opposes, may have other curvatures or shapes, for example, parabolic or approximately frustoconical, so long as the filament is approximately equi-distant throughout its length from the anode face.

In the construction shown in Fig. 5, the anode includes a hollow cylindrical head 28 attached at one end by straps 29 to a stem 30. The cathode includes a pair of live stem wires 3|, which extend axially of the anode into the other end thereof and are then bent approximately radially toward the inner surface of the anode and terminate in end sections 32, which lie substantiallytwo parts. Each filament is connected at its ends to the terminal sections 32 of wires 3| and between its ends to the terminal section 34 of wire 33. Each filament lies in a plane normal to the axis of the anode and both parts of each filament are approximately semi-circular. Accordingly. each part of a filament lying between a terminal section 32 of a wire 3| and the terminal section 34 of wire 33 is approximately equidistantly spaced from the inner surface of the anode. The main portions of wires 3| and 33 are shielded by a shielding means similar to members 23 and 24, shown in Fig. 2, and including a cylindrical member 36 extending into one end of the anode head 23.

In the constructions shown in Figs. 7 to 10, inc., the anode comprises a stem 31 and a hollow head made up of a hemispherical part 33a attached to the stem and a cylindrical part 3812 integral with part 33a and projecting beyond the imaginary rim of the hemisphere. The cathode includes a pair of live stem wires 39, which extend into the open end of the anode head and are bent outwardly and then bent again to form terminal sections 39a extending parallel to the axis of the anode head and lying close to one another. The cathode also includes another pair of support wires 40, which project into the open end of the anode head and are bent outwardly and then bent again to form terminal sections 40a lying parallel to the axis of the anode head and close to one another. Another support wire 4| projects into the open end of the anode head between wires 40 and is bent to form a radial portion 4|a terminating in a loop 42.

The filament of the tube is in four parts, which may be formed of a continuous length .of wire. One end of the filament wire is connected to the terminal section 39:; of a wire 39 and has a part 43 approximately semi-circular in form and lying in a plane normal to the axis of the anode head. The other end of this part of the filament is bent to extend along the terminal section 4011 of a wire 40 and is secured thereto in any suitable manner. From the section 40a, the next part 44 of the filament extends in approximately semi-circular form and in a plane to and through loop 42, the plane of this part of the filament lying at about 45 to the axis of the anode head. The third part 45 of filament extends from loop 42 to the terminal section 400. of the other wire 40 and is then bent to extend along the terminal section 40a and is secured thereto. Part 45 of the filament is approximately semi-circular in form and lies in approximately a plane at about 45 to the axis of the anode head and thus at about 90 to the plane of part of the filament. The fourth part 46 of the filament extends from terminal section 40a to terminal section 39a of the second wire 39. Between the terminal sections, the filament part 46 lies substantially in a plane normal to the axis of the anode head and is semi-circular in form. With this arrangement, the filament parts 43 and 46, which lie in the cylindrical part of the anode head, are approximately equi-distantly spaced from the interior of the anode head throughout their lengths between their points of anchorage to terminal sections 39a and 4011. Similarly, the filament parts 44, 45 lying within the hemispherical part of the anode head are approximately equi-distant from the inner surface of the head throughout their lengths between their points of anchorage at terminals 40a and loop 42'. The tube of Fig. '7 includes shielding means,

6 of which only the cylindrical part 41 is shown and which is generally similar to the shielding means 23, 24 of Fig. 2.

In all forms of the tube described, the space charge is kept at a minimum, partlybecause of the uniform spacing of all parts of the filaments from the anode and partly because the total length of filament employed may be such that the filament current density may be kept relatively low. With the space charge limited, the forward voltage drop is kept correspondingly low, so that the energy of the ions bombarding the filament is such that they have little eflect on the emitting surface. In those forms of the cathode in which two or more filament parts are employed, the parts of the filament are spaced angularlv from one another to such an extent that their mutual shielding effect will not cause distortion of the electrostatic field sufiicient to produce any unbalance of stresses on the filament parts.

In a rectifier tube of the high voltage type, the potential gradient under high inverse voltage between the edge of the anode and the cathode structure is likely to cause cold emission of electrons from the edge of the anode and such elec- I trons are pulled with high energy toward the cathode structure. If the shielding means, such as that made of parts 23, 24 in Fig. 2, were not employed, some of the cold-emitted electrons would pass between the stem wires and would be focused thereby and continue across the tube to bombard the glass wall in their path. This would result in either overheating of the glass at the point of bombardment or liberation of gas or secondary electrons or both from the glass. The shielding means enclosing the wires and extending into the concavity of the anode head intercepts such electrons and thus protects the envelope.

The new rectifier tube, when provided with a thoriated tungsten filament, may be employed to unusual advantage in X-ray equipment because of its low consumption of power for filament heating. In a typical X-ray apparatus of present construction, it is common practice to utilize four rectifier tubes and a high voltage transformer to supply the X-ray tube with fullwave rectified voltage of the required value. The rectifier tubes and transformer are ordinarily installed in an oil-filled enclosure and the heat from the filaments of the tubes is dissipated by transmission through the oil to the wall of the enclosure and by radiation and air convection from the outer surface of the wall. So long as the enclosure for the rectifier tubes and transformer was relatively large, the dissipation'of the heat from the tube filaments without excessive temperature rise presented no difiiculty, but, more recently, it has become desirable to reduce the size of the enclosure, so that the X-ray apparatus may be readily transported as, for example, in carrying out mass population X-ray surveys. Such a reduction in size of the enclosure could not be achieved with prior tubes because of the amount of filament heat to be dissipated, but, when new tubes with thoriated tungsten filaments are used, the heating current is substantially cut down and the enclosure may be made relatively small and compact.

I claim:

1. An electronic rectifier tube, which com' prises an evacuated envelope and an anode and a cathode within the envelope in coaxial relation, the anode having a concave face of the form of a, surface of revolution and the cathode includingspacedsupports on opposite sides of the common axis of the anode and cathode and a filament anchored to said supports only, the portion of the filament between said supports lying in a plane and being of the form of a substantially circular arc of not to exceed 180 and substantially concentric with said anode face, said portion of the filament being opposed to and substantially equidistantly spaced throughout its length from said anode face and lying in that plane passing through the center of the arc of the filament, in which said portion of the filament is at the minimum spacing from the anode face, the chord joining the ends of said portion of the filament being longer than the radius of said anode face.

2. An electronic rectifier tube, which comprises an evacuated envelope and an anode and a cathode within the envelope in coaxial relation, the anode having a concave face of the form of a surface of revolution and the cathode including spaced supports on opposite sides of the common axis of the anode and cathode and a filament lying substantially entirely within the concavity of saidface and anchored to said supports only, the portion of the filament between said supports lying a plane and being of the form of a substantially circular arc of not to exceed 180 and substantially concentric with said anode face, said portion of the filament being opposed to and substantially equidistantly spaced throughout its length from said anode face and lying in that plane passing through the center of the arc of the filament in which said portion of the filament is at the minimum spacing from the anode face, the chord joining the ends of said portion of the filament being longer than the radius of the anode face.

3. An electronic rectifier tube, which comprises an evacuated envelope and an anode and a cathode within the envelope in coaxial relation, the anode being hollow and having an inner face of the form of a surface of revolution and the cathode including at least a pair of filaments each anchored at spaced points and out of contact with each other between said points, the portion of each filament between its points of anchorage lying in a plane and being opposed to and substantially equidistantly spaced throughout its length from the inner face of the anode, said planes intersecting in a line normal to the axis of the inner face of the anode, the chord joining anode having a concave face ofthe form of a surface of revolution and the cathode including spaced supports on opposite sides of the common axis of the anode and cathode and a thoriated tungsten filament anchored to said supports only, the portion of the filament between said supports lying in a plane and being of the form of a. substantially circular arc of not to exceed 180 and substantially concentric with said anode face, said portion of the filament being opposed to and substantially equidistantly spaced throughout its length from said anode face and lying in that plane passing through the center of the arc of the filament, in which said portion of the filament is at the minimum spacing from the anode face, the chord joining the ends of said portion of the filament being longer than the radius of said anode face.

6. An electronic rectifier tube, which comprises an evacuated envelope and an anode and a cathode within the envelope in coaxial relation, the anode having a concave substantially hemispherical face and the cathode including spac'ed supports on opposite sides of the common axis of the anode and cathode and a thoriated tungsten filament anchored to said supports only, the portion of the filament between said supports lying in a plane and being of the form of a substantially circular arc of not to exceed 180 and substantiallyconcentric with said anode face, said portion of the filament being opposed to and substantially equidistantly spaced throughout its the ends of each of said filaments being longer than the radius of said inner face of the anode. 4. An electronic rectifier tube, which comprises an evacuated envelope, an anode and a cathode within the envelope in coaxial relation, the anode being hollow and having an inner face of the form of a surface of revolution, the cathode including a pair of supporting members extending inwardly from the wall of the envelope approximately coaxially of the anode and having spaced free end portions within the anode offset radially from the center of the anode and a filament anchored to said free end portions of the members,

' the portion of the filament between its points of anchorage lying in a plane and substantially equidistantly spaced throughout its length from the inner face of the anode, and shielding means enclosing the supporting members from the wall of the envelope to within the anode.

5. An electronic rectifier tube, which comprises an evacuated envelope and an anode and a oath ode within the envelope in coaxial relation, the

length from said anode face and lying in that plane passing through the center of the arc of the filament, in which said portion of the filament is at the minimum spacing from the anode face, the chord joining the ends of said portion of the filament bein longer than the radius of said anode face.

7. An electronic rectifier tube, which comprises an evacuated envelope and an anode and a cathode within the envelope in coaxial relation, the anode having a hollow substantially hemispherical head and the cathode including spaced supports on opposite sides of the common axis of the anode and cathode and a filament anchored to said supports only, the portion of the filament between said supports lying in a plane and being of the form of a substantially semi-circular are substantially concentric with said anode face, said portion of the filament being opposed to and substantially equidistantly spaced throughstantially of a hemisphere provided with a cylindrical flange projecting from the edge of the hemisphere and the cathode including at least a pair of filaments one within the hemispherical part of the anode head and the other within the cylindrical part of said head, each filament being anchored at spaced points with the portion of the filament between said points lying in a plane and substantially equidistantly spaced throughout its length from the inner face of the part of the anode head, within which the filament lies.

9. An electronic rectifier tube, which comprises an evacuated envelope, an anode comprising a stem sealed through the wall of the envelope and a head mounted on the inner end of the stem and having a concave surface facing away from the stem, a cathode within the envelope coaxial with the anode and including a pair of stem wires sealed through the wall of the envelope and extending inwardly toward the anode approximately along an-axis of the anode, the inner ends of the wires lying inward from the plane of the rim of the concave surface of the anode and extending radially from the center of said anode, and a filament connected at its ends to the inner ends of the stem wires and extending from one side of an axis of the anode to the other, the filament being substantially uniformly spaced throughout its length from said concave surface, and shielding means interposed between the anode and the wall of the envelope adjacent the stem wire seals and including an extension enclosing the stem wire and terminating within the rim of said concave surface.

10. An electronic rectifier tube, which comprises an evacuated envelope, an anode comprising a stem sealed through the wall of the envelope and a head mounted on the inner end of the stem and having a concave surface facing away from the stem, and a cathode within the envelope coaxial with the anode and including a pair of stem wires sealed through the wall of the anode approximately coaxial with the anode, the inner end portions of the wires lying within the concavity of the anode head and being oilset radially from the center of the anode to terminate at one side of the axis of the anode, a supporting element carried by the wall of the envelope and extending toward the anode substantially coaxial with the anode, the inner end portion of the element lying within the concavity of the anode head and being offset radially from the center of the anode to terminate at the side of the axis of the anode opposite to that where the ends of the stem wires lie, and a thoriated tungsten filament lying within said concavity and connected at its ends to the ends of the stem wires and between its ends to the end of the element, each portion of the filament between the end of a stem wire and the end of the element lying substantially uniformly spaced throughout its length from the concave surface of the anode head andsaid portions of the filament lying in intersecting planes defining a considerable angle.

envelope and extending inwardly toward the 11. An electronic rectifier tube, which comprises an evacuated envelope and an anode and a cathode within the envelope in coaxial relation, the anode having a hollow head of the form substantially of a hemisphere provided with a cylindrical flange projecting from the edge of the hemisphere and the cathode including a pair of filaments within the hemispherical part of the anode head and another pair of filaments within the cylindrical part of said head, each filament being anchored at spaced points with the portion of the filament between said points lying in a plane and substantially equidistantly spaced throughout its length from the inner face of the part of the anode head, within which the filament lies, said portions of the filaments within the cylindrical part of the head lying in a common plane and said portions of the filaments within the hemispherical part of the head lying in planes at approximately right angles to one another and at approximately 45 to said common plane.

12. An electronic rectifier tube, which comprises an evacuated envelope and an anode and a cathode within the envelope in coaxial relation, the anode having a head of hollow cylindrical form and the cathode including a filament anchored at spaced points and lying within the anode head, the portion of the filament between said points lying substantially equidistantly spaced throughout its length from the inner face of the anode and in a plane approximately normal to the axis of the anode head.

BROR K. M. MAGNUSSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,613,503 Daumann Jan. 4, 1927 1,628,982 Hulsizer May 17, 1927 1,715,326 Hendry May 28, 1929 2,350,270 Atlee May 30, 1944 2,422,141 Skehan et al June 10, 1947 2,422,142 Skehan et al. June 10, 1947 FOREIGN PATENTS Number Country Date 188,108 Great Britain Nov. 9, 1922

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