US3806753A

US3806753A - Electric discharge tube comprising a directly heatable cathode

Info

Publication number: US3806753A
Application number: US00329198A
Authority: US
Inventors: Warmerdam J Van
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1972-02-17
Filing date: 1973-02-02
Publication date: 1974-04-23
Anticipated expiration: 1991-04-23
Also published as: GB1414392A; DE2304771A1; FR2172354A1; FR2172354B1; DE2304771B2; JPS557655B2; JPS4890669A; CA990778A; NL7202069A; DE2304771C3; CH552281A

Abstract

In a reticulated grid cathode which has a flexible connection at one end to the conductor for the filament current present inside the cathode, the flexible connection consists of a number of thin metal strips of the same thickness. The strips constitute an approximately semi-circular arc and each consist of two strips of preferably the same thickness. In order to minimize the bending stress occurring in the strips, the diameter of the arc formed by the strips is chosen to be as large as possible relative to the radial dimensions of the cathode.

Description

United States Patent [191 Van Warmerdam 1 ELECTRIC DISCHARGE TUBE COMPRISING A DIRECTLY I-IEATABLE CATHODE [75] Inventor: .Ioannes Cornelis Van Warmerdam,

Emmasingel, Eindhoven,

Netherlands [73] Assignee: U.S. Philips Corporation, New

' York, NY.

[22] Filed: Feb. 2, 1973 [21] Appl. No.: 329,198

[30] Foreign Application Priority Data Feb. 17, 1972 Netherlands 7202069 [52] US. Cl 313/341, 313/278, 313/348 [51] Int. Cl HOlj 1/15, I-I0lj 19/08 [58] Field of Search 313/341, 278, 348

[56] References Cited UNITED STATES PATENTS 2,928,978 3/1960 Morton 313/341 [451 Apr. 23, 1974 FOREIGN PATENTS OR APPLICATIONS 987,461 3/1965 Great Britain 313/341 Primary Examiner-Archie R. Borchelt Assistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or FirmFrank R. Trifari [5 7] ABSTRACT In a reticulated grid cathode which has a flexible connection at one end to the conductor for the filament current present inside the cathode, the flexible connection consists of a number of thin metal strips of the same thickness. The strips constitute an approximately semi-circular arc and each consist of two strips of preferably the same thickness. In order to minimize the bending stress occurring in the strips, the diameter of the are formed by the strips is chosen to be as large as possible relative to the radial dimensions of the cathode.

4 Claims, 3 Drawing Figures PATENTEDAPR 23 m4 3" sum 1 BF 2 ELECTRIC DISCHARGE TUBE COMPRISING A DIRECTLY HEATABLE CATHODF,

The invention relates to an electric discharge tube comprising a cylindrical directly heatable cathode which is constructed from wires of thoriated tungsten or another high-melting-point metal which are wound according to left and according to right helical lines and extend on either side of a same cylinder surface, said wires being welded together at least at a number of the intersections, said cathode being rigidly connected at one end to a supporting cylinder which extends coaxially with the cathode and being connected at its other end to a metal conductor present inside the cathode by means of a number of thin metal strips extending radially but axially at the ends.

The above-described cathodes constructed from metal wires are known by the name of reticulated grid cathodes. It is known from the U.S. Pat. No. 2,928,978 to connect one end of such a cathode by means of flexible strips to the current conveying conductor present inside the cathode. An object of this flexible connection is to prevent that the reticulated grid cathode is deformed by thermal stresses resulting from the lagging of the variations in length of the supporting cylinder relative to that of the cathode when the filament current is switched on and off. For that purpose it should be possible for the thermal expansion of the cathode to take place in an unhindered manner in an axial direction which requires the connection between the cathode and the said conductor to be extremely flexible and thus to be formed from strips which are as thin as possible. On the other hand, said strips may not be too thin since they are traversed by the filament current for the cathode and thus should have a minimum electric resistance.

The cathode of the above-described patent has a length of approximately 6 cm and a diameter of approximately cm. The flexible strips which connect said cathode to the interior conductor have an S- shaped bent portion. Owing to the small distance between the conductor and the cathode, the strips are very sharply bent, that is to say over a very short distance. As a result of this, the flexibility of the construction in the axial direction is comparatively small although this will hardly be a drawback for cathodes of the above-described dimensions. In cathodes, however, having a length of more than 200 mm, much higher re quirements are imposed upon the flexibility of the suspension construction and the known construction can no longer be used.

Deformation of the cathode at the operating temperature as a result of its weight can be prevented by a correct choice of the length and thickness of the cathode wires, the number of welded intersections, the diameter of the cathode and the number of cathode wires. The problem is definitely more difficult to obtain a connection between the cathode and the inside conductor which in addition to a sufficient flexibility also has a very low electric resistance.

From U.S. Pat. No. 3,560,791 a construction is known the flexible connection of which between the cathode and the central conductor is formed by a cap which is constructed from a number of thin circular dishes which are stacked one on the other and are rotated through a given angle relative to each other, said dishes each comprising leaf springs which extend radially towards the cathode and are bent at right angles at that area. The rigidity of such a construction, however, is still too large to use it in the reticulated grid cathode in question having a length of more than 200 mm. Another drawback of this construction is in addition that, taken in the radial direction to the center of the dishes, the leaf springs enclose each other for an ever increasing part, as a result of which the thermal dissipation at that area is impeded to a considerable extent.

It is an object of the invention to provide a construction of the type described in the preamble in which the connection between one end of the reticulated grid cathode and the interior conductor has a low electric resistance, while nevertheless the flexibility in the axial direction is so large that deformation of the cathode as a result of the thermal stresses when the filament current is switched on and off is minimized.

In a cylindrical reticulated grid cathode which is flexibly connected at one end to a metal conductor present centrally inside the cathode and in which said connection is formed by a number of thin metal strips of equal thickness extending radially but axially at the ends, according to the invention the two ends of the said strips extend over a restricted length in the same sense in the axial direction and the part present between said ends constitutes an approximately semi-circular arc the diameter of which, measured between the two parallel ends, is equal to 0.2 to 0.4 times the diameter of the cylindrical cathode.

The construction according to the invention has particularly favourable properties. The shape in which the strips are bent give the construction a great flexibility in the axial direction. Another advantage of the construction according to the invention over that mentioned in the above described patent specifications is that when the length of the cathode varies, the strips roll up and down on the elongation of the connection surfaces during which movement radial forces hardly occur, provided the radius of curvature of the are formed by the strips is chosen to be as large as possible relative to the radial dimensions of the cathode. In that case, actually, the bending moment occurring in the strips is as small as possible. The upper limit imposed upon said radius of curvature with a given cathode diameter depends upon the required number of strips which must be capable of terminating on the circumference of the filament current conductor present centrally inside the cathode. Actually, with a given width of the strips, the minimum number is fixed with the electric resistance still just permissible which is represented by the parallel arranged strips.

A reduction of the electric resistance of the strips, without the said favourable properties of the construction being lost, is realised if the strips, taken from the narrow end secured to the cylinder having the smaller diameter, change into a part having a larger width via a gradual transistion.

Since the rigidity of the strips is proportional to the third power of the thickness of the strips, it is particularly favourable to compose each individual strip from two sub-strips or strip elements of preferably the same thickness. The flexibility then becomes better by approximately a factor four, while the electric resistance of the strips does not vary. A construction of each individual strip from more than two such strip elements has the drawback that the heat dissipation of the strips enclosing each other on either side would be too strongly impeded.

The invention will be described in greater detail with reference to the drawing, of which FIG. 1 is a perspective view of an embodiment of a suspension construction for a cathode according to the invention;

FIG. 2 is a longitudinal sectional view of the cathode suspension of FIG. 1, and

FIG. 3 shows a strip in the stretched condition.

The reticulated grid cathode 1 shown diagrammatically in FIG. 1 is constructed from 30 wires of thoriated tungsten extending according to a left helical line and 30 wires extending according to a right helical line, said wires being welded together at the intersections. The cathode wires have a thickness of 0.45 mm and constitute a cathode having a length of 257 mm and a diameter of 78.8 mm. At one end, the cathode 1 is welded to the outer ring of a circular rectangular metal channel 2 and at its other end to a circular ring 3, said ring constituting one end of the hollow metal supporting cylinder 4. Inside the hollow cylinder 4 and the cathode 1, the hollow metal cylinder 5 extends coaxially and forms part of the filament current circuit of the cathode and changes via a dish-like member 6 into a hollow cylinder 7 of smaller diameter. The holes 11 present in the cylinder 5 provide access to a few nonevaporating getters present behind said holes. The thin molybdenum strips 8 are secured to the free end of the cylinder 7 and clamped between the cylinder wall and the strip 9 which likewise consists of molybdenum. From this connection the strips 8 extend initially in the axial direction, then describe an approximately semicircular arc and finally terminate again in the axial direction between the molybdenum strip 10 and the inner ring of the cathode channel 2.

In order to improve the flexibility of the construction, each individual strip consists of two strips which overlap each other entirely and have each a thickness of 0.1 mm.

FIG. 2 is a longitudinal sectional view of the suspension construction for the cathode of FIG. 1. The strips 8 consist of two

strips

12 and 13 which each have a thickness of 0.1 mm. The connection between the cathode 1 and the conductor 7 is constituted by 16 such composite strips. The diameter of the are formed by the strips, measured between the parallel end members thereof, is 23 mm, while the supporting cylinder 7 has a diameter of 22 mm and the cathode 1 a diameter of The construction according to the invention furthermore consists of a screening cap 14 which for clarity of the drawing is not shown in FIG. 1. By means of the to the metal sleeve 15. The sleeve 15 is welded to the cylinder 7 at 18. The screening cap 14 is at cathode potential and prevents any electron emitted by the strips 8 and contributing to an undesired anode dissipation from reaching the anode.

FIG. 3 shows a tape 8 in the stretched condition. At 21 the strip has a portion which narrows over a length of 12 mm which portion changes at 22 into a section having a length of 28 mm and a width of 6 mm and at 23 changes into a section having a length of 8 mm and a width of 3.5 mm.

1. An electric discharge tube, comprising:

a. a support cylinder;

b. a cylindrical directly heatable cathode which is constructed from wires of a high-melting-point metal, said wires being wound according to left and according to right helical lines and extending on either side of a common cylinder surface defined thereby, said wires being welded together at least at a number of the intersections therebetween, said cathode being rigidly connected at one end to said support cylinder and extending coaxially with said support cylinder;

c. a metal conductor present inside said cathode; and

d. plural thin metal strips extending radially between and connecting said metal conductor and the other end of said cathode, said strips extending axially at the respective ends thereof, end portions of each of said strips extending in substantially the same axial direction and the part of each said strip present between said ends thereof constituting a substantially 3. An electric dicharge tube as claimed in claim 1, wherein said strips individually have a narrow portion, a wide portion and an intermediate transition portion which gradually merges said narrow and wide, said wide portion being disposed at said cathode grid and said narrow portion being disposed interiorly thereto.

4. An electric discharge tube as defined in claim 2, wherein said strip elements are substantially equal in thickness.

t l l t t

Claims

1. An electric discharge tube, comprising: a. a support cylinder; b. a cylindrical directly heatable cathode which is constructed from wires of a high-melting-point metal, said wires being wound according to left and according to right helical lines and extending on either side of a common cylinder surface defined thereby, said wires being welded together at least at a number of the intersections therebetween, said cathode being rigidly connected at one end to said support cylinder and extending coaxially with said support cylinder; c. a metal conductor present inside said cathode; and d. plural thin metal strips extending radially between and connecting said metal conductor and the oTher end of said cathode, said strips extending axially at the respective ends thereof, end portions of each of said strips extending in substantially the same axial direction and the part of each said strip present between said ends thereof constituting a substantially semi-circular arc having a diameter as measured between said parallel end portions, equal to about 0.2 to about 0.4 times the diameter of said cylindrical cathode.

2. An electric discharge tube as claimed in claim 1, wherein each of said strips consists of two strip elements which substantially entirely overlap each other.

3. An electric dicharge tube as claimed in claim 1, wherein said strips individually have a narrow portion, a wide portion and an intermediate transition portion which gradually merges said narrow and wide, said wide portion being disposed at said cathode grid and said narrow portion being disposed interiorly thereto.