US2202687A - High-voltage discharge tube - Google Patents

Description

Patented May 28, 1940 UNITED STATES eres-er FFICE to N. V. Philips' Gloeilampenfabrieken,

Eindhoven, Netherlands Application November 23, 1937, Serial No. l76,102 In Germany December 16, 1936 8 Claims.

My invention relates to high-voltage discharge tubes, and more particularly to an envelope construction for such tubes.

As my invention is particularly advantageous for use in X-ray tubes, I shall describe the same in this connection. However, it is not limited thereto but is equaliy applioable to other types of high-voltage tubes, such as rectifiers and. corpuscular ray tubes.

Discharge tubes for very high Voltages, i. e. voltages of the order of 290 kilo volts and higher, must have insulating envelope portions of large size. This increases the size of the tubes and makes them relatively fragile and subject to breakage, and as they are comparatively costly,

this is a serious disadvantage. Furthermore, the portability of the tubes, which is particularly important in the case of irradiation tubes, is obviously greatly impaired by their large size.

The main object oi my invention is to over come the above difiiculty, and to provide highvoltage discharge tubes of small size and of strong Construction.

In accordance with the invention I provide, at least at one end of the tube, an outwardly-open fold in the insulation part of the tube envelope. This old, which is of annular shape, surrounds the tube axis, and extends into the vacuum space of the tube While remaining at a considerable distance from the voltage drop space between the high-voltage eleotrodes. The space formed by the fold is preferably filled with a solid insulation. The insulating Capacity of this space should not be decreased by conductive inserts.

The invention is particularly advantageous with a discharge tube whose Wall comprises in known manner a central portion which has the same potential throughout during operation of the tube. such a portion, which hereinafter and in the clains will be designated as an equipotential portion, may be formed by a metal ring to whose edges the insulating parts chat withstand the potential difierences between the electrodes are sealed, or it may be constituted by metal coatings which electrostatically screen a portion of the glass Wall, so that this portion will have a uniform potential throughout.

The use of such an equipotential portion en sures that there will be a stable distribution of the dielectric load over the insulating wall parte, and thus allows these parts to be made for a Voltage which correspo'ds 'to this distribution and is generally lower than the Voltage otherwise to be taken into account. I have found that in this case the use of folds arranged according to the invention results. in a much greater decrease in the tube length than with tubes having no equipotential wall portion. i

In constructions in which an electrode is supported froni the insulating envelope portion by a sea] oi large surface for instance by a metal disc fused into the insulating portion, and in which there is danger of the seal being damagecl by thermal conduction or dissipation of heat from an active part of the electrode, I arrange the seal near the end of the tube and thus as far as possible from the point at which the heat is generated. In prior tubes, such scaling of the electrodes to the tube wall would cause a cons derable reduction in length of the insulating portion. In tubes according to the invention, however, the entire insulating length measured along both sides of the old is efiectve as a leakage path between the seal and the periphery of the insulating portion.

In order that my invention may be clearly understood and readily carried into effect, I shall describe the same in more detail with reference to the accompanying drawing in which,

Figure l is a sectionized View of an X-ray tube embodying the invention and Fig. 2 is a sectionized View oi a portion of an X-ray tube illustrating another embodiment of the invention.

The X-ray tube shown in Fig. 1 has an envelope comprising an equipotential portion` in the form of a metal ring i, for instance of ferro-chrome, to the ends. of which are sealed insulating portions 2 and S oi glass. Portions 2 and 3 are provided with folds and 5 respectively which are open to the outside of the tube. The folds extend into the vacuum of the tube but are spaced a considerable distance from the Voltage-drop space between the electrodes. Because of this the projecting edge of the folds will remain free from dangerous electric charges.

Glass portion 3 extends to form a reentrant part 55 to the end of which is sealed a cathode structure i projecting beyond the old 5 within which is disposed an incandescible filament 20 provided with supply leads !sii-il. It will be noted that portion 3 has an insulation length between ring i and cathode structure 'l of more than three times its overall length in the direction ol the axis of the tube.

Glass portion 2 diliers from portion 3 in that it does not form a reentrant part, but a disc ll, for instance of chrome-iron, is sealed at 21 to the edges of old l. A metal rod lil is Secured to the inner surface of disc ti. This rod projects beyond. the fold G and carries on its lower end a target 9 in the form of a disc of refractory material, such as tungsten, which is inclined to cause the X-ray beam to pass through a glass window 8 sealed in ring i. A metal rod !8 Secured to the outer surface of disc ll serves as a terminal for the anode.

As the seal 2! is located at the end of the tube it will not be intensively heated by the dissipation of heat from. disc 9, which is heated to a high temperature during the operation of the tube. However, because of old i the insulation length of portion 2 will still be more than twice the overall length of portion 2 along the axis of the tube.

With the Construction illustrated, X-ray tubes for operation on voltages above 300 kv. can be made with an overall axial length oi less than 30` crns.

If the anode is of the cooled type, it is unnecessary that the seal between the anode body and the insulating portion be located at the extreme end of the tube and thus portion 2 of Fig. 1 may be formed with a reentrant part. such a construction is illustrated in Fig. 2, in which the insulating portion 23 is similar in shape to portion 3 of Fig. 1. Sealed to the lower end of the reentrant part of portion 23 is a ring 2 3 of ferrochrome secured to an anode body 25, for instance of copper. Anode body 25 carries a target 'rit of reiractory material such as tungsten and the cooling of the anode is eected by means of a heat absorbing and radiating body &i whose end fits into a cavity of body 25.

Referring now to Fig. 1 members 2? and 28 of solid insulating material such as "Bakelte have portions !2 and !3, respectively, extending into iolds 5 and l respectively. Portions !2 and !3 prevent a disruptive discharge occurring between the opposing walls of the folds, which are at a high potential difference during operation and are not separated by a vacuum space.

Member El has a central projecting portion i i which extends into the space formed within reentrant part l5 and surrounds leads and ll. Portion M serves to prevent disruptive discharges in the space between the surface of reentrant part !5 and leads iii and i'l.

To prevent as far as possible the occurrence of any harmful ionization in the air spaces, which are unavoidably present between the solid members 21 and 28 and the glass portions and l, between projecting part l l and lead wires iii-i l, or between member 28 and rod lt, I prefer to fill these spaces with an insulating cement For this purpose I prefer to use a hardening nonshrinking insulating material such as litharge.

It should be noted that in constructions accord ing to the invention it is essential that the space within the fold, i. e. the space occupied by por-- tions I 2 and l3 in Fig. 1, is surrcunded by a vacuum space. In Fig. 1 an annular vacuum space is formed by the outer tube wall and the outer side of the fold. Because of this a perfect insulation will be obtained as the leakage path from the lower edge of fold t to the outer wall 2 is considerably lengthened While I have described my invention in connection with specific examples and applications, I do not wish to be limited thereto, but desire the appended clains to be construed as broadly as possible in View of the prior art.

What I claim is:

1. A high-voltage discharge tube comprising an envelope having an insulating portion forming at one end of the tube an annular fold surrounding the axis of the tube and open toward the outside of the tube, and electrodes within said envelope and spaced apart to form a voltage-drop space, said fold extending into the vacuum space of the tube and being spaced a considerable distance from the voltage-drop space, and non-ionizable insulating material completely filling the space within the fold.

2. A high-voltage discharge tube comprising an envelope having an equipotential portion and an insulating portion sealed thereto, said latter portion iorming at one end of the tube an annular fold surrounding the axis of the tube and open toward the outside of the tube, and electrodes within said envelope including an electrode carried by said insulating portion, said electrodes being spaced apart to form a voltage-drop space, said old extending into the Vacuum space of the tube and being spaced a considerable distance from the voltage-drop space, and non-ionizable insulating material completely filling the space within the old.

A high-voltage discharge tube comprising an envelope having an insulating portion forzning at one end of the tube an annular fold open toward the outside of the tube, electrodes within said envelope including an electrode carried by said insulating portion, said electrodes being spaced apart to form a Voltage-drop space, said fold extending into the vacuurn space oi the tube, and non-ionizable insulating material completely fill: ing the space within said fold.

4. A high-voltage discharge tube comprising an envelope having an insulating portion forming at one end of the tube an annular fold open toward the outside of the tube, electrodes within said envelope including an electrode carried by said insulating portion, said electrodes being spaced apart to form a voltage-drop space, said fold ex tending into the vacuum space of the tube and a member of solid insulating material extending within the fold and forming spaces with the surface thereof, and a non-ionizable material completely filling said latter spaces.

5. A high-voltage discharge tube comprising an envelope having an insulating portion forming at one end of the tube an annular fold surrounding the axis of the tube and open toward the outside of the tube, and a reentrant portion surrounded by said fold, and electrodes within said envelope including an eleotrode oarried by said reentrant portion, said electrodes being spaced apart to form a voltage-drop space, said fold extending into the vacuum space of the tube and being spaced a considerable distance from the voltagedrop space, and a non-ionizable insulating material completely filling the space within said fold.

6. A high-voltage discharge tube comprising an envelope having an insulating portion forming at one end of the tube an annular fold surrounding the axis of the tube and open toward the outside of the tube, electrodes within said envelope including an electrode carried by said insulating portion, said electrodes being spaced apart to form a voltage-drop space, said fold extending into the vacuum space of the tube and being outside the voltage-drop space, a member oi solid insulating material within said fold and iorming thin interspaces with the surface thereof, and insulating cementing material filling said interspaces.

7. A high-Voltage X-ray tube comprising an envelope having two end portions of insulating material, an anode and a cathode each ca'ried by one of said portions and spaced apart to form a voltage-drop space, each of said end portions having an annular told surrounding the axis of the tube and. extending into the vacuum space of the tube, said anode and cathode extending each beyond the fold of that portion by which they are carried, and a non-ionizable insulating material completely filling the spaces within said folds 8. A high-voltage X-ray tube comprising an envelope having an equipotential portion, and

two end portions of insulating material sealed to the ends thereof, an anode and a cathode spaced apart within said envelope to form a voltage-drop space and each carried by one of said end portions, each of said end portions having an annular fold open to the outside of the tube, said folds extending into the Vacuum space of the tube and lying outside the voltage-drop space, and a nonionizable insulating material completely filling the spaces within said folds.

ALBERT BOUWERS.

Images