US2119070A

US2119070A - High voltage discharge tube

Info

Publication number: US2119070A
Application number: US759299A
Authority: US
Inventors: Bouwers Albert
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1934-01-26
Filing date: 1934-12-26
Publication date: 1938-05-31
Anticipated expiration: 1955-05-31
Also published as: GB434319A; FR783324A; DE633298C; NL41746C

Description

May 3l, 1938. A. BoUwERs HIGH VOLTAGE DISCHARGE TUBE Filed Deo. 26, 1934 @13s-Pixar;

4ms/er noun/tas Arron, r.

Patented May 31, 1938 UNITED STATES PATENT OFFICE HIGH VOLTAGE DISCHARGE TUBE hoven, Netherlands Application December 26, 1934, Serial No. 759,299 In Germany January 26, 1934 3 Claims.

This invention relates to high voltage discharge tubes and more particularly to discharge tubes having equipotential wall portions.

In many types of high voltage discharge tubes,

5 for example X-ray tubes and rectifying tubes, the envelope comprises equipotential portions, for example a metal portion or a non-conductive portion coated with conductive material, to which are edgewise joined portions of insulation material, for instance of glass.

The insulating portions adjoining the equipotential portion are as a rule so shaped as to form a substantially colinear continuation of the equipotential portion, for instance, when the equi- 16 potential is a cylinder the adjoining insulating portions are also cylindrical, having substantially the same diameter as the equipotential portion. Even if there is in some cases a diierence in the diameter of these adjoining portions, this differ- 20 ence is slight.

In tubes of such constructions the insulating portions in the vicinity of the equipotential portion assume a high electric charge and the voltage drop per unit length of insulation, i. e., poten- 25 tial gradient or concentration of electric lines of force, is much greater in the vicinity of the equipotential portion (and especially at the junction of the insulating and equipotential portions, where it has its maximum), than at the more re- 80 mote parts of the insulating portions.

Thus, to prevent in such a tube the potential gradient at the edge of the equipotential portion exceeding a value which causes a rupture of the insulation at this. point, the insulating portions 85 have to be made quite long, 'whereby at the more remote parts of the insulating portions the poten tial gradient is much smaller than the insulation could stand at these portions. In spite of this, however, the danger of rupture at or in the vicnity 40 of the edge of the equipotential portion remains.

I have found that by properly shaping the insulating portion, especially at the junction and vicinity of the equipotential portion, the potential gradient is there greatly reduced, eliminating the 45 danger of rupture and whereby at the same time, for a given operating voltage, the length of the insulating portions and thus the overall length of the tube can be materially reduced, or with the same overall length a much higher voltage tube 50 can be obtained.

In accordance with my invention, I so shape the insulating portions that their intersection with any plane passing through the axis of the tube results in a curve, along which the potential 55 gradient is substantially constant. Furthermore,

(Cl. Z50-35) the potential gradient occurring along these curves, and thus along the insulating portions, is much smaller than the potential gradient existing in the vicinity of the equipotential portion of prior art tubes. 5

According to a preferred embodiment of my invention, applicable to a high voltage discharge tube whose envelope comprises a central metal sleeve, the insulating portions form bulged-out vitreous members which are sealed at one end to 10 the metal sleeve and at the other end carry the electrodes, and which together with the sleeve form the closed container of the tube. The insulating portions are joined to the central metal sleeve by sealing them with the edge of an inwardly-curved end portion to the outer surface of the metal sleeve at a distance back from the edge of the sleeve.

In order that my invention may be more clearly understood it will be more fully described in reference to the accompanying drawing, in which:

Figure 1 is a partly sectionized view of a portion of a discharge tube of prior art construction, showing the electric iield distribution in the vicinity of the junction between the equipotential and insulating portions.

Fig. 1a. is an enlarged view of a portion of Fig. 1.

Fig. 2 is a graph showing comparative curves for the voltage distribution along the length of insulating portions made according to the prior art and according to the invention.

Fig. 3 is a partly sectionized view of an X-ray tube having insulating portions shaped in accordance with the invention.

Referring to Figure l, the wall of the tube, for instance an X-ray tube, comprises an equipotential metal portion, for instance a chrome iron sleeve I, to the edges of which are sealed two insulating portions 2 of vitreous material, for instance of glass., (only one of which is shown) and which have the same diameter as the sleeve I, thus forming a colinear continuation thereof. The tube is provided in the usual way with electrodes, the figure showing only the anode 4 which is carried by a re-entrant part 3 of the insulating 45 portion 2, to which it is fused.

With a high potential difference applied between the electrode 4 and the metal sleeve I, as exists. during the operation of the tube, there is set up at and in the vicinity of the edge of the sleeve I, an intensive electric eld whose system of equipotential surfaces--ie., the consecutive surfaces between which the same potential difference existsintersects the plane of the drawing, which also passes through the axis of the tube, in

curves which are indicated by the group of dotted lines 5. The equipotential surfaces, may be considered as surfaces of revolution of which the lines 5 are the generatrices. Between the sleeve l and the electrode 4 these surfaces have the shape of concentric cylinders, whereas beyond the sleeve I, they bend around the edge of the sleeve I and increase their mutual distance in a fanlike manner.

As shown on an enlarged scale in Fig. la, the distances between adjacent equipotential surfaces, as measured along the insulating portion 2, and indicated as ai, a2, as, etc. is a minimum close to the edge of portion l and increases with increasing distances from this edge. Conversely, the potential gradient along the insulating portion 2 is a maximum at the edge of sleeve l and greatly decreases with increasing distances therefrom.

If a curve G is drawn from the edge of the sleeve i so as to intersect the curves 5 in points which are substantially equally spaced apart, the potential gradient along this curve will be substantially constant; and at the same time considerably smaller than the potential gradient along the cylinder 2 in the vicinity of the sleeve. As the curve tends to take a direction perpendicular to the surfaces 5, it does not continue beyond the solid portion at the end of which it is tangent to the orthogonal trajectory of the surfaces 5.

According to my invention I provide insulating portions of such a shape as to follow closely the general shape of the solid portions of the curve i3 and the dash-dot line. The advantage of such an arrangement will be explained in connection with Fig. 2.

1n Fig. 2 the abscissa axis represents the length of the insulating portion as measured from the edge of sleeve i, and the ordinates represent the potential difference existing between the sleeve l and individual points of the insulating portions.

The curve a represents the voltage distribution along the insulating portion 2 of Fig. l, for a total voltage difference of the value E1 applied between the sleeve l and the electrode 4. The slope of the Curve represents at any point the potential gradient at this point, and it is again clearly apparent that the potential gradient is very high at the edge of sleeve l (corresponding to the point O), where it assumes its maximum value measurable by the tangent drawn to the curve at this point, e. g. 10 kv. per cm., which may be considered as the maximum permissible value for most kinds of glass surfaces under normal conditions. With increasing distance from the edge of sleeve i the potential gradient decreases, and over a considerable portion of the insulation is practically zero.

if the insulating portions were given the shape oi a body of revolution, the intersection of which with the plane of the drawing, is represented by the curve G-at least so far as it is drawn in iull line-and again assuming that a voltage difference El exists between sleeve l and electrode 1 -the voltage distribution would be represented by the straight line g of Fig. 2.

in this case the potential gradient would be the same at any point along the insulating portion and would be considerably smaller than that existing in the vicinity of sleeve l in the construction of Figure l. Thereby the electrical stress to which the insulating portion is subjected would be the same throughout its entire length and its insulating capacity would be fully utilized.

In practice, it is impossible to draw a curve connecting the sleeve I and the electrode 4 along which the potential gradient is constant throughout its entire length. However, insulation portions shaped as shown in Fig. 3 as 8 and 9, and later more fully to be described, will result in a voltage distribution which follows the curve b, giving substantially the same advantages as the voltage distribution represented by straight line g.

Thus a discharge tube having insulation portions of the shape shown in Fig. 3 has a substantially uniform potential gradient along a considerable portion of the insulating portions and the maximum value oi the gradient will be considerably smaller than is the case of the tube of Fig. 1.

The result is that with a tube according to the invention the insulating portions-not being subjected to a much higher potential gradient in the vicinity of the equipotential section than at the other portionscan be made much shorter for the same effective operating voltage, the respective lengths, as appears from Fig. 2, being Z1 (for Fig. l) and l2 (for Fig. 3). Or if the maximuni potential gradient is used in both types of tubes, a tube made according to Fig. 3 and having the same insulating length as the tube made according to Fig. 1, can stand a much higher voltage. In the latter case the voltage distribution of a tube according to Fig. 3 is represented by the curve c, which has the same initial potential gradient as curve a, i. e., the same tangent d, and will permit the use of a voltage E2 which is much higher than E1.

The tube schematically shown in Fig. 3 is assumed to be an X-ray tube for very high operating voltages, for instance, a deep therapy tube. The envelope comprises a central metal cylinder l and glass members 8 and 9.

The glass members are sealed with their inner edges to the two ends of cylinder 'l and bulge out in a mushroom-like form. The inner end surfaces of the glass portions 8 and 9 are inwardly curved and are sealed to the cylinder l, preferably by means oi an annular flange il, extending perpendicularly from the cylinder' l at a point somewhat back from the edge of the cylinder. Thereby 'the glass portion approaches the flange il substantially a plane perpendicular to the axis of the tube. This arrangement provides for an easier method of sealing, and also assists in obtaining the desired field distribution. As will be noted, the maximum diameter P of the glass members il is greatly in excess of the diameter q of the cylinder l.

In order to avoid disruptive discharges, the inner edges of the cylinder 'i are preferably surrounded with a heavy .layer of glass, shown in dotted lines at i5 and iii.

At the outer ends, the metal members 8 and 9 are provided with re-entrant portions 25J and 2i which carry the anode and cathode il respectively.

The cylinder 'i is provided in known manner with a ray-emitting window I2.

The tube receives its operating voltage from the secondary winding of a high-tension transformer 25 whose primary winding is connected to a lsuitable A. C. supply, and a battery serves to heat the cathode of the tube.

Tubes made according to the invention, as stated, can be of much smaller overall lengths than tubes according to the prior art if using the same operating voltage; for instance, for an operating Voltage of 200 kv. a tube made according to Fig. l will have an overall length of 20 inches, whereas a tube according to the 1nvention will have a length of 12 inches.

While I have shown the invention as applied to an X-ray tube, it may be similarly applied to other high Voltage discharge tubes, for instance, rectier tubes, and instead of providing the equipotential portion as a metal member sealed to the insulating member, the equipotential member may be formed of glass provided with a conducting layer or otherwise.

It should be also Well understood that the shape of the insulating portion given in Fig. 3 is merely typical and may be deviated from without greatly affecting the results.

Various other modifications of my invention will present themselves to one skilled in the art, and therefore I do not wish to be limited to the exact examples and applications shown and described, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:

1. In an X-ray tube, an envelope comprising a central metal cylinder having an annular metal flange spaced back from each edge thereof, two

insulating portions whose width perpendicular to the axis of the cylinder is greater than the diameter of the cylinder, each of said insulating portions carrying at one end the electrodes of the tube, the other end being inwardly curved and edgewise sealed to said annular flange.

2. A high-voltage electric discharge tube comprising electrodes, and an envelope having a central metal cylinder surrounding a portion of said electrodes and provided with an annular metal flange spaced back from each edge of the cylinder, and two insulating end portions, each of said portions having a width in a plane perpendicular to the axis of said cylinder at the edge .thereof substantially greater than the diameter of the cylinder, said end portions carrying said electrodes and insulating same from said cylinder, the surface of one end of each of said insulating portions being inwardly curved and edgewise sealed to one of said flanges.

3. In a high-voltage discharge tube, an envelope comprising a central metal cylinder having an annular metal flange spaced back from each edge thereof, and two insulating portions whose width perpendicular to the axis of the cylinder is greater than the diameter of the cylinder, each of said insulating portions carrying at one end the electrodes of the tube, the other end being inwardly curved and edgewise sealed to said annular flange.

ALBERT BOUW'ERS.