US2107597A - High voltage discharge tube - Google Patents

Description

Feb. 8, 1938. A. BOUWERS ET AL HIGH VOLTAGE DISCHARGE TUBE WM W M w WV 4 w O 343W 1 a W W-&

Filed June 26, 1933 the discharge Patented Feb. 8, 1938 PATENT OFFICE 2,107,597 HIGH VOLTAGE DISCHARGE TUBE Albert Bouwers and Jacob Harmannus van der Tuuk, Eindhoven, Netherlands, assignors, by

mesne assignments, t

lampenfabrieken, Dutch Company Application June 26, 1933,

N. V. Philips Gloei- Eindhoven, Netherlands, a

Serial No. 677,708

7 In Germany June 27, 1932 7 Claims.

The present invention relates to high voltage discharge devices and more particularly to novel means to prevent the electric charges deposited on the inner tion of the device. 7

Our invention is particularly useful in connection with X-ray tubes operating at very high voltages and with rectifier tubes which operate 10 mainly on pure electron. conduction.

The drawbacks resulting from electric charges accumulating on the inner wall of'the envelope of a discharge tube, especially on its portion surrounding the discharge path, for instance in the deterioration of this envelope portion, are well recognized and various means have been suggested to avoid same.

The most eificient means to avoid or minimize this influence is by surrounding the discharge path by means of screens and/or by providing an equi-potehtial portion in the tube envelope where it surrounds the discharge path. The best results so far have been obtained with constructions using equi-potential waist sections around path, as described in the copending application of Albert Bouwers, Ser. No. 5108,

filed January 27, 1925, patented February 27, 1934, Patent No. 1,949,347, which constructions are especially adapted to be used for X-ray tubes 30. having a cylindrical shape and in which the voltage existing between the cathode and anode of the tube is properly divided between the two halves of the tube. 7

However, if X-ray tubes are operated at very high voltages (100 kilovolts and higher) as is, for instance, required for deep therapy, neither the use of screens nor of an equi-potential conductive waist section fully prevents electrons from leaving the: discharge path and depositing 40' on the screen or the waist section. If screens are used the charges formed on the screen may cause sparking within the tube, which affects the proper'operation of the'tube and may cause its destruction. If an equi-potential metallic waist section is used, the charges impinging on the waist section prevent the'latter from automatically assuming a potential which divides the electrode voltage in half, or if a fixed potential is impressed on the waist section to obtain 0 such a division, the waist section is caused to heat up to an undesired extent.

In the 'copendingapplication of Leopold Volkel, Ser. No. 561,766, filed September 8, 1931, now Patent #1,927,75, means have been proposed to 3 prevent the deleterious influence of wall charges wall of the discharge device con- 5 tainer from interfering with the proper operain the case of very high voltage tubes. The

means there suggested neutralize or minimize the effect of such charges without, however, actually preventing the occurrence of such charges.

The present invention provides for means by which the above drawback is entirely obviated and in whichthe formation of objectionable electric charges on either the tube wall or on a protective shield is altogether avoided.

It is well known that v in discharge tubes, for instance, X-ray tubes, in addition to the primary electrons emitted by the cathode and travelling toward the anode, there are also generated secondary electrons. These secondary electrons are liberated at the front portion of the anode due to the primary electrons impinging on same. The secondary electrons leave the anode at an initial speed which substantially equals that of the impinging primary electrons and travel in the general direction of the cathode.

Our invention is based on the realization that the charges on the tube wall or screen are caused solely by the secondary electrons and that a geometric relation of the electrodes can be obtained which altogether prevents the secondary electrons from reaching the wall.

In devices of the type under consideration the primary electrons are under the influence of a strong electric field which drives them from the cathode towards the anode; furthermore,- by some means, for instance by a focussing device, these electrons are confined within a narrow beam so as to impinge on a relatively small area of the anode surface, referred to as the focal spot.

Due to such confinement within a narrow beam, the primary electrons do not leave the discharge space and do not impinge on the tube wall.

On the other hand, the secondary electrons are not focussed, but leave the anode surface under various angles, and while they are emitted from the anode at a high initial speed, in their further travel are slowed down and deflected by the field directed from the cathode toward the anode. Consequently the secondary electrons, instead of reaching the cathode, may leave the discharge space and follow a path which ends on the wall surrounding the discharge space.

The present invention provides for means to prevent any secondary electron from reaching the tube wall and to cause all these electrons to return to the anode.

According to one embodiment of the invention, as used, for instance for very high voltage X-ray tubes, the discharge space is confined be-- tween two plane and parallel surfaces of the anode and cathode, with the anode extending perpendicularly to the cathode ray beam to such an extent that even those secondary electrons which follow the longest possible path are intercepted by the anode surface.

The term anode as used herein should also include metallic members, which are in direct electrical connection with the anode proper and form wholly or partly the frontal surface of the anode structure, and similarly the term cathode should also include such metallic members, which are in direct electrical connection with the cathode proper and form' wholly or partly the frontal surface of the cathode structure.

We have found that if in such an arrangement the anode surface is made s'uficiently large, the paths of all of the secondary electrons are bent back by the action of the electric field existing in the space between the anode and the cathode so that they fall back on the anode surface. We have furthermore found that in an arrangement as just stated, the anode structure should extend in a direction perpendicular to thecathode ray-beam at least four times the distance existing between the anode and cathode surfaces plusthe width of the focal spot on the anode, and that preferably the paths of all secondary electrons should fall within a homogeneous field and that therefore the cathode should extend in this direction to the same extent as does the anode.

In the case of very high voltages the distance between the anode and cathode surfaces has to be considerably increased to prevent the liberation of auto-electrons from unheated portions of the cathode, which auto-electrons otherwise would unfavorably influence the operation of the tube. As for preventing the secondary electrons from leaving the discharge space the width of the anode and cathodewould have to be, as stated, at least four times the distance between the electrodes plus the width of the focal spot of the primary electrons on the anode, this would result in a very large tube diameter. We have found that the lateral extension of the electrode surfaces can be considerably reduced by curving the anode and cathode surfaces, preferably by giving the opposing anode and cathode surfaces concave and convex shapes respectively, and forming same as two concentric spherical caps.

The influence upon the path of the secondary electrons of a construction according to our invention, will be more fully described in connection with the examples given hereafter; however, we wish to point out that our construction is radically different from such known construc tions which provide for metallic screens which either surround as cups, the frontal part of the anode, or which surround part of the cathode ray-beam. In such known constructions, for instance the filament was deeply sunk into a metallic vessel, and a diaphragm was disposed in the proximity of the anode so as to concentrate the electrons on a limited surface of impact. Even if the electrode distance of such tubes was comparatively small, the requirements met by the present invention were not satisfied, and separate means had to be provided to prevent the wall from being struck by the electrons.

In the drawing forming part of the specification:

Figure 1 is a schematic diagram showing the paths of the secondary electrons in the case of a conventional anode and cathode Strm ll i a schematic diagram showing the paths of the secondary electrons for a cathode and anode structure according to the invention in which the plane-parallel anode and cathode surfaces extend in a direction perpendicular to the cathode ray-beam and to a distance which is greater than four times the inter-electrode distance plus the width of the focal spot.

Fig. 3 is a schematic diagram indicating the paths of the secondary electrons in a construction in which the width of the cathode is small compared to that of the anode.

Fig. 4 is a partly sectionized side elevation of an X-ray tube having a metallic equipotential waist section and having electrodes constructed in accordance with one embodiment of the invention.

Fig. 5 is a perspective detailed View of the cathode structure of Fig. 4.

Fig. 6 is a partly sectionized side elevation of a rectifying tube having electrodes constructed according to a modified embodiment of the invention. I

Fig. '7 is a schematic diagram illustrating the paths of the secondary electrons when the anode and cathode are shaped as concentric'spherical caps according to another form of our invention.

Fig, 8 is a fractional sectionized side elevation of the cathode and anode structure of an X- ray tube embodying the principle shown in Fig. '7,

Referring to Fig. 1, which illustrates a conventional cathode and anode structure in their relative size and disposition, i denotes the frontal surface of the anode and 2 denotes the frontal surface of the cathode, between which surfaces lies the discharge space; or in other words, the space in which the potential drop of the tube occurs. g h

The electrical field set up in the discharge space by this potential difference between the electrodes causes the electrons emitted by the cathode to strike the anode. The electrons impinging on the anode cause'the latter to heat up and to emit secondary electrons which are Fig. 2 is dislodged from the surface I at an initial velocity.

which, according to well-known physical laws, cannot exceed the velocity of the impinging electrons. These secondary electrons may leave the anode under any angle. Electrons leaving the anode in the direction of the tube axis travel straight towards the cathode, but being opposed by the electric field, lose their speed and while coming close to the cathode cannot reach the same but fall back on the anode. Electrons leaving the anode under various angles, for instance, an electron. starting in the direction indicated by the arrow 3 follow curved paths, for instance the path indicated by the line 4. As it appears, under the influence of the electric field existing in the discharge space the path of such electrons is bent, but as the electron leaves the interelectrode space, the influence of the field decreases and the electron lands on the tube wall 51. This results in the formation of the above referred to objectionable wall charges.

If, in accordance with the invention the frontal surfaces of both electrodes are extended in a direction perpendicular to the axis of the oathode ray or primary electron beam to an extent as above stated, the path of the secondary eleo- In Fig. Z the anode and cathode have planoa which the potential parallel frontal surfaces l and 2 respectively. The cathode ray beam is focussed on the anode, and the width of the focal spot on the anode is 2a,'whereby in case of a circular focus, a represents the radius of the focal spot. With an interelectrode distance of b and by giving the anode and cathode a width in excess of 2a +4b according to the invention the conditions will be as follows:

As those secondary electrons which are liberated from the anode at the periphery of the focal 7 spot can travel the farthest, only the path of these electrons need to be considered, these being represented in the section shown in Fig. 2 by the electrons liberated at a point 50.

As appears from the above, the secondary electrons liberated by the impinging at 50, may leave the anode surface under any angle.

Assuming first that the surfaces l and 2 extend infinitely in a direction perpendicular to the tube axis (respectively the axis of the cathode ray beam) it will be evident that the electric field in which the secondary electrons move is homogeneous throughout. Under such conditions the forces affecting the secondary electrons are analogous to those affecting a trajectile which travels in the homogeneous field of gravity. Thus similarly to a trajectile, the maximum distance (in a direction perpendicular to the tube axis) which an'electron dislodged from the .anode surface and travelling in the homogeneous electric field can cover, is obtained for electrons leaving the anode surface at an angle of For electrons which leave the. anode at an angle which is greater or smaller than 45, the 'distance covered by the electrons will be less as is shown for the paths represented ,by curves 6 and l. v

Computations and tests show, that under the conditions above assumed, a secondary electron at an angle of 45 relathe anode, covers a distance which is equal to 21); b being the distance between .the opposing electrode surfaces between difference exists. Thus if the width of both the anode and cathode surfaces, or in other words their extension in the direction perpendicular to the axis of the electron beam, somewhat exceeds 2a+4b, the paths of all secondary electrons will fall entirely within the homogeneous electric field existing in the inter-electrode space, in the same manner as if these electrodes had an unlimited extension.

If the cathode surface 2 is made smaller than the anode surface I, as shown in Fig. 3, the field arrowed line 8. To insure in this case that the paths of all of the secondary electrons be interfor the case in which the field in the inter-electrode space is homogeneous throughout.

It will be thus seen that the smallest anode diameter, at which all of the secondary electrons return to the anode, and thus the smallest tube of the cathode is equal diameter, can be obtained if the frontal surface to that of the anode.

The X-ray tube shown in Fig. 4 comprises an envelope having two glass portions l9 and 20, each having a re-entrant portion 4| and 42 re- Between the portions l9 and 20 is provided a central metallic sleeve I! preferably of ferro-chromium, fused to the glass portions. The function of this metallic sleeve is fully described in the above-mentioned patent of Albert Bouwers. A ray-transmitting window I8 is sealed into the metallic sleeve IT.

The anode H projects from one end of the tube and is preferably supported by means of a metal ring 43, preferably of ferro-chrom'ium, which ring is sealed to the re-entrant portion 4| and to which the anode H is hermetically sealed, for instance by Welding.

The portion of the anode surface surrounded by the metal sleeve 43 may be reduced in diamface of the'sleeve 43.

The frontal surface I5 of the anode is made oblique with regard to the perpendicular drawn to the tube axis, so that the X-rays generated at the anode surface l5 are directed through the ray window l8. The angle which the anode surface'forms with the perpendicular to the tube axis is, as a rule, small, for instance 20 or even less.

the opposing frontal surfaces l4 and !5 is homogeneous, the

A stem 44 is sealed to the re-entrant portion 42 through which stem are sealed two lead-in wires wire 45 is directly connected to one end of the filament l2, whereas the As will appear from the figure, due to the specific shape of the cap IS, a portion of the X- rays emitted from the anode surface l5 are intercepted by the right-upper portion of the cap I3. To fully utilize the entire X-ray beam directed toward the window [8, we prefer to render that portion of the cap l3 which so intercepts the X-rays, readily transparent to such rays.

parent to X-rays. (See also 5). The member 2|, for instance, may be made of thin metal foil, for example, of aluminum foil or thin ferrochromium sheet.

Instead of providing such a separate member 2i, it is also possi le. to provide a single-piece cap it, in which that portion which intercepts the X-ray beam is thinned down, for instanceby, filing or grinding.

It should be understood that a slight absorption of the X-rays bysuch a portion of the cap 53 is not objectionable, especialiy if the invention is used in connection with very high voltage tubes, (100 kilovolts and above), for which it is primarily destined. In such case the useful rays are rays of such penetrating power that they are not absorbedby the interceptingportion of the cap l3, and theabsorption of softer rays by same is not objectionable. I

The X-ray tube illustrated in Fig. 4, as stated, isprimarily adapted for use in connec 'on with very high voltages, in which case the provision of an equi-potential waist section does not suffice to prevent the accumuiation of wall charges.

In the device of Fig. erthe requirements previously set forth, namely, that the width of the electrodes is more than four times the interelectrode distance, plus the width of. the focal spot of the primary electron beam, is met and consequently all of the secondary electrons return to the anode surface, and thus the formation of wall charges is altogether prevented. In former constructions the width of the electrodes was for example 35 or 40 tance being i2 millimeters or more for a voltage of 290 kilovolts. These tubes show the disadvantage of considerable wall chargeswhich required a strong insulation and finally caused rupture of the tubes. We now use for example for the same voltage anodes and cathodes having a frontal surface of 60 millimeters diameter, the focal spot having an extension of 8 millimeters and the inter-electrode distance being 10 millimeters. The wall charges are then prevented to a high extent.

The application of our invention, however, is not limited to X-ray tubes for very high voltages, but can be applied also to other tubes and'tubes for lower voltages.

For instance Fig. 6 illustrates our invention as applied to a rectifier tube of the high vacuum type or one which may have a gas filling but within such a low pressure range that gas ionization cannot take place.

The rectifier comprises a vitreous envelope 22 which has a spherical middle portion. Cm the anode end the vessel is closed by a ferrochromium disc 29, which on the outer side carries a lead-in wire and to the inner Side of which is welded or otherwise secured the anode support 28 carrying the anode 23. The anode is provided with a circular plate 26 which is preferably upwardly rounded at its edge.

The cathode structure is supported on a reentrant portion 49 of the vitreous envelope 22, to which is attached a cylindrical sleeve 25 preferably of ferro-chromium fused to the reentrant portion 49. Placed Within the Vessel 25 is the electron emitting filament 24. The Sleeve 25 is provided near its lower end with a bottom wall 56, through which is insulatingly and hermetically passed a lead-in wire 30', to which is connected one end of the. filament 24. The other end of the filament 24 is connected to the sleeve 25 to which is also connected a second leadin wire 30. The upper end of the sleeve 25 forms an aperture through which the primary electron to'the sleeve 25,

millimeters, the electrode dis-.

beam is focussed towards the anode. Connected or integral therewith is a plate 21, whichis parallel with the anode plate 26, has the same diameter and is similarly provided with an outwardly bent rim.

Again it will be noted that the frontal surfaces of the anode and cathode extend in the direction perpendicular tov the electron beam to an extent which is more than four times the inter-electrode distance, plus the widthof the focal spot of the primary electron beam on the anode, and thus all of the secondary electrons liberated at the anode again return to the anode; This construction, in which the equi-potential waist section is omitted, is especially applicable to discharge tubes which operate at comparatively low voltages.

The outward bending of the edges of the plates 26 and 2'! is to prevent point to point discharges between the edges of these plates. The plates 26 and 2'! preferably consist of ferre -chromium, which material has a comparativelysmall electron emissivity. v

When using ferro-chromium the plate 21 and the sleeve 25 can be made integral, as a ferrochromium sleeve is well adapted to be fused to the re-entrant glass portion 49 v In Fig. '7 we have diagrammaticaily shown an arrangement which permits the reduction of the width or diameter of the electrodes by curving the frontal surfaces of the electrodes. As will appear from figure, the frontal anode surface I and ajfrontal cathode surface 2' are given a concave and convex shape respectively, these surfaces forming preferably portions of two concentric spheres. The proper dimensions the surfaces, to meet the requirements of the present invention can be determined in a simple manner. As shown in the drawing, the frontal surfaces of the anode and cathode are drawn from a common angular opening of the anode at which the anode intercepts the paths of ali of the secondary electrons, is, determined by the following formula:

which formula can be mathematically computed and in which 7 is the radius of the spherical frontal portion of the cathode, and d the radial distance between the two electrode surfaces.

To take care of the width of the focal spot, the opening angle or of the beam of the primary electrons has to be added to the angle 9, derived from the above formula. Thus in practice the minimum opening angle of the anode will be 9'=6+a.

In case the X-ray beam has other than circular cylindrical contour, the angle or is determined by the largest width of the beam at the anode.

Fig.8 gives a preferred construction for carrying out the principle illustrated in Fig. 7. The cathode structure comprises a filament 32 surrounded by a focussing device 3| whose upper portion is spherically rounded. An aperture 33 is provided for the passage and focus-sing of the primary electrons. For the supply of the filament current two lead-in wires 51 and 52 are provided of which the wire 5! is connected to a partition member 53 of the focussing device cos 9- 3!; to which member one end of the filament 32 is also connected, while the lead-in wire 52 is led through the partition 53 but insulated therefrom.

The anode 3!] is cut-off face 54 into provided with an obliquely which is inserted a disc 36 of highly refractory material, for instance of tungsten. The anode extends beyond its oblique face portion 54 by means of a cylindrical ring portion 38, in the end of which is inserted a concave anode mirror 34, which is concentric with the spherical portion of the focussing member 3|, and the opening angle of which isdetermined by the above given formula. Between the anode mirror 34 and the oblique face 2 of the anode a chamber 55 isformed.

The anode mirror is provided with an aperture 35 through which the focussed electron beam passes and impinges on the tungsten in- The X-rays generated on the surface of this insert emerge through an opening 33 provided in the extending portion 38 of the anode, which Window may be covered by a foil member 39.

It should be noted that the secondary electrons hich are liberated by the electrons impinging on the insert 35 are prevented from reaching the tube wall. A large portion of these elec-. trons are kept within the chamber 55 between the surface 54 and the mirror 34, whereas those secondary electrons which may emerge through the aperture 35 are forced, by the electric field existing between the surfaces 3! and34, to land on the surface 34.

Similarly those secondary electronswhich are liberated by primary electrons which, instead of passing through the aperture 35 towards the anode, impinge near the edges on the surface 34are also caused by said electric field to return to the mirror 34.

Thus it ,will be seen that all secondary electrons, whether emitted by the anode surface or by the anode mirror surface, will be prevented from reaching the tube wall or from causing ionization within the discharge vessel.

Preferably the plate 34 consists of a good heatconducting metal and is given a suificient thickness to prevent its undue heating by the secondary electrons impinging on same. In this construction we make tubes adapted for voltages exceeding 400 kilovolts having a cathode with a radius r of 46 millimeters and a distance from the anode d of 25 millimeters.

While we have described our invention by means of specific embodiments and of specific applications, we do not wish to be limited to same, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What We claim is:

1. An X-ray tube for high voltages comprising an evacuated envelope, an anode member and a cathode member in said envelope, said cathode member comprising an electron-emitting filament and a metallic diaphragm surrounding said filament and serving to direct the primary electrons emitted by said filament towards the anode member, said'metallic diaphragm having a plane frontal surface and said anode having a plane frontal surface opposing said first frontal surface and parallel thereto, said anode emitting under the impact of the primary electrons, secondary electrons whose paths are interrupted by the frontal surface of the anode and emitting an X-ray beam which forms an angle not exceeding 20 with its frontal surface, a ray-window in said envelope through which said X-ray beam passes, said metallic diaphragm having a portion interposed between said anode surface and said window, said portion being substantially transparent to the rays of the X-ray beam.

2. A high-voltage discharge tube comprising an evacuated envelope, two electrode members spaced apart within said envelope to form a discharge space and having coextensive plane-parallel opposing frontal surfaces of conductive material and of substantially smaller area than the cross-sectional area of the envelope in the planes of said surfaces, one of said members comprising means to emit primary electrons which impinge upon said second member to dislodge secondary electrons from a surface area thereof, said frontal surfaces being perpendicular to the direction of travel of said primary electrons and having directions substantially equal to the width of said secondary emission surface area plus four times the distance between said surfaces.

3. A high voltage X-ray tube comprising an evacuated envelope having reentrant parts, an

terial parallel to said anode frontal surface and at least coextensive therewith, said cathode structure comprising means including any incandescible member to emit and to direct toward said anode frontal surface a beam of primary electrons impinging upon said anode frontal surface to form a focal spot, said anode frontal surface having a portion surrounding said focal spot and of a width at least twice the distance between said frontal surfaces.

4. A high-voltage discharge tube comprising an envelope, an anode structure within said envelope and having a plane frontal surface of conductive material, and a cathode structure within said envelope and having a plane frontal surface of conductive material parallel to said anode than the cross-sectional area of the envelope in the plane of the surface, said cathode structure comprising means includto be heated to incansaid anode structure a area of the anode frontal surface to dislodge secondary electrons therefrom, said anode frontal and of a width at least twice the distance between said frontal surfaces.

5. An X-ray tube comprising an evacuated envelope, an anode structure having a. plane frontal surface, and a cathode structure comprising means including an electron-emitting filament and a metallic apertured diaphragm for emitting and directing an electron beam toward said anode structure, said diaphragm having a plane frontal surface opposing said anode frontal surface and parallel thereto and at least coextensive therewith, said anode frontal surface having a width in any direction equal to at least four times thedistance between said frontal surfaces plus the width of the electron beam in the same direction at the anode frontal surface, said envelope having a portion insulated for high tension from both said anode and cathode struc- 6 tures and defining a ray window for the exit of X-rays from the tube. v

6. An X=ray tube for high voltages, comrising an evacuated envelope, an anode structure, and a cathode structure comprising an electronemitting filament, and a metallic diaphragm surrounding said filament and in electrical connection therewith, said' anode and cathode structures having parallel opposing surfaces spaced apart to form a discharge space, a Window for the exit of X-rays in a portion of said envelope surrounding said discharge space, said anode structure directing a beam of X-rays through said window, said diaphragm having a portion intercepting a portion of said beam and made of a material substantially transparent to the rays of said X-ray beam,

7. An X-ray tube for high voltages comprising an, evacuated envelope, an anode structure, and a cathode structure comprising an electronemitting filament, and a metallic diaphragm surrounding said filament and having a plane frontal surface, said anode structure having a plane frontal surface opposing said diaphragm frontal surface and parallel thereto, said anode structure under the impact upon its frontal surface of electrons emitted by said filament being adapted to emit X rays striking said metallic diaphragm, said diaphragm having a portion pervious to said X-rays and including part of its frontal surface. I

ALBERT BOUWERS. JACOB HARMANNUS VAN DER TUUK.

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