US1995478A - High potential electric discharge tube - Google Patents

Description

March 26, 1935. c. C. LAUR lTSEN HIGH POTENTIAL ELECTRIC DISCHARGE TUBE Filed Nov. 12, 1930 2 Sheets-Sheet 1 PUMP [72 van for C/zar/e-s C. Lauri-1 5s);

March 26, 1935. c. c. LAURITSEN HIGH-POTENTIAL ELECTRIC DISCHARGE TUBE Filed Nov. 12, 1930 2 SheetsSheet 2 C/zcgr/es 'C. Laur/fsen A ey Patented Mar. 26, 1935 UNITED STATES 1,995,478 HIGH POTENTiAL ELEgTRIC DISCHARGE TUB Charles C. Lauritsen,

Pasadena, Calil'., assignor to California Institute of Technology, Pasadena, Calif., an California educational corporation of Application November 12, 1930, Serial No. 495,087 11 Claims. (Cl. 250-35) This invention relates to high potential discharge tubes operating under high vacuum conditions.

Such tubes can be utilized for any of several well understood purposes; for example, for rec tification of alternating electromotive forces, or for the production of extremely high frequency radiations.

scribe a tube adapted for the production of Roentgen ray radiations, which are of this high order of frequency. However, the invention is generally adapted to other uses.

Discharge tubes of this character customarily consist of two electrodes, spaced from one another, and enclosed in an evacuated vessel. A

relatively high difference of potential is maintained between these electrodes, and an intense electrostatic field is thereby created in the evacuated space between them.

The negative electrode, or cathode, can be provided with a filament, which may be heated by an external source of current, and which, when so heated, serves as a source of electrons.

These electrons are liberated within the zone of action of the electrostatic field between the electrodes, and by the interaction of this field and the charges borne by the electrons, the latter are propelled rapidly towards the anode, or target.

The impact of electrons on the target gives rise to Roentgen rays, whose characteristics depend on the velocity'with which the electron stream impinges upon the anode, or target. The higher the potential difierence maintained between the electrodes, the higher this velocity, and the higher the frequency of the radiation produced. v

With the structures customarily employed,

however, it is impractical to employ between the I electrodes differences of potential in excess of 40 several hundred thousand volts, on account of problems of insulation. Flash over between electrodes externally to the tube, and accumulated stress resulting in perforation of the insulating envelope, are especially fruitful causes of breakdown, and can not be avoided by the expedient of enlarging the dimensions of the structure.

The reason for such breakdowns is that, in applying potentials between the electrodes sumcient to produce high frequency radiations, in-.

50 tense electrostatic stresses are developed between the electrode structures and the air surrounding the tube. v

' These stresses, when-below a certain value, are distributed with reasonable uniformity in the 55 surrounding air, and in the glass or other insu- In the present instance I shall de-' lator from which the tube must be partially constructed.

When. the stresses are increased, however, to a point where the surrounding air becomes ionized an unstable condition arises, and the charged or 5 ionized air may redistribute the exterior stresses with extreme irregularity, thus imposing on some portions of the insulation electrical potential difierence s suflicient to cause rupture.

Irregular distribution of stresses may also ex- 1o ist on the interior walls of such tubes, due to imperfect evacuation, and the presence of residual gases. Such irregularities may be partially oilset by care in pumping, but have never been entirely eliminated in practice. Furthermore, 15 the emission of electrons from the conducting structures. in the tube to the exterior causes asevere stress on the wall of the tube enclosing the electrodes. I

It is one of the objects of my invention to pro- 20. vide a structure which will prevent such irregular distribution of stresses, thereby permitting extremely high potentials to be employed on discharge tubes of practical dimensions.

It is another object of my invention to shield the envelope or tube from the bombardment of the electrons torn ofl the electrode structures under the intense electrostatic stresses.

It is a further object of my invention to make possible the operation of such discharge tubes at 0 potentials far in excess of those heretofore practical; and particularly, of the order of a million volts. I have found that when such extremely high potentials are employed, radiations of substantially the frequency of the gamma radiation from radium may be obtained from the tubes embodying my invention.

Such tubes can therefore be employed as the full equivalent of radium inthe treatement of disease, or for therapeutic or other purposes.

On account of the extreme rarity and cost of radium, the expense of employing it in any 7 considerable quantity is prohibitive.

My tube, on the contrary, can be readily constructed atrelatively low expense, and operated to emit radiations at an intensity far greater than could be obtained from any quantity of radium heretofore isolated and collected in one place.

I accomplish the beneficial results of my invention by the us'eof 'a novel structure of conducting shields, which distribute the stresses in the insulating portion of the tube, and the surrounding external air, and prevent the concentration of such stresses in amounts capable ofcausing rupture or flash over. Furthermore, the

electrostatic stresses on the envelope are reduced by making the tube sectional, so that only a small portion of the whole electrostatic stress is impressed on each section.

My invention possesses many other advantages, and has other objects which may be made more easily apparent from a consideration of one embodiment of my invention. For this purpose I have shown a form in the drawings accompanying and forming part of the present specification. I shall now proceed to describe this form in detail, which illustrates the general principles of my invention; but it is to be understood that this detailed description is not to be taken in a limiting sense, since the scope of my invention is best defined by the appended claims.

Referring to the drawings:

Figure 1 is a sectional view of a tube constructed in accordance with my invention, the supporting structure being for the most part omitted, and some of the elements being shown diagrammatically;

Fig. 2 is an enlarged sectional view of the electrode structures, the upper one being shortened to reduce the size of the figure;

Fig. 3 is an enlarged top plan view of the upper electrode; and

Fig. 4 is an enlarged detail section showing the structure of the exterior envelope or vessel.

In Fig. 1 there is shown a part of a supporting structure 11 having an aperture 12 into which the bottom section 13 of the tube can extend. This supporting structure can be the roof of a chamber in which the radiations are utilized. The bottom section is shown as tubular, being made from metal, and supporting a bottom electrode structure 14. This electrode structure will be described in detail hereinafter; it is sufficient to note now that the bottom of section 13 is closed, forming the end of a tubular evacuated vessel in which both electrodes are disposed. In order to hold this tube section 13 in aperture 12, a base 15 can be welded or otherwise secured to the top of this section, which base is supported in any appropriate fashion on support 11, as by feet 16. A pump 1'7 can also connect to this lower section, which can either be continuously operated, or the tube can be sealed off, as desired.

The remainder of the tube structure can be formed by a series of glass tubular sections 1'7, 18, 19, 20, superimposed one on top of another. In order to disclose the actual sizes used in an operative tube heretofore made, it may be stated that each section 1'7, 18, 19, 20 was about twelve inches in diameter and about two feet high. Interposed between the abutting ends of the contiguous sections there arepreferably one or more compressible gaskets or rings such as the rubber rings 21 (Fig. 4). A pair of such rings are shown, between which there is a fiat metal member 22 made from sheet iron or steel, and for a purpose to be described hereinafter. Shellac can be used at these joints; and the preponderance of pressure externallly of the tube serves to maintain the seals between sections 1'7, 18, 19, 20 tight.

The lowermost section 17 rests on base 15,.there being interposed the rubber gasket 21. The top section 20 is covered, with a metallic diaphragm member 24 (Figs. 1 and 3 Between this member and section 20 there is interposed a rubber ring 21. The upper electrode structure 25 passes 4 through this member 24 and is welded or is otherwise sealed in the diaphragm 24. i I

The structure of this electrode will be detailed hereinafter. It may here be noted that structure extends downwardly so as to be spaced withinthe desired distance'of the lower electrode structure 14, this distance being chosen to accord with the desired operating potential and other characteristics of the tube. Due to the high de-- gree of vacuum maintained, the electron stream has its sole source in the heated cathode and may be maintained between closely spaced electrodes if desired. The external dimensions of the tube, however, must be sufficient so that the surround.- ing air will be able to sustain the applied potentials without flash over.

I shall now describe the upper electrode structure 25, which in this instance is the cathode. It is formed, as shown most clearly is Fig. 2, of an outer tubular member 2'7 the upper part 28 of which is thickened to produce rigidity. There is a reentrant tubular portion 29 forming an annular water jacket 30 in which a cooling medium can be circulated as by the aid of conduits 31, 32.

The cathode proper is located near the bottom of the reentrant portion 29, and is shown as a spiral filament 33, arranged to be heated by an electric current. Thus one end 34 can be fastened in a metal plug 35 engaging threads inside of inner tube 29. The other terminal 36 can connect to a post 36' passing through the insulation 38 in plug 35 and can connect with a lead-in 39 passing through insulation plugs 40, 41 at the bottom and top respectively, of the reentrant portion 29. These plugs can be made from pyrex or other equivalent insulation. It is apparent that an energizing current can be passed through filament 33 by connecting the terminals of an appropriate source to the structure 25 and lead-in 39, respectively.

In order to make it possible to align the structure 25 properly in the tube, this structure is provided with a number of radial arms 42, 43, 44, 45, the extremity of each of which carries an adjusting screw 46 threaded therein. The ends of screws 46 contact with the rigid rim 4'7 of diaphragm 24; and by adjusting the lengths of these screws below the arms'42 to 45, the structure 25 can be moved angularly about one or more diameters of diaphragm 24, said diaphragm readily permitting this motion.

The lower electrode structure 14 can be inthe form of a hollow metallic member, carrying at its upper surface, a tungsten target 4'7 opposite the cathode 33. This structure 14 can also be water cooled, as by the aid of conduits 48, 49. Structure 14 is preferably grounded, whereby the operator of the tube can safely manipulate it, as the high potential parts are mainly outside of the chamber in which the lower, operating end of the tube is located.

It is apparent that if a high direct or alternating current potential be impressed across the structures 14, 25, this potential may cause a rupture of one or more of the glass sections 1'7, 1'8, 19, 20, due to concentration of electric stress from the stem 2'7 of structure 25, through the intervening vacuum of the tube, through the glass, and to ground. It is for the purpose of preventing such occurrences that use is made of metal shields so arranged that there is no undue or disruptive stress anywhere in the system.

To effect this result, there are utilized one or more metal shields, such as 50,. 51, 52, and 53, which surround the stem 2'7 but are out of electric contact therewith. These shields are tubular,

. and have rounded over ends, such as indicated at 54, to reduce all sharp corners. They are also out of electrical contact with each other, and are spaced axially along the stem 27. They are shown in this instance as overlapping, by a slight telescoping, but this is not an absolute essential. To facilitate such telescoping without providing electrical contact between them, they are alternately large and small in diameter.

Theseshields 50, 51, 52, 53, can aptly be termed floating as they need not be in electrical contact with either electrode. In order to support these shields in proper spaced relation, .use is made of the rings 22 heretofore mentioned. These rings are fastened to the external surface of the shields, and the edges of the rings are clamped between the contiguous ends of the glass sections 17, 18, 19, 20; the ring 23 of the lowermost shield 50 is, however, merely placed on top of base 15.

These floating shields serve effectively to distribute the electric stresses around the stem 27 uniformly so that no concentration through any localized spot of the glass walls is possible. Ap-

parently each individual shield can take up a definite potential dependent upon its position, which may be difierent from that of the adja cent shields, the variation in potential from shield to shield providing the desired effect. Indeed, since the ends of these shields are close together, electron emission can take place between them to distribute-the stress; but this emission of course is-not needed where the shields are connected to definite sources of potential through resistances or by taps.

These shields also interrupt the flow of electrons to the glass sections 17, 18, 19, 20, from stem 27. The potential difference between contiguous shields is only a fraction of that between the electrodes proper. Since a glass section is interposed between the conducting rings 22 connected to these shields, it is seen that no greater stress exists across any one section than the potential difference between the shields.

In order further to equalize stresses external of the tube, there may be utilized external extensions 55, 56, 57, 58 of the shields 51 to 53. These extensions can be in the form of sheet metal pieces, each having an aperture the edge of which overlaps the edge of corresponding ring 22. These pieces extend a considerable distance radially, and can be insulatingly supported in any appropriate manner at their outer edges, as by the ,aid of insulator stands.

These external shields are thus respectively in contact with the internal shields. They serve to distribute the electric field or stress uniformly in the external region surrounding the tube. It is due to these shields that safeguarding against disruption can be provided at potentials neighboring and in excess of a million volts.

I claim:

1. In a high potential electrical discharge tube, an evacuated vessel a pair of electrode structures in said vessel, one of said structures having a conducting stem, and a series of overlapping tubular shields surrounding the stem, said shields being electrically unconnected to each other or to either electrode.

2. In a high potential electrical discharge tube, an evacuated vessel, a pair of electrode structures in said vessel, one of said structures having a conducting stern, a series of spaced shields in the vessel surrounding the stem, said shields progressively extending over the stem and an external shield for each of the shields in the vessel and electrically connected respectively with the said internal shields."

3. In a high potential electrical discharge tube,

an evacuated vessel, a pair of electrode structures .in said vessel, one of said structures having a conducting stem, a series of overlapping tubular shields surrounding the stem, said shields being electrically unconnected with respect to each other as well as with respect to either electrode, and an external shield for each of the shields in the vessel and electrically connected respectively with said shields.

4. In a high potential electrical discharge tube, an evacuated vessel a pair of electrode structures in coaxial relation in said vessel, and spaced apart axially one of said structures having a conducting stem, and a series of mutually electrically insulated shields in the vessel surrounding the stem, said shields having a common axis and progressively extending over the stem.

5. In a high potential. electrical discharge tube, a series of tubular sections forming an evacuated vessel, an electrode structure having a conducting stem extending into the vessel, another electrode structure in the vessel, and a series of unconnected tubular shield structures progressively extending along the stem and surrounding the stem, each of said shield structures having a supporting portion held between contiguous ends of a pair of sections, and being electrically insulated from each other .as well as from the electrode structures.

6. In a high potential electrical discharge tube, an evacuated vessel, a pair of electrode structures in said vessel, one of said structures having a stem, and means inside of said vessel, said means surrounding and extending axially along the stem, said means having parts occupying substantially different axial positions, said means substantially entirely covering the stem, and said parts cooperating respectively with difierent portions of the stem for distributing uniformly, electrical stresses in the vessel externally of the said stem.

, 'I. In a high potential electrical discharge tube, an evacuated vessel, a pair of electrode structures in said vessel, one of said structures having a stem, means inside of said vessel, said means surrounding and extending axially along the stem, said means having parts occupying substantially different axial positions, said means substantially entirely covering the stem, and said parts cooperating respectively with different portions of the stem for uniformly distributing electric stresses in the vessel externally of the said stem, and supplemental means for uniformly distributing electric stresses externally of the vessel.

8. In a high potential electric discharge tube, an evacuated vessel, a pair of electrode structures in the vessel, one of said structures having a stem, said vessel having a series of sections forming a wall ofinsulating material, enclosing said structures, and means internally of the vessel, said means surrounding. and extending axially along the stem, said means having parts occupying substantially different axial positions, said means substantially entirely covering the stem, and said parts cooperating respectively with different por tions of the stem, for ensuring substantial uniformity of electrostatic forces acting from one end of each section to the opposite end.

9; In a high potential electric discharge tube, an evacuated vessel, 9. pair of electrode structures inthe vessel, said vessel having a series of sections forming a wall of insulating material, enclosing said structures, and means internally of the vessel for ensuring substantial uniformity of electrostatic forces acting from one end of each section to the opposite end, comprising a series of tubular shields each interposed between one of the electrode structures and the wall and electrically unconnected to each other or to either electrode.

10. In a high potential electric discharge tube, an evacuated vessel, a pair of electrode structures in the vessel, said vessel having a. series of sections forming a wall of insulating material, enclosing said structures, and means internally of the vessel for ensuring substantial uniformity of electrostatic forces'acting from one end of each section to the opposite end, comprising a series of tubular shields, each interposed between one of the electrode structures and the wall, and so arranged that each extends over an axial distance that corresponds to a part of one section and a part of the contiguous section, so as to overlap a pair of adjacent sections, and electrically unconnected to each other or to either electrode.

11. In a high potential electric discharge tube, an evacuated vessel, a pair of electrode structures in the vessel, one of said structures having a stem, said vessel having a series of sections forming a wall of insulating material, enclosing said structures, and means internally of the vessel, said means surrounding and extending axially along the stem, said means having parts occupying substantially different axial positions, and cooperating respectively with substantially difierent portions of the stein, for ensuring a substantial uniform field strength adjacent one of the electrode structures and axially thereof.

CHARLES C. LAURITSEN.

Images