US2845559A - Structure for high voltage tube - Google Patents

Description

July 29,y 195s J. LEMPERT 2,845,559

` STRUCTURE FOR HIGH VOLTAGE TUBE l Filed oct. 9. 195s 2 sheets-sheet 1 ff-d 27% Joseph Lemprt.

ToRNEY v i 'July 29, "1951s J. LEMPERT 2,845,559

STRUCTURE FOR HIGH VOLTAGE TUBE Filed Oct. 9. 195s v 2 sheets-sheet 2 Fig. 55

UnitedStates Patent O STRUCTURE FOR HIGH VOLTAGE TUBE Joseph Lempert, West Elmira, N. Y., assignor to Westmghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 9, 1953, Serial No. 385,069

Claims. (Cl. 313-58) My invention relates to high voltage electron tubes and in particular relates to tubes so designed as to prevent breakdown across the gaps between the electrodes inside the tube. Typical of the tubes towhich my invention is applicable are high voltage rectiers and high voltage X-ray tubes -operating with potentials in the order of thousands of volts between their electrodes.

In prior art tubes of the foregoing types, voltage breakdown between the electrodes frequently manifests itself as high' current pulses of short duration, the magnitude of the current depending on the amount of impedance in the tube circuit. In those cases where a high impedance in series with the tube electrodes is tolerable, the current pulse may be limited to values which are not too injurious, but in certain modern uses like X-ray tubes for taking radiographic exposures from the order of 0.1 second to exposures as short as 0.1 microsecond or less duration an impedance of size suicient to adequately protect the tube is out of the question because for the short period of time in which current ows, extremely high current values are needed to produce the pictures desired.

In accordance with the principles of my invention, I subdivide one or more electrodes in the tube into sections, one of which may be looked at as forming a kind of shield, in case of arc-over for the other section which it partly surrounds. In normal operation, the discharge between the electrodes passes through the main or shielded sections of the electrodes and traverses the low impedance path desired for normal operation of the tube. Impedances Iof suiciently high value are interposed between each shielding section and the electrode which it protects, and when voltage breakdown between the tube electrodes starts, the current transfers to the shielding electrodes and has to traverse the high impedances which are then in series with them. In this way, energy supply to the arc-over has to pass over high-impedance paths which prevent its rise to values destructive to the tube and its electrodes.

One object of my invention is, accordingly, to provide a novel type of structure for electrical discharge tubes which shall render them immune to injurious effects of breakdown between their electrodes.

, Another object is to provide a novel type of structure for electrodes yof electrondischarge devices.

Another object is to provide a new and improved type of high voltage electron tube which shall be capable of connection to supply channels of relatively low impedance for normal operation but in which arc-over currents are forced to traverse high impedance circuits.

Another object is to provide an improved type of X-ray tube capable of conducting large instantaneous currents between its electrodes, yet in which breakdown discharges between the electrodes are limited to nondestructive values.

Other Iobjects of my invention Will become apparent upon reading the following description taken in connection with the drawings, in which:

Patented July 29, 1958 Figure 1 is a schematic diagram and a view partly -in longitudinal section of a high voltage X-ray tube and supply circuit therefor which embody the principles of my invention;

Fig. 2 is a longitudinal sectional view of a high voltage rectifier tube designed to employ my invention.

Fig. 3 and Fig. 4 are schematic views in longitudinal section of a less expensive form` in which my invention may be applied to tube electrodes; and

Fig. 5 is a schematic diagram showing my invention applied to an X-ray tube of type standard with roentgenologists and operating at 60 to 125 kilovolts.

Referring in detail to Fig. 1, an X-ray tube comprises an electron-emissive cathode 1, which may be of tungsten, and an anticathode 2 supported from opposite ends of a tube 3. The latter comprises a mid-portion 4 of cylindrical glass sealed at its opposite ends to rings 5 and 6, which may comprise the sealing cobalt-nickeliron alloy well known under the trade-name Kovar. The ring 5 is also sealed to a stem 7 through which are sealed two leads 8, 9 which support and supply heating current to the cathode 1. A shield 11 is supported on the lead 9 and extends downward over the stem 7 to shield the latter from any electric gradient between the cathode 1 and the anticathode 2 or the ground. A shielding-section 12 Aof cylindrical contour is connected at one end to the ring 5 and has an opening or window 13 in its other end looking toward the anticathode 2. The margin of window 13 is rolled over as indicated so that no sharp edges are presented toward either anticathode 2, cathode 1 `or the ground. Other portions of shielding-section 12 are likewise curved for the same reason.

The anticathode 2 may comprise a copper block having an inclined end-face in which is set a plate 14 of tungsten, and supported at its other end by a cylinder 15 which may have a Kovar edge sealed to a re-entrant glass cup 17, the other edge of which is sealed to ring 6. The edges of anticathode 2 are all rounded for the reasonpointed out in describing shield-section 12.

A shield-section 16 for the anticathode is supported from seal-ring 6 and has all its corners rounded like shield-section 12. The anticathode 2 is electrically connected to its shield-section 16 through a high impedance of, for example, 500,000 ohms which maybe in the form of a sleeve 18 ofy high resistance material surrounding the cup 17. The shield-section 12 for cathode 1 is connected to the latter through a high impedance which may likewise have the form of a sleeve 19 surrounding stem 7.

The X-ray tube 3 is supplied by a conventional directcurrent voltage source 21 which may comprise two similar transformer secondares 22, 23 delivering each a voltage of the order of 60,000 and powered by a primary winding (not shown). The opposite ends of windings 22 and 23 are interconnected by a pair of like capacitors respectively connected in series with rectiriers 26 and 27 which are oppositely poled. These networks are well known in the art as means to charge the capacitors 24, 25 each to approximately the voltage of windings 22 and 23, with their voltages additive to around 125,000 Volts. The negative end of one set of condensers 24, 25 is connected to cathode 1 through a resistor 28 of say 150,000 ohms and its positive end is grounded, while the negative end of the other set of condensers 24, 25 is grounded and its positive end is connected to the antiwhich passes through resistors 28, 29 but avoids high resistors 1S, 19. All elements of the cathode except for hot lament are shielded from high electric field by shield electrode 12, which is at cathode potential until a high voltage breakdown occurs. Similarly the shield 16 on the anode side of the tube shields anode 2 from high electric fields. High'voltage breakdown, which is produced by the high electric fields must then occur between these electrodes. Thus, should the space within tube 3 break down, because of a field emission from the cathode shield, the discharge will naturally go to the shield of opposite potential, thus relieving cathode 1 of any destructive effect incident to subsequent arc-over. However, the arc-over current can reach shield- sections 12 and 16 only by fiowing through high impedances 18 and 19; hence it will be limited by them to a much smaller value than that it could attain if it traversed only resistors 28, 29, cathode 1 and anticathode 2. The presence of shield- sections 12, 16 thus protects the cathode 1 and other elements of tube 3 from injury due to electrical breakdown in interior of tube 3 and greatly minimizes the probability of destruction of the tube by puncture of the tube envelope.

Fig. 2 shows a high voltage electron-discharge rectifier to which the principles of my invention have been applied. This comprises a glass-walled container 31 having two sections 32, 33 sealed together by a metal ring 34 which supports a shield-section 35 for the anode 36. The latter is a metallic tube or rod closed at its inner end and having an annular collar 37 sealed to the container section 33. The anode 36 is connected to the shield-section 35 through a high impedance 38.

The cathode 41 comprises a filament of tungsten or thoriated tungsten supported on in- leads 42, 43 sealed through a re-entrant stem which is continuous with the container section 32. The cathode 41 is surrounded by a cylindrical shield 45 supported on the stem 44 and connected to inlead 43.

The tube of Fig. 2 applies the shielding principles of my invention to only one electrode, although those principles might, of course, also be applied to its cathode in the same way as they were applied to the cathode in Fig. l. The application of shielding to one electrode only is, of course, simpler and less costly than its application to two or more electrodes, and yet it obtains much of the advantage of the more expensive structure.

Fig. 3 shows another form in which my invention may be applied to an electrode of a high voltage X-ray or other high voltage tube. In this figure the cathode 1 may be like that of Fig. l but the press 51 now encloses an inlead 52 in addition to the heating- current inleads 8 and 9. The shielding section 53 may be like that in Fig. l except that its base is not flared and that it is supported at the end of inlead 52. The shield 11 may be omitted in Fig. l and has been omitted in Fig. 3. The inlead 52 is connected through a resistor 54- to one inlead such as 8.. The vacuum-tight envelope 55 has a continuous glass wall at the point where Fig. 1 is provided with seal 5.

Fig. 4 shows still another structure in which my invention may be embodied. Fig. 4 differs from Fig. 3 in that the shielding-section 53 is supported on an inlead 56 passing through the wall of container 57 instead of through the seal 58.

Fig. shows my invention applied to an X-ray tube 60 powered by a full-wave rectifier of a type frequently employed in roentgenology to supply voltages of 60 to 125 kilovolts. This power supply comprises a transformer secondary 61 having its mid-point grounded. One end of secondary 61 is connected to the anode of a rectifier 62 and to the cathode of a rectifier 63. The other end of secondary 61 is connected to the anode of a rectifier 64 and to the cathode of a rectifier 65. The free ends of rectifiers 62 and 64 are both connected to the anticathode 66 of X-ray tube 60 and the free terminals of rectifiers 63 and 65 are connected together to a cathode heater-lead for tube 60. Tube 60 may have a cathode structure of the type described in Fig. 3, for example, and its cathode heater is supplied with current from the secondary 67 of a filament transformer of conventional type. The shielding section 53 is connected to one side of secondary 67 through a resistor 54 as previously described in connection with Fig. 3.

I claim as my invention:

1. In combination, a vacuum-tight envelope, a first electrode projecting inward from the wall of said envelope and supported therefrom, a cooperating electrode projecting inward from the wall of said envelope and supported therefrom toward said first electrode, a first shielding conductive wall about said first electrode and having a window facing said cooperating electrode, said first conductive wall supported from the wall of said envelope and the intervening wall of said envelope providing a high insulation between said first conductive wall and said first electrode, a second shielding conductive wall supported from said envelope wall and embracing the path through said window between said electrodes, the distance separating said shielding conductive walls being less than that separating said electrodes, and a pair of high impedances of about 500,000 ohms each positioned external of said envelope for respectively connecting said first shielding conductive wall with said first electrode and said second shielding conductive wall with said cooperative electrode to provide a high impedance shunt breakdown path for abnormal currents.

2. In combination, a vacuum-tight envelope, a first electrode projecting inward from the Wall of said envelope and supported therefrom, a cooperating electrode projecting toward said first electrode and supported from said envelope wall, a shielding conductive wall about said first electrode and having a window facing said cooperating electrode, the distance separating said cooperating electrode from said shielding conductive wall being less than that separating it from said first electrode, the envelope wall providing high insulation between said electrodes and said shielding conductive wall and a high impedance of about 500,000 ohms positioned exterior of said envelope connecting said shielding conductive wall to said first electrode to form a high impedance shunt circuit between said coperating electrode and said shielding conductive wall for abnormal currents.

3. In combination, a vacuum-tight envelope, a first electrode having a thermionically-emissive surface thereon projecting inward from the wall thereof, a cooperating electrode projecting toward said first electrode, a shielding conductive wall about said cooperating electrode and having a window facing said first electrode, the distance separating said cooperating electrode from said shielding conductive wall being less than that separating it from said first electrode, and a resistance of about 500,000 ohms positioned exterior of said envelope and connecting said shielding conductive wall to said first electrode to provide a high impedance path for abnormal currents between said electrode and said conductive wall.

4. An X-ray tube comprising a vacuum-tight envelope, a first electrode having a thermionically-emissive surface thereon projecting inward from the wall thereof, a cooperating electrode projecting toward said first electrode, a first conductive wall about said first electrode and having a window facing said target electrode, a second conductive wall embracing the path through said window between said electrodes, the distance separating said conductive walls being less than that separating said electrodes, means for applying a voltage between said first electrode and said cooperating electrode, and a. pair of impedances positioned exterior of said envelope of about 500,000 ohms each respectively connecting said first conductive wall with said first electrode and said second conductive wall with said cooperative electrode to provide a high impedance shunt circuit for abnormal currents.

5 v 6 5. In combination, a vacuum-tight envelope, a first ating electrode to provide high impedance external circuit electrode projecting inward from the wall thereof, a co between said conductive walls for abnormal currents. operating electrode projecting toward said first electrode, a first conductive wall about said lirst electrode and hav- References Cited 111 the 51e 0f this Patent ing a window facing said cooperating electrode, a second 5 UNITED STATES PATENTS conductive wall embracing the path through said window between said electrodes, means for applying a voltage 1598150 Mulvany et al' Aug' 31 1926 1,603,603 Holden Oct. 19, 1926 between sald rst electrode and said cooperating elec- 2,516,663 Zunlck July 25, 1950 trode, a pair of lngh lmpedances positioned exterior of 2,525,205 Caslmir Oct. 10, 1950 said envelope of about 500,000 ohms each respectively 10 connectin said first conductive wall with said first e1ec 2531583 Ott Nov' 28' 1950 g 2,617,046 Douma et a1. Nov. 4, 1952 trode and said second conductive wall with said cooper-

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