US3397337A - Flash X-ray dielectric wall structure - Google Patents

Description

3, 1968 A. s. DENHOLM 3,397,337

FLASH X-RAY DIELECTRIC WALL STRUCTURE Filed Jan. 14. 1966 INVENTOR ATTORNEY United States Patent 01 Bee 3,397,337 Patented Aug. 13, 1968 3,397,337 FLASH X-RAY DIELECTRIC WALL STRUCTURE Alec S. Denholm, Lexington, Mass., assignor to Ion Physics Corporation, Burlington, DeL, a corporation of Delaware Filed Jan. 14, 1966, Ser. No. 520,690 8 Claims. (Cl. 313250) ABSTRACT OF THE DISCLOSURE A flash X-ray tube comprising an evacuated envelope containing a field emission cathode and an opposing anode wherein the wall forming the envelope is composed of insulating ring segments rhombic in cross-section alternated with hollow conical frustums forming ring shielding equipotential planes.

This invention relates generally to X-ray tubes and more particularly to cold cathode X-ray tube structures.

X-rays, their manner of generation, and the tube structures designed to produce them have long been known. Conventional X-ray tubes generally comprise an evacuated housing, a window in the wall of the housing, and a thermionic cathode contained within the housing together with an anode or target. The anode is positioned such that electrons emitted by the cathode and accelerated through an electric potential will strike and generate X-rays. The X-rays thus generated pass through the window and out of the tube where they are used for various known purposes. Such tubes generally have source sizes of about 1 mm. and a beam current of only a few milliamps which means that the exposure time required to X-ray most objects must be in the order of seconds. For static objects, this is quite satisfactory, but for many high speed phenomena this is entirely inadequate.

Certain applications require that the exposure time be on the order of a hundredth of a microsecond. To achieve such exposure times, it is necessary that the X-ray beam current be in excess of 1,000 amperes. If one attempted to produce such a current in a typical X-ray tube, a current density of more than 100,000 amperes per square centimeter would be required. Standard X-ray tubes cannot achieve such currents. To overcome this problem, the so-called flash X-ray tubes were developed.

The flash X-ray tube in its simplest form comprises an evacuated envelope, containing a field emission or cold cathode and an opposing anode. Such devices require no input energy to the cathode but need only the presence of a strong electric field at the cathode. This phenomenon is such that a high density stream of electrons can be obtained from the unheated cathode when the electric field is sufficiently intense. Theory predicts that the current of such devices can be on the order of 100 million amperes per square centimeter of cathode surface. This is about 1 million times greater than the current that can be obtained from thermionic cathodes. Typically, such devices in which the emitting cathode is in the form of a 0.040 inch tungsten wire have provided currents of 10,000 to 20,000 amperes for as long as 2 nanoseconds. Such devices can be made to produce even greater currents by using multitip cathodes.

Because of the high fields necessary to such devices, high voltages are required and a phenomenon known as dielectric surface flash-over in vacuum occurs which limits the voltage capable of being applied to the device.

The present invention is directed toward the control and prevention of such breakdown in flash X-ray tubes.

Since the radiation output of a flash X-ray tube increases approximately as the cube of the applied voltage,

the flash-over phenomenon limits the voltage capability of such devices and the achieving of the radiation theoretically available in such devices is prevented.

The present invention is thus directed toward the control and prevention of this flash-over phenomenon and permits devices of minimum size for a given applied voltage to be built. Apart from a general desire for compactness, size is an important consideration for reasons of inductance, which should be small in the fast discharge circuits normally associated with such devices.

Broadly speaking, fiashover control is accomplished by forming the walls of the tube from dielectric ring segments interposed with equipotential planes which are arranged at an angle to the longitudinal axis of the tube.

The present invention has a further attractive feature in that the ring segments in equipotential planes are selfstacking so that the tube may be manufactured without use of complicated jigs or other supporting structures. Other advantages and features of the invention will become apparent from the following specification, taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a cut-away view of a flash X-ray tube built in accordance with the present invention;

FIGURE 2 is a section through one portion of the wall of the tube of FIGURE 1; and

FIGURE 3 shows, in detail, a different embodiment of the equipotential planes of the invention.

Referring now to FIGS. 1 and 2, there is shown various views of an evacuated flash X-ray tube 10, employing a preferred embodiment of the present invention. The tube 10 comprises a cap plate 11 which carries a cathode support shank 12, an anode support ring 14, having a dished anode 15 of suitable target material, affixed thereto, and a plurality of dielectric rings 17 and equipotential planes 18 which form the walls of the tube.

More specifically, the cathode support shank 12, which can be cylindrical or conical throughout its entire length, has at its apex a sphere 19 which prevents unwanted field emission at the termination. At the pole of the sphere, opposite to the cone 12, is at least one needle-like protuberance 20 which is separated from the planar surface of the anode 15 by a specified distance X. Application of a sufficiently high voltage between the shank 12 and the anode 15 by a suitable power supply (not shown) causes the needle 20 to emit electrons, shown as arrow 22, by the phenomenon known as field emission. These electrons are directed by the applied field to anode 15 which they strike with sufficient energy to cause the production of X-rays, both in the forward direction 27, and in the reverse directions 23, 24, 25. The X-rays 27 which pass through the anode 15 and out of the tube are used for conventional purposes.

If the X-rays in the reverse direction 23, 24, 25 strike the dielectric wall sections 17 with sufiicient energy, they create electrons which can lead to electrical breakdown by an electron cascade process down the dielectric surface.

The present invention is directed toward inhibiting and controlling this cascading mechanism by forming the equipotential planes 18 as the frustum of a hollow core, whose upper surface 28 is at an angle 0 to the internal diameter of the dielectric rings 17. The planes 18 have an internally extending portion 29 which shields the dielectric rings 17 from X radiation (depending on the thickness) and from target material evaporated from the anode. Usually the best angle 0 is 45, although in special cases other angles greater or lesser than this angle may be preferred. The electric field geometry produced by having the angle 0 at 45, rather than at prevents electrons from cascading down the surface.

Further benefits can be obtained by forming the anode 15 in a shape of a dish having a side wall 30. Anode 15 may be joined to the anode support 14 by any convenient means such as brazing or welding. The side wall 30 may be made to meet, at any convenient angle 0, the planar center surface of the anode. This angle however is preferably greater than 45 and less than 90 and is limited only by mechanical requirements.

As shown in FIG. 2, all of the X radiation in the reverse directions 23, 24 and 25 strikes either the anode side wall 30 or the equipotential plane sections 29 which are made sufficiently thick to prevent significant penetration of the radiation. Thus the radiation does not strike the dielectric rings 17. The planes 18 thus not only protect the rings 17 from X radiation but also from evaporated target material. Furthermore, by forming the planes 18 in a conical shape, a self-nesting structure is obtained which eliminates the need for jigging during manufacture of the tubes.

Still further for reasons related to inductance and radial field consideration, the cathode shank diameter and the internal diameter of the tube should both be as large as possible. The present invention provides a tube which has a larger internal diameter than the tubes of the prior art but has the same external dimensions. Thus a tube embodyin the present invention will, by virtue of its larger internal diameter, have better radial field characteristics and can accommodate a larger diameter cathode shank which promote better inductive characteristics.

Also a tube formed in accordance with the present invention has a wall strength greater than that provided in the prior art. The described tube can also be made in demountable form as shown in FIG. 3. In this view the equipotential planes 18 are made in two separate sections 32 and 33 which are separated by a soft compressable O ring 34. The entire structure is nested as previously described and when the tube is evacuated the exterior ambient pressure forces the dielectric rings 17 against the sections 32 and 33 and the O ring 34 forming a vacuum tight seal. To take this tube apart all that is necessary is that the vacuum inside the tube be broken. This structure thus eliminates the need and the necessity of forming a dielectric to metal seal between the planes 18 and the rings 17.

Variations in the invention can be made. For example, the portion 29 of planes 18 could be caused to be made to curve and parallel the longitudinal axis of the tube which would reduce radial field effects and provide additional shielding of the rings 17.

Having now described at least one preferred embodiment of the present invention and since other variations and modifications may now become apparent to those skilled in the art, it is desired that the invention be limited only by the following claims.

What is claimed is:

1. A sealed off, self supporting, high voltage, high 1361111111 electric device comprising ring like insulating members, with interposed metallic members, said insulating members being rhombic in cross-section, and said ."netallic members being the frustum of a hollow cone and adapted to nest with said insulating members, thereby forming the self supporting walls of said device.

2. The device of claim 1 wherein there is provided at its respective ends cathode and anode assemblies.

3. The device of claim 2 wherein said metallic members have an inwardly projecting portion adapted to shield the inner walls of said insulating rings from said anode.

4. The device of claim 1 wherein each of said metallic members are in the form of two concentric hollow conical frustums and each of said frustums being separated from the other by a compressable O ring.

5. The device of claim 3 wherein the bottom and top surfaces of said insulating rings intersect planes normal to the longitudinal axis of the device at an angle greater than 0 degrees and less than 90 degrees.

6. The device of claim 5 wherein said angle is 7. The device of claim 1 wherein there is provided at its respective ends cathode and anode assemblies, said metallic members have an inwardly projecting portion adapted to shield the inner walls of said insulating rings from said anode, and the top and bottom surface of said rings intersect planes normal to the longitudinal axis of the device at an angle of approximately 45 degrees and said metallic members are intimately bonded to said rings.

8. A sealed off, self supporting, high voltage, high vacuum electrical device cold cathode, flash X-ray tube, comprising a cap plate carrying a cold cathode, an anode support ring, an anode afiixed on said support ring, a wall holding said support ring and said cap plate in fixed separation with said cathode being separately maintained from said anode, means for applying to said cathode and said anode a voltage sufiicient to cause a pulsed field emission of electrons with an energy suflicient to cause the production of X-rays when they strike said anode, said walls comprising ring like insulating members alternated with interposed metallic equipotential plane members, said insulating members each being rhombic in cross section and said metallic members each being the frustum of a hollow cone having a central aperture, each metallic member being adapted to nest with said insulating members and form the self supporting walls of said device, said metallic members and said support ring having a portion extending into the region enclosed by said insulating members interposed between said anode and said insulating members to intercept every line of sight path from the center of said anode and said insulating members to shield said insulating members from X-rays and control the dielectric surface fiashover of said insulating members between said metallic members.

References Cited UNITED STATES PATENTS 2,501,882 3/1950 Trump et al 313-63 2,569,154 9/1951 Donath 313-313 X 2,729,748 1/1956 Robinson 313313 X 3,065,374 11/1962 Rockwell 313-250 X 3,303,372 2/1967 Cager 313-336 X 3,309,523 3/1967 Dyke et -al 313- X 3,308,323 3/1967 Van de Graaff 313-250 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

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