US3388275A - Cathode provided with ion-producing material for decreasing space charge - Google Patents

Description

June 11, 1968 H BETTENHAUSEN E A 3,383,275

CATHQDE PROVIDED WITH ION-PRODUCING MATERIAL FOR DECREASING SPACE CHARGE Filed June 8, 1966 SWITCH CAPACITOR BAN K TUBE HIGH VOLTAGE SUPPLY PIE United States Patent 3,388,275 CATHODE PROVIDED WITH ION-PRODUC- ING MATERIAL FOR DECREASING SPACE CHARGE Lee H. Bettenhausen, Harold M. Epstein, and Arnold M. Plummer, Columbus, Ohio, assignors to The Battelle Development Corporation, Columbus, Ohio, a corporation of Delaware Continuation-impart of application Ser. No. 511,947, Dec. 6, 1965. This application June 8, 1966, Ser. No. 556,054

Claims. (Cl. 313-55) ABSTRACT OF THE DISCLOSURE Dose rates in a flash X-ray tube are increased significantly by coupling to the cathode 23, 123, etc., a nonconducting conical focusing member 22, 122, etc., that provides a plasma to neutralize the space charge and increases the density of fast electrons that flow to the anode 18.

This invention relates to electron field emission and, more particularly, to a method and means for increasing the discharge rate of electrons from a cathode so as to improve the electron emission characteristics thereof. The invention relates additionally to a method of increasing Xray dose rates in a flash X-ray system by means of the aforesaid cathode. This application is a continuous-1npart of copending application Ser. No. 511,947, filed Dec. 6, 1965, now abandoned.

It is often desirable to free a large number of electrons from a cathode surface in a short time interval. The number of electrons that may be freed within a stated time interval at a given voltage and area of cathode is limited by the space charge. One application requiring the production of a large quantity of electrons within a stated short time interval (e.g., as short as l0 seconds) is in the production of high dose-rate flash X-rays. These X-rays have a greater intensity than those produced from the continuous beam of electrons available in an ordinary X-ray machine. The short, intense flash X-ray produced upon impact of the intense current with a target finds use in X-ray photography of high speed phenomena and in the study of transient radiation effects. It has also been suggested as a means for inducing changes in the properties of materials.

To produce a flash X-ray pulse, known systems have employed a plurality of capacitors or other energy storage devices to induce rapid flow of electrons. The usefulness of these devices has been found to be hampered by the previously described space charge limitation characteristic of cathodes and existing at the various levels of potential employed. In other words, for a given field, only a known limited current can be drawn from a metallic emitter.

Accordingly, it is an object of this invention to increase the current density of devices emitting electrons.

It is a further object of this invention to increase the X-ray dose rates available in a flash X-ray system.

It is a still further object of this invention to provide an improved flash X-ray device.

It is still another object of this invention to increase the space charge limited field emission current in a flash X-ray system.

The foregoing and other objects of the invention are attained in the method and apparatus described and illustrated herein.

In the drawings:

FIG. 1 is a schematic representation of an electrical system for producing flash X-rays according to the present invention.

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FIG. 2 is a cross section of a tube such as is represented schematically in FIG. 1.

FIG. 3 is a cross section of an alternative embodiment of the cathode assembly of an electrode system.

FIG. 4 is a cross section of another alternative embodiment of a cathode assembly of an electrode system.

FIG. 5 is a cross section of still another alternative embodiment of a cathode assembly of an electrode systern.

FIG. 6 is a cross section of a cathode that may be employed in a cathode assembly.

The present invention includes within its scope the formation of a plasma to neutralize the space charge contiguous to the emitter face of a cathode to create a large current density. Plasma can be formed by the coupling of an electrical nonconductor to the cathode of an electrode system to form a cathode assembly. At least a portion of the coupled nonconductor extends outwardly from the cathode in the direction of the opposite electrode. The aforesaid outwardly extending portion is provided with at least one wall defining a right cone having an axis substantially perpendicular to the emitter surface of the cathode.

Referring to FIG. 1, a schematic representation is shown of an electrical system including a tube provided with the cathode assembly according to the present invention. A high voltage power supply serves a bank of capacitors communicating with the tube. For the case where a high voltage is needed to operate the tube in a short time interval, a switch is interposed between the power supply and the tube to allow the voltage to build up to the desired level.

An embodiment of a tube 10 of this invention suitable for production of flash X-rays is illustrated in FIG. 2. In tube 10, a centrally located cathode assembly 25 is fixed in a holder 12 and is suitably spaced from a thin metallic anode or target 18 supported around its periphery by a base 16 fixed in holder 12 and secured by a clamp 14 served by a lead 15. Anode 18 may typically comprise iron having a thickness of 2 mils or tungsten having a thickness of 1 mil, or 3 mils thick aluminum faced with 2 microns of gold or other similar materials known to the art. An opening defining a vacuum manifold 20 surrounds the periphery of cathode assembly 25 and defines a passage to the space separating the cathode assembly 25 and target 18. Cathode assembly 25 comprises a cathode 23 and an electrically nonconducting sleeve 22 slidably engaging the outer longitudinal portion of cathode 23 to completely surround the same. The upper portion 24 of the inner surface of the sleeve 22 slopes inwardly away from cathode 23 in the direction of anode 18 forming a truncated right cone whose axis is perpendicular to the emitter face of cathode 23 so that the area of the opening formed by sleeve 22 is smaller than the area of the face 21 of cathode 23. At the end of cathode assembly 25 opposing anode 18, cathode assembly 25 engages a dielectric switch 27 furnished with leads 29 and communicating with a conductive transition piece 31 having leads 33 running from a power supply (not shown). The transition piece 31 is provided with an axial bore 32 to allow vaporized material to escape from the area occupied by dielectric switch 27 upon activation of the same.

In the operation of tube 10, a high voltage is allowed to build up in transition piece 31 of evacuated tube 10. When the voltage reaches the desired level (e.g., 75,000 volts) a current is induced in dielectric switch 27 to cause the same to disintegrate and thereby become conductive and connect the potential to cathode 23. (See Technical Specifications of an Exploding-Wire-Triggered Solid Dielectric Switch (SIT-P78), Henry Huber, Stevens Institute, January 1963; Report AD 299077 on Contracts AT(30-1)28l3 and DA-36039SCS7242. Abstracted in Nuclear Science Abstracts, volume 18, No. 7, Apr. 15, 1964; Abstract 10343.) At the aforesaid high voltages, a decrease of the space charge occurs in the area of field concentration at the interface of cathode face 21 and the upper portion 24 of the inner surface of the nonconductor or sleeve 22 of FIG. 2. While the exact nature of the effect is not known with certainty, it is believed that various components of the high voltage field cause electrons leaving the cathode surface to re-encounter the surface of the slightly inwardly sloping wall 24 of multiplier sleeve 22. The electrons gain SUillCiIlt energy in the field to cause the ratio of secondary electron emission to be greater than unity upon impact with surface 24 of sleeve 22. It is postulated that this initial electron cascade along the inwardly sloping wall of the sleeve causes a plasma sheath to be created along the sleeve surface. The space charge at the interface of the cathode and the plasma will be decreased by neutralization with the positively ionized plasma so as to allow higher current densities for a given field strength. It is necessary to have a positive ion current equal to about .002 of the electron current to neutralize the space charge. The electron interaction with the nonconducting sleeve is obtained with a fraction of the voltage drop, so that the remaining electrical potential may be used for electron acceleration. The electrons accelerated from the cathode assembly strike target 18 to produce the intense flash X-ray.

The positive ion current required to neutralize space charge is low enough that the probability of a collision between an accelerating electron and a positive ion is negligible and the entire electron energy is delivered to the anode target as kinetic energy. This causes an explosive blowoif of material from the anode moving at very high shock velocities. When this fairly dense plasma reaches the cathode, the probability for collisionless electron acceleration decreases considerably and the tube starts to short out. Times on the order of a microsecond or two may elapse between initiation and effective shorting for the present device.

Another phenomena which complicates the tube discharge is the pinch effect on the neutralized plasma. This should occur only after a fairly dense plasma is in existence or during the late phases of the discharge. This type of pinch may result in the collapse of the flux lines in a manner tending to divide the plasma into sausage-like links. This squeezing-off of the current can result in induced voltages greater than the initial voltage, giving rise to bursts of high energy X-rays.

The factors affecting the formation of the plasma during the useful part of the current pulse and its characteristics are not known with certainty. It is known that the presence of the plasma is essential to the improved current density according to the invention by neutralization of the space charge during the useful part of the current pulse.

Alternative cathode assemblies for producing the i1nproved emission characteristics according to the invention are shown in FIGS. 3 through 5. Referring to FIG. 3, there is shown a cathode assembly 125 similar to that of FIG. 2 but having a small portion of sleeve 122 overlapping the emitter face of cathode 123. The upper portion 124 of the inner surface of sleeve 125 again slopes inwardly from the cathode surface to define circular opening at its base. The effect of the cathode assembly of FIG. 3 is to provide a smaller conical portion or tunneling channel for electrons.

FIG. 4 illustrates a cathode assembly 225 wherein a cathode 223 is coupled to a truncated conical component of a nonconductor 222. The outer walls of nonconductor 222 slope outwardly away from the face of cathode 223.

In FIG. 5 cathode assembly 325 is illustrated wherein a conical nonconductor 322 is shown coupled with cathode 323 and is provided combined features of those shown in FIGS. 2 or 3 and 4. The outer walls of nonconductor 322 slope inwardly away from the emitter face o thode 323 so as to avoid flow of electrons along these inwardly sloping faces. A co-axial conical passage defines the tunneling channel previously described in connection with FIG. 3. Under some operating conditions, the coupled nonconductor of FIGS. 4 and 5 may be less desirable than the nonconductor sleeve of FIGS. 2 and 3, especially where electrons might be discharged from the unprotected walls of the cathodes of FIGS. 4 and 5.

The nonconductors 22, 122, 222, and 322 of the oathode assemblies of FIGS. 2 through 5 have at least one feature in common. At least one portion of the nonconductor extending outwardly from the planar face of the cathode defines the shape of a right cone having an axis substantially perpendicular to the emitter face of the cathode. The conical portion is provided with either an outwardly (e.g., FIG. 4) or an inwardly (e.g., FIG. 2) sloping wall that defines the path of electron flow. In any event, a portion of nonconductor is thus interposed between the anode and cathode. Generally it has been found that the slope of the wall defining the How path of the electrons should be relatively small (e.g., 10 to 30).

An electrically nonconducting material of Lucite has been found to be satisfactory for the nonconductor. Numerous other suitable nonconducting materials may also be used. It may also be desirable to coat the surface of the nonconductor with semiconductor materials such as those used in the dynodes of photomultipliers or to mix these materials with the bulk material of the insulator. The cathode can be fabricated from OFHC copper, aluminum or the like. In FIG. 6, a bimetallic cathode 423 is shown comprising, say, tungsten or stainless steel emitter face 421 integral with larger aluminum body 424. Similarly, water can be circulated through a portion of the cathode where the generation of excessive heat is likely.-

To assure neutralization in the area immediately adjacent the emitter face of the cathode where electron densities are greatest and to avoid the formation of large electrostatic forces, it is important to have adequate ionization of the plasma. One method for increasing ionization is to increase the temperature of the plasma. A sloping nonconductor wall (e.g., as in FIG. 2) probably serves to constrain the plasma thereby acting to elevate the temperature of the plasma. In another method for increasing ionization, materials having a low ionization potential such as cesium and uranium are intentionally introduced to the plasma. This can be done simply by coating these materials on the cathode. In the assemblies of FIGS. 2 through 5, additional ionized material may be available from the vapor blown off the nonconductor walls. Thus, from a consideration of electron density at the cathode, adequate control of the plasma is achieved by varying the geometry of the nonconductor wall and its properties as well as varying temperature or introducing low ionization potential materials to regulate the positive ion density of the plasma.

EXAMPLE 1 In an operation with an electrical system similar to that of FIG. 1 and tube assembly similar to that of FIG. 2, the cathode assembly operated at over 1 10 amp./ cm. at 75,000 volts. The dose rate of the X-rays produced upon impact of the electrons with the target was significantly higher than could be obtained for the voltages used by known devices operating under the metallic cathode space charge limitations.

For a cathode of tungsten, electrons in an apparatus of the type described escape by Schottky emission which can be characterized in terms of current density by:

Where J is the emitted current density, F is the electric field, (p is the work function and K is 8.0 10- For the electric field applied at 75,000 volts and cathode temperatures on the order of about 5000 K., currents in the 10 amp./cm. range as achieved herein can be expected.

However, for the condition where space charge is not neutralized, the field emission is reduced to:

Thus, at 75,000 volts under the conditions of Example 1, current density obtainable from a device operating under the space charge limitation would be less than 100 amp./ cm. Elevating the temperature of the cathode would produce only slightly higher currents.

EXAMPLE 2 A foil was placed at a distance of 100 mils from the anode target to measure the dose of X-rays produced upon collision of electrons with an anode target under the conditions of Example 1 using 58,000 volts. The dose of the X-rays was about 10,000 rads (silicon). This compares with an expected dose of about 10 rads (silicon) characteristic of a device operating at the same voltage under cathode space charge limitations. Dose rates several orders of magnitude greater than those achieved in the present device at 58,000 volts would be expected at higher practical voltages that can be achieved such as 240,000 volts.

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention may be made within the principles and scope of the invention. For example, although formation of plasma has been shown by means of a coupled nonconductor, any means of creating a plasma, such as a conventional plasma gun, may be used together with the cathode.

What is claimed is:

1. An electrode system comprising a cathode assembly spaced from an anode, said assembly comprising a cathode coupled to means for increasing the electron emission of said cathode assembly with a fraction of the total voltage drop, said means comprising a member, consisting essentially .of a nonconducting material, having at least one portion extending outwardly from the emitter face of said cathode, at least a portion of said outwardly extending portion defining a right cone having an axis substantially perpendicular to the emitter face of the cathode.

2. The electrode system of claim 1 wherein said nonconducting material is Lucite.

3. The electrode system of claim 1 wherein said member includes also a semiconductor material.

4. The electrode system of claim 1 wherein said member comprises a sleeve member having a longitudinal axis, a portion of said sleeve member engaging the outer longirtudinal walls of said cathode and coaxial therewith and another portion of said sleeve member comprising said outwardly extending portion, having walls sloping inwardly away from the face of said cathode to define a conical tunneling channel for the flow of electrons from said cathode assembly.

5. The electrode system of claim 4 wherein said sleeve member includes an annular step having an inner surface contiguous to a portion of the emitter face of said cathode.

6. The electrode system of claim 1 wherein said member comprises a truncated conical component coupled to the face of said cathode, said conical component having a cross-section in a plane transverse to the axis of said cathode increasing with distance from the face of said cathode.

7. The electrode system of claim 1 wherein said member comprises a truncated conical component having a cross-section in a plane transverse to the axis of said cathode decreasing with distance from the emitter face of said cathode and provided with a coaxial conical passage defining a right cone having walls sloping inwardly away from the emitter face of said cathode to define a tunneling channel for the flow of electrons from said cathode assembly.

8. In an electrical system for the production of X- rays wherein electrons emitted from a cathode strike an anode target to produce X-rays, the combination with said cathode of a cathode assembly spaced from said anode, including a coupled nonconducting material having at least one portion extending outwardly from the emitter face of said cathode, at least a portion of said outwardly extending portion defining a right cone having an axis substantially perpendicular to the face of said cathode.

9. A flash X-ray device comprising:

a cathode assembly comprising a cathode coupled to a nonconducting material, said nonconducting material having at least one portion extending outwardly from the face of said cathode, at least a portion of said outwardly extending portion defining a right cone having an axis substantially perpendicular to the face of said cathode;

a target anode spaced from said cathode assembly;

and a power supply connected to said cathode and said anode to provide a potential difference therebetween.

10. An electrode system comprising:

a cathode;

means for forming a plasma in the area immediately contiguous to the emitter face of said cathode; and an anode spaced from said emitter face of said cathode; said means for forming a plasma comprising a member, consisting essentially of a nonconducting material, having at least one portion extending outwardly from the emitter face of said cathode, at least a portion of said outwardly extending portion defining a right cone having an axis substantially perpendicular to the emitter face of the cathode.

References Cited UNITED STATES PATENTS DAVID J. GALVIN, Primary Examiner.

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