Field Effect Electron Gun Having At Least A Million Emitting Fibers Per Square Centimeter

Abstract

An electron gun uses a field effect emitter in a vacuum tube, providing the dvantages of cold cathode emission at atmospheric temperature in addition to a simplified control system. Temperature independent emission is achieved by using an oxide-metal composite emitter for releasing electrons. The anode and emitter can be shaped to produce a desirable electric field and current path for special electron tubes.

Description

BACKGROUND OF THE INVENTION

An electron gun is an electrode structure for producing a specified number of electrons at a specified velocity with provisions for controllably introducing the electrons into an interaction space. Electron guns are used in many electron tubes such as television picture tubes, traveling wave tubes, klystrons, X-ray tubes, and other electronic devices. The electron gun as embodied in simple cathode-ray tubes were used by early experimenters to determine the ratio of charge to mass of an electron, and contributed to basic research in many ways. Typically, electrons are boiled off the cathode and are accelerated toward the anode with an energy depending on the difference in potential between the anode and cathode. Although some of the electrons strike the anode the majority pass through a hole in the anode and into an interaction region or drift space which contains parallel deflection plates. If no potential is applied across the deflection plates, the electrons travel in a straight line and strike a fluorescent screen. If a potential is applied across the plates the electron stream is deflected, being focused at a different point on the screen.

The basic electron gun employs an electron emitter and an accelerating anode. In modern electron tubes, a focusing anode concentrates the electron beam such that essentially all the electrons will go through a hole in the accelerating anode, improving efficiency of the device and eliminating heat related problems generated by the electrons striking the electrode. Some gun assemblies are designed along concentric circles in order to maintain a uniform field and to allow the use of a larger emitter which reduces the current density requirements for the emitter. Two extremely important parameters of an electron gun are the number of electrons in the beam, which must be constant and controllable, and the energy of all the electrons, which must be nearly the same for efficient operation. Prior art thermionic emitters must have the temperature level and voltage controlled, the electron energy being a function of temperature as well as of the potential applied between emitter and anode. Thermionic emitters must be operated at saturation in order to avoid current changes with even small changes of anode voltage. With various absolute temperatures, the current available from a thermionic emitter is a function of the applied potential. Even small changes in emitter temperature result in changes in electron emission. Thus, both the anode potential and the emitter temperature must be extremely well regulated to provide constant current.

SUMMARY OF THE INVENTION

A unique field effect electron gun utilizes an oxide-metal composite emitter to operate and control electron flow without thermionic emission or interference. The oxide-metal composite emitter is a field effect electron emitter that operates at ambient temperatures. The number of electrons emitted is a function of the electric field. The electric field developed between emitter and anode can be shaped by the emitter and anode structure to direct current where desired. Therefore, the nature of emission of the electron gun allows variety to be readily engineered into the gun structure for specific applications.

An object of this invention is to provide an electron gun having only a single parameter involved in controlling electron emission.

Another object of this invention is to provide an electron gun having a faster response time than conventional, prior art electron guns.

Still another object of this invention is to provide an electron gun of rugged and simple structure that eliminates heat transfer problems by providing ambient temperature emission of electrons to the exclusion of heated electrode emission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a conventional electron gun and typical associated components.

FIG. 2 is a typical graph of thermionic current at various emitter temperatures.

FIG. 3 is a diagrammatic view of a field effect electron gun and associated components.

FIG. 4 is a diagrammatic sectional view of a compact field effect electron gun.

FIG. 5 is a typical embodiment of an electron gun with a field effect emitter.

FIG. 6 is a diagrammatic sectional view of an electron gun structure for generating a hollow electron beam.

FIG. 7 is a current density graph for the electron gun of FIG. 6.

FIG. 8 is an enlarged section of an emitter structure employable in the electron gun of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical electron tube which employs an electron gun includes the conventional thermionic gun and associated components shown in FIG. 1. With the envelope and non-essential structure being omitted, FIG. 1 discloses a thermionic cathode 10 encompassing a heater electrode 12. Electrode 12 is coupled to a heater transformer and control system 14 for maintaining a substantially uniform cathode temperature. An accelerating anode 16 adjacent cathode 10 accelerates electrons emitted from the cathode into an intersection or drift space 18. In the drift space the electrons may be acted on by deflection plate potentials before impinging on a collector 20. The accelerating potential between anode 16 and cathode 10 determines the electron current for any given operating temperature. Accelerating anode 16 may be a typical grid screen as shown in the drawings or may be of the more modern gridless, beam focusing design to reduce electron interception and resulting grid current. FIG. 2 shows several typical curves of current available from a thermionic emitter as a function of applied voltage at various absolute temperatures. As a rule, all thermionic emitters have the same general curve and show the Edison effect at the low end, the straight line portion and then the saturation portion at high voltages. As noted in FIG. 2, the emitter must operate at saturation to avoid current changes with changing anode voltages. In designing an electron gun using a thermionic emitter, it is necessary to consider several variables. The power supplied to the heater to raise the temperature of the thermionic emitter to its operating point must be computed. Heat losses by conduction and radiation as well as cooling produced by the electrons leaving the emitter must be considered. As shown in FIG. 2, the current density is a function of both temperature and accelerating potential, and is very sensitive to at least one of these variables at any point selected. The accelerating potential determines the energy of the electrons and is selected to give the proper energy transfer in the interaction space. For a constant current, the anode potential and the emitter temperature must be well regulated.

In FIG. 3 the thermionic cathode and related control circuits associated with the electron tube have been replaced with a field effect electron gun structure 22. In an electron gun using the field effect electron emitter, the spacing between the emitter and accelerating anode is computed from the anode potential B+, which may be variable and the field required for the desired current from the emitter. Emitter temperature is not a parameter to be considered and heat losses are not relevant since the emitter operates at ambient temperature and is not temperature sensitive. Electron gun 22 comprises accelerating anode 16 spaced apart from a field effect emitter 24. Emitter 24 is joined to and supported by conductive backing plate 26. Backing plate 26 is connected to conductor 28 which provides the cathode voltage B therethrough to field effect emitter 24. An efficient space saving embodiment of the field effect electron gun is shown in FIG. 4. The accelerating anode 16 is fixed to the emitting surface of emitter 24 for accelerating electrons into the interaction space.

Field effect emitter 24 comprises a metal-oxide composite wherein a plurality of electrically conductive fibers project through an oxide matrix or filler. As shown in FIG. 5, metal rods (fibers) 34 project through oxide filler 36 to form cathode emitter 24. Rods 24 can be more than a million for each square centimeter of surface area, normally terminating in a place substantially parallel with surface 35. Backing plate 26 is electrically connected to one surface of the emitter. The emitting surface can be etched with a suitable chemical etch to allow emitting rods 34 to project through the oxide matrix a predetermined distance. Rods 34 can be etched below oxide surface 35 to form a void 38 therebetween, as shown in FIG. 8. If the anode of the gun is to be placed directly in contact with oxide 36 (as shown in FIG. 4) the rods are etched to a desired uniform level below the oxide surface, as shown in FIG. 8.

The beam edge shown in FIG. 5 is determined by the edges of anode 16 and emitter 24. If a particular beam pattern is required anode screen 16 is modified in a manner as typified in FIG. 6. This structure changes the direction of the electric field developed between emitter and anode providing a central aperture. For the anode structure of FIG. 6 a hollow beam is projected from the anode toward the collector with little or no current flowing through the anode central aperture due to the electric field effect. The current density decreases rapidly with increased distance between the anode surface and emitter surface since a small change in field strength involves a large change in current density, as shown in FIG. 7. A hollow electron beam can be formed using a solid emitter when this type of field effect electron gun is used because of the field dependency of the electron emission The collector in an electron tube such as a klystron or traveling wave tube serves to catch the electrons and complete the electrical circuit through the associated power supplies and back to the emitter. No change in the collector design is required by substituting emitters since the same electron velocity and interaction space is used.

The basic field effect electron gun comprises a flat field effect emitter attached to a backing plate and a flat accelerating anode. Electrons are emitted from the emitter when a positive potential, with respect to the emitter, is applied to the accelerating anode. The electrons are accelerated by this anode potential in a straight line and the majority pass through the accelerating anode screen to the collector.

The kinetic energy of an electron can be written as E = eV where e is the charge, V is the voltage across which the electron moves and E is the kinetic energy. The potential of the accelerating anode with respect to the emitter is V. From classical physics, E is equal to one-half Mv.sup.2 where M is the mass of the electron and v is the velocity thereof. The two equations can be combined providing an expression for V, V = Mv.sup.2 /2e. Thus the anode potential is computed in a manner well known in the prior art.

The number of electrons of current density J for a field effect emitter is a function of the electric field and can be determined for a given group of emitters. The field is given by the expression F = V/S where F is the field and S is the spacing between emitter and anode. The gun current I is written as I = JA where A is the area of the emitter. By knowing the required current density J, the field required is given from experimental data for the emitter. By then knowing both the field F and the potential V, determined hereinabove, the spacing between anode and emitter is found.

In operating the field effect electron gun there is no thermionic emission required. The gun begins emitting electrons at the prevailing ambient temperature as soon as the emitter-anode, emitting voltage is applied thereto. The electron density is determined by the electric field intensity between the emitter and the anode, the field intensity being easily varied by changing the anode potential.

There are several unique properties of the field effect electron gun. Both the electron beam current and the electron velocity are controlled by a single parameter, the potential of the anode with respect to the emitter. Operation begins immediately upon application of the anode potential and there is no warm-up time involved. Special beam shaping, such as the hollow beam, can be accomplished by simple design methods. No emitter temperature control is required. Advantages include reduced cost since heater and heater axillary equipment are not required. The useful life is increased since the generation of electrons no longer depends on a high temperature reaction. The gun produces electrons immediately upon application of the anode potential. The overall efficiency is increased since no heater power is required. No gas is generated when the gun is turn on as in the conventional electron gun, eliminating possible gas ionization and resultant failure of conventional electron guns.

Obviously many modifications and variations to the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

I claim:

1. An improved electron gun assembly for use in evacuated electron tubes and comprising: a field effect emitter having a planar emitting surface for emitting electrons in a vacuum at prevailing ambient temperatures, said emitter having at least a million emitting fibers disposed in parallel per square centimeter of emitting surface; a conductive backing plate adjoining respective first ends of said fibers for conducting an electrical potential thereto; an insulating oxide matrix encompassing separating said fibers, respective second ends of said fibers terminating in said emitting surface below the surface plane of said oxide; and a flat surfaced, shaped accelerating anode the surface of said oxide matrix and in parallel spaced apart relationship for enhancing electron flow from said emitter, said anode and said emitter having circular surfaces and said anode further having an apertured center for providing a hollow electron beam when subjected to an electric field.