US3243640A

US3243640A - Space-charge neutralized electron gun

Info

Publication number: US3243640A
Application number: US257180A
Authority: US
Inventors: Arie L Eichenbaum
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1963-02-08
Filing date: 1963-02-08
Publication date: 1966-03-29
Anticipated expiration: 1983-03-29

Description

March 29, 1966 A. EICHENBAUM SPACE-CHARGE NEUTBALIZED ELECTRON GUN Filed Feb. 8, 1963 JMKMM United States Patent 3,243,640 SPACE-CHARGE NEUTRALIZED ELECTRON GUN Arie L. Eichenbaum, Levittown, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Feb. 8, 1963, Ser. No. 257,180 16 Claims. (Cl. S15- 3.5)

This application is a continuation-impart of my application, Serial No. 38,780, tiled June 27, 1960, now abandoned, assigned to the same assignee.

The present invention relates to electron beam tubes, and particularly to a beam tube having a space charge neutralized electron gun capable of producing a stable electron beam of high current density at moderate cathode temperature or a stable electr-on beam of moderate currrent density at very low cathode temperature.

The current density :of -a beam consisting essentially of electrons continuously emitted from .a ilat surface is limited to about 8 amperes per square centimeter (a/ cm.2). Previous attempts to increase the beam current density led to development -of convergent guns of the well known Pierce and Heil types. The beams produced by these guns, while having high current densities, are known to be very noisy and also subject to excessive beam scalloping. Attempts to avoid these difficulties have led to the development of hollow cathode guns. While the beam noisiness :and scalloping are reduced with such guns, it was found that space charge limitati-ons prevented beam current densities in excess of about 8 a./cm.2. It is known that much higher electron current densities can be produced in the electron stream of a triode by introducing positive ions into the stream in suicien-t quantity to neutralize the negative space charge -of the electrons. Due to lower mobility, and usually higher individual positive charge, the number of positive ions required for neutralization is much smaller than the number of electrons emitted by the cathode. APositive ions have been produced by several methods, including (l) ionization of gas atoms as a result of collision or impact by the electron-s in a beam, (2) ionization of allcali metal vapor atoms by contact with a heated metal surface having a work function higher than the ionization potential of the gas atotms, and (3) direct thermionic emission of positive ions from a heated lion source. The rst of these methods is not sui-table for the production of stable high density beams or low velocity electron beams (i.e. lower than the ionization potential of the gas) useful for low-noise purposes. Therefore, either of the other two methods, which involve positive ion emission, is used in practicing the invention. The second method involving contact ionization is carried out by causing alkali metal atoms, Isuch as cesium, to contact a heated cathode of tungsten, for example, or by causing the cesium atoms to contact an auxiliary heated tungsten electrode.

An object of the present invention is to provide an electron tube having .a space charge neutralized electron gun capable of producing a stable, high density beam (at least 20 a./cm.2) .at moderate cathode temperatures (at least l000 K.). Another object is to provide an electron tube having a space charge neutralized hollow cathode electron gun capable of producing a stable, relatively dense electron beam (at least 100 rma/cm?) at low cathode temperatures, of the order of 700 to 1000" K., for use in very low noise applications.

These land other objects are accomplished in accordance with the invention by providing fa thermionic cathode, having a relatively large emissive surface, land a beam forming electrode, having a single small beam aperture with an area that is small compared to the area of the emissive surface of the cathode, located in frront of the 3,243,640 Patented Max'. 29, 1966 ice cathode, heating the emissive surface of the cathode to electron emitting temperature, introducing suiiicient positive ions into the space between the emissive surface and the aperture other than by ionization by electron impact to neutralize the space charge yof the electrons, and extracting the electrons emitted from the emissive surface through the small aperture at low velocity. A plasma, or substantially neutral mixture of charged particles, is formed rand maintained in this space which makes it possible to bias the Iapertured beam forming electrode at a low positive potential relative to the potential of the emissive surface :and still draw out substantially all of the emitted electrons through the beam aperture. Under these conditions, the electron current issuing from the aperture is substantially :a thermal current; that is, its electron temperature is substantially the same as the cathode temperature. In .other words, the velocity spread yof the electrons in the beam is substantially equal to the velocity spread of the electrons at the emissive surface of the cathode. This thermal current issuing from the aperture at low velocity can be accelerated to high velocities without substantially increasing the electron temperature or beam noise, and hence, is hi-ghly suited for iuse in beam ampliers such as klystrons .and traveling wave tubes where high s-ignal-to-noise ratios are desi-red. The positive ions may be produced by contact ionization of alkali metal vapofr atoms, either by the electron emissive surface of the cathode or by the surface of ,a separate metal element spaced from that surface. This is considered an ion emission process. Alternatively, the positive ions may be supplied by la separate thermionic positive ion emitter. Suitable means are provided for constraining or focusing the beam to prevent spreading thereof after emerging from the aperture electrode. For high vacuum use of the beam, suitable means, such as a liquid air trap, is provided for reducing the gas pressure in the beam path beyond the :apertured electrode to a good vacuum.

In the accompanying drawing:

FIG. l Iis an `axial sectional view of an electron beam tube incorporating the present invention;

FIG. 2 is a transverse sectional view taken on the line 2 2 of FIG. 1;

FIG. 3 is a graph to be used in explaining the operation of the tube of FIG. l;

FIGS. 4 and 5 are axial sectional views of two modifications of the electron gun structure yof FIG. l; and

FIG. 6 is an axial sectional View of another embod-iment of the invention.

The beam tube shown in FIGS. l and 2 comprises a vacuum tight envelope or bulb 1 including a cup-shaped end portion 3 containing an electron gun 5 embodying one form of the invention. The gun 5 comprises a hollow tubular thermionic cathode 7 having a relatively large cylindrical internal electron emissive surface 9 coaxial with the central axis of the envelope. The cathode 7 is closely surrounded by a coil 11 for heating the cathode surface 9 to electron emitting temperature. A beamforming plate electrode 13 having a small central aperture 15 is located closely adjacent to the upper open end of the cathode 7, with the aperture 15 coaxial with the cylindrical emissive surface 9. The area of the aperture 1S is small compared to the area of the emissive surface 9.

The gun 5 further includes a positive accelerating electrode 17 having a beam aperture 19 spaced from and coaxial with the aperture 15. The aperture 19 is somewhat larger than the aperture 15. The electrode 17 is made of magnetic material and is cup-shaped with a tubular Wall portion 20 extending back over the cathode 7 to substantially shield the latter from external magnetic fields. The cathode 7, heater 11, plate electrode 13 and accelerating 3 electrode 17 are provided with external leads for the application of suitable operating potentials, a set of which is shown on the drawing as an example.

A small amount of cesium, or a compound containing cesium, is introduced into the envelope portion 3 during manufacture in any suitable manner. For example, the envelope portion 3 may originally include a tubular appendage containing a number of pellets of cesium generating material which are heated by RF to cause them to explode and inject cesium into the envelope portion 3, after which the appendage is pinched off as shown at 21. During operation of the tube, the heat from the heater 11 may be sufficient in many cases to heat the envelope portion 3 to vaporize the cesium and maintain the desired cesium vapor pressure. However, it is preferred to independently heat the envelope portion 3, as by means of a heater coil 23 surrounding the same, to control the cesium temperature and vapor pressure. The stem portion of the bulb is kept at a temperature somewhat higher than the adjacent cylindrical portion by the heat refiected downward from the hot cathode region. This eliminates the necessity for separately heating the stem portion and also reduces leakage currents between the tube leads by preventing the condensation of cesium thereon. The excess cesium in the tube is indicated schematically by the numeral 25 in FIG. l.

As shown in FIG. 1, the tube envelope 1 further includes an end portion 27 which may contain any desired structure for utilizing the high density beam from gun 5, as schematically illustrated by a drift tube 29 axially aligned with the beam path. For example, the drift tube 29 may be the helix or other delay line for traveling7 wave interaction with the electron beam. Since the interaction region of the tube must normally be a high vacuum region, a portion of the beam path between the gun and end portion 27 is surrounded by suitable means, such as a liquid air trap 31, for condensing the cesium vapor thereon to reduce the gas pressure in the interaction region to a good vacuum. The trap 31 is sealed to the envelope portions 3 and 27 to form part of the vacuum envelope of the tube.

The entire tube is surrounded by means such as a magnet coil 33 for establishing an axial beam focusing magnetic field along the beam path beyond the aperture 19. The space within the cup-shaped magnetic member 17 is substantially shielded thereby from the field of coil 33. While the magnetic shield 17 causes most of magnetic ux from coil 33 to by-pass the space within the hollow cathode 7, some of the flux will thread through this space and thereby assist in directing the electrons through the exit aperture 15.

In the operation of the tube, the cathode 7 is heated to the desired temperature, by the heater 11 and an external current source 35, to cause the surface 9 to emit a copious flow of electrons. These electrons are extracted through the small apertures and 19 by a low positive accelerating potential on the plate electrode 13 and/or the field of a relatively high positive potential on electrode 17. In order to overcome space charge effects and concentrate and pass most of the electrons emitted by the large area hollow cathode through the small aperture 15, sucient positive ions are provided within the hollow cathode to neutralize the space charge of the electrons. This is done by heating the gun enclosure, as by means of the heater coil 23, to a temperature suicient to vaporize some of the cesium and maintain the desired cesium temperature and vapor pressure. The cesium vapor atoms diffuse throughout the gun enclosure including the interior of the hollow cathode. The surface 9 of the cathode 7 is made of a high work function metal such as tungsten. Some of the cesium atoms which come into contact with the hot tungsten surface 9 give up electrons to that surface by the phenomena of contact ionization and become free positive ions. This is because the ionization potential of cesium, which is 3.9 volts, is less than the electron work function, 4.5 volts, of pure tungsten. It is believed that the contacting vapor atoms share electrons with the surface, and hence, they are effectively part of the surface. The heat of the surface causes the ions to be emitted therefrom leaving the shared electrons in the surface. 1f the cathode temperature is high enough relative to the cesium temperature, none of the cesium atoms or ions remain on the tungsten surface. In this case, the effective work function of the emissive surface 9 is that of pure tungsten, 4.5 volts. At lower cathode temperatures for a given cesium temperature, some of the cesium atoms are adsorbed by the tungsten surface, which reduces the effective work function of some areas of the tungsten surface while leaving other areas of the tungsten surface bare for contact ionization of cesium atoms.

Data on the emission of electrons from cesiated tungsten cathodes were published by Langmuir and Kingdon, Physical Review 21, p. 380, 1923; and Taylor and Langmuir, Physical Review 44, p. 423, 1933. In these experiments, the emission due to adsorbed layers of cesium on tungsten Wire was measured as a function of the vapor pressure of saturated cesium vapor (given by the bulb temperature) and the tungsten cathode temperature. FIG. 3 shows typical curves obtained by Taylor and Langmuir for cathode temperatures between about 500 and 2200 K., and bulb temperatures of 253, 270, 290, 313, 340, 372, and 412 K. The graph also includes lines indicating the fractional coverage, 0, of tungsten by cesium, for values of 0, .2, .4, .5, .55 and .67. It can be seen that, for each bulb temperature, as the cathode temperature is lowered from a relatively high value, the emission first decreases to a minimum at a low value of 6, then increases to a maximum at about 9:.55, and then decreases again. All of the curves merge at the 9:0 line, so that high electron emission can be obtained at high cathode temperature for each value of bulb temperature. It is believed that at the 0:.55 maximum the tungsten is covered by substantially a monomolecular layer of cesium atoms. At this value of 0 the effective work function of the cesiated tungsten surface is about 1.8 volts.

The tube of FIG. l can be operated with any combination of cathode and bulb temperatures deterimning values of 0 between 0:0 and 9:1. The highest beam densities are obtained with cathode temperatures above 2200 K. and at 9:0. In such higher temperature operation, electrons are emitted copiously by the cathode and the entire tungsten surface is available for contact ionization of the cesium (0:0). For a particular emission rate, in this case determined solely by the cathode temperature, the supply of positive ions is regulated by adjusting the bulb temperature to obtain maximum beam current through the aperture 15, which means best obtainable neutralization of the space charge. Under these conditions the mixture of electrons and positive ions within the cathode 7 constitutes a plasma that extends substantially to the exit aperture 15. If the effective potential of the beam forming electrode 13 is the same as the cathode 7, and the accelerating electrode has a substantial positive bias, the plasma will extend to a region just beyond the aperture 15, and a high density, thermal electron current will be drawn from the boundary of the plasma by the applied fields. If the effective potential of electrode 13 is substantially higher than the cathode potential, the plasma boundary will be depressed below the aperture 15, but the current extracted through the aperture will still be substantially thermal. On the other hand, conventional electron guns without space charge neutralization produce either low temperature beams with very low densities or high density beams with very high electron temperatures. In one tube designed for high temperature operation and having a tungsten cathode similar to that of FIG. 1, except that it was directly heated, with a length of about mills and an inner diameter of about 500 mils, and a plate aperture 15 of about 30 mils diameter, thus having a ratio of emissive area to aperture area of about 400 to l, I have obtained a beam having a current density at the aperture of about 40 amperes/cm-2, at a cathode temperature of about 2200" K. This is considerably lower than the usual operating temperature of tungsten laments, which is about 2500 K. A similar tube having an area ratio of 1000 to 1 operated with the cathode at 2200 K. should have a beam current density of about 100 amperes/cm-2. A similar tube having a ratio of emissive area to aperture area of about 140 to 1 operated at a cathode temperature of about 2400 K. has produced a beam current density of about 70 amperes/ cm?, In both of these high temperature examples, the plate electrode 13 was operated essentially at cathode potential. Preferably, the active cathode area should be at least two orders of magnitude (of the order of 100 to 1) greater than the aperture area.

The particular metals, tungsten and cesium, described above, are merely examples. Instead of tungsten, the cathode may be made of tantalum (work function 4.07 volts), molybdenum (4.3 volts) or oxidized tungsten (9.2 volts), all of which have relatively high work functions are suitable for use in the tube at the desired temperatures. Other alkali metals suitable for use instead of cesium are rubidium (ionization potential 4.17 volts) and potassium (4.34 volts). Of course, the alkali metal used must have an ionization potential lower than the particular cathode material selected. Although the curves in FIG. 3 apply only to tungsten and cesium, similar curves may be drawn for other combinations, such as nickel and 'cesium, for example, since the reactions between the alkali metals and the cathode materials are similar, with respect to both contact ionization and reduction of cathode work function.

For many purposes, such as in low noise traveling Wave tubes, it is desirable to operate at very low cathode temperatures, with lower current densities. However, at low cathode temperatures the ionization eciency of the cathode is low, not only because of the low temperature itself but also because the cesium coverage interferes with the production of ions by contact ionization at the cathode surface, which tends to result in incomplete neutralization of the electron space charge. Therefore, in low temperature operation, electrode 13 should be operated at a small positive potential (e.g., one Volt, as shown in FIG. l) to provide a small accelerating eld near the exit aperture 15, which eld assists in extracting the beam. Even with this small accelerating eld the electrons emerging from the aperture 15 constitute a thermal current since the velocity spread in the electron stream is equal to that at the cathode.

I have operated an electron gun constructed as shown in FIG. l, with a tungsten cathode cylinder having a length and diameter of about 500 mils, and a plate 13 having an aperture 15 of about 30 mils diameter, thus having a ratio of active cathode area to aperture area greater than 1000 to l, at a cathode temperature of about 760 K. (only 487 C.) and a cesium pressure determined by a bulb temperature of about 313 K., obtaining a beam current density of about 100 ma./cm.2, which is ample for most low noise applications. As in the case of the higher temperature operation described above, tantalum, molybdenum, and oxidized tungsten can be used instead of tungsten for the cathode. Also, lower melting point emissive materials such as nickel (work function 5 volts) or a sintered mixture of powders of tungsten and barium oxide (2.7 volts, average) can be used for low temperature operation in FIG. 1. Also, rubidium or potassium can be used instead of cesium with any of these alternative cathode materials. As an example, in a tube having a nickel cathode 7 operated at about 825 K. in the presence of cesium, a beam current density of about 2a/cm.2 was obtained.

Since the positive ions are produced at the cathode surface 9 in FIG. l, the ion density within the cathode will not be uniform throughout the electron paths. Thus, it

6 may not be possible to obtain complete neutralization of the space charge under all conditions of operation.

FIG. 4 shows a modification of the electron gun of FIG. 1 wherein the positive ions are produced at the surface of a separate ionizing element, shown as a coil 37 of tungsten, for example, coaxially mounted Within the hollow cathode 7, instead of at the surface 9 of the cathode. Any of the alkali metals cesium, rubidium and potassium may be provided within the envelope as in FIG. l to be contact ionized by the coil 37. For example, for use in a 1/2 x 1/2 cylindrical cathode, the coil 37 may be made of 10 mil diameter wire with 40 mils spacing between turns and have a diameter of mils. Preferably, coil 37 is biased about 2 volts positive with respect to the cathode 7, to accelerate the electrons toward the coil 37 and accelerate the positive ions toward the cathode 7,

and a positive potential of about one volt is applied to the plate 13 to establish a small axial eld component toward the exit aperture 15.

The structure of FIG. 4 is particularly suitable for low noise operation, in which case the cathode is maintained, by coil 35, at a very low temperature and the coil 37 is maintained, by a current source 39, at a higher temperature. The supply of positive ions for neutralization of the space charge is controlled by both the temperature of the coil 37 and the vapor pressure of the cesium vapor (by coil 23). Since the coil 37 is for providing ions only, it is preferably operated -under conditions for maximum ion production, i.e. with 0:0. For example, a gun having a tungsten cathode 7 and an aperture 15 having an area ratio of about 1000 to l, a tungsten coil 37 and cesium vapor at a cathode temperature of about 760 K., a coil temperature of about 1600" K. and a bulb temperature of about 313 K., produced a low temperature low velocity electron beam having a current density at the aperture 15 of about 150 ma./crn.2, which was 50% better than current density produced by the gun of FIG. l under similar conditions without the coil 37. In this example, the electron emission from the cesiated tungsten cathode is a maximum in the low temperature region, for the particular cesium pressure used (see FIG. 3).

Since the structure of FIG. 4 does not depend upon contact ionization at the cathode surface 9 for the production of positive ions, any suitable cathode material can be used instead of cesiated tungsten or the other cathode niaterials listed above, such as thoriated tungsten (work function 2.6 volts) or oxide coated cathodes (about 1.1 volts). It will be understood that the structure can also be operated under conditions of higher cathode temperature, to produce very high density beams, as described for FIG. 1.

FIG, 5 shows a further modification of the gun of FIG. 1, in which the positive ions are thermionically emitted directly by a positive ion source 41 into the path of the electrons to be space charge neutralized. The source 41 may comprise a heater coil 43 embedded in or completely covered by a fused mass 45 of a material known as eucryptite, Li2OAl2O32SiO-2, having the following composition:

a-type, from which no ion emission takes place, and a high temperature -type which emits positive lithium ions. The a-type is converted into the -type at 972 C.i20 C.

As an example, the coil 43 may be a coil of 8 mil diam. Wire of rhodium-platinum alloy with 40 turns per inch, coated with eucryptite material 45 to a diameter of 150 mils, for use in a 1/2" x 1/z cathode cylinder 7. The temperature of the eucryptite material 45 is adjusted, in the range from 1100 to l500 K., by varying the current in the coil 43 by an external current source 47, to produce 7 the maximum beam current density beyond the apertures and 19.

In operation, the ion emitter coil 43 should be biased at a positive potential somewhat higher than the potential of coil 37 in FIG. 4, because the mass 45 is insulating, in order that the surface of mass 45 will have the desired potential of about 2 volts. The plate 13 is preferably biased at about one volt positive, as in FIG. 4.

The emissive surface 9 of the hollow cathode 7 in FIG. 5 may be made of any suitable electron emissive material, as in the embodiment shown in FIG. 4. For example, a gun having a barium oxide coated cathode 7 at a temperature of 1100" K., with a ratio of active cathode area to aperture area of about 1000 to 1, should produce a high density beam having a current density of at least 100 a./cm.2. For low temperature use, a similar gun having a cesiated tungsten cathode 7 at a cathode temperature of 760 K. with a bulb temperature of 313 K., where 6:.55, should give a low temperature beam with a current density of about 300 ma./cm.2. It will be understood, that the bulb heater coil (23 of FIG. 1) may be omitted in the modification of FIG. 5, except in the case where an alkali metal vapor is used to lower the work function of the cathode material,

FIG. 6 shows another modification of the electron gun in FIG. 1 in which the positive ions are produced at th-e surface of an ionizing element separate from the cathode. In this embodiment of the invention, the electron gun comprises a thermionic cathode 51, a combination beamforming and ion-producing electrode 53 having a heater coil 55 and a central aperture 57, an apertured heat shield 59 having a skirt portion 61 surrounding the electrode 53, and a cup-shaped apertured accelerating electrode 63 within which the other electrodes are mounted.

The cathode 51 comprises a circular electron emissive surface 52, facing electrode 53, having an effective work function from 1 to 3 volts. The lower values (1-2 volts) apply to cathodes such as oxide-coated cathodes and cesiated-tungsten cathodes operating in the temperature range 800 to 1200 K., while the higher values (2-3 volts) apply to impregnated cathodes and L-cathodes operating in the temperature range 1100 to 1500 K. The surface 52 may be flat as shown, or slightly concave for focusing purposes.

The surface 54 of electrode 53 facing the cathode 51 is designed to supply sufficient positive ions to the space between the cathode 51 and the aperture 57 to neutralize the space charge of the electrons emitted by the surface 52. For example, the ions may be introduced by contact ionization of alkali metal vapor atoms, such as cesium, by the heated surface 54, as in FIGS. 1 and 4. In such case, the surface 54 is made of a material such as tungsten, tantalum, hafnium, molybdenum, niobium or carbon having a work function higher than the ionization potential of the alkali metal vapor used. Electrode 53 is spaced from the cathode surface 52 a sufficient distance to form a substantial plasma region therebetween. For example, the emissive surface 52 may have a diameter of 250 mils, the aperture 57 may have a diameter of 30 mils, and the spacing between the surface 52 and aperture 57 may be 80 mils, in which case the ratio of active cathode area to aperture area is about 70 to 1.

The cup-shaped accelerating electrode 62 is made of magnetic material in order to substantially shield the interior space from external magnetic fields as in FIG. 1.

In operation, the cathode is heated by a conventional heater (not shown) to a temperature producing copious emission of electrons, and the surface 54 is heated by heater 55 to a temperature in the range 1100-1500 K. to produce efficient production of ions by Contact ionization. The vapor pressure of the alkali metal vapor, preferably cesium, is controlled, eg., by the envelope temperature as in FIG. l, to maintain an adequate supply of atoms for ionization. When the cathode 51 and electrode 53 are operated at nearly the same potential, the electrons and ions mix to form a plasma in the region between the emissive surface 52 and the aperture 57 from which a high density thermal electron current can be extracted through the aperture. With zero bias applied to the electrode 53, the latter is slightly negative with respect to the cathode, due to the fact that the effective work function of surface 54 is slightly higher than that of the cathode. Thus, the surface 54 repels the electrons in the plasma. The potential at the exit aperture 57 is slightly higher than surface 54 due to accelerating fields provided by

electrodes

59 and 63. On the other hand, the positive ions produced by the negative surface 54 are attracted by the negative space charge which results in efficient mixing of the ions with the electrons to form the plasma. Preferably, the effective potential of the surface 54 is only slightly negative (a few tenths of a volt) relative to emissive surface 52, in which case the plasma region is generally cone-shaped with its apex in the aperture 57, the potential at the aperture 57 is zero or only slightly positive, and the aperture current is substantially thermal. The bias potentials V1, V2 and V3, on

electrodes

53, 59 and 63, are chosen to produce this condition. In a tube having the electron gun of FIG. 6, with substantially the dimensions given in the above example, and with a barium-impregnated tungsten cathode operated at about 1400" K. and having a work function of about 2.2 volts, and an ion-producing surface 54 of niobium operated at about 1350 K. with

electrodes

53, 59 and 63 operated at about 0, +4, and +4 volts, respectively, a beam current density at aperture 57 of 28 a./cm.2 was obtained. By measuring the current drawn by each of the electrodes, it was determined that about of the total electron current from surfaces 52 and 54 was drawn through the aperture 57. In this example, the niobium surface 54 included portions covered by cesium having a work function of about 2 volts. Thus, these portions emit electrons which also enter the plasma region and contribute to the extracted beam current. The positive ions are produced by contact ionization at the bare portions of the surface 54, which portions have an ionizing work function of about 4 volts. The effective work function of the surface seen by the electrons and ions at an appreciable distance therefrom may be about 3 volts, for example, in which case the effective potential of the surface 54 relative to the cathode surface 52 would be (-3)-(-2.2), or .8 volt, if V1 were zero.

In each of FIGS. 1, 4, 5 and 6 the bias potential on the beam forming electrode is preferably not greater than about 1 volt, and in any case not greater than the ionization potential of the alkali metal vapor used, which is about 4 volts for cesium.

The aperture in the beam forming electrode in each embodiment need not be completely unobstructed, as shown in the drawings. For example, a fine mesh grid may be mounted across the aperture, to maintain the same potential across the aperture plane.

What is claimed is:

1. An electron gun comprising:

(a) a plurality of electrodes including (l) a thermionic cathode having a relatively large electron emissive surface, and

(2) a beam forming plate electrode adjacent to said cathode having a single aperture spaced from said surface and having an aperture area that is small compared to the area of said surface;

(b) means, including one of said electrodes, for introducing positive ions into the space between said surface and said aperture other than by ionization by electron impact, for neutralizing the space charge of electrons emitted by said surface and thereby forming a plasma in said space; and

(c) means, including an apertured positive accelerating electrode coaxially positioned adjacent to said apertured beam forming electrode, for extracting a dense,

substantially thermal electron current from said plasma through said aperture.

2. An electron gun comprising:

(a) a plurality of electrodes including l) a thermionic cathode having a relatively large electron emissive surface, and

(b) means for introducing positive ions into the space between said surface and said aperture for neutralizing the space charge of electrons emitted by said surface and thereby forming a plasma in said space, said means comprising (l) an ion emitter having a relatively high work function, and

(2) means for maintaining alkali metal vapor atoms having an ionization potential lower than said work function at a surface of said ion emitter for contact ionization thereby; and

(c) means, including an apertured positive accelerating electrode coaxially positioned adjacent `to said apertured beam forming electrode, for extracting a dense substantially thermal electron current from said plasma through said aperture.

3. An electron gun comprising:

(a) a thermionic cathode having a relatively large active surface, one portion of said surface having a low effective Work function for emitting electrons at relatively low temperature-s, another portion of said surface having a high work function;

(b) a beam forming electrode adjacent to said cathode havin-g a single aperture spaced from said surface and having an aperture area that is small compared to the area of said surface;

(c) means for maintaining alkali metal vapor atoms having an ionization potential lower than said high work function at said other portion of said surface for producing positive ions by contact ionization thereby, to neutralize the space charge of said electrons and thereby form a plasma in the space between said surface and said aperture; and

(d) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a dense substantially thermal electron current from said plasma through said aperture.

4. An electron gun comprising:

(2) a beam forming electrode adjacent to said cathode having a single aperture spaced from said surface and having an aperture area that is small compared to the area of said surface;

(b) means for introducing positive ions into the space lbetween said surface and said aperture for neutralizing the space charge of electrons emitted by said surface and thereby forming a plasma in said space, said means comprising 1) an ion emitter adjacent to fbut spaced froml said cathode, and (2) means for heating said ion emitter to ionemitting temperature, and (c) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a dense substantially thermal electron current from said plasma through said aperture. S. An electron gun comprising: (a) a plurality of electrodes including (1) a thermionic cathode having a relatively large electron emissive surface, and

(b) means for introducing positive ions into the space between said surface and said aperture for neutralizing lthe space charge of electrons emitted by said surface and thereby forming a plasma in said space, said means comprising (l) an ion emitter adjacent to but spaced from said cathode and having a relatively high work function,

(2) means for maintaining alkali metal atoms havin-g an ionization potential lower than said work function at a surface of said ion emitter for contact ionization thereby, and

(3) means for heating said ion emitter to contact ionizing temperature; and

(c) means, including an apertured positive acceleratingv electrode coaxially positioned close to said apertured -beam forming electrode, for extracting a dense, substantially thermal electron current from) said plasma through said aperture.

6. An electron gun comprising:

(a) a thermionic cathode having a relatively large electron emissive surface;

(b) a ybeam forming electrode adjacent to said cathode having 1) a single aperture spaced from said emissive surface and having an aperture area that is small compared to the area of said surface, and

(2) a surface of high work function surrounding said .aperture and facing said emissive surface;

(c) means for introducing positive ions into the space between `said emissive surface and said aperture for neutralizing the space charge of electrons emitted Iby said surface thereby forming a plasma in said space, said means comprising:

(1) means for maintaining alkali metal vapor atoms having an ionization potential lower than said work function at said surface of said beamforming electrode for contact ionization thereby, and

(2) means for heating said surface to contact ionizing temperature; and

(d) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a dense, substantially thermal electron current from said plasma through said aperture.

7. an electron gun comprising:

(a) a plurality of electrodes including (l) a thermionic lcathode having a relatively large electron emissive surface, and

(2) a beam forming electrode adjacent to said cathode having a single aperture spaced from said surface and having an aperture area that -is small compared to the area of said surface;

(b) means for introducing positive ions into the space between said surface and said aperture for neutralizing the space charge of electrons emitted by said surface and thereby forming a plasma in said space, said means comprising (1) an electrode adjacent to but spaced from said cathode and comprising a body of a material which emits positive ions when heated, and

(2) means for heating said -body to ion-emitting temperature; and

(c) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a dense, substantially thermal electron current from said plasma through said aperture.

8. An electron beam tube comprising an envelope containing:

(a) a thermionic cathode comprising a hollow cylinder open at least at one end and having a relatively large internal electron emissive surface of high work function;

(b) a beam forming plate electrode disposed adjacent to said open end and having a single aperture coaxial with said cylinder, the area of said aperture being at least two -orders of magnitude smaller than the area of said surface;

(c) means for maintaining alkali metal vapor atoms having an ionization potential lower than said work function at said surface for producing positive ions by contact ionization thereby, to neutralize the space charge of electrons emitted by said surface and thereby form a plasma in the space between said surface and said aperture; and

9. An electron beam tube comprising an envelope containing:

(a) a plurality of electrodes including,

(1) a thermionic cathode comprising a hollow cylinder open at one end and having a relatively large internal electron emissive surface, and

(2) a beam forming plate electrode disposed adjacent to said open end and having a single aperture coaxial with said cylinder, the area of said aperture being at least three orders of magnitude smaller than the area of said surface;

(b) means, including an ion emitter, for introducing positive ions into the space between said surface and said aperture, for neutralizing the space charge of electrons emitted by said surface and thereby forming a plasma in said space; and

(c) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a relatively dense thermal electron current from said plasma through said aperture.

10. An electron beamI tube comprising an envelope containing:

(a) a thermionic cathode comprising a hollow cylinder open at least at one end and having a relatively large internal electron emissive surface;

(b) a beam forming plate electrode disposed adjacent to said open end and having a single aperture coaxial with said cylinder, the area of said aperture being at least three orders of magnitude smaller than the area of said surface;

(c) means for introducing positive ions into the space between said emissive surface and said aperture for neutralizing the space charge of electrons emitted by said surface, said means comprising:

(1) an ion-emitter mounted coaxially within said cylinder and spaced from said aperture, and

(2) means for heating said ion emitter; and

(d) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a relatively dense thermal electron current from said plasma through said aperture.

11. An electron beam tube as in claim 10, wherein said ion emitter is a coil of high work function metal and said ion introducing means further comprises means for supplying alkali metal vapor atoms having an ionization potential lower than said Work function at the surface of said coil for contact ionization thereby.

12. An electron beam tube asin claim 10, wherein said ion emitter comprises a body of -eucryptite material l2 which emits positive lithium ions when heated above about 1l00 K.

13. An electron gun comprising:

(a) a plurality of electrodes including (1) a thermionic cathode having a relatively large electron emissive surface, and

(2) a beam forming electrode adjacent to said cathode having a single aperture spaced from said surface and having an aperture area that is small compared to the area of said surface,

(b) means, including an ion emitter, for introducing positive ions into the space between said surface and said aperture for neutralizing the space charge of electrons emitted by said surface; and

(c) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a relatively dense substantially thermal electron current from said plasma through said aperture, said accelerating electrode being made of magnetic material and comprising a tubular shield portion surrounding said cathode and said beam forming electrode.

14. An electron beam tube comprising:

(a) an electron gun for producing an electron beam along a predetermined path, including (1) a plurality of electrodes disposed along said path and comprising (A) a thermionic cathode having a relatively large electron emissive surface, and

(B) a beam forming electrode adjacent to said cathode having a single aperture spaced from said surface and having an aperture area that is small compared to said emissive area,

(2) means, including an ion emitter, for introducing positive ions into the space between said surface and said aperture, for neutralizing the space charge of electrons emitted by said surface and thereby forming a plasma said space, and

(3) means, including an apertured positive accelerating electrode coaxially positioned close to said apertured beam forming electrode, for extracting a dense, substantially thermal electron current from said plasma through said aperture;

(b) means coupled to said path in a region beyond said electron `gun for interaction with said beam; and

(c) condensing means, interposed between said electron gun and said interaction means, for removing gas from said beam path and thereby maintaining a high vacuum in the interaction region of the tube.

15. An electron beam tube as in claim 14, wherein said condensing means comprises a liquid air trap surrounding said beam path.

16. An electron beam tube comprising an envelope containing:

(a) a thermionic cathode comprising a hollow cylinder open at least at one end and having a relatively large active internal surface, one portion of said surface having a low eifective work function for emitting electrons at relatively ylow temperatures, another portion of said surface having a high work function;

(b) a beam forming plate electrode disposed adjacent to said open end and having a single aperture coaxial with said cylinder, the area of said aperture being at least two orders of magnitude smaller than the area of said surface; v

(c) means for maintaning alkali metal atoms having an ionization potential lower than said high work 3,243,640 13 14 function at said other portion of said surface for References Cited by the Examiner producing positive ions -by contact ionization there- UNITED STATES PATENTS by, to neutralize the space charge of said electrons e d th b f 1e eh e between said 2,798,181 7/1957 Foster 313-1611 Sgrfaeeern sffprtm e spac 5 2,841,726 7/1958 Kmechui 313-230 X (d) means, including an apertured positive accelerating 218831560 4/1959 Beam et@ 313"320 X electrode coaxially positioned close to said aper- 3,021,472 2/1962 Hermqulst 313-2305( tured Ebeam forming electrode, for extracting a dense, substantially thermal electron current from said plas- HERMAN KARL SAALBACH Primary Exammer ma through said aperture. lo S. CHATMON, JR., Assistant Examiner.

Claims

1. AN ELECTRON GUN COMPRISING: (A) A PLURALITY OF ELECTRODES INCLUDING (1) A THERMIONIC CATHODE HAVING A RELATIVELY LARGE ELECTRON EMISSIVE SURFACE, AND (2) A BEAM FORMING PLATE ELECTRODE ADJACENT TO SAID CATHODE HAVING A SINGLE APERTURE SPACED FROM SAID SURFACE AND HAVING AN APERTURE AREA THAT IS SMALL COMPARED TO THE AREA OF SAID SURFACE; (B) MEANS, INCLUDING ONE OF SAID ELECTRODES, FOR INTRODUCING POSITIVE IONS INTO THE SPACE BETWEEN SAID SURFACE AND SAID APERTURE OTHER THAN BY IONIZATION BY ELECTRON IMPACT EMITTED BY SAID SURFACE AND THEREBY FORMING A PLASMA IN SAID SPACE; AND (C) MEANS, INCLUDING AN APERTURED POSITIVE ACCELERATING ELECTRODE COAXIALLY POSITIONED ADJACENT TO SAID APERTURED BEAM FORMING ELECTRODE, FOR EXTRACTING A DENSE, SUBSTANTIALLY THERMAL ELECTRON CURRENT FROM SAID PLASMA THROUGH SAID APERTURE.