US2622225A - Electron beam device and system employing space charge neutralization - Google Patents

Description

D 1952 E. e. I ER ELECTRON BEAM DEVICE SYSTEM EMPLOYING SPACE CHARGE NEUTRALIZATION Filed D60. 31, 1948 INVENTQR may; fiIZ Fr 1'- {W ATTORNEY Patented Dec. 16, 1952 UNITED S'E'T ELECTRON BEAM DEVICE AND SYSTEM EMPLOYING SPACE CHARGE NEU- TRALIZATION Ernest G. Linder, Princeton, N. J., assigncr to Radio Corporation of America, a corporaticn ofDelaware This invention relates generally to high current electron guns and more particularly to such guns incorporated in microwave apparatus, such as oscillators and modulators.

In applying electron guns to microwave devices and their associated circuits, it has been most desirable in the past that the electron beam be fine, dense and concentrated. These qualities have been somewhat improved by the use of various electron focussing devices particular forms of field-free arrangements.

However, while arrangements have been proposed for shielding the beam from extraneous fields, for example, by enclosing it in a metal envelope, the only solutions which have been devised for eliminating the space charge field of the beam itself have required that the beam be surrounded by a gaseous medium at a critical and substantial pressure. Nor has the prior art taught utilizing concentration and dispersion of the beam as a step in the method of using th phenomenon of ion trapping for new and useful purposes of generating, modulating, amplifying, and mixing of currents.

The principal object of the invention is to provide a method of and apparatus for controlling the density of a high current density electron beam.

Another object of the invention is to provide a method of and apparatus for concentrating a stream of electrons into a fine and dense beam.

Another object of the invention is to provide a method of and apparatus for concentrating an electron beam that it maypass through restricted apertures.

Another object of the invention is to provide a method of and apparatus for concentrating and dispersing an electron beam as it approaches a restricted aperture.

Another object of the invention is to trap posi. tive ions, formed by the collision of the electrons in a beam with the residual molecules of the mediurn'through which the beam passes, by establishing electric fields at boundaries within the medium.

Another object of the invention is to control the density of an electron beam asit approaches an aperture to control the portion of electrons in the beam that pass through the aperture and thereby accomplish current modulation.

Other objects will be apparent from the description of the invention as hereinafter set forth in detail and from the drawings made a part h eq ie'whicb ur diagrammatic sketch of cne embodiment of the invention; Fin

are 21 is a diagrammatic sketch showing the optimum concentrating effect on a focussed electron beam of a space within the beam that is free of space charge; Fig. 2b is a similar sketch showing the efiect of the magnetic field 'on a convergent electron beam; Figure 3 is a" diagrammatic sketch of the invention applied to a refiex- Klystron; Figure 4 is a modification of the arrangement of Figure 3 showing a paramagnetic ring within the Klystron tube to concentrate the magnetic field Within a definite region of the tube; and Figure 5'is a diagrammatic sketch of the external circuits applied to Figure 2a to'accomplish modulation.

Similar reference characters are applied to similar elements throughout the drawings.

Figure 1 illustrates an electron gun in which the numeral l indicates a cathode, shown "as spherical in shape, which is the source of a focussed electron beam. Paths of individual electrons are shown at 2. Y The electrons pass through the region shown generally at 3, through an apt-2rture 4 in a cup-shaped electrode id to one side of electrode 5, where they are repelled or collected according to the use made of the gun. The side walls 4b of electrode 6a extend back to 'an electron-permeable grid 8. Theelectrode la and grid 8 form a hollow conductive member that is substantially closed on all sides and serves as a Faraday cage to shield the interior'thereof from external electric fields.

Juxtaposed to the other side of electrode 5 is a source 6 of a magnetic field which permeates the region 3. Themagnetic fiux lines are in general parallel to the paths o f the electrons in the beam, as indicated by the dotted lines 1. The dimensions and shape of the magnetic field sourcemay be chosen so that the flux field may have the proper configuration. Electrode 8, in the form of a grid, is positioned near cathode i and conforms generally in shape to spherical cathod' l Electrode '8 and cathode l have a common center of curvature, which is located at, or near, aperture t. The electrodes 1, 4a, 5 and 8 are located'within an envelope c of magnetic field permeable material containing a gaseous medium. I

Unless grid 8 is used for modulation or amplification, its potential ismaintained the same as that of electrode id and under such conditions electrode 8 maybe connected to the sidewalls ib of electrode 4a as shown inFig. l.

Referring to Figure 2a, the Q QQF Z PS emit from spherical cathode" l tend to follow"the straight radial paths 2 and focus at the aperture inelectrode la'a'nd would do so excep't'for repulsive effects within the space charge of the electrons within the beam and the electrons produced by ionization of molecules in the gaseous medium by the electrons in the beam.

However, in operation, the space charge eflects cannot be disregarded as they affect to a great extent the degree of concentration of the beam which in turn affects the use that can be made of the beam. When the invention is applied to an oscillator, maximum accuracy in the concentration of the beam at aperture 4 i desired so that the number of electrons which can pass therethrough and therefore the output energy of the oscillator will be as high as possible;

When the invention is applied to a modulator, maximum variation in the accuracy of said concentration is desired to obtain maximum variations in the output electron current passing through the aperture 5.

The space charge effects are overcome by placing substantially equal potentials on electrodes 4a and 8.

Electrodes 4a and 8 provide a field-free space therein, except for the field due to the electron charge in the beam. This beam field performs two functions: (1) it attracts and traps into the beam ions formed by ionizing collisions and (2) it repels the electrons created by the said collisions as they are for the most part of low velocity. This process continues until the ions present in the beam neutralize the space charge field of the beam. A trapping equilibrium is thereby established. It has been found in practice that gas pressure of the medium is not critical and may be as low as that of the residual gas remaining after evacuation. Gas pressures affect the time of the device to reach a state of equilibrium, but even with wide range of gas pressures the variations in build-up time is only a matter of a very few microseconds. The beam space thereby becomes actually field-free. The electrons in the beam also respond to the flux of the magnetic field impressed upon region 3. The path of least resistance for an electron in a magnetic field is that path that does not cross the magnetic lines of force. The electrons, therefore, tend to follow the flux paths and the magnetic field tends to keep the beam of electrons from spreading.

The effects of the magnetic field and the electric field set up by the potentials on electrodes 4a and 8 cooperate with each other to produce improved results. The optimum effect obtainable with the electric field alone is shown in Figure 2a in which figure it Will be seen that the angle of convergence of the electrons, as they approach the aperture 4 in electrode 4a, is wide for short spacing of cathode I from electrode 4a. On the other hand as shown in Fig. 2b, the magnetic flux lines I are substantially perpendicular to the cathode surface in the region near the cathode I and become substantially parallel and closer to each other in the region of aperture 4. Due to the influence of the magnetic field the paths of the electrons passing through and beyond the aperture are practically parallel and are closer to each other, with attending increase in density of the beam, permitting the use of smaller apertures. The magnetic field prevents the cross-over of electrons near the aperture 4 shown in Fig. 2a.

Figure 3 discloses the application of the invention to a reflex-Kylstron oscillator, The device includes a conductive envelope 9 enclosing a gaseous medium. A reflector is supported by a .4 member ID made of a magnetic-field permeable insulating material, such as a suitable ceramic. Lugs H extend away from envelope 9' and in these lugs are threaded screws I2. These screws [2 hold magnet core 6 in position and, by the adjustment of screws 12, magnetic core 6 may be centered in the axis of envelope 9.

The cathode I is heated by electric currents which may be supplied to cathode I over wires M by connecting any suitable source to the terminals 13. Positive potentials are applied to the cavity resonator I5 and envelope 9, which constitute the equivalent of electrode 4a of Figure l, and to grid 8 through wires l6 and ll, respectively. Reflector negative potential is applied to electrode 5 over a lead l8.

The electron beam passes through the apertures of cavity resonator I5 into the drift space between cavity resonator l5 and reflector electrode 5 where the electrons are bunched. The bunched electrons are reflected back into cavity resonator I5. Power is drawn from cavity resonator 15 by loop I9 and coaxial transmission line 20.

The device disclosed in Figure 4 is similar to that in Figure 3, except that a paramagnetic ring 21 is positioned within envelope 9' to shape the electromagnetic field from core 6 into the desired configuration and thus effect a longer narrow section of the electron beam.

In the devices shown in Figures 3 and 4, a substantially field-free region for trapping positive ions is formed by the conductive envelope 9. the cavity resonator l5 and the grid 8.

In testing the devices herein disclosed and in obtaining their operational characteristic curves, various values of potentials were applied to the grid 8 and electrode 4a with various values of magnetic fields, and Without any magnetic field impressed on region 3. It was found for the devices tested that the positive potential on grid 8 should be substantially equal to that on electrode 441 to provide a field-free region therebetween. By impressing a potential on grid 8 higher than that on electrode 4a, the positive ions will move toward electrode 4a. This direction of movement of the positive ions is generally preferred as the concentration of the positive ions near the aperture 4 assures more effective trapping of the ions in the beam as it approaches electrode 40. and thus a finer and more concentrated beam as it passes through aperture 4, which is desired.

It was also found that when the positive potential on grid 8 was lowered with respect to electrode 4a, the positive ions were withdrawn or swept from region 3 and the beam was dispersed. The electrons passing through aperture 4 were thereby decreased, causing a decrease in the value of the oscillating current.

The variation of potential on electrode 8, due to superimposing on electrode 8 the potentials set up by the modulating current, causes some variations of the acceleration of the electrons and the density of the electrons in the beam. These latter variations are minor compared with the variations in the size and density of the beam caused by the effect of the existence or the neutralization of the excess space charge, which is controlled by the varying potential on electrode 8.

The device may therefore be used as a modulator by impressing upon grid 8 a varying potential from a modulating current source, such as shown at 22, Figure 3. A constant positive potential is placed on grid 8 by electric source 23, which is connected between source 22 and the ground orianeutralpotential point, such as one oiithei'wires; lllthatzsupplies currentt cathode; l-. Reierringto Figure 5 there is shown therein theconstruction arrangements of Figure 2a and the'circuit arrangements to modulate the currentin a: steady electron focussed beam originatingat cathode I and to collect the electrons at electrode 5 as. amodulated current. Electrodes 4a and, 85 are maintained at substantially the same average positive potentials by electric sources 24 and; 23;. respectively. Superimposed upon; the

potentialof electrode 5 due to source 23; is a potential varying in accordance with characteristics of themodulating current. This is accomplishedby connecting source 22 to the primary of transformer 25, the secondary of which is connected; in series between electrode 8 and source 23: The, load or output circuit of the device consists of collector electrode 5, load re-.

sistance 25', electric source 2'? and ground 28. Electric sources 23 and 24 are grounded at 28.

The operation of the device is as previously described. The varying potential on electrode 8 varies the extent to which the region 3' is completely fieldefree, i. e., varies the field configuration of the ion-trapping substantially field-free space and, therefore, the accuracy with which the. beam is brought to a sharp focus at the aperture 4. Obviously this controls the portion of electrons of the beam that pass through aperture 4, to be collected by electrode 5. The varying number. of electrons collected by electrode 5 constitutes the modulated current which is utilized. by passing it through load 25.

In general, the cathode, may be positioned much fart-her away from the cavity resonator l5 than in the conventional refiex-Klys ron tube as the space charge limitations in such tubes do not occur in the case of the structures described herein. Because of this, the time of transit of electrons returning toward the cathode is longer, practically complete debunching occurs, and harmful hysteresis due to multiple transits are eliminated.

There is thus disclosed a method of and an apparatus for controlling the density of an electron beam as it approaches an aperture to concentrate it by neutralization of the excess space charge for the use of the beam in an oscillator structure or to concentrate or disperse the beam in accordance with the characteristics of a varying current to modulate the beam and thereby produce a modulated current.

I claim as my invention:

1. Apparatus for producing a high current density electron beam comprising a hollow conducting member substantially closed on all sides to shield the interior thereof from external electric fields, at least a part of one wall of said member being electron permeable, the opposite wall of said member having a small beam aperture, a gaseous medium within said member, and means including a cathode of large area relative to said aperture located adjacent to said member for projecting a convergent focussed beam of electrons through said electron-permeable Wall part and said beam aperture to produce positive ions in said medium which are trapped within said convergent beam for neutralizing the space charge thereof.

2. Apparatus according to claim 1, wherein said cathode and said electron permeable wall part lie in two spherical surfaces having a common center on the axis of said convergent beam.

3. Apparatus according to claim 1, wherein said le tronerm able, W l: ar s ctri ally sulatedas to; directcur-rents; from the remainden of; said; hollow conducting member, and saidapparatus includes means for applying a variable voltage tosaid wall part.

i. Apparatus for producing oscillationsirr a medium including means for projecting; electronsalong such paths that they tenduto produce a focussed electron beam, means providing a substantially field-free region through which said paths extend, a cavity resonator including. opposing-conductive Walls each having :an opening which surrounds the axis of said beamand-is, spaced from the other to define. an interaction gap, a. gaseous medium within said region, and electron reflector means positioned in said axis and on the side of said cavity resonatoroppofite to said electron projecting means, whereby oscillationsiare produced insaid cavity resonator and whereby positive ions formed by the ionization of the medium are trapped by said electrons in said region and neutralize the space charge therein.

5. Apparatus according to claim 4, including electrode means, forming a part of said secondnamed means, for varying the field configuration of said substantially field-free region.

6. Apparatus for producing a high-currentdensity electron beam comprising: a source supplying a large diffuse electron current, means, adjacent said source for accelerating the elece trons away therefrom along paths which tend to converge at a given point to form anapex, means, between said source and said point for providing a substantially field-free region therebetween, and a gaseous medium within said region whereby in the operation of the apparatus positive ions. of'said medium are entrapped by electrons within said region to neutralize their negative space charge, and meansv adjacent said. region for impressing upon said beam a magnetic field the flux lines of which are substantially parallel to said paths of the electrons.

7. Apparatus for producing a variable. density electron beam comprising: a source supplying a large diffuse electron current, means adjacent said source for accelerating the electrons away therefrom along paths which tend to be convergent at a given point to form an apex, an electrode having a beam-limiting aperture at said point, means between said source. and said point for providing a substantially field-free region therebetween, a gaseous medium in said region whereby in the operation of the apparatus positive ions of said medium are entrapped by electrons within said region to substantially neutralize their negative space charge, an electrode partially bounding said region for varying the field configuration of said substantially field-free region whereby said entrapment and said neutralization will vary in accordance with the variations of the field configurations and the electron current which passes through said aperture will also vary in accordance therewith, and means adjacent said region for impressing upon said beam a magnetic field the flux lines of which are substantially parallel to said paths of the electrons.

8. Apparatus according to claim 1, wherein said cathode includes an electron-emissive surface which is concave toward said aperture, and said apparatus further comprises means adjacent said hollow conducting member for impressing upon said beam a magnetic field the flux lines of which are substantially perpendicular to said cathode surface in the region near said cathode and become substantially parallel and closer to each other in the region near said aperture, whereby cross-over of electrons near said aperture is prevented.

9. Apparatus for producing oscillations in a medium including: means for projecting electrons along such paths that they tend to produce a focussed electron beam, means providing a substantially field-free region through which said paths extend, a cavity resonator including opposing conductive walls each having an opening which surrounds the axis of said beam and is spaced from the other to define an interaction gap, said last-mentioned means including a gaseous medium within said region, electron reflector means positioned in said axis and on the side of said cavity resonator opposite to said electron projecting means, whereb; oscillations are produced in said cavity resonator and whereby the positive ions formed by the ionization of the medium are trapped within the said beam and neutralize the space charge therein, and means adjacent said resonator for impressing upon said beam a magnetic field the magnetic flux lines thereof being parallel to the paths of the electrons of said beam.

10. Apparatus for producing varying microwave oscillations in a medium including: means for projecting electrons along such paths that they tend to produce a focussed electron beam, means providing a substantially field-free region through which said paths extend and including a gaseous medium, an electrode having a beamcurrent-limiting aperture adapted to limit the current of electrons which leave said region in accordance with the focus attained by said beam and an electrode partially bounding said region for varying the field configuration thereof to control the focus attained by the beam by controlling ion entrapment in said region, a cavity resonator including opposed walls each having an opening which surrounds the axis of said beam and is spaced from the other to define an interaction gap, electron reflector means positioned in said axis and on the side of said cavity structure opposite to said beam producing means, whereby said oscillations are produced in said cavity and whereby the positive ions formed by the ionization of the medium are variably trapped Within the said beam and variably neutralize the space charge therein and said oscillations are modulated in accordance with the said varying configuration of said field, and means adjacent said resonator for impressing upon said beam a magnetic field, the said magnetic flux lines thereof being parallel to the paths of the electrons of said beam.

11. Apparatus for producing oscillations in a gaseous medium including: means for producing a focussed electron beam in a substantially fieldfree region in said medium, a cavity resonator opposite walls of which have aligned apertures in the axis of said beam, electron reflector means positioned in said axis and on the side of said cavity resonator opposite to said beam producing means, and magnetic means adjacent said reflector means for impressing upon said beam a magnetic field, the said magnetic flux lines thereof being parallel to the paths of the electrons of said beam, whereby oscillations are produced in said cavity.

12. The method of producing oscillations comprising: providing a source of moving electrons in a medium and focussed into a beam, projecting said beam through a field-free region that includes a resonant cavity, bunching said electron beam, reflecting said bunched beam in the reverse direction into said cavity, whereby the positive ions formed by the ionization of said medium are trapped within said beam and neutralize the space charge therein, and simultaneously impressing upon said beam and said bunched beam a magnetic field, the said magnetic flux lines thereof being parallel to the paths of the electrons of said beam and said bunched beam.

ERNEST G. LINDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,289,952 Zowrykin July 14, 1942 2,320,860 Fremlin June 1, 1943 2,401,945 Linder June 11, 1946 2,403,795 I-Iahn July 9, 1946 2,404,279 Dow July 16, 1946 2,460,332 Manifold "new"--- Feb. 1, 1949,

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