US2348133A - Method and apparatus for developing electron beams - Google Patents

Description

H. A. IAMS METHOD AND APPARATUS FOR DEVELOPING ELECTRON BEAMS Filed Sept. 29

INVZE/NTOR 1y I A] 5 B a air:

ATTORNEY trons.

Patented "Ma a, 1944 METHOD AND APPARATUS FOR 13E- VELOPING ELECTRON BEAMS Y I HarieyA. Iams, Summit, N. .L, asslgnor to Radio Corporation of America, a corporation of Dela.-

ware

Application September 29, 1942, Serial No. 460,080

- (01. 315-13) My invention relates to electron discharge de- 14 Claims.

vices of the cathode ray type and particularly to methods and apparatus for developing electron beams.

In conventional cathode ray tubes, beam tubes, and other discharge devices, it is necessary to producea cathode ray beam preferably of high current density and of small cross-sectional area. With present available electron sources, such as indirectly heated cathodes, the current density drawn from such a source is inherently limited and it is necessary to concentrate electrons from a large area into a beam of relatively small crosssection. Due to distortional eflects, such as spherical aberration of electron lenses, the maximum current density of an electron beam is limited when such focusing arrangements are utilized. Consequently, the current intensity of such a beam, which is dependent upon its'crosssectional area is likewise limited. Furthermore, in tubes utilizing electron beams, it is often'necessary that the beam be of small diameter over a considerable portion of its length so that targets at- .diflerent distances from the electron source may be scanned without appreciable change in resolution; that is, in apparent crosssectional area of the beam. Furthermore, in certain'ultra-high-frequency oscillator tubes, it is often desirable to pass a, large current through a series of small apertures without excessive loss of electrons.

In addition to the above-mentioned applications for electron beams, it may be desirable to develop beams of electrons having segregated longitudinal velocities, by which I mean that certain grou of-the electrons may have longitudinal velocit higher or lower than other elecupon entering ankelectr'ostatic deflection field is deflected by an amount depending upon the ve-- tron groupsof the beam. An electron beam vide means'for developing electron beams of high current density which may be passed through a series of apertures without excessive loss of electrons and it is a still further object to provide apparatus for developing electron beams wherein the electrons of the beam have 'segre-' gated longitudinal velocities. It is a still further object to. provide electron beams of high current density which may be focussed atdiflerent points along the beam path. These and other objects, features and advantages of my invention will become apparent when considered with respect to the following description and the accompanying drawing wherein:

Figure 1 is a longitudinal view in partial perspective of an electron discharge device incorporating structure made in accordance with my invention, and- V Figure 2 is a similar view of a modified structure which may be used in the device shown in Figure 1:

Since my ,invention relates specifically to methods and means for developing electron beams,- I will refer specifically to a tube of the cathode ra beam type although it will be ap-- preciated that my invention is equally applicable to any type of tube requiring high densityelec-= tron beam formation. In accordance with my invention I utilize-a plurality of electron sources to develop electron beams wherein the electrons have diflerent velocities andJIpombine' the plurality of beams to form a single electron beam.

More particularly I direct a plurality of electron beams wherein the electrons of each beam have difierent longitudinalvelocity into a magnetic field parallel with the beams crossed by an electrostatic field and I so control the longitudinal velocity of the electrons in each beam with re- 'spect to the magnetic and electrostatic field tron beam comprises electrons of different velocity, the electrons will be deflected by an amount depending on the velocity, and to obtain such-action it is desirable to develop a single electron beam having diilerent groupings of .velocity so that a single deflection means provides variable deflection of the several groups of elec- It is an object of my invention to provide a strength that upon emergence oi: electrons from the crossed fields, the. electrons are combined in a single beam. Still further in accordance with my invention I choose the velocity of the electrons in the various beams such that, upon emergence from the crossed electrostatic-magnetic fields, the electrons of the combined beams method and apparatus for developing cathode ray beams 01 high current density. It is a further. object to. provide an apparatus for develfoping beams of high current density and low orms-sectional area. It is 'anotherobiect to prooccupy a minimum cross-sectional areajanci I so proportion the beam-forming and utilization structures that electrons repeatedly form a minimiulrln beam cross-sectional area; along the beam pa Referring to Figure 1 I have shown a tube having an envelope I incorporating a target 3' frequency multiplier type may be used, it being appreciated that my invention does not relate to the specific target structure but that vai-ious targets may be utilized without departing from the scope of my invention. In accordance with. my invention I provide a plurality of electron guns 5, I, and 9 which may be of the type described in my U. S. Patent No. 2,288,402. Each of the electron guns is ofiset and arranged to direct its beam in a plane with the beams of the other guns. -A single potential source, such as the battery H, or other direct current source, may be utilized to obtain operating potentials for the electron guns as shown-in Figure 1. The electron gun 5 is representative and has a cathode l3, a control electrode l5 and an anode Il operated with a cathode-to-anode potential less than the potential applied between the cathode and the anode of the electron gun 1. Similarly the potential between these electrodes of the gun I is less than .that applied between the cathode and the anode of the electron gun 9. The cathodes ,of the three electron guns shown are representative of any plurality of electron sources and in accordance with my invention I direct the electron beams from the plurality of sources into crossed electrostatic-magnetic fields developed between oppositely disposed beam directing plates Iii-2| in combination with a longitudinal magnetic field developed by a magnetic coil 23 which extends over-at least the length of the plates Iii-2i in the direction of beam propagation. More particularly the plates |9--2l may be of the type described in my Joint U. S. Patent No. 2,213,175 wherein the-edges thereof are fiared outwardly at each end in the direction of the beam propagation to produce a gradually increasing and decreasing electrostatic field in this direction. The plates |9-2l are preferably positioned on opposite sides of and equidistantly from the .plane in which the electron beams are developed and are maintained at fixed potentials with respect to the most positive anode of the electron guns, one being slightly positive and the other negative thereto. by a potential source 25 connected to provide potential balance with respect to the most positive electron gim anode. Adjacent the exit end of the tudinal magnetic field developed by the coil 22 causes electrons to be directed within the plates in a plane parallel to the plates such as in-the plane midway between the plates. ,Furthermore, the electrons are displaced in this plane by a distance measured in centimeters (cm) equal to H V where E is the electric field between the plates in volts per cm., L is the axial length'of the electrostatic field between the plates in cm., H is the axial magnetic field in gausses, and V is the beam velocity in volts. Consequently, since the electrons from the gun 5 are or lower velocity than those from the electron guns I or 8, the displacement of the beam from the gun 5 in a plane between the plates will be greater than the displacement of the beam from the gun I. Similarly the displacement of the beam from the gun I will be greater than that of the beam from the gun 9. Therefore, in accordance with my exit end of these plates. Consequently. the electhe electron guns under the influence of the magnetic field follow helical paths which diverge and then converge to form points of focus along the beam trajectory. The distances betweenthese points of focus, that is, the length of a single helix or pitchpf the helical electron path may be varied by varying the strength of the *magnetic field or by varying the velocity of the beam directing plates I provide a beam limiting apertured electrode 21 having a beam limiting aperture 29 preferably centrally positioned with respect to the plane midway between the beam directing plates. The cross-sectional area of the beam limiting aperture 29 is preferably that of the minimum cross-sectional area desired of the electron beam, the electrode containing this aperture preferably being operated at the same potential as the average potential of the beam directing plates. I may provide in accordance with my invention a further apertured electrode 3! having a similar aperture 33 axially aligned with the aperture 29 and with the magnetic field electrons as represented by the following equation:.

gen- L H V where L is equal to the length of-a single helix;

developed by the coil 23 'to further limit and define the electron beam.

In operation of the structure thus far described, it'will be apparent that the velocity of the electrons from the gun.- 5 is less than that of the electrons from the. gun I which in turn is 'less than the velocity of the electrons from the gun 9. Each of the electron beams from this plurality of electron sources is directed into the crossed fields in a plane preferably midway between the beam directing plates. The action of the electrostatic field developed between the directing plates in combination with the longithat is, distance between points of focus in centimeters, H is the magnetic field strength in gausses; and V is the velocity of the electrons measured in volts. In accordance with my inof potentials applied to the electron gun anodes for a given longitudinal magnetic field strength. I have shown only the average path, that is, the axial path of electrons from each of the guns for the sake of clarity rather than showing the helical paths with the various points of focus .along these paths although it will be appreciated that thenumber of points of focus may be'controlled by the relationship between beam velocity and magnetic field strength. Since the magnetic field strength developed by the coil 23 is common to each of the electron beams from the gun structures, the, electron velocityof the vari--- -ous beams may be adjusted to obtaincoincidence at the limitingaperture '29 although it may be found that the electron beams are not,

focused with their points of focus at this aper-m ture. It is, therefore, desirable to so mount the f beam limiting apertured electrode 21 and similarly the electrode 3|, if such is used, such that the electrode may be shifted in the direction of the beam displacement produced within the directing plates. It is unnecessary to provide shifting normal to this direction for beam alignment because the beams may be laterally displ ted by moving the magnetic field with respect .the tube. It will be appreciated, however. ince each of the beams for coincidence must be developed and displaced in substantially a single plane, that the alignment of the electron guns with their emitters and their apertures in this plane must be quite accurate. I

. I have shown the beam limiting apertured electrodes 21 and 3| as being separated along the axial path of the combined beams and as being operated at different potentials. However for op-' an integral number of points of focus between the electrodes. If the beam limiting apertured electrodes 21 and 3| are operated at different potentials as shown, the pitch length of the helices in centimeters is determined by the expression:

L2 V1+V2 where V1 is the velocity expressed in volts of the electrons entering theaperture 29 and V3 is the I electron velocity of the electrons entering the aperture 33. The combined beam flowing through the apertures 29 and 33 is composed of electrons having diflerent group velocities since the cathodes of the electron guns are of different potentials with respect to both of the electrodes 21 and.

3!, Therefore, ,in accordance with a further teaching of my invention I provide a potential difference between any two electrodes, such as 'the apertured electrodes 21 and 3| throughout which the beam passes, and so space the electrodes with respect to the potentials and the magnetic field in which the electrodes are immersed such that points of focus of each com- 35 and 31.

for combining eiectron'beams of diiferent velocity to form a single beam may be utilized for any purpose wherein arhigh intensity or high density beam .is' desired.

For example, use may be made of my invention in oscillator tubes either of the deflection type or velocity modulation type and the latterutilizing either internal or external resonant cavity circuits. My inventionis of particular use, however, where it is desired to provide magnetic deflection of a beam having individual electron group velocities. Thus, referring again to Figure 1 the combined electron beam embrging from the aperture 29 or. the aperture 33, when the electrode 3| is utilized, may be directed toward'the target 3 comprising a luminescent screen and be'deflected thereover to form conventional, oscilloscope or television scanning rasters by the deflection coils In accordance with 'my invention I utilize an electron beam having different velocity group electrons and these electrons when directed into the magnetic fields, such as developed by the coils -31, tend to follow the lines of flux produced by the combined axial magnetic field and the deflecting fields. Thus, when the magnetic fields are sufficiently strong, all of the velocity groups are deflected together over the path 39 to form a single bright scanning pattern.

If it should be desired, the beam may be separated into its component parts by passing it through one or more electrostatic fields. us, either one or both sets of deflection coils 3M1 may be replaced with electrostatic deflection ,ponent beams is difierent, the electrostatically deflected beam will be deflected in accordance with. the velocity and follow the paths shown in Figure 1 at 4|, 43, and in light clashed detail. However, the same principles apply between the beam limiting apertured electrode and the target as between the electrodes 21 and 3| so that it is ponent beam of the combined beam occur at each of the apertures. The above parameters may be chosen by application of the above expressions for motion of the electrons in combined electrostatic and magnetic fields. As a simplified example, assuming a velocity of 1, 2, and 3 units for the three constituent beams, it would be necessary to provide at least three loops of focus for the lowest velocity beam, two loops of focus for the medium velocity beam and one loop of focus for the highest velocity .beam between the two electrodes. However, any integral number of this relationship may be provided between the apertured electrodes while still maintaining a point of focus at each aperture. Therefore, it will be appreciated thatthe number of loops of focus of the lowest electron velocity group or beam should desirable for a minimum cross-sectional beam diameter of either a single magnetically deflected beam or each of the electrostatically deflected beams at the target, to provide a number of loops of focus such that all of the electrons pass through a point of focus at the target. The velocity of the electrons may be controlled between the beam limiting apertured electrode and the target by one or more electrodes preferably in the form of a conductive wall coating 41 applied to .the wall of the envelope l between the targbt 3 and the nearest adjacent beam limiting apertured electrode, Therefore, the said distance between the target and the nearest beam limiting apertured electrode is preferably chosen with respect structure made and operated in accordance with this teaching of my invention wherein the electron source is continuous and extends in the plane parallel to or midway between the beam directing plates 19-21. For example, I may utilize a directly heated electron emitting cathode or electrode wherein the potential drop produced by electrically heating the cathode 40 provides lectrons having different emission velocities over he length of the cathode. For simplicity a filanent having a uniform potential drop along its ength may be used; for maximum concentration L non-uniform potential distribution may be deaired to strictly satisfy the requirements of beam notion set forth in the above expressions. In .his arrangement a single anode 42 may be posiiioned between the cathode 40 and the directing plates l9-2l to direct the streams of electrons within the crossed electrostatic and magnetic lelds between the plates. With this construction ahe electrons from the more positive end of the :athode are displaced to a greater degree than those from the opposite or negative endof the :athode 40. Withthis arrangement the electron streams within a certain velocity range follow between the plates converging paths within a triangular sheet-like area determined by the length of the cathode 40 and bounded on opposite sides by the highest and lowest velocity electrons, Obviously, the anode 42 may be subdivided into individual portions, each effective over a portion of the cathode length, this structure being desirable where it is desired to separately modulate electrons of difierent velocity by use ofa grid electrode or electrodes interposed between the anodes and the cathodes.

'While I have described my invention with particular'reference'to a tube incorporating spaced apertured electrodes, it will be appreciated that other electrode structure may be utilized between these described electrodes such as for utilizing my invention in velocity modulation tubes and that while I have specifically described a tube incorporating a luminescent screen and certain uses to which such a tube may be applied, it will be further appreciated that any type of target may be utilized depending upon the particular application desired without departing from the scope of the appended claims.

I claim:

1. Apparatus for developing a high intensity electron beam comprising means to develop a crossed-electrostatic and magnetic field of substantially constant magnitude, means to develop and direct into said field a plurality of electron streams displaced one from another in a plane parallel to the lines of force of said, magnetic field, and means to vary the velocity of the electrons comprising said streams to converge said streams into an electron beam of high intensity.

2. Apparatus for developing a high intensity elertron beam comprising means to develop a crossed electrostatic and magnetic fieldof substantially constant magnitude, means to develop a plurality of electron streams of different velocity displaced one from another in a plane parallel to the lines of force of said magnetic field, and means to direct said streams into said crossed field to converge said streams into an electron beamof high intensity. 3. Apparatus for developing a high intensity of electrons comprising a pair of oppositely disposed plates to develop .an electrostatic field. means to develop a magnetic fleld having lines of force transverse to said electrostatic field. means to develop and direct a plurality of electron streams into said fields and means to control the velocity of said streams to cause convergence thereof into a single-high intensity electron beam.

4. Apparatus for developing a high intensity electron beam comprising means todevelop a plurality of electron streams, means [to develop the electrons of each beam a magnetic field, means to direct said streams at difiereht velocities into said field in a plane substantially parallel to the lines of force thereof, means over a portion of the electron stream paths within said field to develop an electrostatic field having lines of force transverse to said streams and to said magnetic field to cause convergence of said streams in accordance with the velocity thereof and form a high intensity electron beam.

5. Electron discharge apparatus comprising electron emitting means to develop a plurality of displaced electron streams, an electron receiving target in alignment with said emittin means, means for producing a magnetic field having its lines of force parallel to the line of alignment of said emitting means and said target, means to vary the velocity of said streams whereby the velocity of said streams progressively increases in the direction of displacement of said streams, and means for deflecting said' streams in said direction in accordance with their velocity including means for producing an electrostatic field having its linesof force crossing and normal to the lines of said magnetic field and normal to said direction of displacement.

6. Electron discharge apparatus comprising electron emitting means to develop a plurality of electron streams displaced one from another in a single plane, an electron receiving target oppositely disposed from and substantially normal to said plane, means for producing a magnetic field having its lines of force parallel to said plane and intersecting said target, means to accelerate one of said streams to a predetermined velocity, means to accelerate the remaining streams to a velocity proportional to the displacement thereof from said one of said streams, and means for deflecting said streams in said plane in accordance with their velocity including means for producing an electrostatic field having its lines of force crossing and normal to the lines of said magnetic field and normal to said plane. 7. Electron discharge apparatus comprising a plurality of cathodes displaced one from another in a common plane to develop a corresponding plurality of electron beams, target means oppositely disposed from said cathodes to receive the electrons of said 'beams, means for producing a magnetic field having lines of force parallel to said plane of displacement, means to accelerate in the direction of said field in proportion to the displacement of one beam from another whereby the beams sepparated along said plane by the greatest distance have the greatest difference, in velocity, and means to develop 'an electrostatic field having lines of force normal to said magnetic lines of force and to said plane to deflect said beams in accordance with their velocity and produce a single electron beam of high intensity.

8. Apparatus for-developing a high intensity electron beam comprising a fila'inentary cathode to develop a plurality of electron streams displaced one from another in a common plane, laid streams originating at difierent points along said cathode, target means oppositely disposed from said cathode to receive the electrons of said streams, means including said cathode and i an anode to vary the velocity of the streams in proportion to the displacement of points of origin one from another along said cathode whereby the streams separated along said plane by the greatest distance have the greatest difference in velocity, means for producing a magnetic field having lines of fprce parallel to said plane, and means to simultaneously deflect each field transverse to said magnetic field and said of said streams each by an amount inversely direction to cause convergence of said streams proportional to their velocity comprising sepaof electrons with one another thereby combining rated electrostatic plates for forming an elecsaid streams into a high current electron beam. trostatic field having lines of force normal to 5 12. The method of developing a high intensity and coextensive with a portion of said magnetic electron beam comprising developing a plurality field to produce a single electron beam of high oi! electron beams having diiferent velocity in the intensity. direction of beam propagation, directing said 9. Electron discharge apparatus comprising beams into a crossed magnetic and electrostatic an evacuated envelope, a target in said envelope l0 field in th direction of said magnetic field and adapted to be bombarded by electrons, means varying the velocity of at least one of said beams, to develop a magnetic field having lines of force to cause convergence with another of said beams extending substantially normally to said target, thereby combining said beams into a beam of means to develop an electrostatic field transhigh intensity.

verse to said magnetic field, means. including a 13. The method of developing a high current cathode to develop and direct a plurality of elecelectron beam comprising developing a crossed tron streams into said fields, means to control electrostatic and magnetic field, developing a the velocity of said streams to cause convergence plurality of streams of electrons, directing said thereof into a single high intensity electron electrons into said crossed fields in a direction beam, an apertured electrode with its aperture parallel to said magnetic field and varying the aligned with and in the path of said single beam velocity of the electrons comprising said streams and means to separate electrons from said beam to cause convergence of said electrons into an in accordance with the velocity thereof into a electron beam of high current intensity.

number of streams corresponding substantially 14. The method of developing'a high current to the plurality of streams developed and directbeam comprising developing a magnetic field, deed into said fields. veloping an electrostatic field transverse to and 10. Electron discharge apparatus as claimed extending over a limited length of said magin claim 9 including means to deflect said riumnetic field, developing a plurality of electron her of electron streams over said target. streams, directing said streams into said elecll. The method or developing a high current trostatic field in a direction substantially parallel electron beam comprising the steps of 'developto the lines of force of said magnetic field and ing a plurality of electron streams having diivarying the velocity of said streams of eleci'erent velocity, directing said streams into a trons to cause convergence thereot'at the termimagnetic field having linesof force in the direcnus of saidelectrostatic field.

tion of said velocity, developing an electrostatic as HARLEY A. IAMS.

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