US2431688A - Velocity modulation electron discharge apparatus - Google Patents

Description

D669 2, 1947 5 F N iifl ifig v VELOCITY MODULATION ELECTRON DISCHARGE APPAR TUS Filed Dec. 14;, 1945 CENTER OF SEX-M :2 g A PEAK DENSITY AT cc P \32 7 Ol- [Q6 Fug 2 m5 ,PEAK DENSITY AT 8-5 g 3l ,Pz-:AK DENSITY AT A/\ --"TJNMODUL.ATED BEAM DENSITY EDGE OF BEAM- EpcE OF BEAM BEAM CROSS SECTION IQVENTOR EUGENE FEENBERG 10mm E Patented Dec. 2, 1947 VELOCITY .MQDULATIQN ELE ThQN mscnAro rrAnerus Eugene Feenberg New York, -Y., assignon to Sperry Gyroscope Company, Inc., a corporation o ew York Application December 14, 1943, fierial 1N0, '5 1 1, 2 20 Claims. (Cl. 315:6)

The present invention relates to the art ln cluding ultra high frequency electron discharge devices, especially of the velocity modulation type in which electrons of an electron beam are periodically varied in velocity at an ultra highfrequency, and are then permitted or caused to -velocity group or form bunches by the action of the faster electrons overtaking the slower electrons. Ultra high frequency energy of the'i-rrput frequency or a harmonic thereof may then be extracted from the bunched or Variable current beam. :Such devices serving as amplifiers or oscillators are discussed in Varian Patent No. 2 242,275, issued Ma 20, 1941, and, when serving .as modulators or frequency multipliers, are disclosed in Varian and Hansen Patent No. 2,281,935, issued May 5, 1942.

During the bunching action in such devices, certain effects take place which tend to impair the ability of the bunched or density-modulated beam to give up high frequency energy to the output circuit. One of these effects is caused by the mutual repulsion of the similarly charged and unneutralized electrons of the non-uniform beam. This repulsion causes a delay in the for mation of the bunches by electrons at the .cen.- ter :of the beam with respect to similar bunch formation at the edges of the beam. That is, a longer drift distance is required at the .center .of the beam to produce a given electron density by velocity grouping than is required at .the edges of the beam. Also, for maximum energy extracs tion from ,the beam, the electron density in :a predetermined plane transverse to the beam at the location of the energy extraction circuit is greater at the edges of the beam than ,at the cen: ter, and so the ability of the centrally located electrons to deliver high frequency energy to the output circuit is reduced, and the maximum value of the instantaneous beam current .or average electron density across the beam cross-section is lowered, leading to lowered output. This eifect is especially noticeable in the case of frequency multipliers or harmonic generators of the type shown in Patent No. 2,281,935 mentioned above, since the sharpness or peak current in? tensity of the bunch, and the consequent harmonic frequency content of the beam current passing through the output circuit grids, are materially reduced.

According to the present invention this non.- uniform bunching effect is substantially prevent.- ed or minimized by modifying the action of :the velocity modulating grids or electrodes so as to act d i eren ly n t e fien ral ele trons .or :the

2 beam. By increasing the velocity modulating effect upon the central electrons with respect to the electron's at the edges of the beam, and by the proper amount, the non-uniformity of the bunches arriving at theoutput electrodes can be made substantially zero. Thus, the entire electron beam passing through the output grids will have equal electron density over its entire crosssection. -By properly positioning the output grids along the team, "as choice of the drift tube length; t e i eh si' n ii ca be ma e o a the '4 timum value to produce amplification of e u tim "e in r'rl e i e u nc m plication, in the manner mentioned in the abovediscussed Pater 12,2 12275 and 2,231,935. Accordingly, it is an object or the present invention topi ie tlor minimize the non-uniiormity of the electron bunches in velocity rnodulation and -velocity grouping electron discharge d v g V,

' It is another object of the present invention to overcome the harmful efiects of longitudinal debunching in an electron beam having variable electron density therealong.

It is a further object of the present invention to provide improved velocity modulation electron discharge apparatus inwhich electrons at the center of the beam are velocit modulated differently from those atthe edges of the beam.

:It is a further object of the present invention to provide improved velocity modulation electron discharge apparatus of the frequency multiplier type in which the'ha-rmf u-l e fiects of nonuniform electron bunches or longitudinal debunching are minimized or substantially prevented.

It is another object of the present invention to provide improved electron beam velocity modulating apparatus utilizing a pair of electronpermeable velocity modulating electrodes having diflerent spacings rat difierent portions of the electron beam passing 'therethrough.

.Other objects and advantages will become apparent from the following --specifica-tion, taken in connection with the accompanying drawings, he ein Fig. l is a schematic diagrammatic representation of a conventional form of velocity modulation frequency multiplier apparatus useful in explaining the principle of :the present invention.

"Fig. *2 shows a graph of the peak electron density distribution across the drift tube atvaricus locations along the drift tube of the device of le Figs. 3, A and 5 show similar diagrammatic mesh grids or formed by radial or 'parallel slats,

or may be omitted entirely in some forms of the device. A beam of electrons is projected by means of an accelerating battery or other accelerating voltage source l8 from a cathode I 9 through the first or velocity modulating gap defined by grids I4 and I5, then through the drift space within tube l3, then across the output or energy extracting gap defined by grids l6 and I1, and is finally collected on the grounded conducting wall 2| of the device, or by a separate collecting electrode. The entire apparatus may be evacuated or enclosed in an evacuated envelope, or may have only the electron path evacuated.

Ultra high frequency energy of the operating angular or radian frequency w, to which resonator II is tuned, is supplied to resonator H by way of a suitable input coupling such as a concentric transmission line 22 and a coupling loop 23 coupling line 22 to the electromagnetic field within resonator H. In thisway resonator H is excited to oscillate at the frequency w, and a corresponding alternating electric field isimpressed across grids l4 and 15. This field periodically alters the velocity of the electrons of the beam emitted from cathode l9 and thus velocity modulates the electron beam. The velocity-modulated electrons then travel down the length'of drift tube l3, wherein the faster electrons overtake the slower electrons, so that some portions of the beam have increased electron density, while other portions have decreased electron density. In this way the beam becomes density-modulated or bunched and contains a high harmonic content by the time it reaches grid IS. The passage of this bunched beam between grids l6 and I! then delivers ultra high frequency energy of a harmonic no of the frequency w to the resonator l2 tuned to this harmonic frequency. Energy may be extracted from resonator 12 by a similar coupling loop 25 and output transmission-line 24 to supply any desired type of load. The device thus far described therefore serves as a frequency multiplier for ultra high frequencies, as is described in Patent No. 2,281,935. If resonator I2 is tuned to the fundamental frequency w, the device serves as an amplifier, as in Patent No. 2,242,275. In such case, if desired, the device may also serve as a regenerative oscillator by coupling or feeding back energy from resonator [2 to resonator II. In this case the input connection 22 may be omitted. In devices of the type just described, the mutual repulsion of the electrons of the beam produces an effect known'as longitudinal debunching. That is, the tendency of the faster electrons to overtake the slower electrons is opposed to some extent by the mutual repulsion of the similarly charged electrons. This effect has been found to be more marked at the center of the beam than at the edges of the beam, 50 that the formation of the bunches,

or the increasing of electron density in the beam, is delayed at the center of the beam with respect to the edges of the beam. This phenomenon is believed caused by the induction of image charges on the inner surface of the drift tube which travel down the drift tube along with the electron bunches and tend to neutralize the electron charges at the edges of the beam, so as to prevent longitudinal dispersion of these outermost electrons. At the center of the beam, however, the effect of the image charges is greatly reduced, so that the longitudinal debunching between the similarly charged electrons is not prevented to any substantial extent.

This debunching phenomenon is indicated in Fig. 2, which shows the peak electron density distribution across the drift tube at several locations A-A, BB and 6-0 along the drift tube, at instants when the electron density at these points is maximum. Thus, at location A-A along drift tube I3, in Fig. 1, the velocitymodulated electrons have had but little opportunity to become bunched, and accordingly the peak electron density is only slightly larger than the average electron density which, in turn, is equal to the electron density of the beam before electron velocity modulation thereof. As shown by curve 3| of Fig. 2, this peak density is substantially the same over the entire cross-section of the drift tube.

At the location BB, the bunching process has proceeded further, so that the peak electron density is substantially greater than the average electron density. Also, the efiects of longitudinal debunching are beginning to become apparent, so that the electron density at the center of the beam is slightly less than that at the edges of the beam, as shown by curve 32.

Curve 33 shows the electron density distribution at line C--C within the catcher resonator I2, where the bunching process is at a maximum so that the peak electron density is greatly increased. However, the peak electron density is much greater at the edges of the beam than at the center of the beam because of the longitudinal debunching effect, leading to decreased eificiency, because the peak beam current, averaged across the beam, is lowered by the nonuniform bunching.

The loss in efficiency resulting from this nonuniform bunching process is considerably greater in frequency multipliers, such as shown in Fig. 1, than in simple amplifiers of the type shown in Patent 2,242,275. Also, the decrease in efficiency becomes greater for high multiplication ratios. Since such frequency multipliers may have multiplication ratios as high as 10 to 20, the harmful effects of non-uniform bunching are multiplied to that extent.

In accordance With the present invention, this loss in efficiency is minimized by substantially preventing non-uniformity of the bunch at the catcher or output grids I6 or IT, either in amplifier or oscillator devices such as in Patent No. 2,242,275, or in frequency multiplier devices such as in Fig. 1. For this purpose the electrons at the center of the beam are acted upon more strongly by the input wave than the electrons at the edges of the beam, so as to produce a greater amplitude of velocity modulation and thus overcome the opposing debunching tendency due to charge repulsion. The tendency for the electrons to bunch depends in part on their transit time through the velocity-modulating grids. An increase in this transit time in devices of the present type produces a decreased tendency to bunch,- while a decrease in transit time produces sharper bunching. In the present invention, the debunching at the center of the beamis substantially prevented by increasing the tendency of the center electrons to bunch, in relation to the outer electrons. This may be done in the manner shown in Fig. 3, which illustrates the velocity modulating portion of the device of Fig. 1, modified in accordance with the present invention.

Thus, instead of utilizing the customary parallel grids l4 and I5 as in Fig. 1, or other electrodes, these grids are now made to have variable spacing across the electron beam. As shown in Fig. 3, the grid M may remain substantially planar. Grid l5, however, is made to bow inwardly, so that the separation between grids l4 and i5 at the center of the beam is less than that at the edges of the beam. Preferably, grid i5 is made axially symmetrical with respect to the axis of the beam. Inthis way the transit time for the central electrons is reduced, leading to sharper bunching to overcome the tendency to debunch. By proper choice of the grid separation or gap as a function of radial distance from the beam center, the bunch arriving at the energy extracting gap may be made substantially uniform. Grids l6 and ll, therefore, may remain of the conventional type disposed in parallel planes.

I have discovered that a highly preferred form of the invention has a modulating grid separation s at a distance y from the beam axis which obeys the following relation:

where 83, is the grid spacing at the edge of the beam, y is the distance from the beam axis, and k is an experimentally determined constant. Where the electron beam nearly fills the drift tube, an approximate value for the factor k; can be given by the formula k- 2 lOL Vl9 2 A a e dun in which L is the length of the field-free drift space; is the transit angle in radians through the velocity modulating gap at the edge of the beam; A is the wave length; a is the radius of the beam; is the ratio of electron velocity to the velocity of light; V is the unidirectional accelerating voltage; and I is the average electron beam current. It will thus be seen that the preferred form of grid i5 is essentially in the form of a paraboloid of revolution having an axis concentric with that of the beam.

Instead of bowing grid l5 inwardly, as shown in Fig. 3, it will be clear that grid l5 may be retained planar, as shown at E5 in Fig. 4, While grid i4 is here bowed inwardly as shown at M" to provide the required variable spacing.

Also, both grids I l and 15 may be made nonplanar, as shown in Fig. 5, wherein both grids W and W are bowed inwardly while maintaining the desired variation in spacing over the cross-section of the beam. This is preferable, since then no radial acceleration is imparted to the electrons.

It should be understood that the drift space in tube 53 is substantially free of any portion of the alternating electromagnetic field existing within the resonators H and i2 due to the oscillations therein. For this reason this drift space is known as the field-free drift space, and is designated as such herein.

Preferably, the grids used in the present invention are of the mesh or lattice type, such asknit or stocking grids, to permit bowing to the proper curvature. However, as an approximation, grids formed by generally radial, but com cally disposed slats may be used.

Since many changes could be made in the drawings shall be interpreted as illustrative and.-

not in a limiting sense.

What is claimed is:

1. High frequency apparatus comprising a pair of adjoining electron permeable electrodes defining a gap therebetween, a tuned circuit connected to said electrodes and adapted when excited, to impress an alternating electric field between said electrodes, a second pair of electron permeable electrodes defining a second gap, a second tuned circuit similarly connected to said electrodes, conductive means connecting one electrode of each of said pairs together and providing a fieldfree drift space between said gaps, means aligned with said electrodes for projecting an electron beam successively through one of said gaps, said field-free space and the other of said gaps, said first pair of electrodes having a spacing varying parabolically from the center of the path of said beam to the edges of said beam path.

2. High frequency apparatus comprising means for producing an electron stream, first resonant circuit means along the path of said stream and adapted to produce an oscillating electric field at a predetermined point of said stream path to modulate the velocity Of the electrons of said stream, means along said stream path for causing said electrons to form into bunches, and means along said stream path for extracting energ of a frequency harmonically related to the frequency of said field, said first tuned circuit means including a pair of electron-permeable electrodes along said stream path defining a ap through which the stream is passed and having non-uniform spacing across the cross-section of said stream which varies parabolically with respect to radial distance outward from the center of the path of said stream.

3. Apparatus as in claim 2, in which both of said electrodes are non-planar.

4. Apparatus as in claim 2, in which one of said electrodes is disposed substantially i a i e plane and the other of said electrodes is formed as a paraboloid of revolution coaxial with said beam.

5. High frequency apparatus comprising a first cavity resonator having a pair of electron-permeable walls, a second cavity resonator tuned to a harmonic of the resonant frequency of said first cavity resonator and having a further pair of electron-permeable walls, and means aligned with said wall for projecting an electron beam successively through said resonators by way of said walls, said first pair of walls having a spacing across the cross-section of said beam varying substantially parabolically with respect to radial distance from the center of the path of said beam.

6. High frequency apparatus for frequency multiplication comprising means for producing an electron beam, means along the path of said beam for velocity modulating said electron beam at a predetermined frequency, and means along said beam path including a resonant circuit tuned to a harmonic of said predetermined frequency for extracting harmonic frequency energy from said modulated beam, said velocity modulating means including a pair of smoothly curved grids in said beam path having a spacing increasing curvilinearly from the center of the beam outward, thereby minimizing loss inefiiciency due to longitudinal debunching of said modulated electron beam at said energy extracting means.

'7. High frequency apparatus comprising means for producing an electron beam, means along the path of said beam for velocity modulating said electron beam at a predetermined frequency, means along said beam path for causing said modulated beam to become bunched, and means along said beam path including a resonant circuit tuned to a harmonic of said predetermined frequency for extracting harmonic energy from said bunched beam, said velocity modulating means including a pair of smoothly curved grids in said beam path having a spacing increasing curvilinearly from the center of the beam outward, thereby minimizing non-uniformity of said electron bunches at said energy extracting means.

8. A velocity modulation electron discharge apparatus comprising a cavity resonator having a pair of juxtaposed electron permeable walls defining a gap through which an electron beam may be passed, said permeable walls having a spacing varying parabolically with respect to radial distance from the center thereof.

9. Apparatus as in claim 8, wherein one of said electrodes is substantially planar and the other of said electrodes is substantially in the form of a paraboloid of revolution having its axis perpendicular to said first electrode.

10. A velocity modulation electron discharge apparatus comprising a cavity resonator having a pair of juxtaposed electron-permeable walls defining a gap through which an electron beam may be passed, both of said walls being non-planar and having a relative spacing varying parabolical- 1y with respect to radial distance from the center thereof.

EUGENE FEENBERG.

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

UNITED STATES PATENTS

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