US2983842A - Electrode system - Google Patents

Description

May 9, 1961 G. w. HRBEK ELECTRODE SYSTEM Filed June 23, 1959 L'Illllli. IIIH I- ELECTRODE SYSTEM George W. Hrbelr, `Elk Grove Village, Ill., assigner to Zenith Radio Corporation, a corporation of Delaware Filed June 23, 1959, Ser. No. 822,267

8 Claims. (Cl. S15-16) The present invention relates to an electrode system for use in conjunction with a magnetic beam-focusing arrangement to achieve a Brillouin-flow electron beam.

Such a beam is characterized by the fact that all of the electrons therein have the same axial velocity, independent of radial distance from the center of the beam, and is further characterized by the fact that the beam itself advances in a fashion resembling a solid rod moving endwise while concurrently rotating about its longitudinal axis. Brillouin-flow represents a condition of balance attained between the space charge and centrifugal forces acting on the electrons on the one hand and the focusing force of a homogeneous unidirectional focusing field on the other. Expressed a little differently, the balance of forces is such that the electrons of the beam have substantially zero component of radial motion.

The condition of Brillouin-flow is an objective or state of perfection that cannot be attained as a practical matter in tube structures but it is highly desirable to approach this condition as closely as possible for optimum operation in a number of electron beam devices. An electron beam type of parametric amplifier, for example, may be constructed as a low noise device and if optimum noise figures are to be realized, the condition of Bri-llouin-ow should be established to the highest practical degree.

Most previous efforts to achieve Brillouin-flow have featured structural arrangements designed to produce an electron beam initially having no rotational velocity and to introduce the beam into the focusing magnetic field at an ang-le critically related to the field configuration. This is a highly unsatisfactory approach for electron beam devices of small physical size, particularly if the diameter of the vacuum envelope is a substantial fraction of its axial length, since the magnetic field must terminate within the electron gun.

It is appreciated that, as a theoretical matter, if a point electron source is provided at the axis of a homogeneous magnetic focusing eld, the desired condition may be realized because each electron would then cross the correct number of lines of force in travelling a given distance radially of the beam path and would therefore be subject to the -appropriate skewing or rotational force of the focusing field; this state of `affairs will lead to Brillouinflow. Previous efforts to adopt this principle have failed for a variety of reasons. It is most difficult, if not impossible, to provide an emitter small enough to represent a point source and, if that could be accomplished structurally, it is well understood that its role as a point source would at best be a transient matter. Accordingly, electrode systems have been proposed in which various techniques are employed to select the electrons originating from a small region of a larger cathode structure. Prior structures have not succeeded to the extent attainable with the arrangement described herein and some suffer additionally lfrom the fact that they require the electrode system to be undesirably long.

It is therefore a principal object of the present inven- 2,983,842 Patented May 9, i961 tion to provide a new and improved electrode system for producing an electron beam characterized by exhibiting a close approach to Brillouin-flow.

It is another specific object of the invention to provide an electrode system of simplified construction which, in conjunction with a homogeneous unidirectional magnetic field, achieves a closer approximation of Brillouin-electron llow than arrangements of the prior art.

It is a specific object of the invention to provide a new and improved electrode system for use in an electron beam type of parametric amplifier.

An electrode system constructed in accordance with the present invention is intended to cooperate with a homogeneous unidirectional magnetic focusing field to produce and project a Brillouin-flow electron beam along a path coaxial of the focusing field. The system cornprises an electron gun including a cathode and an anode having an aperture coaxial of the beam path and established Iat a positive potential relative to the cathode to accelerate an electron stream issuing therefrom and to project that stream along the path in the region beyond the anode. A decelerating electrode, having a circular aperture coaxial of the path, is positioned in the magnetic field in the region beyond the lanode and maintained substantially at cathode potential to form in the center of its aperture a virtual cathode region and simulate a point electron source located on the beam path. Finally, an electrostatic lens system is positioned coaxially of the path beyond the decelerating or retarding electrode to collimate electrons issuing from the point source into a Brillouin-flow beam having substantially zero component of radial motion.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

Figure 1 is a schematic diagram of an elect-rode system embodying the present invention; and

Figure 2 is -a longitudinal cross-sectional view of an electron beam type parametric amplifier tube employing the electrode system of Figure 1.

Referring now more particularly to Fi-gure l the electrode system there represented is especially suited for use in an electron beam type of parametric amplifier in that, in conjunction with a homogeneous unidirectional magnetic Afocusing field, it -achieves a more perfect approach than predecessor arrangements to establishing Brillouinelectron ow along a beam path which is coaxial with the lfocusing field. The focusing `field is, for convenience, represented symbolically by the arrow H and, as a matter of practice, is usually established by means of a solenoid arranged coaxially of the beam path 12 and coupled to a `direct current source of adjustable magnitude to facilitate the `development of a homogeneous unidirectional yfocusing field of desired value. The electrode system for producing and projecting an electron beam along path 12, coaxially of the focusing field, comprises an electron gun which may be similar in construction and operation to that conventionally employed in a cathode ray type image reproducing device. It includes a cylindrical cathode 10 having an end cap l1 coated with emissive material. A vfocusing electrode 13, is positioned across the beam path and spaced a short axial distance from emitting surface 1l. It, in turn, is followed by an anode 15, similarly disposed across the beam path and likewise spaced axially a short distance from focus electrode 13. Both the focus electrode and anode are centrally apertured so that their apertures are coaxial of the beam path and the aperture 14 of the focus electrode is larger than aperture 16 of the an0de.,`Both apertures are smaller than the diameter of the cathode emittingV surface.

Next beyond anode 15 is a decelerating or retarding electrode 17 having a circular aperture 18 coaxial of the beam path. This electrode is ifollowed by a series of centrally apertured lens electrodes positioned at successive points transversely of the beam path to complete an electrostatic lens system for collimating electrons into a desired beam. This lens system includes an electrode 19 having an aperture 20, an electrode 211 having an aperture 22 and, finally, an electrode 23 apertured at 24. The final two electrodes are maintained at the same operating potential by a conductive connection 25 extending therebetween. The beam path 12 terminates in a collector electrode 26. v

The operating potentials to be supplied to the several electrodes of the system vare represented by appropriate legends and may be derived from any conventional power supply. it wll be `observed that cathode is maintained at a reference potential such as ground and that Idecelerating electrode 17 is established at substantially cathode potential. The potential VF of the lfocusing electrode may be slightly negative and theY potential VA of anode is positive relative to the cathode. The poten tial VL, of lens electrode 19 is positive relative to the cathode but much less positive than anode 15. Finally, lens electrodes 21 and 22 are Iat a potential Vm which ispositive but less positive than electrode 19.

Assuming that the electrode potentials have beenV ap'- propriately established, an electron stream issuing from cathode 10 is accelerated and at the same time focused `under the conjoint influence of focus electrode 13 and anode 15 into a sharply constricted stream,'sornetimes referred to as a cross-over, in the vicinity of anode aperture '16. This focusing eiect is essentially that of an immersion lens and results in a stream having an envelope as indicated at 30. The stream as thus focused and accelerated is projected along beam path 12 in the region beyond anode 15 in the direction of collector 26.

As the beam passes anode 15 it enters upon a decelerating or retarding eld, since decelerating electrode 17 is maintained very close to cathode potential. It has been Idiscovered that most of the incident electrons approaching from anode 15 are repelled or rejected as indicated at 30' and a virtual cathode region is consequently formed at the center of aperture 18. Only 'a fraction of the electrons, those approaching retarding electrode 17 close to the axis or beam path 12, pass beyond into the lens system; they emerge as from `a simulated point electron source located on beam path 12.l The electrostatic lens system comprised of electrodes 17, 19, 21 and 23 collimatesV electrons issuing from this effective point source into a Brillouin-dow beam having substantially zero component of radial motion. In particular, electrodes 17, 19, 21 create a ldivergent electrostatic lens 31 effectively located at aperture 20 so that the elec-` tron stream initially diverges as indeed it must to attain the desired beam diameter. Since therstream issues, in effect, from a point source located on the axis of a symmetn'cal and homogeneous focusing eld, all of the diverging electrons cross the same number of field lines in traversing a given radial distance and therefore all yare subject to the same skewing or rotational effect of the focusing field. Thus one of the conditions essential to Brillouin ow is assured. Y

Lens electrodes 19, 21 and 23, in turn, constitute a convergent electrostatic lens effectively located at aperture 22 which arrests the spread of the beam at the desired beam .radius r. It terminates the 4divergent action and straightens out the beam envelope in a direction parallel to beam path 12. Simultaneously, a marginal portion of the beam is stripped oi and intercepted by the solid portion of electrode 21 surrounding collimating aperture-` 22. In this manner, by -appropriate adjustment of the lens potentials and focusing eld, an electron beam exhibiting a close approximation to Brillouin-flow is projected along path 12 toward electron 26.

The electron beam type of parametric amplifier to which the desired electrode system is especially suited is a low current density device. Accordingly, it is feasible in the described arrangement to realize a simulated point electron source in the region of aperture 18 of retarding electrode 17 as aforesaid. It hasbeen discovered that the use of this electrode system in an electron beam type of parametric -ampliiier materially improves the noise ligure.

`One embodirnent of. the electrode system that has been constructed and operated With highly satisfactory results employs the following dimensions and operating potentials. It should be understood, however, that this recitation is merely by way of illustration rather than limitation on the structure. Cathode 10 has a diameter of 0.132V inch and its emissive coating is spaced from focus electrode 1.3 by a distance of 0.010 inch. The focus electrode has `a thickness of 0.005 inch and a circular central aperture having. a diameter of 0.020 inch. Anode 15 is spaced from focus electrode 13 by a distance of .010 inch and the remaining electrodes, speciiically lens electrodes 17, 19 and 21 have an axial spacing of .030y inch. The central aperture of anode 15 is 0.010 inch While electrodes 17 and 19 have an aperture of .030 inch. Final lens electro-des 21 and Z3 have an axial separation of .052 inch and apertures of .016 inch. The operating potentials for the several electrodes are as follows: cathode at ground potential, focus electrode 13 at -2 volts, anode electrode 15 at +270 volts, decelerating electrode 17 at cathode potential, lens electrode 19 at +41 volts and lens electrodes 21 and 23 at -I-lO volts. The collector 26 is operated at 40 volts. The strength of the magnetic iield H is approximately 152 gausses and the center frequency is 425 megacycles per second.

A practical form of parametric amplifier employing an electrode system of the type represented in Figure l is illustrated in Figure 2. The amplier, per se, is the subject of a copending application of Glen Wade, Serial No. 747,764, filed July 10, 1958, and assigned to the same assignee as the present invention. The tube assembly is disposed within an envelope 40 through the opposite ends 41 and 42 of which suitable electrical connecting leads project. Disposed near end 42 is-the portion of the electrode system of Figure 1 up to and including linal lens electrode 23. As explained, it includes cathode 10 supported by a ceramic wafer 45 from a metallic annulus 46 through which an end of cathode .10 yfreely projects and on which end is disposed the cap exteriorly coated with electron emissive material 11. A heater 48 is provided to raise the emitter to an emissive temperature. VSpaced beyond cathode 10 is another metallic annulus 49 forming with annulus 46 a cage substantially conning cathode 10 except for the exposed coating 11. Spaced in front of the cathode is focus electrode 13 having its circular aperture aligned with the axis of cathode v10 Vand beam path 12. Next beyond the metallic wafer forming focus electrode 13 is accelerating anode 15 comprising a metal disc also having a central aperture coaxial of beam path 12 tok accelerate and project electrons into the region therebeyond. Successively spaced beyond accelerator 15 are the decelerating electrode 17 and the remaining lens electrodes 19, 21 and 23, all of which are centrallyapertured and supported with their apertures centered on the beam path`1'2.

Just beyond the described electron system is a modulator 50 for impressing signal energy on the electron beam travelling path `12. In the next succeeding portion of the beam path in the direction of the end 41 of the envelope is a signal modulation expander 51`which, in turn, isY .followed by a demodulator 52 Vfor removing signal energy-from the beam'.` j j assasaa On beyond demodulator 52 is an electrode 53 having an aperture centered on beam path 12 and which during operation serves the function of a suppressor electrode. Finally, the electron beam is collected by anode 26 which is disposed transversely of beam path 12 behind the aperture in suppressor 53. The entire assembly is supported within a confining envelope by means of four ceramic rods 54 symmetrically disposed about beam path 12 and extending through all of the various apertured electrodes and through appropriate insulating discs 55 in which the modulator, expander and demodulator electrodes are secured. The different electrodes are separated by suitable ceramic washers 56 encircling ceramic rods 54 between the different electrodes; discs 55 are separated by similar ceramic sleeves 58. The entire assembly is held tightly together by means of compression springs 59 acting b'etween a collector mounting plate 60 and a Washer 61 pinned to ceramic rods 54. Of course, suitable internal leads are brought out from the various electrodes to the terminals projecting through the base presses.

An explanation of the operation of the modulator, expand'er and demodulator is unnecessary for an understanding of the present invention which is directed solely to the electrode system and, in conjunction with the mag-- netic field, establishes Brillouin-Bow of the electro'n beam. Suiice it to say for the present that input signal energy impressed upon the beam in modulator 50 develops an electron wave representing the signal intelligence and at the same time undesired noise carried by the beam is removed therefrom. In expander 51, the signal energy is parametrically amplified by subjecting the electrons to a time-variable inhomogeneous pumping eld. Demodulator 52 includes an electron coupler which is usually identical with modulator 50 and serves to extract the amlified signal energy from the beam for delivery to a loa Oi course, a homogeneous magnetic eld is required for the operation of the tube and is usually produced by a. solenoid which encompasses the tube envelo'pe, being arranged coaxially therewith. The solenoid is of suiicient length that the electron gun including cathode 10, focus electrode 13 and anode 15 are within the focusing eld as is the simulated point electron source established by decelerating electrode 17. It has been found that the electrode system operating lwith the potentials specified hereinabove produces a beam which is a close approximation of a condition of Brillouin-How. Accordingly, interception of the beam by modulator parts and other electrodes o'f the tube is negligible and the noise factor iS significantly improved over similar devices having predecessor types of electrode systems.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

I claim:

1. An electrode system for producing and projecting -an electron beam along a path extending coaxially of a homogeneous unidirectional magnetic field to achieve Brillouin electron flow comprising: an electron gun including a cathode and an anode having an aperture coaxial of said path and at a positive potential relative to said cathode to accelerate an electron stream issuing therefrom and to project said stream along said path in the region beyond said anode; a decelerating electrode positioned within said magnetic eld, having a circular aperture coaxial of said path, positioned in said region beyond said anode and maintained substantially at the potential of said cathode to form a virtual cathode region and simulate a point electron source located on said path; and an electrostatic lens system positioned coaxially of said path beyond said decelerating electrode to collimate electrons issuing from said point source into a Brillouin# flow Ibeam having substantially zero component of radial motion. v

2. An electrode system for producing and projecting an electron beam along a path extending coaxially of a homogeneous unidirectional magnetic eld to achieve Brillouin electron ilow comprising: Aan electron gun including a cathode, a focus electrode, and an anode, said focus electrode and said anode individually having an aperture coaxial of said path and said anode being at a positive potential relative to said cathode to accelerate an electron stream issuing therefrom and to project said stream along said path in the region beyond said anode; a decelerating electrode positioned within said magnetic field, having a circular aperture coaxial of said path, positioned in said region beyond said anode and maintained substantially at the potential of said cathode to form a Virtual cathode region and simulate a point electron source located on said path; and an electrostatic lens system positioned coaxially of said path beyond said de celerating electrode to collimate electrons issuing from said point source into a Brillouin-flow beam having substantially zero component of radial motion.

3. An electrode system `for producing and projecting an electron beam along a .path extending coaxially of a homogeneous unidirectional magnetic eld to achieve Brillouin electron ow comprising: an electron gun including a cathode, a focus electrode and an anode, said focus electrode and said anode individually having an aperture coaxial of said path and having a potential in relation to that of said cathode to accelerate and focus an electron stream issuing therefrom into a stream sharply constricted in the vicinity of said anode aperture and to project said stream along said path in the region beyond said anode; a decelerating electrode positioned within said magnetic field, having a circular aperture coaxial of said path, positioned in said region beyond said anode and maintained substantially at the potential of Said cathode to form a virtual cathode region and simulate a point electron source located on said path; and an electrostatic lens system positioned coaxially of said path beyond said decelerating electrode to collimate electrons issuing from said point source into a Brillouin-flow beam having substantially zero component of radial motion.

4. An electrode system for producing and projecting an electron beam along a path extending coaxially of a homogeneous unidirectional magnetic iield to achieve Brillouin electron flow comprising: an electron gun including a cathode and an `anode having an aperture coaxial of said path and -at a positive potential relative to said cathode to accelerate `an electron stream issuing therefrom and to project said stream along said path in the region beyond said anode; .a decelerating electrode positioned Within said magnetic eld, having a circular aperture coaxial of said path, positioned in said region beyond said anode and maintained substantially at the potential of said cathode to form a virtual cathode region and simulate a point electron source located on said path; and an electrostatic lens system including a plurality of centrally apertured lens electrodes positioned coaxially of said path beyond said decelerating electrode to collimate electrons issuing from said point source into a Brillouin-flow beam having substantially zero component of radial motion.

5. An electrode system for producing and projecting an electron beam along a path extending coaxially of a homogeneous unidirectional magnetic field to achieve Brillouin electron ow comprising: an electron gun including a cathode and an anode having an aperture coaxial of said path and at a positive potential relative to said cathode to yaccelerate an electron stream issuing therefrom and to project said stream along said path in the region beyond said anode; a decelerating electrode positioned within said magnetic eld, having a circular aperture coaxial of said path, positioned in said region beyond said anode and maintained substantially at the potential of said cathode to Aform a virtual cathode region and simulate a point electron source located on said path; and an electrostatic lens system including a plurality of centrally apertured lens electrodes positioned coaxially of said path beyond said decelerating lelectrode to constitute therewith a divergent lens succeeded by a stronger convergent lens for collimating electrons issuing from said point source into a Brillouin-110W beam having subs-tantially zero component of radial motion.

6. A11 electrode system for producing and projecting an electron beam along a path extending coaxially of a homogeneous unidirectional magnetic field to achieve Brillouin electron flow comprising: an electron gun including a cathode, a focus electrode and an anode, said focus electrode and said anode individually having an aperture coaxial of said path and having a potential in relation to that of said cathode to accelerate andV focus an electron stream issuing therefrom into a sharply constricted stream in the vicinity of said anode aperture and to project said stream along said path in the region beyond said anode; a decelerating electrode positioned within said magnetic field, having a circular aperture coaxial of said path, positioned in said region beyond said anode and maintained substantially at the potential of said cathode to form a virtual cathode region and simulate a point electron source located on said path; a rst centrally apertured lens electrode positioned coaxially of Said path beyond said decelerating electrode and positive relative thereto but less positive than said anode; and at least one additional centrally apertured lens electrode positioned coaxially of said path beyond said iirst lens electrode to `constitute therewith an electrostatic lens system for collimating electrons issuing from said point source into a Brillouin-dow beam having Vsubstantially zero component of radial motion.

7. The method of developing a Brillouin-110W electron beam comprising: developing a homogeneous unidirec tional magnetic iield coaxially of an electron beam path; accelerating an electron stream from an emitting source toward an apertured anode positioned within said field coaxial of said path and projecting said stream along said path in the region beyond said anode; subjectingn said stream to a .decelerating iield in said region beyond said anode to establish'a virtual cathode region and simulate a point electron source located on said. path; and subject? ing electrons issuing from said point source to an electrostatic lens system to collimate said electrons into a Brillouin-flow beam having substantially zero component of radial motion. t

8.'An electrode system for producing and projecting an electron beam along a path extending coaxially of a homogeneous unidirectional magnetic eld to achieve Brillouin electron flow comprising: .an electron gun positioned within said iield including a cathode, a focus electrode and an anode, said focus electrode and said anode individually having an aperture coaxial of said path and having a potential in relation to that of said cathode to accelerate and focus an electron stream issuing therefrom into a stream sharply constricted in the vicinity of said anode aperture and to project said stream along said path in the'regionbeyond said anode; a decelerating electrode, having a circular aperture coaxial of said path, positioned in said region beyond said anode and maintained substantially at the potential of said cathode to form a virtual cathode region and simulate a point electron source located on said path; and an electrostatic lens system positioned `coaxially of said path beyond said decelerating electrode to collimate 'electrons issuing fromrsaid point source into a Brillouindio'w beam having substantially zero component of radial motion.

References Cited in the ile of this patent Y UNITED STATES PATENTS 2,161,316 Rogowski June 6,Y 1939 y2,306,663 lSchlesinger Dec. 29,1942 2,309,220 Skellett Jan. 26, 1943 2,505,261 Syrdal Apr. 25, 1950 2,852,716 Laiienty Sept. 16, 1958 2,888,606 Beam May 26, 1959

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