US3746915A

US3746915A - Traveling wave tube with planar equiangular spiral slow wave circuit

Info

Publication number: US3746915A
Application number: US00253043A
Authority: US
Inventors: F Sherburne; L Jasper; Santis C De
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1972-03-15
Filing date: 1972-05-15
Publication date: 1973-07-17
Anticipated expiration: 1990-07-17

Abstract

This invention concerns a more compact, lower-cost, lowervoltage, broader-band, traveling wave amplifier that has a cylindrical electron gun between closely spaced, parallel, flat surfaces of a pair of ceramic disks and a ring collector joined to the disks concentric with the electron gun. At least one of the flat surfaces has a printed slow wave circuit in the form of at least one tightly wound equiangular spiral arm between the electron gun and the collector. Though a high power amplifier needs focusing means, a small amplifier made according to the invention for operation at low power needs no electron beam focusing between electron gun and collector. The invention can be combined with a phased array to serve as an amplifier in place on the array; one such amplifier is provided for each radiator element. The invention can serve as amplifier and radiator by designing the spiral arm for radiation.

Description

nu smears' [451 ,any i7, i973 Mnited @taies Patent t191 jasper, ,lin et al,

..315/3.6 3/1954 Tiley.....................................315/5 [541 rnAvELrNc WAVE ruimt wim PLANAR 3,571,651 EQUIANGULAR srinni, Stow WAVE 2672572 cmcuir [75] Inventors: Louis J. Jasper, Jr., Neptune City;

Primary Examiner--Rudolph V. Rolinec Char. M D S n N t Assistant Examiner-Saxfield Chatmomjr.

es e m Si eP une; Attorne -Harr M. S' `t Ed d ll Frederick 1B. Sherburne, Oceanport, et al y y dragovl Z War J Ke y al1 of NJ.

[73] Assignee: The United States oli America as [57] ABSTRACT This invention concerns a more compact, lower-cost, lower-voltage, broader-band, traveling wave amplifier represented lby the Secretary ot the Army, Washington, D.C. that has a cylindrical electron gun between closely [.22] Filed' May l5 w72 spaced, parallel, flat surfaces of a pair of ceramic disks [2l] Appl. No.: 253,043 and a ring collector joined to the disks concentric with the electron gun. At least one of the flat surfaces has a printed slow wave circuit in the form of at least one tightly wound equiangular spiral arm between the electron gun and the collector. Though a high power amplifier needs focusing means, a small amplifier made according to the invention for operation at low power needs no electron beam focusing between electron gun [56] References Cited UNITED STATES PATENTS and collector. The invention can be combined with a phased array to serve as an amplier in place on the ar- 3,305,752 2/1967 315/5 X 3,258,702 6/l966 3,153,742 l0/l964 2,617,961 ll/l952 ray; one such amplifier is provided for each radiator el Hart..........

ement, The invention can serve as amplifier and radia tor by designing the spiral arm for radiation.

Kluver 4 Claims, 4l Drawing Figures Patnted July 17, 1973 I 3,746,915

2 Sheets-Sheetl Patented yJuly 17, 1973 3,746,915

-2 Sheets-Sheet 2 f F/G; 3

FIG. 4

BACKGROUND OF THE INVENTION In any traveling wave amplifier device, a beam of electrons gives up energy to a high frequency signal to amplify the Sianalfhesianalis nlgpaaatsiillgnaaitcuitous path at essentially free space velocity, but the advance of the circuitous path in a direction normal to the path is a small fraction ofthe length ofthe circuitous path. The signal that is propagated along the circuitous path is accompanied by a wave propagated normal to the path with a slow phase velocity that is a fraction of the free space velocity of the signal along the circuitous path. The beam of electrons is propelled normal to the circuitous path at essentially the phase velocity of the slow wave and along intense electric field regions of the slow wave. The electron beam is accelerated by the electron gun to the slow-wave phase velocity before introduction into the interaction region and then traverses the interaction region. The interaction region is equipotential. Magnetic or electrostatic focusing is applied to the electron beam along the interaction region. During interaction between the beam electrons and the propagated slow wave, interacting electrons are velocity-modulated causing bunching of electrons along the transiting beam and a general slowing of the beam. When operating properly, there is transfer of energy from the electron beam to the signal, amplifying the signal. The gain is related to the length of the interaction region in wavelengths and the beam current density that interacts with the propagated wave.

A common form of slow-wave guiding means for a traveling wave tube is a wire helix that conducts the high frequency signal energy and concomitantly propagates a slow electromagnetic wave produced by the signal energy along the helix at a phase velocity substantially matched by the velocity of the electron beam. The wire helix surrounds the electron beam path. In another type of traveling wave tube, a structure which guides the wave crosses back and forth across the path of an electron beam, but provides an unobstructed path for the electron beam along electric field regions of the propagated signal wave. As the signal propagates along the helix or waveguiding structure, the electron beam, in synchronism with the longitudinal phase velocity of the signal, interacts with the electric field ofthe propagated slow wave in such manner that the electron beam becomes velocity-modulated and bunched and the signal grows in strength.

Known types of traveling wave tube amplifier devices have one or more disadvantages which include high cost, complex design, comparatively narrow band, operate at high voltage and are larger and heavier than desirable. Improvement of any one or more of these characteristics is generally desirable. More particularly, there is need for lighter weight, broadband, lower voltage, efficient, economical expendable batteryoperated low-power units for expendable jammers or other expendable high frequency electronic equipments. Also, there is need lfor improved modulator and amplifier devices that can be used in place in phased array antenna assemblies. 'Also there is need for a device that can be used as amplifier, modulator and radia- (01.

SUMMARY OF THE INVENTION An object of this invention is to provide a more compact traveling wave tube amplifier device.

A further object is to provide a lighter weight, lowpower traveling wave tube amplifier device.

A further object is to provide a low voltage traveling wave tube amplifier device.

A further object is to provide a traveling wave tube device with a high perveance electron gun.

A further object is to provide a printed circuit traveling wave tube amplifier device.

A further object is to provide a superior, high-power traveling wave tube amplifier device.

A further object is to provide a more economical and more efficient traveling wave tube amplifier device.

A further object is to provide a generally superior broadband traveling wave tube amplifier device.

A further object is to provide a traveling wave tube device for use in place in a phased array antenna assembly as a superior amplifier.

A further object is to provide a traveling wave tu be for use in an antenna assembly as combination amplifier and modulator, or as amplifier and antenna element.

A traveling wave tube amplifier device according to this invention includes a pair of dielectric disks and a ring member sealed to the perimeters of both disks. The disks are spaced apart a fraction of an inch. A cy` lindrical electron gun is supported between the disks and with its axis normal to the flat parallel faces of the respective disks. The ring member serves as electron beam collector. Slow-wave propagation circuit means including at least one flat equiangular spiral arm conductor is printed on the inner surface of at least one of the disks between the electron gun and the collector. Coaxial connections are provided to the inner and outer ends of the spiral conductor. If beam focusing is needed in the interaction region, it is provided in the form of periodic permanent magnet (PPM) means against the outer sides of the disks or by use of the Einzel lens effect, i.e. electrostatic focusing using the spiral conductors as lens means.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a plan view of the device shown in FIG. 2;

FIG. 2 is a section of a short circular traveling wave tube according to this invention taken along line 2-2 of FIG. l;

FIG. 3 is a section taken on line 3-3 of FIG. 2 showing a flat equiangular spiral conductor slow wave cir cuit and anode; and

FIG. 4 shows an interlaced double spiral for use in this invention.

The embodiment shown in FIGS. 1 and 2 is a short cylindrical traveling wave tube device 8 that includes a pair of parallel dielectric disk plates l0, 1l of alumina ceramic or other suitable dielectric material, spaced apart as little as inch in a low power tube, and a conductor ring l2 joined to both plates and together confining a short cylindrical vacuum chamber 13. A cylindrical electron gun 14 is supported between the plates for propelling a high perveance sheet beam radially, over fully 360 around the electron gun to the collector ring. A microperveance of 50 is reasonable because this arrangement lends itself to high-current lowvoltage operation. A flat equiangular spiral arm conductor slow-wave circuit l5, as shown in FIG. 3, is printed on the plate to provide a slow radiallypropagated wave with a substantially circular wavefront when a high frequency signal is coax-coupled into the inner end of the spiral and is coax-coupled out of the other end of the spiral. The equiangular spiral conductor increases in width from the inner end to the outer end. Under the proper operating conditions, the radial electron beam and the radially propagated traveling wave are in synchronism and interact; the signal energy conducted through the spiral extracts energy from the beam and is amplified while transiting the spiral. The plate heat-sinks the printed spiral conductor.

Electron gun 14 includes a cathode 16, a heater 17 within the cathode, an electrode 18 in the form of a cylindrical wire grid surrounding the cathode, and accelerating

electrodes

19, 20 printed on the two plates and surrounding the electrode 18. Cathode 16 is in the form of a short circular cylinder 16a of emitting material with non-emitting conductive circular flanges 16b and 16C at its ends. The flanges 16b and 16C, at the same potential as the emitting cylinder 16a, shield against cathode emission from the emitting cylinder ends directly toward the plates 10, 11. The diameter and length of the cathode is designed so that the cathode surface is large enough to supply the required beam current. Accelerating

anodes

19 and 20 are identical flat conductive rings printed on the plates 10, 11. Wire grid electrode 18 is a nonintercepting grid or it is formed of very thin wire and has comparatively large spaces to offer minimal obstruction to the electron beam. The cathode, wire grid, and accelerating anodes are coaxial with one another and with the ring 12. Direct current power supply means for and connections to cathode 16, wire grid 18, accelerating

anodes

19, 20 and ring 12 are omitted from the drawing. Similarly AC or DC power supply means for the heater 17 are omitted. The collector ring 12 is channel shaped in cross section. One advantage of the channel-shaped collector is that it captures more of the beam electrons that might strike the plates 10, 11 since after the beam passes through the substantially equipotential field in the interaction region, beam spread increases. Another advantage of the channel shape is that the collector 12 can be heat-sinked by the plates 10, 1l.

The choice of electrodes and the number of electrodes in the electron gun and their design are not part ofthis invention. The electron gun requires an accelerating anode. A control electrode, a beam shaping electrode and an additional accelerating anode may be included in the electron gun design. In the disclosed embodiment, the electrode I8 can be used either to switch off` the beam for pulse operation or to otherwise modulate the beam. Alternatively, the electrode 18 may be used as the accelerating anode. The printed

anodes

19, 20 may be replaced by a second wire grid electrode or electrode I8 may be replaced by another pair of printed annular electrodes coaxial with accelerating

anodes

19, 20. In operation, the pair of

electrodes

19, 20 are connected in common and are at the same potential. Any combination of the well known design options for electron guns may be adapted to this invention.

Though the structure shown in FIG. l has the electron gun at the center and the collector at the perimeter. this invention also contemplates a structure where the electron gun forms the perimeter and the collector is at the center, for specialized applications.

The traveling wave tube device in FIG. 1 has a slow wave circuit on only one of the plates; the opposed plate is provided with a printed flat annulus 22 of conductive material having the same radial dimensions as the slow wave circuit l5. In operation, the slow wave circuit 15 and opposed conductor annulus 22 are at the same DC potential relative to the cathode. Alternatively, if there is slow wave circuitry on each of the plates l0, l1, the slow wave circuitry on both plates are identical, and are maintained at the same DC potential during operation. By applying power DC potentials to the cathode, wire grid, accelerating anodes, the slow wave circuitry and the collector, a high perveance sheet beam is obtained within the radial span of the slow wave circuitry. For low power and/or high frequency operation, no magnetic nor electrostatic focusing is needed, provided the radius of the ring l2 is limited to a few inches. For high power and/or low frequency operation, electrostatic or magnetic focusing is required. The spacing between the plates is small in order that a large percentage of the electron beam current interact with the slow wave(s) propagated by the spiral(s); interaction occurs close to the slow wave circuit(s). This spacing is as small as possible consistent with the length of the cathode.

The slow wave propagating structure 15, shown in FIG. 3, is a printed equiangular spiral conductor of constantly increasing width between inner and outer ends. Since the amplification process increases the power of the signal as it progresses along the spiral from the inner end to the outer end, the increasing width accommodates the increasing signal power. The equation describing the outer edge ofthe spiral arm is pl P06005 and the equation describing the inner edge of the spiral arm is where e represents the base of natural logarithms, p and rb are polar coordinates and -ais a constant and is a fraction on the order of 0.01,-p0 and 0 are the initial radial and phase coordinates. The equations are related as follows The constant k is a measure of the angular width of the spiral arm. The constant -adetermines the tightness of the spiral; a lower value for the constant -aproduces a tighter spiral. If the spiral is tight, the slow wave propagated by the spiral has a circular wave front. The wave front becomes elliptical if -ais increased sufficiently; phase velocity ofthe propagated wave is directly proportional to -a. The radial electromagnetic wave propagated by the spiral circuit is very slow with a phase vclocity on the order of l/5O the velocity of light. For such slow phase velocity, accelerating voltage can be low, i.e. under 500 volts. Also slow phase velocity means small circuit wavelength. Since gain is proportional to the number of circuit wavelengths, a high gain device can be fabricated in a small or compact package.

The length L of the spiral, defined as the distance from origin to the outermost length of the spiral arm,

as measured along the center of the spiral arm can be expressed approximately as L= 1li/112+ 1)(p-Po) The characteristics of the spiral structure are specified by the three variables; spiral arm length, constant p0 and constant k.

To realize frequency-independent performance, the length of the spiral arm must be at least one wavelength at the lowest frequency; for interaction with a radial electron beam, as described, the length of the spiral arm should be many circuit wavelengths. As a practical matter, the number of circuit wavelengths would be on the order of 20.

ln FIG. 4 there is shown an alternative slow wave propagation structure The structure includes identical interlaced

spiral arms

15a and 15b having equal values of p0 at their origins but with their origins angularly separated 180 relative to the axis. Their inner ends are conductively joined by printed bridging conductor 15C. Either one of the spirals can be described by the relationships given above. The outer and inner edges p3 and p4 of the other of the two spiral arms are defined asl p3 :pueda-n) P4 Pii"(b" d) kpn The outer ends of the spiral arms terminate 180 apart. However, it is feasible to extend one of the spiral arms 180 in order to use one output connection to the other ends of both spiral arms. Comparing two slow wave circuits of the same inside and outside radii, one slow wave circuit having one spiral arm and the other slow wave circuit having two spiral arms and both slow wave circuits having the same' total number of turns, the value -afor the double spiral circuit is twice that for the single spiral circuit and the phase velocity of the wave propagated by the double spiral circuit is twice that propagated by the single spiral circuit which requires an accelerating voltage which is greater by a factor of 4.

A successful method that was used to print the conductor spirals and annular electrodes shown in FIG. 3

on dielectric substrate included the following steps.

The spiral curves p1, p2 etc. were plotted on drawing paper on a larger scale than required for the structure. A computer was used in the plotting of the spirals. Circles were drawn for the annular electrode(s). Then for each spiral arm and for the annular electrodes, the space betweennthe inner and outer edges were blacked in and the drawing was photographed to provide a transparency with the image reduced to the proper size. Each dielectric substrate had a layer of conductor deposited on one surface by one of the known prior art techniques that include sputtering, vacuum deposition, plasma deposition, etc. Then using the transparency, the conductor-overlaid dielectric substrate was photoetched. The printed circuit aspect of the invention contributes reproducibility and economy. Electrical connections, not shown, arevbrought out through holes formed in the ceramic and subsequently sealed.

An equiangular spiral arm has advantages over other kinds of spirals in this invention. ln the equiangular spiral, phase velocity is independent of radial distance;

where v, is phase velocity c is the velocity of light -adetermines the tightness of the spiral as stated previ- Phase velocity dependent on r is undesirable.

A significant property of the equiangular spiral is its log-periodicity. The ratio ofthe arm widths for any two consecutive turns is a constant. Because of the logperiodic property, very wide frequency (multi-octave) bandwidths are attainable. The spiral circuit is essentially frequency independent over a wide frequency bandwidth and exhibits frequency independent impedance characteristics.

Where the equiangular spiral as described is designed for considerable gain and is many wavelengths long, electrons that interact with a slow wave propagated by the spiral may give up enough energy to slow sufficiently to fall out of synchronism. Phase taper can be designed into the spiral by decreasing the value -awith increasing radius just enough to maintain synchronism between the interacting electron beam and the propagated slow wave.

The described embodiment of the invention may be made with one spiral arm on one of the plates 10, 11 or with two or more spiral arms on the one plate. If there are two or more spiral arms, their inner ends are angularly separated 2 1r divided by the number of spiral arms. Alternatively each of the two plates may have one spiral arm or an equal number of spiral arms. All of the spiral arms are essentially identical. As stated previously, if the spiral arms are many wavelengths long, the spirals may be terminated contiguously and joined together, using phase matching techniques well known in the art, so that only one output connection is used for the outer ends of the plurality of spiral arms. During operation a small percentage of the beam electrons may strike the two plates. Those electrons that strike the plates return through the spiral conductors and no significant electric charge accumulates on the ceramic.

A characteristic of the radial electron beam is that the current density decreases with increasing radial distance. This characteristic simplifies the focusing problem. The beam spreads as it transits outward reducing the perveance per unit distance or unit perveance. The spreading of the beam counteracts debunching of the beam as the beam gives up energy to the wave. Debunching is undesirable because it is accompanied by degradation in gain and thus a reduction in signal strength.

Bandwidth on the order of several octaves is obtainable with this invention. Outside radius of the slow wave circuit determines upper frequency cutoff', inside diameter and maximum width ol' the spiral arm deter mines the lower frequency cutoff. Circuit radii' ol'iiboiii 1.5, 3, and 4.5 inches have upper frequency cutoffs nl approximately 2000, 1000, and 50() megahertz, respectively. Bandwidths are about 1000-2000, 500-1000, and l00500 megahertz, respectively.

The spiral arms have antenna properties and will radiate through the ceramic when the circumference of the spiral circuit approaches 2A., where )t0 is free space wavelength. ln order to extend the higher frequency cutoff range and block radiation from the device,

copper shield disks

30, 32 may be secured to the outer sur` faces of the ceramic disks l0, 1l. The copper shields are needed only where radiation would be a problem. However, the device can be made to operate with both amplifier and radiation properties by designing the spiral arm(s) for enhanced radiation and by omitting one or two of the two copper shields. lri this form, the clevice can be used as combined amplifier and radiator element on an antenna array, replacing the conventional radiator element.

l lf beam focusing is required to resist beam spreading because of high beam current level or because outside diameter of the slow wave circuit is large, periodic permanent magnet (PPM) means or Einzel lens electrostatic focusing may be used. A PPM, not shown, can take the from form a washer-like flat annulus of about the same inside and outside radii as the slow wave circuit and have a set of concentric ring zones of successively opposite polarity in the axial direction. It can be formed in a ceramic matrix using well known prior art techniques. Each PPM would be supported coaxally with the spiral structure against the outer faces of the respective plates 10 and 11 or copper shields 30, 32 where such shields are used. DC and RF connections can be brought out through holes drilled in the PPM if the connections cannot be brought out conveniently from within or outside of the annular PPM. lf Einzel lens electrostatic focusing is used, it requires a plurality of spiral arms on each plate and also requires that the spiral arms not be conductively joined at their inner nor outer ends. Assuming two spiral arms on each plate, each of the two spirals of each plate are at selected upper and lower DC voltages that are equally displaced from the selected voltage of the slow wave circuit. For example, if the mean voltage of the slow wave circuit is to be 200 volts positive to the cathode, one of the spirals of each plate would be at 175 volts and the other of the spirals of each plate would be at 225 volts thereby providing the desired mean voltage of 200 volts. Matching impedance means is needed when using this focusing arrangement; matching impedance means may be printed on each of the plates 10, l1.

Based upon Traveling Wave Tubes" by Pierce published by VanNostrand 1950, pages 16 and 252-255, the theoretical gain from a traveling wave tube device is approximately G=A+BCN w 21r X frequency v, Vn/(500) X C= phase velocity of circuit wave c velocity of light V0 voltage of electron beam I., beam current P RF power The impedance of the electron beam is equal to V/1l The interaction impedance K Er2/2P The dominant mode ofinteraction that occurs in this invention is characterized by circular wave fronts. Only the dominant mode grows in amplitude since it is the only mode with circular wavefronts. Hence, there is no problem with higher harmonics in this invention.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and describedl for obvious modifications will occur to a person skilled in the art.

What is claimed is:

1. A radial interaction traveling wave tube for a predetermined band comprising:

a circular collector electrode and two flat members joined together to form a sealed space;

means supported by the flat members within the sealed space coaxial with the collector electrode for radiating a 360 degree electron beam toward the collector electrode; spiral conductor means printed on the inside surface of one of said flat members for propagating an electromagnetic wave, coaxial with the collector electrode and said means propagating microwave energy producing a circular wavefront, the radial width of the spiral along its length being greatest at the outer end and being of constantly increasing width from its inner and to its outer end, one of the edges of the spiral describing a curve according to the relationship p1=p0ea and the other of the edges of the spiral describing a curve according to VtheYY relationship p2=p0ea il where e is the base of natural logarithms and p0, p1, p2, QS and Q50 are polar coordinates, and p0 and da@ are coordinates of the inner end of the spiral; a being a constant on the order of 0.01 and manifesting a tighter or looser spiral as the magnitude selected for a is decreased or increased respectively; the length of said spiral being a plurality of circuit wavelengths at the lowest frequency signal of said predetermined band, whereby said conductor has the configuration of an equiangular spiral and propagates circular wavefronts for signal frequencies within the predetermined band.

2. A traveling wave tube as defined in claim l further including a second equiangular spiral conductor interlaced with said first-mentioned equiangular spiral conductor and wherein said spiral conductors are conductively connected at their inner ends.

3. A radial interaction traveling wave tube as defined in claim l further comprising a second spiral conductor, identical to and coaxial with thc first-recited spiral conductor, printed on the inside surface of the other member.

4. A radial interaction traveling wave tube as defined in claim 3 wherein said flat members are ceramic and further including a copper shield secured to the outer side of at least one of the flat members.

u l lr :if

Claims

1. A radial interaction traveling wave tube for a predetermined band comprising: a circular collector electrode and two flat members joined together to form a sealed space; means supported by the flat members within the sealed space coaxial with the collector electrode for radiating a 360 degree electron beam toward the collector electrode; a spiral conductor means printed on the inside surface of one of said flat members for propagating an electromagnetic wave, coaxial with the collector electrode and said means propagating microwave energy producing a circular wavefront, the radial width of the spiral along its length being greatest at the outer end and being of constantly increasing width from its inner and to its outer end, one of the edges of the spiral describing a curve according to the relationship Rho 1 Rho 0e a and the other of the edges of the spiral describing a curve according to the relationship Rho 2 Rho 0e a( ) where e is the base of natural logarithms and Rho 0, Rho 1, Rho 2, phi and 0 are polar coordinates, and Rho 0 and 0 are coordinates of the inner end of the spiral; ''''a'''' being a constant on the order of 0.01 and manifesting a tighter or looser spiral as the magnitude selected for ''''a'''' is decreased or increased respectively; the length of said spiral being a plurality of circuit wavelengths at the lowest frequency signal of said predetermined band, whereby said conductor has the configuration of an equiangular spiral and propagates circular wavefronts for signal frequencies within the predetermined band.

2. A traveling wave tube as defined in claim 1 further including a second equiangular spiral conductor interlaced with said first-mentioned equiangular spiral conductor and wherein said spiral conductors are conductively connected at their inner ends.

3. A radial interaction traveling wave tube as defined in claim 1 further comprising a second spiral conductor, identical to and coaxial with the first-Recited spiral conductor, printed on the inside surface of the other member.