US2834909A

US2834909A - Traveling wave electron discharge device

Info

Publication number: US2834909A
Application number: US437461A
Authority: US
Inventors: William L Beaver
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1954-06-17
Filing date: 1954-06-17
Publication date: 1958-05-13
Anticipated expiration: 1975-05-13
Also published as: FR1137686A; GB804437A

Description

May 13, 1958 w. L. BEAVER 2,834,909

TRAVELINGWAVE ELECTRON DISCHARGE oavzca IN V EN TOR.

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TRAVELING WAVE ELECTRON DISCHARGE mzvxcr;

Filed June 17, 1954 4 Sheets-Sheet 4 IN VEN TOR.

ATTOEIVE'V United States Patent TRAVELING WAVE ELECTRON DISCHARGE DEVICE William L. Beaver, Palo Alto, Calif., assignor to Varian Associates, San Carlos, Calif., a corporation of California Application June 17, 1954, Serial No. 437,461

4 Claims. (Cl. SIS-3.6)

This invention relates generally to electron discharge devices and, more particularly, to electron tubes wherein interchange of energy takes place between an electron beam and electromagnetic waves traveling in the region traversed by the beam, i. e., a tube of the traveling wave type, and the invention is especially directed to a novel electron discharge device of this type that is especially suitable for use as an ultra-high frequency oscillator and wherein the phase velocity of the electromagnetic waves acts contra to the velocity of the electron beam. This type of tube is sometimes called a backward wave device and in accordance with the present invention, a bifilar helix, i. e., two interwound helices are employed having their turns in close proximity but insulated from each other not only so that the turns of the helices can act as a two conductor transmission line type of slow wave structure but also so that said two helices may have different D. C. potentials applied thereto for effecting electrostatic focusing of the electron beam in use.

Heretofore, in tubes of the backward wave oscillator type it has been common to employ magnetic focusing to insure the passage of the beam within the slow wave structure of the tube. However, where large variations in beam voltage are desired, as where a wide tuning range is desired, the use of magnetic focusing of the Brillovin type is objectionable because of the large perturbations in the beam resulting from the wide changes in accelerating voltage employed. This results .in reduced coupling between the beam and the circuit giving rise to rapid variations of output power with frequency. It also results in the existence of potential depressions in which positive ions will be trapped resulting in additional perturbations of the beam. These difliculties are eliminated by .using magnetic focusing of the confined flow type but at the expense of using a substantially larger magnet. With either type of magnetic focusing, the magnet size and weight may be a serious handicap.

One feature of the present invention is to provide a novel electron discharge device of the traveling wave tube type which is especially suitable for use as a backward wave oscillator, the said device employing a bifilar helix wherein D. C. voltages are applied to the two helices in opposite sense whereby electric fields are maintained between adjacent turns so that electrons of the beam are alternately accelerated and decelerated longitudinally, the radial component of the D. C. fields being more effective on the slow electrons to thereby produce a net inward acceleration serving to overcome space charge forces and maintain the beam within the con- Another feature of the present invention is to provide a novel electron discharge device of the above character that can be operated without the necessity of using a focusing magnetic field and which can be varied as to frequency over a wide range by varying the accelerating voltage while still maintaining optimum focusing conditions of the beam.

Another feature of the present invention is to provide a novel electron discharge device of the above character which is of relatively compact construction and light in weight owing to the elimination of magnetic focusing coils, the bifilar helix employed providing a high shunt impedance to the beam and hence improving the tube characteristics.

Another feature of the present invention is to provide a novel electron discharge device of the above character having impedance means associated with the slow wave structure for eliminating forward traveling waves and hence preventing the production of standing waves within the tube, the said tube employing novel means for removing energy therefrom.

Another feature of the invention is to provide a novel electron discharge device of the above character that may employ either a solid electron beam or a hollow electron beam and which may be used as an amplifier as well as an oscillator.

Other features and advantages of this invention will become apparent from the specification, taken in connection with the accompanying drawings wherein the invention is embodied in concrete form.

Referring now to the drawings,

Fig. 1 is a longitudinal sectional view of a backward wave oscillator employing the principles of the present invention,

Fig. 2 is an enlarged fragmentary view of a portion of the output end of the slow wave structure,

Fig. 3 is a sectional view taken along the line 33 of Fig. 1 looking in the direction of the arrows,

Fig. 4 is a part sectional view taken along line 4-4 of Fig. 1 looking in the direction of the arrows,

Fig. 5 is a view similar to Fig. 1 of a somewhat modified structure,

Fig. 6 is a sectional view taken along lines 66 of Fig. 5, I

Fig. 7 is a sectional view taken along the lines 7-7 of Fig. 5,

Fig. 8 is a sectional view taken along line 88 of Fig. 5,

Fig. 9 is a longitudinal sectional view of a tube employing magnetic focusing,

Fig. 10 is a longitudinal sectional view of a modified form of tube employed as an amplifier,

Fig. 11 is a longitudinal sectional view similar to Fig. 9

but of a somewhat modified structure, and

Fig. 12 is a sectional view taken along line 12-12 of Fig. 11.

Similar characters of reference are used in the above figures to designate corresponding parts.

Referring now to Figs. 1 through 4, the reference numeral 1 designates a novel electron discharge device of this invention as a whole whereinv 2 designates a cathode shown as of the focusing type employing a focusing electrode 2', which cathode is indirectly heated by a filament 3 energized from a source 4. The electrons emitted from cathode 2 are accelerated by a positive potential applied to an anode 5, shown in the form of a. cup-shaped member carried by the base 6, which positive potential is shown supplied from battery 7 through'leads 8 and 9. The focused beam leaving cathode 2 passes through a truncated opening 10 in the anode 5 and thence moves into the interior of a bifilar helix consisting of

helices

11 and 12. The

helices

11 and 12 are of equal diameters and wound with equal pitch, the windings of the helices being interleaved though insulated from' each other so that the turns of the two helical conductors are parallel as shown in Fig. 1. These helices are shown supported within a tubular insulating member or envelope 13 as of glass or ceramic, the tube 13 in turn being shown supported by insulating washers 14 carried within a tubular insulating casing 15 which also may be of glass or ceramic.

The casing 15 is shown secured in sealed relation at one end as by a tubular member 16 to the anode 5 and has'its other end closed by a collector structure 17. The interior of anode 5 and insulating casing 15 is evacuated and the electron beam from cathode 2 after passing within the

bifilar helices

11, 12 passes on to collector 17 where the same is collected. The collector 17 is shown connected by a lead 18 to the positive side of a battery or voltage source 19, the negative side of which is connected by a lead 8 to battery 7. With this arrangement the collector 17 is at a higher potential than the anode 5 to assist in drawing electronsthrough the bifilar helix structure but it is to be understood that this collector may be at a different potential or at the same potential as the anode, if .desired. The outer end of helix .11 is shown extending radially outwardly at 11' through casing 15 and is connected by a lead 20 to battery 19 at a potential point thereon lower than that of collector 17. The outer end of helix 12 also extends radially outwardly through the casing 15 as shown at 12' and is connected by lead 21 to the source 19 at a potential point lower than that of helix 11. The inner ends of the helices are shown extending outwardly at 11" and 12" as a. parallel wire transmission line of characteristic impedance approximately matching the wave impedance of the helix (see Fig. 2) and are connected to spaced

metallic rings

22 and 23 which are carried by the inner wall of casing 15 and are capacitive coupled through the wall of this casing to

flanges

24 and 25 provided at the opposite sides of a waveguide 26, whereby alternate potentials from the

leads

11 and 12 will serve to excite'a traveling wave within guide 26 in use which wave will pass downwardlyas shown by the arrow in Fig. l to a utilization circuit.

The

rings

22 and 23 are approximately a quarter wave length long in the axial direction at the velocity of the waves in the dielectric. The combination of rings and flanges then act as quarter wave chokes to confine the radio frequency energy to the interior of the waveguide. The short-circuited end of the waveguide 26 is made approximately a quarter wavelength long at the'center of the frequency range so as to provide a high impedance shunting the ring-flange assembly and having minimum effect on impedance match. The actual lengths of the rings and flanges and the short-circuited section of waveguide 26 are determined by experiment so that the impedances of the chokes and the short-circuited waveguide add in such fashion that a good match from waveguide to helix is obatined over the broadest possible band. The

height of the waveguide at the coupling flanges is made such that the impedance of the waveguide equals the wave impedance of the helix near the center of the desired frequency range. The waveguide is then gradually tapered at 26' up to standard height.

The voltage source 19 serves to apply differential volt- I ages to the

helices

11 and 12 so that there are maintained electric fields between the successive turns of the helices as especially shown by the arrows in Fig. 2, the said fields being reversed every half pitch. These fields are similar to that of two conductor transmission line wound in helical form. These fieldsact to alternately accelerate and deceleratethe electrons of the beam, the D. C. radial tude toward the left of the tube and is discharged through component of these fields being more efiective on slow electrons than on the fast electrons so that a net inward fields of the structure. In other words, the electron beam is acted upon by an electric field pattern that has both longitudinal and helically symmetric radial components. The longitudinal component causes the velocity of electrons in the axial direction to vary. When the electron velocity is slower than the average the radial component of electric field produces an inward force tending to crowd the electrons into a more condensed beam and when the electron velocity is larger than the average, the radial field produces an outward force which tends to somewhat enlarge the beam. Because of the velocity variation, however, the electrons spend more time in the region of inawrdly acting forces and hence acquire a net inward momentum from the focusing field which keeps the beam well focused along its path and serving to overcome the outward momentum of the electrons due to space charge forces.

In order to eliminate standing waves and the production of forwardly traveling waves a quantity of resistance material such as aquadag 30 is shown coated upon the outer extremity of the tube 13 adjacent the collector 17 so that any energy tending to travel towards the right in Fig. l is absorbed by this material. As the voltage signal wave travels along the structure from right to left as used in Fig. 1 the field configuration travels with it much like that of a folded line type of backward wave oscillator. The oscillation frequency is varied by adjusting the accelerating voltage such that the electron velocity causes the electrons to move through a pitch distance of the winding in one cycle less the phase delay of the wave traveling one pitch distance. This is because the electrons are moving contra to the direction of the amplified wave. The inherent radio frequency field and improved output of the tube with simplicity of construction. Energy thus built up is conveyed by leads 11" and 12" through

plates

22 and 23 to waveguide 26 and from thence to a utilization load.

Owing to elimination of magnetic coils, a tube of this invention can be made quite compact and light in weight. Also by varying the'adiustment of potential contact 27, the accelerating voltage can be varied thereby readily varying the frequency output of the tube over an exceedingly wide range without the necessity of using any mechanical tuning means whatsoever. In a typical tube, it has been found that by varying the accelerating voltagefrom 500 volts to 2000 volts a frequency shift of from 8 to 12.5 K. M. C. was.obtaiued representing more than 50% frequency shift.

The tube structure of Figs. 5 through 8 is similar to that in Fig. 1 with the exception of that instead of etnwaveguide 26. Figs. 5, 6 and 7 show a means of canploying an insulating

choke system

22, 23, 24 and 25 in a tube with'metal envelope. The operation is the same as previously described except that in this embodiment, which may also be used in the tube of Fig. 1, the waveguide 26 is terminated in a cylindrical surface beyond assspos sulating casing 15 of the tube which coil serves to focus the beam within the

bifilar helices

11, 12. In this figure the terminals 11' and 12 of these windings are shown connected by leads 33 and 33' and lead 34 to a common point on the potential source 35 so that the bifilar helix in this case does not aid in focusing the beam. Actually the terminals 11' and 12' could be connected to spaced points on the source 35 so as to obtain the advantages of both electrostatic and magnetic focusing, if desired. The inherent radio frequency advantages of the bfilar helix are useful with either electrostatic or magnetic focusing. The output of the tube shown in Fig. 9 is conveyed from a circular waveguide 36 to a concentric line 37 which line extends longitudinally within the tube focusing coil towards the right as shown in Fig. 9 and leads to a suitable utilization circuit. Instead of using a concentric line 37, a waveguide could be coupled to the guide 36, if desired. The operation of this tube is similar to those previously described, the use of aquadag or other resistance material serving to eliminate standing waves and undesired modes of operation.

In the form of invention shown in Fig. 10, a cathode 37 is employed producing an annular beam of electrons which passes within the ,

bifilar helix structure

11, 12. In this form of tube, electrostatic focusing is used and to connected through ring 23 to a choke coil 38 wrapped around an insulating sleeve 39 mounted on the insulating tube 13, the other end of the choke coil 38 connected through terminal 40/ carried by thoughmsulated from the metallic cylindrical casing 41 to a potential source 19'. Similarly terminal 11' of helix 11 is connected through a ring member 42 to a choke coil which in turn is connected to a terminal post that is in turn connected to the potential source 19 at a point differing from the point of connection of terminal post 40 40, whereby the

helices

11 and 12 are held at dilfering D. C. potential to effect the desired focusing action.

The tube shown in Fig. 10 may operate as an amphfier in that energy to be amplified is supphed through a waveguide 44 capacitive coupled to spaced rings 42 45 and 42' which are connected to terminal leads l2' and 11' of the two windings. This energy is progressively built up within the tube and is discharged in amplified condition through waveguide 45. This mode of operation is known as backward wave amplification, in which condition the beam current is kept below the starting current for backward wave oscillations which have been previously described. By increasing the beam current above the backward wave oscillation threshold and providing a matching load on waveguide 44 this tube may also be used as a backward wave oscillator. 0n the other hand, by putting the input signal into waveguide 45 the tube may operate as a traveling wave tube and deliver amplified energy out through waveguide 44. This tube could act as a regenerated amplifier by interconnecting the wave- 80

guides

44 and 45 with a suitable feedback connection.

In another form of invention shown in Figs. ll and 12 magnetic focusing is employed using a magnetic coil 47 surrounding the metallic casing 48. In this form of the invention the

bifilar helices

11 and 12 contained within the insulating tube 13 have their left-hand terminals 11" and 12 connected directly to the sides of the waveguide 49 that extends longitudinally within the evacuated casing 48 and outward therethrough for connection at 50 to a suitable utilization circuit. A sealing window 51 is provided at the end of the guide 49. The right-hand terminals of the helices, i. e 11' and 12' are shown wrapped around the exterior surface of the tube 13 with aquadag painted thereon to thereby eliminate forwardly traveling waves.

This tube also serves as a backward wave oscillator operated in the manner described in connection with Fig. 9. Although the tubes of Figs. 9 and 11 are shown as employing magnetic focusing, as previously pointed out, these tubes could be electrostatically focused if desired or employ a combination of electrostatic and magnetic focusing. This could be accomplished in Fig. 9, for example, by inserting the potentialsource 31 in lead 33'.

The novel tube of this invention is of simpler construction than prior tubes of this type and has higher impedance and hence improved output in use. The impedance along the bifilar helix is superior to that of both a single helix and to an interdigital slow wave structure. Since the R. F. fields are concentrated near the helices it is desirable to have most of the beam close to the helices as especially exemplified in the structure of Fig. 10. The novel tube of this invention is not only of reduced size and weight but maintains optimum focusing over a wide frequency range. Also ion trapping is reduced because ions can drain to the helix of lower potential.

Since many changes could be made in the above construction of the novel electron discharge device of this invention and many apparently widely different embodi- 5 ments of this invention could be made without departthe accompanying drawings shall be interpreted as illusaccomplish this the terminal 12" of the helix 12 is shown 30 native and not in a limiting sense.

What is claimed is:

1. In an oscillation device of the backward wave type, in combination, a wave amplifying device including an electromagnetic wave transmission path comprising a pair of conductors wound into the form of a bifilar helix having the individual helices thereof of the same pitch and diameter/with the conductors of the two helices interleaved though insulated from each other for carrying signal currents in phase opposition, a source of electrons, means for forming electrons from said source into a concentrated stream traveling at a suitable velocity longitudinally of and in the proximity of said helix and in a direction contra to the phase velocity of the traveling wave thereon, means applying differential direct current voltages to said conductors, an output waveguide, means coupled respectively with said conductors for conveying theout-ofiphase signal components of said individual helices for supplying a microwave output to said waveguide, and means for varying the velocity of said electron stream to thereby vary the frequency of said microwave output, the electrons of said stream moving through a pitch distance of the helices in one cycle less the phase delay of the wave traveling one pitch distance.

2. An oscillation device of the backward wave type as defined in claim 1 wherein said coupling means comprises spaced ring members of substantially a quarter wave length long coupled respectively to' said pair of conductors at the ends thereof adjacent said electron source, said ring members being capacitively coupled to opposite sides of said output waveguide.

3. A backward wave electron discharge device comprising an emitter, an accelerating electrode, a bifilar helix structure positioned in front of 'said accelerating electrode for receiving the beam leaving said emitter, said structure having the individual helices thereof of the same diameter and interleaved though insulated from each other, a collector beyond said structure for collecting the beam after leaving the latter, an output waveguide at the end of; said helix structure adjacent said emitter having coupling flanges on its opposite sides, spaced rings coupling the ends of the individual helices of said structure to the coupling flanges at opposite sides of said output waveguide, and means connected to the individual helices of said structure adjacent said collector for applying differing fixed voltages to said helices,

7- 9 whereby the signal wave traveling along said helix structure moves oppositely to said electrons.

4. An electron discharge tube as defined in claim 3 having an evacuated insulating envelope containing said bifilar helix structure and wherein said spaced ring mem- 5 bers and said waveguide coupling flanges are approximately a quarter wave length long and serve as quarter wave chokes to confine radio frequency energy to the interior of said waveguide.

References Cited in the file of this patent UNITED STATES PATENTS 2,702,370 Lerbs Feb. 15, 1955 2,707,759 Pierce May 3, 1955 2,725,499 Field Nov. 29, 1955 FOREIGN PATENTS 992,048 France June 27, 1951 668,017

Great Britain Mar. 12, 1952