US2289846A - Half-wave velocity modulation tube - Google Patents

Description

July 14, 1942- c. v. LlTToN HALF-WAVE VELOCITY MODULATION TUBE Filed July 16, 1940 U N ww NN# WN l Q si Y mN U N RN q h MN RN L nl n.

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July 14, 1942. c. v. L ITToN HALF-WAVE VELOCITY MODULATION TUBE 2 Sheets-Sheet 2 Filed July 16, 1940 Patented July 14, 1942 A 2,289,846 HALF-WAVE VELOCITY MonULA'rroN TUBE Charles v. Litton, Redwood city, cau-f., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application July 16, 1940, Serial No. 345,706

Claims.

The present invention relates to high frequency tubes and more particularly to such tubes which amplify or generate ultra high frequencies Y by velocity modulating an lelectron stream, permitting the electrons of such stream to become bunched as a result of such velocity modulation,

and thereafter extracting high frequency energy from such hunched electrons.

The present application is an improvement over my copending United States application, Serial No. 330,662, led April 20, 19.40 for "Radial form ultra high frequency tu In ultra high frequency tbes of the velocity modulation type to which this invention relates, the velocity modulation of the electron stream is usually effected by high frequency potentials across a pair of electrodes, such as grids, which are disposed transversely of the electron stream so that the latter passes rst through one and then through theother of these electrodes. A resonant circuit is usually connected between the two electrodes and it has been suggested that such resonant circuit should be constituted by a resonant cavity joined to the two velocity modulating grids and preferably surrounding these. In certain forms of tube previously suggested such resonant cavity has generally the form of two concentric cylinders joined to each other at both ends and arranged so that the electron stream to be velocity modulated passes centrally through the inner cylinder along the common axis of the two concentric cylinders. A pair of grids or other perforate electrodes are mounted relatively close together across the inner cylinder so as to be passed through successively by the electron stream, and between these two grids the inner cylinder is cutaway so as to leave a gap. The frequency of the resonant cavity ls determined by the capacity across the gap so formed (i. e. the capacity between the two portions of the inner cylinder and of the two grids attached thereto) together with the inductance formed by the annular cavity between the inner and outer cylinders.

In accordance with the present invention a velocity modulation ultra high frequency tube is constructed in generally the form of two concentric cylinders connected with each other at both ends, but the direction of the electron stream is transverse to the longitudinalaxis oi the concentric cylinders. The two grids or other velocity modulating. electrodes then lie in the surfaces of the two cylinders respectively, and

. tubes l and 2.

surfaces themselves if these are suitably` slotted or perforated.

Preferably the two concentric cylinders are relatively long in comparison with their diameter and have an over-al1 length of roughly vonehalt wave length at the operating frequency.

Thus these concentric cylinders may be considered as a coaxial transmission line one-half wave length long, short-circuited at bothends, or alternatively as two one-quarter wave coaxial transmission lines each short-circuited at one endv and disposed with their open ends together and connected by the grids. These grids are preferably annular in form and may be formed by grid bars which extend parallelto the axis of the cylinders or by rings perpendicular to such axis. Othercforms of grids, such as meshl grids, may be used, or in some cases the surfaces of the cylinders themselves may bemerely perforated.

'I'he invention may be best understood by reference to the attached drawings in which:

Fig. 1 is a perspective view partially in section of a tube embodying the invention;

Fig. 2 is a section oi' a modified form of tube also embodying the invention; and

Fig. 3 illustrates still another form which may be desirable in certain cases.

Fig. 4 illustrates a vmodiilcation which may bemadeinFlgs. 1, 20rd. Referring now more particularly to Fig. l, I, 2', 3 and 4 are fourA tubes disposed concentrically with respect to one another to define three annular cavities A, B and C, respectively. The cavity C is used as a resonant cavity for the velocity modulation portion of the device, the concentric tubes I and 2 being permanently short-circuited to each other at the right end, and being short-circuited at the left end by conductive annulus 9 which fits snugly but may be adjusted inA position along the length oi' the I'and 2 are cut away and replaced by two annular sets of grid bars 5 and E, the grid bars 5 extending between the two halves of tube I, and the grid bars 6 extending between the two halves of tube 2.

The cavity A is used as a resonant cavity to tune the velocity extraction portion of the device, the tubes 3 and 4 being short-circulted at their right end in permanent fashion and at their left end in adjustable-fashion by a sliding annular member I0. VThe central portions of the tubes 3 and 4 are cut away and replaced by may, if desired, be constituted by the cylinder' two annular sets of grid bars 1 and 8,V the grid The central portions of the tubes bars 1 extending between the two halves of tube. 3, and the grid bars 8 extending between the two halves of tube 4.

of the system although it may also be tuned if desired by one or more sliding annular members not shown.

A cathode II is disposed within the inner grid formed by grid bars 5 and receivesheater current o ver a pair of lead-in wires I2, I3 passing through the insulating plug I4. An annular target electrode I5 is preferably provided which surrounds the central portion of the device, and is sealed thereto by glass seal rings I6, II. Further` glass sealing members I8, I9, 20, 2I and 22 serve to hermetically seal the device which is exhausted to a high degree of vacuum.

In operation, heating current is applied through lead-in wires I2, I3 to heat the cathode II, and the pair of tubes I and 2 is maintained at a high positive potential to draw outward a disk-like stream of electrons from cathode II. The pair of tubes 3 and 4 is also maintained at a high positive potential which may be the same as or different from the potential of the tubes I and 2. The collector electrode I5 is maintained at a potential positive with respect to cathode I I, but preferably far less positive than the potentials -oftubes I,2,3and 4.

If the tube is to be used as an amplifier, high frequency energy to be amplified is coupled over line 35 and loop 36 to the resonant cavity C which is tuned by member 9 `to resonance with such applied energy. The two inner grids formed by bars 5 and 6 thus accelerate and decelerate the disk-like stream of electrons during the time that such electrons are between these two grids. Then, dunng the interval while these electrons pass through cavity B the diiferences in velocity result in a bunching thereof. Finally, when the bunched electrons pass between the two outer grids formed by bars 'I and 8, part of the energy thereof is extracted and serves to set the cavity A into oscillation, this cavity being tuned to resonance by member Ill so as to increase the eiliciency of such energy extraction. The amplified output is drawn from the resonating cavity A over any known coupling means, such' as loop 31, feeding to line 38. If the tube is to be used as an oscillation generator a feedback coupling of suitable phase and strength is further provided to couple cavity A back to cavity C. After passing beyond the outermost grid the electrons are decelerated to a low velocity by virtue of the comparatively low positive potential on electrode I5,and are finally caught by the target electrode I5.

Since the pairvof tubes I and 2 forms essentially a coaxial transmission line short-circuited at both ends, it will be found that the effective length of tubes I and 2 Vwhen adjusted for resonance will be very nearly one-half wave length at the operating frequency. In a similar manner when member I0 is adjusted for resonance it will be found that the 'eifective length of the coaxial line formed by tubes 3 and 4 is very nearly one-V half wave length.

Although it is preferred to make all of the grids have portions extending through from end to 75 end, and only sections are cut out and replaced by grid bars. In this case the active emitting material may be removed from the cathode on the parts opposite' the extending portions.

Fig. 2 represents another embodiment of the invention wherein the grid bars run circumferentially instead of longitudinally, being thus in the form of rings 25, 26, 2l and 28. In the arrangement of Fig. 2, also, the two tubes 2' and 3 have been replaced along part of their length by a single tube designated 3' so as to render the structure more compact. Such a construction is suitable if the same direct current potential is to be applied to the tubes I, 2 and to the tubes 3, 4.

The embodiment of Fig. 2 also illustrates the fact that although 'the resonant cavities are preferably formed of generally cylindrical surfaces `whose axis is transverse to the electron stream,

it is not necessary that these surfaces be perfect cylinders. It will be noted, for instance, that in Fig. 2 the outer tube 4 is reduced in diameter near the central portion while the inner tube I is conversely increased in diameter near the center. The cylindrical members I, 2, 3 and 4 of Figs. l and 2 are preferably circular cylinders, but may, if desired, be elliptical or square cylinders, in addition to the fact that these surfaces may deviate from true cylindrical form, for instance, by variations in diameter as above mentioned in connection with Fig. 2. 'I'he annular members 8 and I 0' of Fig. 2 are of hollow form with rounded corners to increase eiiiciency. Such adjusting members may be provided at both ends of the tube in any of the embodiments of the present invention in order to permit a wider range of tuning. Ordinarily, however, adjustable means at one end of each cylindrical cavity are sufiicieut, since a shift of the electrical center of the cavity such as will be produced by moving the sliders at one end only does not produce any detrimental effects so long as the region of the grids is near enough to the electrical center to undergo a potential variation not greatly smaller than the maximum potential variation occurring at the electrical ce'nter.

It will be noted 4that in Fig. 2 the internal seal constituted by glass rings 20, I9', 22 is nearer the center of the tubes than the corresponding seal formed by members I9, 20, 2|, 22 of Fig. l. In this arrangement the glass seals of lines 35, 38 are unnecessary. If desired, the region of sealing may be moved still closer to the center of the tubes in either Fig. 1 or Fig. 2. For example, the glass seals I9', 20, 22, I6 may be arranged between the point where the tubes I, 2, 3, 4 are broken away and the point where the grid rings commence. In such case, all these glass seal members could be united into one single glass plate sealed to the electrode I5 but not necessarily sealed to the tubes I, 2, 3, 4.

In the case of Fig. 1 the seal member might be correspondingly moved to the broken away space at the centers of the tubes I, 2, 3, 4, and would then have the form of a single glass plate through which the grid bars 5, 6, 1 and 8 would be sealed and the outer edges of which could be integral with the glass ring I6.

Fig. 3 illustrates still another modification of the invention in which a single pair of tubes I and 2 provide a single annular cavity C which is used for tuning both the velocity modulating electrodes and the velocity extracting electrodes. In Fig. 3 the surfaces of the cylinders I and 2 themselves constitute the velocity modulating and velocity accelerating electrodes, each tube having two diametrically opposite perforations for pass# ing the stream of electrons. The portions of the cylinder surfaces adjacent such opening conthe device is especially suitable as a high frea quency generator. The length of the free drift path between the velocity modulating and velocity extracting electrodes is determined by the diameter of tube I; and the length of this path should be correlated with the speed of the electron stream and the working frequency so as to give the desired phase relation between the two pairs of electrodes. Only one line 38a is'coupled by loop 31a to this single cavity device for output purposes. l

In the embodiment of Fig. 3 two annular members 9 and 9a are provided for adjusting the resonant cavity C at both ends thereof so as to permit a wider range of adjustment and for a smaller shift of the electron center during` adjustment. In order to permit such adjustment at both ends two annular seals and 20' are provided in place of the single annular seal 20 of Figs. 1 and 2.

The cathode I I is supported from lead-in wires I2, I3 extending through glass plug I4 which is sealed to a dome-shaped member 30 brazed to the lower surface of tube 2. The target electrode Il is sealed by a single seal ring I6' to a ring 2| brazed on the upper surface of tube 2.

Although the grids of Fig. 1 are shown in th form of parallel bars, the grids of Fig. 2 in the form of parallel rings, and the corresponding electrodes of Fig. 3 are shown as being constituted by the surfaces of the cylinders themselves which are simply perforated, it will be understood that any type of grid may be used interchangeably in be understood that either single-end or doubleend adjustment may be provided in any of the embodiments of the invention.

Although the specific embodiments shown and described are provided with slidable annular plugs l and I0 (or 9', I0' or 9, 9a) to vary the length of the coaxial transmission lines or resonant cavities -A and C, these plugs may be `replaced by flexible annular diaphragms or bellows-like end portions which can be flexed to vary the length of these cavities. In such case the glass seals V 20, 20 and 22 may be omitted. Fig. 4 shows, on

an enlarged scale, such-a modification of the lefthand end of Fig. 1. ,Buch modied form is preferred at wavelengths below about 30 cm. since the' dielectric loss in seal rings 20 and 22 is thereby avoided. I

Alternatively, the tuning of the cavities may be effected by joining the inner and outer cylinders of each cavity (i. e. I and 2 for cavity C or I and l for cavity A) over a tunable impedance such as a variable condenser. v

Although certain embodiments of the invention have been shown and described for, purposes of illustration, it 4will be'understood that adaptations, alterations and modifications thereof by one skilled in the art may be made without departing from the scope of the invention as deiined in the appended claims.

What is claimed is: .a

1. An ultra high frequency tube of velocity modulation type comprising a conductive generally toroidal shell having inner and outer portions of generally cylindrical form and enclosing an annular cavity, said inner and outer portions having openings,1 therethrough in alignment with each other transversely of the axis thereof, means for projecting a stream oi' electrons through said openings, adjustably displaceable short-circuiting means at one end of said toroidal shell between said inner and outer portions for adjustably varying the natural frequency of said cavity, and an insulating seal between said inner and outer portions intermediate said openings and said adjustable short-circuiting means.

2. An ultra high frequency tube in accordance with claim 1, wherein said adjustable short-circuiting means comprises flexible bellows means for varying the volume of said cavity.

3. An ultra high frequency tube of velocity modulation type comprising a conductive generally toroidal shell having inner and outer portions of generally cylindrical form and enclosing an annular cavity, said inner and outer portions having openings therethrough in alignment with each other transversely of the axis thereof, means for projecting a stream of electrons through said v openings, adjusting means at one end of said toroidal shell for varying the natural frequency of said cavity and a second conductive toroidal shell i similar to said'ilrst-mentionedshell and externally concentric therewith and provided with openings in alignment with said first-mentioned openings.

4. An ultra high frequency tube of velocity modulation type comprising substantially coextensive inner and outer concentric conductive cylindrical members having openings therethrough in alignment with each other transversely .of the any of the forms of the invention. It will also axis thereof. means for projecting a stream of electrons through said openings, short circuiting means comprising annular conductive members substantially closing the space between said inner and outer cylindrical members at both ends f of said tube, one of said short circuiting members being adjustably displaceable longitudinally.

and an insulating seal between said inner and outer portions intermediate said openings and each of said short circuiting members, whereby said-tube may be adjusted to resonate at a frequency controllable externally of said tube.

5. An ultra high frequency device of velocity modulation type comprising a rst conductive generallytoroidal shell and a second conductive cuantas, v. rrrron.

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