US3360679A

US3360679A - Electron discharge device having lossy resonant elements disposed within the electromagnetic field pattern of the slow-wave circuit

Info

Publication number: US3360679A
Application number: US346495A
Authority: US
Inventors: Rodney R Rubert
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1964-02-21
Filing date: 1964-02-21
Publication date: 1967-12-26
Anticipated expiration: 1984-12-26
Also published as: SE338622B; GB1075603A; FR1431953A; JPS5241626B1

Description

Dec. 26, 1967 R. R. RUBERT 3,360,679

ELECTRON DISCHARGE DEVICEYHAVING LOSSY RESONANT ELEMENTS DISPOSED WITHIN THE ELECTROMAGNETIC FIELD PATTERN OF THE SLOW-WAVE CIRCUIT Filed Feb. 21, 1964 2 Sheets-Sheet 1 INVENTOR.

' RODNEY R. RUBERT ATTORNEY 1366- 1967 R. R. RUBERT ELECTRON DISCHARGE DEVICE HAVING LOSSY RESONANT ELEMENTS DISPOSED WITHIN THE ELECTROMAGNETIC FIELD PATTERN OF THE SLOW-WAVE CIRCUIT 2 Sheets-Sheet 2 Filed Feb. 21, 1964 FIG. 6

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I20 KV 6O KV.

IO uSEC FIG IO S IOFSEC I lOp. SEC

INVENTOR RODNEY R. RUBERT United States Patent ELECTRON DISCHARGE DEVICE HAVING LOSSY RESONANT ELEMENTS DISPOSED WITHIN THE ELECTROMAGNETIC FIELD PATTERN OF THE SLOW-WAVE CIRCUIT Rodney R. Rubert, Santa Clara, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Feb. 21, 1964, Ser. No. 346,495 21 Claims. (Cl. 315-3.5)

ABSTRACT OF THE DISCLOSURE Undesired electromagnetic wave energy associated with certain frequencies, primarily at the bandedge regions of the operating passband characteristic of a traveling wave type of high frequency electron discharge device and also at frequencies above the operating passband characteristic such as, for example, those associated with higher order modes can be dissipated with the utilization of lossy resonant elements disposed internally of a slow wave circuit at positions removed from the interaction region so as to minimally perturb electromagnetic wave energy associated with frequencies within the operating band.

The lossy resonant elements can be disposed to primarily perturb higher order modes while simultaneously being tuned to another undesired frequency for resonant absorption of energy e.g., the bandedge of the operating passband characteristic. In all cases improved tube stability against undesired oscillations results.

This invention is concerned in general with high frequency traveling wave tubes, and more particularly, with such tubes having oscillation suppression means incorporated therein.

High frequency electron discharge devices such as the traveling wave tube are finding increased usage in applications such as frequency agile radar systems, phased array radar systems and as broadband amplifiers. However, as R.F. powers involved in such systems approach the kilowatt and megawatt level, spurious oscillations such as non-resonant drive induced, pulse induced and higher order mode types and especially oscillations of the resonant circuit type become increasingly deleterious to high efiiciency and stabilized operation. Such resonant circuit oscillations are generally characterized by modes, either higher order or fundamental going into oscillation at that portion of the respective :mode which is characterized by a low or zero group velocity. Such oscillations are the result of numerous factors such as, for example; drive induced oscillations which are caused by operating the traveling wave tube above saturation in order to take advantage of the rather flat power output characteristics of an overdriven tube wherein the overdriven tube iscaused to oscillate due to the slowing down of the electron beam caused by the heavy extraction of beam energy with a resultant synchronization of beam velocity and phase velocity of the RF. energy at the upper band edge of the fundamental mode; pulse induced oscillations which are caused by the sweep of the beam voltage vto its operating point also induce resonant circuit oscillations at the band edges of the fundamental mode. Both the driveinduced oscillations and the pulse induced oscillations generally occur at the band edges of the operating mode or that part of the mode which has a low or zero group velocity.

Other types of spurious oscillations which preclude optimum stability are oscillations such as the higher order mode oscillations. These higher order mode oscillations can be especially troublesome, mainly because the slow- 3,360,679 Ce Patented Dec. 26, 1967 wave interaction circuit is difficult to properly terminate for all mode configurations. Since it is essential that the fundamental mode of operation be well terminated, major emphasis is placed on this mode. The impedance matching terminating configuration resulting in exceptional terminating characteristics for one mode will not in general properly terminate other modes which will in general result in resonant behavior existing in these other modes since diverse field patterns exist for the various modes. The fundamental or lowest order of mode of propagation for any periodic slow wave circuit is characterized by a particular field pattern in a plane transverse to the direction of propagation which field pattern is periodic in amplitude along the axis of propagation. Higher order modes of propagation are herein defined as any modes other than the fundamental mode which are also characterized by a particular pattern in a plane transverse to the direction of propagation which field pattern is independent of position along the axis of propagation and which field patterns are each individually distinct and different from each other.

Stability or the absence of output R.F. energy in the absence of RF. input energy and dependence of frequency and amplitude of the output energy on the frequency and amplitude of the input energy becomes a pronounced factor in limiting power level and overall gain of traveling wave tubes at high power levels. Energy loss to high order modes limits efliciency for the fundamental or operating mode, thereby providing a limitation on power level and gain. The adverse effects of drive induced or pulse induced oscillations are apparent in the input and output portions of non-stabilized traveling wave tubes. Such oscillations can cause carbonization of the insulation utilized on the input portion of the circuit or such oscillations can couple to the output portion of the circuit and are particularly undesirable at this point when the tube is utilized, for example, in a frequency agile radar system since such band edge oscillations can provide undesired identfying signals in addition to the desired signal. The power levels involved in such band edge oscillations can be quite high as, for example, on the order of /3 to /2 the peak power output on the main pulse of the tube.

It is desirable for various reasons, such as flat power output characteristics over the frequency band of operation with constant drive power to operate a high powered traveling wave tube in an overdriven or oversaturated condition. However, as mentioned previously, formerly when traveling wave tubes were operated in overdriven or oversaturated conditions, oscillations resulted. Such oscillations were present both within the operating band and at the band edges. The present invention provides a novel approach to eliminating spurious oscillations such as mentioned previously in traveling wave tubes. The novel solution employed by the present invention to obtain stability in traveling wave tubes is the utilization of lossy resonant elements internal to the slow-Wave interaction circuit of the traveling wave tube. The lossy resonant elements employed by the present invention can take several forms such as, for example, resonant conductive loops having lossy material deposited thereon, resonant cavities either of the Waveguide or coaxial variety having lossy material deposited therein and positioned internally of the slow-wave interaction circuit. The aforementioned lossy resonant elements have the advantage by being positioned internal to the slow-wave interaction circuit of not'radiating energy externally of the tube while simultaneously providing the desired stabilization. The resonant lossy elements disclosed in the present invention are particularly useful in the cloverleaf type slow-wave type of circuit. An explanation of the theoretical aspects of the cloverleaf slow wave circuit can be found in the Proceedingsof the I.R.E., August 1957, pages 1112 to 1118 and o in the I.R.E. Transactions on Military Electronics, April 1961, pages 39 to 45, as Well as in other publications.

It. is therefore, the object of the present invention to provide novel stability techniques for traveling wave tubes.

One feature of the present invention is the provision of a traveling wave tube having novel oscillation suppression means therein.

One feature of the present invention is the provision of a traveling wave tube having at least one lossy resonant element disposed therein for the purpose of suppressing oscillations in said traveling wave tube at at least one frequency.

Another feature of the present invention is the utilization in a traveling wave tube of conductive resonant elements having lossy material deposited thereon wherein said lossy resonant elements are positioned such as to suppress undesired oscillations in said traveling wave tube.

Anotherfeature of the present invention is the provision of a traveling wave tube having a plurality of lossy resonant elements disposed therein internally of the slowwave interaction circuit said plurality of lossy resonant elements being adapted and arranged such as to suppress oscillations in that portion of the operating mode which is characterized by a low or zero group velocity.

Another feature of the present invention is the provision of a traveling wave tube having a plurality of lossy resonant elements disposed therein internally of the slowwave interaction circuit, said plurality of lossy resonant elements being adapted and arranged to suppress higher modes of propagation in said traveling wave tube device.

Another feature of the present invention is the provision of a traveling wave tube having a plurality of lossy resonant elements disposed therein internally of the slow-wave interaction circuit, said plurality of lossy resonant elements being adapted and arranged to suppress oscillations in both higher order modes of propagation and in that portion of the operating mode which is characterized by a low or zero group velocity.

Another feature of the present invention is the provision of a traveling wave tube having a plurality of lossy resonant elements disposed therein internally of the slow-wave interaction circuit, said lossy resonant elements being positioned in said traveling wave tube at portions thereof which are characterized by having low electromagnetic field intensity of'the operating mode.

Another feature of the present invention is the particularization in the aforementioned features of the lossy resonant element as being a conductive loop having lossy material deposited thereon.

Another feature of the present invention is the particularization of the lossy resonant element in any of the-aforementioned features as being a cavity resonator having lossy material deposited thereon.

Another feature of the present invention is the particularization of the lossy resonant element mentioned in any of the aforementioned features as being a coaxial cavity resonator having lossy material deposited thereon.

Other features and advantages of the present invention will become more apparent upon'a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. l is a fragmentary longitudinal cross-sectional view, partly in elevation, of a high power traveling wave tube incorporating certain of the'novel features of the present invention;

FIG. 2 is an enlarged cross-sectional view of the traveling wave tube depicted in FIG. 1 taken along the lines 22 in the direction of the arrows;

FIG. 3 is a fragmentary cross-sectionalview depicting an alternative embodiment of the present invention;

FIG. 4 is a cross-sectional view of the resonant element depicted in the alternative'embodiment of FIG. 3 taken-along the lines 4-4 in the direction of the arrows;

FIG. 5 is a fragmentary cross'sectional view of another alternative embodiment of the present invention showing a coaxial resonator type of lossy circuit element.

FIG. 6 is a cross-sectional view taken along the lines 6-6 in the direction of the arrows of the alternative embodiment depicted in FIG. 5;

FIG. 7 is an illustrative w-fi diagram of the cloverleaf slow-Wave circuit depicted in FIGS. 1, 2, 6;

FIG. 8 is an illustrative graphical portrayal of a voltage pulse and the pulse induced oscillations encountered in non-stabilized traveling wave tubes as the pulse sweeps through its operating range;

FIG. 9 is an illustrative graphical portrayal of an R.F. output pulse in overdriven conditions showing the effects of drive induced oscillations both in the passband and at the band edges without the utilization of the novel oscillation suppression techniques of the present invention.

FIG. 10 is an illustrative graphical portrayal of power v. time of an RF. output pulse with the utilization of the oscillation suppression techniques of the present invention.

Referring now to the drawings, there is shown in FIG. 1 a traveling wave tube 12 of the aforementioned cloverleaf slow wave circuit design 13 having an electron gun portion 14 disposed at the one end thereof, together with accelerating anode portion 15 and a collector structure 16 disposed at the downstream end. R.F. input coupler 17 and cooling means 18 of conventional design are shown in elevation in FIG. 1. Since the particular details of the mechanical features of the tube do not form part of the present invention and can be found elsewhere a detailed description will not be given. For further information on the particular details of a traveling wave tube of the type depicted in FIG. 1, see U.S. patent application Ser. No. 56,415 filed Sept. 16, 1960, by John A. Ruetz et al., assigned to the same assignee as the present invention. Since the general operation of traveling wave tubes is well known, a detailed explanation thereof will not be presented herein. Suffice it to say that the slow-wave interaction circuit structure 13 supports a high frequency R.F. field that interacts with the electron beam produced by the gun portion 14 of the tube such that useful interaction results therefrom.

In brief, the particular slow wave circuit depicted in FIGS. 1-6 comprises a plurality of circular periodic sections of cloverleaf configurations, one of which is shown in detail in FIG. 2 positioned in hollow cylindrical shells 19. The cloverleaf sections each include two metallic end walls 20, common to adjacent sections, each end wall having an annular beam aperture 21 axially positioned therein, which also serves as a capacitive coupling opening between sections. A sinuous orfour-element cloverleaf shaped metallic side wall 22 is brazed between the two end walls 20 of each section. The common walls 20 separating the cavity section are provided with a plurality of radially disposed conductive coupling slots 23 spaced apart every 45 relative to each other such that every other section is in alignment. The particular type of slow-wave section utilized in this traveling wave tube amplifier is described in U.S. patent application, Serial No. 7,481, entitled Conductive Coupling Means and Methods for High Frequency Apparatus, filed February 8, 1960, as a continuation of Serial No. 536,597, filed September 26, 1955 by Marvin Chodorow. The coupling between the slow-wave sections is termed negative mutual inductive coupling, which gives the slow-wave structure a forward wave fundamental mode, and is therefore a higher impedance structure than certain other types of slow-wave structures. High impedance permits the attainment of a high elficiency for this traveling wave tube amplifier.

As can be seen from examination of FIG. 2, the sinuous side walls 22 of each cloverleaf section form a plurality, four in number for the tube shown in FIG. 1, of spaced hollowed-out chambers 25 for each main cloverleaf section. The hollowed-out chambers 25 are 90 spaced rotated with respect to each other. The H-fields of the fundamental operating mode depicted by the dashed lines 26, as can be best seen in FIG. 2, generally follow the curvature of the sinuous side walls 22. It is readily apparent upon examination of the I-I-fields represented in FIG. 2 for the TM mode mode that the intensity thereof is minimal or low for the fundamental mode at the peripheral wall portion 32 of each chamber 25. Whereas higher order modes can generally be Said to have high or maximized H as well as E fields in the vicinity of the leaves or chambers 25 such as shown by the illustrative higher order modes represented by the dot-dash lines 26'. Lossy resonant elements 24 are positioned in each cloverleaf section in chambers 25 as shown in FIGS. 1 and 2. The lossy elements 24 as shown in FIGS. 1 and 2 take the form of U-shaped loops and can be made of any highly conductive material, such as copper, for example.

As mentioned in the introductory remarks in the specification, the lossy resonant elements 24 are utilized to eliminate spurious oscillations in a traveling wave tube such as depicted in FIG. 1. The mechanism by which spurious oscillations are eliminated will be described in more detail hereinafter.

Directing your attention to FIG. 7, there is depicted therein an w-,6 diagram in which the fundamental mode of operation characteristic A is illustrative of the pass band for the cloverleaf circuit depicted in FIG. 1. As can be seen when 1r phase shift between sections occurs the fundamental mode A has a low or zero group velocity and is therefore susceptible to resonant circuit oscillations for this particular mode. These oscillations at the band edge or low group velocity portion of the fundamental mode A are induced when the beam voltage represented by characteristic B sweeps through or is synchronized with the band edges. Such conditions of synchronization between the fundamental mode phase velocity and the beam velocity can occur for a number of reasons. Characteristic B may also be said to be representative of the phase velocity for the fundamental mode at the band edge.

Thus, it is apparent that synchronization between the beam voltage and phase velocity occurs at the band edge of the fundamental mode A when the beam voltage is either sweeping through its transient range to its operating velocity, as for example, characten'stic C, under pulse conditions or when overdriven R.F. energy slows the beam velocity through extraction of beam energy to a point where synchronization between the phase velocity of the RF. energy of the fundamental mode at the band edges and the beam velocity occurs. It is to be noted upon examination of the diagram of FIG. 7 that the band edge is a relatively small portion of the passband of the cloverleaf structure. Thus, if one could suppress propogation in that part of the fundamental mode which is encompassed by the frequency range denoted by, for example, the portion of the passband delineated by D in FIG. 7 one would effectively eliminate the possibility of oscillations occurring over this region.

The present invention provides a novel approach to this problem in the following manner. Since the frequency range wherein band edge oscillations can be induced is relatively small as can be seen upon examination of FIG. 7, it is conceivable that selective loading can be accomplished to load down this range of frequencies. Utilization of an internally'disposed non-radiating resonant lossy element to accomplish the purpose is taught by the present invention. Directing your attention to FIG. 3 there is depicted an alternative embodiment of the present invention. In this embodiment a resonant cavity 27 is formed within the chamber 25 defined by the sinuous side walls 22 of the cloverleaf. Positioning of the cavity 27 is such that higher order modes are maximally effected by physical perturbation thereof While the fundamental mode is minimally effected through physical perturbation. A slot 28 is provided at the central portion of the cavity 27, the cavity 27 is loaded with lossy material, such as, for example, Kanthal alloy A comprising 5% aluminum, 22% chromium, 0.5% cobalt, the balance iron. The lossy coating can be flame sprayed over the interior surfaces of the cavity 27 to a depth of approximately 0.005".

FIG. 5 depicts another alternative embodiment employing the lossy resonant element techniques as broadly disclosed by the present invention. The embodiment of FIG. 5 utilizes a coaxial resonator 29 disposed in the chamber 25 which is defined by the same side walls 22 of the cloverleaf. A lossy material can be deposited on the interior portions of the coaxial resonator in the same fashion as described with regard to FIGS. 3 and 4. Direct your attention once again to FIGS. 1 and 2, the lossy resonant U-shaped loop elements 24 are preferably 0.050" in diameter copper wires sprayed with the aforementioned Kanthal alloy A and bent into hairpin-like or U shapes brazed into the cloverleaf section 25 as shown. In a preferred embodiment the lossy mode suppressors 24 are disposed in substantial longitudinal alignment taken in the direction of the longitudinal axis of the tube. In addition, the planes of the loops 24 are parallel to the longitudinal axis of the tube. In a preferred embodiment the radial extent of each loop is varied in each of the chambers 25 to thereby tune the resonant frequency of the loop to slightly different frequencies in successive chambers 25. The lossy resonant loops 24 have their frequencies tuned to overlap the frequency range where band edge oscillations are expected such as that region defined by d-d in FIG. 7.

In a typical S band tube of the present invention the upper edge of the pass band of the tube was at 2900 megacycles and the band edge oscillations were observed to occur at 302.0 megacycles without the provision of the mode suppressors 24. The mode suppressors 24 were tuned to blank the frequency range of the band edge oscillations and in particular the radial extent of the loops in adjacent sections were 1%", 1%", 1 /2", 1 /3" and 1%" and the band edge oscillations were found to be completely suppressed for a tube such as depicted in FIG. 1.

Although a preferred embodiment utilizes a plurality of lossy resonant loops disposed in substantial longitudinal alignment taken in the direction of the logitudial axis of the tube, it is to be understood that the present invention is not restricted to this particular embodiment or orientation. For example, the lossy resonant elements, such as the U-shaped loops 24 depicted in FIG. 2, may be advantageously utilized in each of the four chambers 25 defined by the sinuous side wall portions 22 of the cloverleaf in each section of the cloverleaf. Furthermore, the plane of the loops, although preferably disposed parallel to the longitudinal axis of the tube, can be varied therefrom without departing from the scope of the present invention. The effect of such a displacement of the plane of the lossy resonant elements is to reduce the coupling between the RF. energy and the loops. Maximum cou pling to a given electromagnetic field configuration occurs when the plane of the loop is parallel to the E- fields and perpendicular to the H-fields.

Experimental evidence has shown that the radial extent of the loops within the cavities primarily determines the resonant frequency of the loop. A simple technique for determining the frequency at which the loop is resonating is as follows: A cold test cavity may be utilized with a signal generator transmitting R.F. energy at the particu lar frequency of interest into the cavity. A standard crystal detector may be disposed at the output portion of the cavity. The output signal from the detector may be applied to an oscilloscope and observed thereon. At a minimum of transmission, at the undesired frequency, it is readily apparent that energy at this frequency is being dissipated within the cavity and not propagated therethrough. Therefore, loops having varying radial extents, element or loop diameters and shapes in a particular chamber or chambers of a cloverleaf section or sections can be employed to pre-determine the particular frequency range which is to be suppressed. For example, assume the desired frequency to besuppressed is at approximately 3000 megacycles. A loop having a particuler diameter and radial extent and configuration may be intubeoper-ation. This technique is obviously extendible to blanket any desired range of undesired frequencies while minimizing perturbation of the operating mode.

If very heavy loading of a particular frequency is desired,then obviously one could position aloop in each of the chambers 25 defined by the side Wall portions 22 of the cloverleaf wherein each of the loops would be of identical physical shape and thus resonant at the same frequency. Alternatively, if a broad spectrum is desired to be blanketed such as frequencies above the passband of the tube, then quite obviously different sized loops reso-v nant at different frequencies can advantageously be positioned in each of the chambers 25 defined by the side wall portions 22 along the entire longitudinal extent of the slow wave circuit. If both the band edge portion of the fundamental mode and higher order modes are desired to be. suppressed, then obviously utilizing the techniques of the present invention, one would advantageously employ resonant loops in such a fashion that the entire spectrum including the upper edge of the fundamental mode and undesired frequencies thereabove would be blanketed. In the preferred embodiment of the present invention, however, the lossy resonant loops are tuned to blanket the band edge of the operating mode and in addition since the plane of the loops is parallel to the E-fields and perpendicular tothe H-fields of higher order modes such as those represented by the, dot-dash lines 26' in FIG. 2 as well as being located at points of maximum field intensity of these modes, physical perturbation of these modes results in the destruction of the resonant circuit and periodic properties thereof for a given mode which precludes oscillation in these modes. It is to be understood that higher order modes can be dissipated by positioning lossy resonant elements in the cavities which are tuned to the frequency of oscillationof these modes and destroying these modes by dissipation techniques such as used to suppress ocillations at the band edge of the fundamental mode as 1 well as by physically perturbing the electromagnetic fields of these modes by positioning the conductive loops, cavities, etc., of the preferred embodiment in the vicinityof maximum electromagnetic field intensity of higher order modes and destroying them through physical perturbation of the fields in a given portion or portions of the higher order mode while simultaneously suppressing the band edgeportion of the fundamental mode through dissipation by'means of energy transfer to the lossy resonant element.

As can be seen upon examinationof FIG. 2, coupling between the R.F.; energy in each cloverleaf section and the resoant loop is primarily inductive. The fundamental mode R.F. energy magnetic field portion thereof as represented by 26 is minimal at the peripheral portion 32 of the cloverleaf side walls .22. Therefore, energy extraction from the fundamental mode is minimal and the operating characteristics of the tube with regard to the fundamental mode are not seriously adversely effected over the. operating portion. However, certain higher order modes such as depicted in FIG. 2 have maximum H-fields in the vicinity of the loop 24 and will therefore, tend to couple very strongly thereto if the lossy resonant element or elements are designed to resonant at the frequency of oscillation of the higher order modes and thus be destroyed through dissipation or to be destroyed through physical perturbation of the mode pattern by the presence of the conductive element even though dissipat-ion is minimal due' to coupling thereto under resonant conditions as mentioned above.

In FIG. 3, as mentioned previously, a resonantcavity 27 is disposed in the chambers 25. The particular dimensions and resonant frequency of the cavity 27 defined by the peripheral wall portion 32 of the side wall portions of cloverleaf 22 and a metallic member such as copper septum 29 can be determined through utilization of a signal generator and detector system as previously explained. The particular frequencies desired to be suppressed can be modified as chosen with regard to the particular problems presented by the chosen mode of operation. A greater perturbation of the fundamental mode takes place when the cavity configuration depicted in FIG. 3 is employed due to increased physical perturbation of the E-fields and the H-fields of the fundamental mode. However, the operating portion of the fundamental mode will not couple into the resonant chamber 27 since chamber 27 is effectively cut off for the fundamental mode. The slot 28 through which energy couples into the resonant chamber 27 is advantageously positioned at the center portion of the septum 29 although obviously it could be varied at will. The slot dimensions 28 Will determine the cut-off frequency of the energy propagated therethrough. A particular example for slot dimensions would be to so dimension the slot so that all frequencies below the upper band edge of the fundamental mode are precluded from propagating therein. The lossy material utilized within the cavity is preferably Kanthal A or any other equivalent lossy material.

FIG. 5 utilizes a coaxial resonator 33 positioned in chamber 25 as defined by side Wall portion 22 of the cloverleaf section. The radial extent of the coaxial resonator 33 is preferably AA at the frequency at which it is designated to suppress. The orientation number and radial extent of the coaxial resonator-s depicted in FIG. 5 can be varied at will in order to blanket a particular frequency spectrum or suppress certain undesired frequencies. The lossy material utilized within the coaxial cavity resonator is preferably Kanthal A or any other equivalent lossy material.

FIG. 8 is an illustrative example of pulse-induced oscillations caused by the beam voltage sweeping through an illustrative operating range including the transient portions thereof. Characteristic F is representative of a typical beam voltage pulse. Characteristics G and H are typical examples of pulse-induced, rabbit-ear, oscillations appearing when a beam pulse is introduced without employing the selective loading techniques of the present invention. It is apparent that since it is, practically speaking, impossible to obtain a zero rise time that the voltage of the beam pulse as it sweeps up will synchronize at some point with the band edge of the fundamental mode and at this point or points in the frequency spectrum a spurious pulse is produced. These rabbit-ear or pulse induced oscillations can occur both at the beginning and end portions of a voltage pulse such as shown in FIG. 8. With utilization of the lossy resonant loading techniques of the present invention such that the band edge of the fundamental mode is blanketed the rabbit-ear or pulse induced oscillations represented by G and H in FIG. 8 are completely suppressed and cannot be observed.

FIG. 9 is illustrative of a typical peak power out (P versus time (t) R.F. output characteristic for a traveling Wave tube such as depicted in FIG. 1..Characteristic z is representative of the RF. power output observed for a 2700 to a 2900megacycle passband over a 6.6 to 8 microseconds duty cycle period. Characteristic i is an idealized condition and is practically speaking unobtainable with conventional cloverleaf circuits even with ideal variable drive energy. Characteristic j is illustrative of a typical R.F. output spectrum when, for example 4 db overdriven conditions exist. Characteristics h, h and h" are representative of spurious oscillations occurring both at the band edges and within the passband itself at around 3020 me. without the resonant lossy circuit loading of the present invention. The h, h" oscillations are representative of band edge overdriven or pulse induced oscillations while h within the passband is representative of oscillations occurring at band edge frequency during overdriven conditions only.

FIG. 10 depicts peak RF. power output (P vs. time (t) again utilizing a 6 to 8 microsecond pulse duty cycle when the resonant lossy loading of the present invention is employed to blanket the band edge of the fundamental mode of operations. It is readily apparent that a flat output characteristic k at 4 db overdriven conditions is obtained without the presence of spuroius oscillations both within the fundamental operating mode and the band edges thereof. The employment of the lossy resonant loading techniques of the present invention has effectively completely eliminated drive induced oscillations with the resultant beneficiial advantage that the flat saturation characteristics of the cloverleaf slow wave circuit can be utilized to maintain high output power over such as, for example, 8 to 9% bandwidths, and better, with a constant drive power. The variation of drive required to maintain the desired stable power output in previous cloverleaf circuits is no longer necessary with the utilization of the lossy resonant circuit structures of the present invention since the tube can be driven past saturation to advantageously utilize the fiat saturation characteristics thereof.

Although the utilization of lossy resonant circuits has been described with respect to the cloverleaf circuit in FIGS. 1-6 it is readily apparent that the techniques of the present invention are applicable to other coupled cavity types of slow wave circuits. For example, a long slot coupled circuit such as described in Stanford University Microwave Laboratory, W. W. Hansen Laboratory of Physics, 1st and 2nd'Annual Report for Period July 1958 to June 1960, entitled, Development of High Power Broadband Tubes and Related Studies, under Air Force Contract AF 301 (602)-1844, published January 1961, pages 93-124 can advantageously benefit from the use of the present invention. Another slow-wave circuit called by terminology, the centipede circuit discussed and described in a paper by M. Chodorow, A. F. Pearce, and D. K. Winslow, entitled, The Centipede High Power Traveling Wave Tube, ML Report No. 695, Microwave Laboratory, Stanford University, May 1960, may advantageously benefit from the use of the lossy resonant circuit techniques of the present invention. Other coupled cavity slow-wave circuits may similarly benefit from the present invention as mentioned previously. In each case the particular type of lossy resonant element utilized is a matter of choice as well as the frequency range or ranges which are desired to be blanketed in order to suppress undesired oscillations. Regardless of the type of slow wave circuit employed, the present invention is easily applied thereto by utilizing a frequency generator, oscilloscope and detector arrangement, as described previously in conjunction with conventional mode configuration identification techniques. See also my copending application Ser. No. 334,496 filed Dec. 30, 1963, together with Robert L. Perry in which the lossy resonant loops of the present invention are utilized in the cloverleaf section of the [novel hybrid tube described therein.

Since many changes would be made in the above construction and many apparently widely different embodiments of this invention could be made without departing 10 from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense;

What is claimed is:

1 A high frequency electron discharge device including a traveling wave interaction region and including: means for forming a stream of electrons, slow-wave interaction circuit means disposed along said stream for electromagnetic interaction with said stream of electrons, and means disposed at the downstream end of said interaction circuit means for collecting said electron beam, said slow wave interaction circuit means being disposed along said stream path for providing cumulative interaction between the stream of electrons and wave of RF. energy moving on said slow wave interaction circuit, said slow wave interaction circuit means being of the coupled cavity type having at least one lossy resonant element disposed therein internally of at least one of the cavities of said coupled cavity slow-wave interaction circuit means, said at least one lossy resonant element being radially displaced from said stream of electrons and asymmetrical with respect to said stream of electrons, said lossy resonant element adapted and arranged to dissipate R.F. energy present on said slow-wave interaction circuit at at least one frequency, said at least one internally disposed lossy resonant element being physically disposed within the electromagnetic field pattern of the operating mode of the device within said at least one cavity.

2. A device as defined in claim 1 wherein said lossy resonant element is a conductive wire loop having a lossy coating deposited thereon.

3. The device as defined in claim 1 wherein said lossy resonant element is a resonant cavity having lossy material disposed therein.

4. A device as defined in claim 1 wherein said lossy resonant element is a coaxial cavity having a lossy coating deposited therein.

5. The device as defined in claim 1 wherein said coupled cavity slow wave interaction circuit means comprises a plurality of coupled cloverleaf type slow wave circuit sections and wherein at least two of said plurality of coupled cloverleaf slow wave circuit sections have lossy resonant elements internally disposed therein.

6. The device as defined in claim 5 wherein said lossy resonant elements are disposed in each of said cloverleaf sections at points of low electromagnetic field intensity of the fundamental mode of propagation.

7. The device as defined in claim 6 wherein said lossy resonant elements are aligned in the same axial plane extending along the longitudinal axis of the slow wave circuit.

8. The device as defined in claim 1 wherein said lossy resonant element is disposed within a cavity of said coupled cavity slow-wave interaction circuit such as to suppress oscillation in primarily by physical perturbation of the electromagnetic fields of said higher order mode.

9. The device as defined in claim 1 wherein said lossy resonant element is resonant at a frequency in the upper bandedge region of the operating mode such as to suppress device oscillations at the upper band edge of the operating mode of the device.

10. The device as defined in claim 1 wherein said slow wave circuit is a cloverleaf type slow wave circuit and wherein a plurality of resonant lossy elements are disposed along the longitudinal axis of said slow wave circuit, said lossy elements being conductive loops having lossy coatings deposited thereon and wherein said loops vary in radial extent taken along the longitudinal axis of said slow wave circuit.

11. A high frequency electron discharge device having means for forming and projecting an electron beam along a predetermined electron beam axis disposed at one end of said beam axis, slow-wave interaction circuit means disposed along said beam axis and electron beam collecting means disposed at the downstream end of said elecas to suppress device oscillations at certain frequencies both within and without the passband of said device, said internally disposed lossy resonant elements being physically disposed within the electromagnetic field pattern of the Operating mode of the device within said cavities.

12. The device as defined in claim 11 wherein said lossy resonant elements are positioned in each of said coupled resonant cavities in a region of minimal field strength for electromagnetic wave energy in the operating mode of the device.

13. The device as defined in claim 12 wherein said lossy resonant elements are resonant within a frequency band to dissipate electromagnetic energy at frequencies in the vicinity of the band edge of the operating mode of said device.

14. The device as defined in claim 13 wherein said coupled cavities include lossy resonant elements tuned to suppress higher order mode resonant circuit oscillations of said device occurring at frequencies above the passband of the operating mode.

15. The device as defined in claim 11 wherein said lossy resonant elements are conductive loops having lossy material deposited thereon.

16. The device as defined in claim 11 wherein said lossy resonant elements are resonant cavities having lossy material deposited therein.

17. The device as defined in claim 11 wherein said lossy resonant elements are coaxial cavities having lossy material deposited therein.

18. A high frequency electron discharge device of the traveling wave tube type having a vacuum envelope and having a coupled cavity type of slow wave circuit disposed therein, said coupled cavity type of slow wave circuit having a plurality of lossy resonant elements disposed therein, and protruding from the cavities side walls into the. interior region of the volume defined by the cavities of said coupled cavity slow wave circuit, said lossy resonant elements being adapted and arranged to absorb electromagnetic wave energy at certain frequencies without radiating electromagnetic energy external to the slow wave circuit said lossy resonant elements being physically disposed within the electromagnetic field pattern of the operating mode of the device.

19. The device as defined in claim 18 wherein said lossy resonant elements are adapted and arranged to suppress oscillations of said device at the band edge of the operating mode of said device primarily through dissipation and wherein said lossy resonant elements are also adapted and arranged to suppress oscillations of said device in a higher order mode primarily through physical perturbation of said higher order mode.

20. A high frequency electron discharge device comprising an electron gun, slow wave interaction circuit and collector means physically attached in an operative relationship such that high frequency electromagnetic energy propagated along said slow wave interaction circuit will cumulatively interact with an electron beam emanating from said electron gun and directed along an elongated control, said device including conductive loops positioned within said slow wave circuit, said conductive loops being radially removed from said central beam axis and individually asymmetrical with respect to said central beam axis, said conductive loops being physically disposed within the electromagnetic field pattern of the operating mode of the device within said slow wave circuit.

21. The device as defined in claim 20 wherein said conductive loops are adapted and arranged to dissipate electromagnetic energy at certain frequencies of order to prevent oscillations in said device from occurring at said frequencies.

References Cited UNITED STATES PATENTS 2,785,381 3/1957 Brown 333-98 2,841,738 7/1958 Pierce 315-35 2,952,795 9/1960 Craig et al 315-35 2,970,242 1/1961 Jepsen 315-539 3,181,024 4/1965 Sensiper 315-35 3,221,204 1 1/1965 Hent et al. 315-35 3,221,205 11/1965 Sensiper 315-35 HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

S. CHATMON, JR., Assistant Examiner.

Claims

1. A HIGH FREQUENCY ELECTRON DISCHARGE DEVICE INCLUDING A TRAVELLING WAVE INTERACTION REGION AND INCLUDING: MEANS FOR FORMING A STREAM OF ELECTRONS, SLOW-WAVE INTERACTION CIRCUIT MEANS DISPOSED ALONG SAID STREAM FOR ELECTROMAGNETIC INTERACTION WITH SAID STREAM OF ELECTRONS, AND MEANS DISPOSED AT THE DOWNSTREAM END OF SAID INTERACTION CIRCUIT MEANS FOR COLLECTING SAID ELECTRON BEAM, SAID SLOW WAVE INTERACTION CIRCUIT MEANS BEING DISPOSED ALONG SAID STREAM PATH FOR PROVIDING CUMULATIVE INTERACTION BETWEEN THE STREAM OF ELECTRONS AND WAVE OF R.F. ENERGY MOVING ON SAID SLOW WAVE INTERACTION CIRCUIT, SAID SLOW WAVE INTERACTION CIRCUIT MEANS BEING OF THE COUPLED CAVITY TYPE HAVING AT LEAST ONE LOSSY RESONANT ELEMENT DISPOSED THEREIN INTERNALLY OF AT LEAST ONE OF THE CAVITIES OF SAID COUPLED CAVITY SLOW-WAVE INTERACTION CIRCUIT MEANS, SAID AT LEAST ONE LOSSY RESONANT ELEMENT BEING RADIALLY DISPLACED FROM SAID STREAM OF ELECTRONS AND ASYMMETRICAL WITH RESPECT TO SAID STREAM OF ELECTRONS, SAID LOSSY RESONANT ELEMENT ADAPTED AND ARRANGED TO DISSIPATE R.F. ENERGY PRESENT ON SAID SLOW-WAVE INTERACTION CIRCUIT AT AT LEAST ONE FREQUENCY, SAID AT LEAST ONE INTERNALLY DISPOSED LOSSY RESONANT ELEMENT BEING PHYSICALLY DISPOSED WITHIN THE ELECTROMAGNETIC FIELD PATTERN OF THE OPERATING MODE OF THE DEVICE WITHIN SAID AT LEAST ONE CAVITY.