US3433999A - Non-resonant stub supports for slow wave circuits - Google Patents

Description

March 18, 1969 R. J. ESPINOSA NONRESONANT STUB SUPPORTS FOR SLOW WAVE CIRCUITS GEZEDSE iv v cow 0 com- MM o a A I v ow m mm M12227 m Y w w a Y m r u\ 0 -o 2 dmwo m P... 0 oo JEN J T n T m 0 GE 2650 m A .021 48.52823 11% woo: Q 4 GEBZMDSE m8? mEwm R /k 8 o 8 w m? ,m m P LE L 0?" m y m- J- n v a J o V GE w m taoma mEwEoIw m 0E N GE n m 6 m8: mam N F 21 AT .m g i r 0 United States Patent ABSTRACT OF THE DISCLOSURE Troublesome spurious oscillations caused by prior art M4 stub supports can be eliminated in ring-andbar type slow wave circuits by dimensioning the stub supports to have a length L defined by where A is the wavelength at the center frequency of the operating band.

This application is a streamlined continuation of application Ser. No. 384,668, filed July 23, 1964, now abandoned.

This invention relates in general to high frequency electron discharge devices and more particularly to such devices incorporating helix-derived slow wave circuits of the ring-and-bar type. These high frequency electron discharge devices are particularly useful in frequency generators, broadband high power amplifiers, etc. A helixderived slow wave circuit of the ring-and-bar type such as shown in US. Patent 2,937,311, by M. Chodorow is an excellent vehicle for propagating fairly high powers of traveling wave electromagnetic energy for interaction with electron beams and subsequent amplification thereof. The thermal capacity of this particular type of circuit such as shown, for example, in US. Patent 2,853,642 by C. K. Birdsall et al. is enhanced to a great extent by the inclusion of a stub support for the slow wave circuit as shown in Birdsall et al. conventionally, the length of the support stubs is made one-quarter wavelength as determined at the center of the operating passband of the circuit. However, as higher and higher electron beam powers are used, stability of tubes utilizing such circuits has become a pronounced factor in power handling capabilities of such tubes.

Higher order circuit modes become increasingly ditficult to stabilize and may seriously affect the performance as the power increases. For example, voltage build up on the slow wave circuit due to high electric fields caused by the buildup of resonant modes and consequent energy loss to these modes limits available energy for the fundamental mode thereby providing a limitation on useful tube operating power level or resulting in destruction of the tube. A slow wave circuit and associated traveling wave tube may be limited seriously unless stabilization techniques are utilized as multi-kilowatt power levels are reached.

An analysis of a few higher order circuit modes such as the ring mode and stub mode and techniques suitable for eliminating or at least reducing the effects of such resonant circuits modes are given in US. patent application Ser. No. 306,570 by Robert J. Espinosa and John A. Ruetz now US. Patent No. 3,335,314 and assigned to the same assignee as the present invention. In that particular application the stub mode was stabilized through the utilization of a tapered stub length along the axial extent.

of the tube. This resulted in increasing the 1,, (start oscillation current) for the stub mode and resulted in greatly increased stabilization of a pulse amplifier utilizing such a tapered technique. The stub mode as can be defined as a higher order resonant circuit mode wherein the length of the support stub determines the upper cutoff frequency of the mode. By way of definition the fundamental or lowest order 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 independent of position along the axis of propagation. Higher order modes of propagation, such as the stub mode, are herein defined as propagation modes other than the fundamental mode which are also characterized by a particular field pattern in a plane transverse to the direction of propagation and which field pattern is individually distinct and different from other modes.

The present invention provides another solution to the problem of stabilizing a high power traveling wave tube incorporating a helix derived stub-supported ring-and-bar slow wave circuit. Through a detailed measurement of the high frequency passbands of stub-supported ring-andbar circuits and through analysis of the tube operating data, it has been determined that the stub length is found to be a determinative factor in the stabilizaion of a stubsupported ring-and-bar slow wave circuit. The tapered stub technique described in the aforementioned application Ser. No. 306,570 has been found to be quite adequate to stabilize a high power traveling wave tube incorporating a stub-supported slow wave circuit. However, when such a tube is driven to saturation over its operating band a reduction in power output is observed at the middle of its operating band. This indicates that the tapered technique disclosed previously is not exactly optimum as far as complete stabilization of the stub mode is concerned.

The present invention provides a solution to such depressed saturated power output problems encountered with the tapered stub-support previously mentioned by a deviation from the conventional /4 stub lengths used in all prior art stub-supported ring-and-bar circuits. This deviation which can take place either above or below the conventional quarter wavelength stub length, although preferably below for band width and cost considerations, as well as enhanced stability properties will be described in more detail hereinafter. The present invention has determined that at approximately /6 stub lengths as determined at f (center frequency of. the operating band) the stub mode can be moved up in frequency sulficiently to prevent second harmonic interaction of the signal fre quency and beam which might otherwise induce stub mode oscillations. The present invention also teaches a preferred range of stub lengths, such as the following: h to A, and A to A which will encompass the desired useful stu'b parameters for stabilization.

The present invention further teaches, in regard to stub supports having inductive and/or capacitive perturbations therebetween that deviations from a /4 as expressed in terms of free space wavelength may be rather pronounced and a plane electromagnetic wave traveling in the perturbed region at some velocity other than the velocity of the light still falls within our teachings and distinguishes over the prior art quarter wavelength teachings while providing stub modesuppression when expressed as follows:

Said stubs having length dimensions falling within the following limits:

)\/l6 L i and y h L )\/3 where L is the stub length and is the physical length of the path taken along the surface of the stub as measured from the end of the stub at the outside diameter of the ring circuit to the stub base or root and c=c/f where f is center frequency of the passband of the stub supported circuit and c is the velocity of a plane electromagnetic 3 wave in the dielectric medium or mediums between said stubs averaged over the length of the stub L.

It is therefore, a principal object of the present invention to provide a novel slow wave circuit and improved high frequency electron discharge devices.

A feature of the present invention is the provision of a helix-derived slow wave circuit having novel supporting techniques therefor.

Another feature of the present invention is the provision of a helix-derived ring-and-bar type slow wave circuit having a stub support of approximately 6 as measured at the center of the operating frequency band of the circuit.

Another feature of the present invention is the provision of a stub-supported, helix-derived slow wave circuit of the ring-and-bar type wherein the stub lengths fall within the where L is the stub length and t is determined at 1 (center frequency of the operating band of the circuit).

Another feature of the present invention is the provision of a stub-mounted helix-derived slow wave circuit of the ring-and-bar type wherein the stubs have length dimensions falling within the following limits:

where L is the stub length and is the physical length of the path taken along the surface of the stub as measured from the end of the stub at the outside diameter of the ring circuit to the stub base or root and \=c/f where f is center frequency of the passband of the stub supported circuit and c is the velocity of a plane electromagnetic wave in the dielectric medium or mediums between said stubs averaged over the length of the stub L.

Another feature of the present invention is the incorporation of any of the slow wave circuits mentioned in the aforementioned features in a high frequency electron discharge device utilizing travelling wave electromagnetic interaction therein.

These and other features and advantages of the present invention will become more apparent after a perusal of the following specification taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a longitudinal cross-sectional view, partly in elevation, of a travelling wave tube incorporating a novel slow wave circuit of the present invention;

FIG. 2 isa cross sectional view taken along the lines 22 of FIG. 1;

FIG. 3 is an illustrative w-fl diagram of the slow wave I circuits depicted in FIGS. 1 and 2;

FIG. 4 is an illustrative graphical portrayal of peak power output vs. frequency of a slow wave circuit such as depicted in FIGS. 1 and- 2 with x.,/ 4 stub lengths; and

FIG. 5 is an illustrative graphical portrayal of peak power output vs. frequency of a slow wave circuit incorporated in a traveling wave tube such as depicted in FIGS. 1 and 2 wherein stabilization techniques of the present invention are utilized.

Referring now to FIG. 1 of the drawing, there is shown a traveling wave tube 7 having an electron gun structure 8, conductive body 9, collector 10, anode portion 11, helix-derived ring-and-bar type slow wave circuit 12 defining a series of interaction gaps 13 between spaced rings 14, supporting stubs 15 and RF. input and output coupling means 16 and 17, respectively. The traveling wave tube depicted in FIG. 1 is axially aligned along a central axis of propagation 7. An electron beam 18 emanating from the electron gun portion 8 as shown by dotted lines travels along the central axis to the collector 10. Since the interaction between an electron beam and RF. energy on slow wave circuits is well known, no further explanation thereof for purposes of amplification is required. A conventional circuit sever 19 is disposed between slow wave circuit sections and the circuits are terminated by a pair of dissipative loads 20, 21 in order to absorb unwanted R.F. energy in a manner well known in the art. Preferably the conductive body 9 is provided with a pair of cooling channels 22, 23 as best seen in FIG. 2 for the purposes of increasing thermal dissipation and thus increasing power handling capabilities of the circuit. Conventional loading ridges 24, 25 such as described in US. patent application Ser. No. 295,605 by John W. Sullivan filed July 15, 1963, now US. Patent No. 3,142,777, are advantageously incorporated for purposes of broadbanding as described in more detail in the aforementioned Sullivan application. The support stubs 15 are preferably tapered per the techniques taught in the aforementioned Sullivan patent application.

In FIG. 3 an illustrative w-B diagram of the pass bands of the circuit depicted in FIG. 1 is given. V=c is a characteristic of the velocity of light while V=u is the operating beam voltage or beam velocity characteristic. The characteristic labeled fundamental mode is the operating mode of the tube utilizing a stub-supported ringand-bar circuit as shown in FIGS. 1 and 2. Dotted characteristic A shows the resultant fundamental mode characteristic when a short stub of approximately A /G is utilized. Characteristic B shows the depression of the fundamental mode when a large or longer length stub support is utilized, as for example, x Dotted characteristic A shows the resultant stub mode characteristic when a short stub of approximately /6 is utilized, showing that the stub mode is not intercepted by u Dotted characteristic B shows the resultant stub mode characteristic when a stub length of approximately 71 A is used, showing that the stub mode occurs far above 21r but is still intercepted by a therefore, there is still a possibility of stub mode interaction if some other means of oscillation suppression is not used. The characteristic labeled stub mode depicts a mode which, as evidenced by the characteristic, is very dispersive and has a very low group velocity, thus showing the resonant circuit characteristics thereof. It is evident that a M 4 stub length as determined at i will cause stub mode oscillations at second harmonic frequencies as shown by the intersection of u and the stub mode characteristic (see diagram in FIG. 3).

The present invention, when utilizing a /6 stub length, results in moving the stub mode up in frequency high enough to prevent synchronous interaction with the operating beam velocity as evidenced by the characteristic labeled stub mode of the short stub circuit. Examination of fundamental mode characteristics A and B shows the perturbation thereof or deviation from the quarter wavelength stub length fundamental mode. The fundamental characteristic for a /6 stub, since it is slightly less dispersive relative to the quarter wavelength characteristic, provides advantageous results with regard to operating tube bandwidth as well as increased stabilization of a travelling wave section incorporating the short stub technique as depicted herein.

The slow wave circuit depicted in FIGS. 1 and 2 when utilizing the following parameters, stub length equal x 6, ring exterior radius equal to .075 stub apex angle equal to 22 /2 waveguide height of approximately MI 4, distance between loading ridge and exterior ring radius of 025%, when utilized in conjunction with a tube which had a saturated gain of db across a 16% frequency bandwidth at a beam velocity of approximately 36 db and 14 amps beam current resulted in the exemplary saturated R.F. output characteristics depicted in FIGS. 4 and 5 with and without, respectively, the stub length control techniques in regard to stabilization as taught by the present invention. Examination of FIG. 4 which depicts exemplary saturated output characteristics for a A /4 stub length even with a tapered stub stabilization technique as described in Us. Ser. No. 306,570 shows a severe drop in power output at f,,. When a x /6 stub length is employed it is quite evident that the depressed power output at i is eliminated. Quite obviously, upon examination of FIG. 5, no reduction in bandwidth or where L is the stub length in a plane transverse to the Z axis and A is determined at f (center frequency of the band of frequencies to be propagated along the circuit).

It is to be noted that the short stub is preferred as it easily eliminates any possibility of correspondence between signal energy with the operating band and frequencies at which the stub mode propagates including second harmonics of any frequency within the operating band. Furthermore, n does not intercept the short stub characteristic thus assuring freedom from noise excitation of the stub mode.

The long stub length 7 easily prevents any correspondence between signal energy at frequencies within the operating band as well as second harmonics of any frequency within the operating band and frequencies at which the stub mode propagates 2) but does not assure complete stabilization since noise excitation by the beam is still possible as evidenced by the crossover between the M and the stub mode of long stub circuit (B) characteristics.

However, just the elimination of stub mode oscillations by second harmonics of frequencies within the operating band provides a tremendous boost for the tube designer when considered of and by itself.

As mentioned previously, in regard to stub supports having inductive and/or capacitive perturbations therebetween such that deviation from a /4 as expressed in terms of free space wavelength may be rather pronounced, a plane electromagnetic wave traveling in the perturbed region at some velocity other than the velocity of light still falls within our teachings and distinguishes over the prior art quarter wavelength teachings while providing stub mode suppression when expressed as follows:

Said stubs having length dimensions falling within the following limits:

where L is the stub length and is the physical length of the path taken along the surface of the stub as measured from the end of the stub at the outside diameter of the ring circuit to the stub base or root and \=c/f where i is center frequency of the passband of the stub supported circuit and c is the velocity of a plane electromagnetic wave in the dielectric medium or mediums between said stubs averaged over the length of the stub L.

Quite obviously the stub supporting stabilization techniques described herein are improvements over the stub mode stabilization techniques disclosed in aforementioned U.S. Ser. No. 306,570.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing 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. In a high frequency electron discharge device for propagating electromagnetic energy within a band of frequencies, said device including a helix-derived slow wave circuit of the ring-and-bar type, the improvement comprising a plurality of conductive stub support members attached to said slow wave circuit and spaced along the axial extent thereof, said stubs having length dimensions L which fall within the following limits:

where L is the stub physical length, =free space wavelength at i (center frequency of the operating band of frequencies of said device).

2. In a high frequency electron discharge device for propagating electromagnetic energy within a band for frequencies, said device including a helix-derived slow wave circuit of the ring-and-bar type, said slow wave circuit having a plurality of conductive stub support members attached thereto and spaced along the axial extent thereof, wherein said stubs have length dimensions which are approximately A /6, where A is free space wavelength at f (center frequency of the operating band of frequencies of said device).

3. In a high frequency electron discharge device for propagating electromagnetic energy within a 'band of frequencies, said device including a slow wave circuit comprising a plurality of spaced conductive rings interconnected by conductive stubs, the improvement comprising said stubs having a length as determined at f (center frequency of said band of frequencies) such that there is no correspondence between the frequencies within the operating band and any second harmonics of any frequency Within the operating band and the frequency at which the stubs are A2 A where A is free space wavelength.

4. A high frequency electron discharge device for propagating electromagnetic energy within a band of frequencies, said device including a vacuum envelope, gun means for generating a beam of electrons at an upstream end of said envelope, collector means for collecting said electrons at a downstream end of said envelope, and disposed along the path of said beam, a helix-derived slow wave circuit, said slow wave circuit having a plurality of conductive stub support members attached thereto and spaced along the axial extent thereof, said stubs having length dimensions L falling within the following limits:

where L is the stub physical length as measured between the outside diameter of the slow wave circuit and the base or root and where f (center frequency of said band of frequencies) and c=average velocity of a plane electromagnetic wave in the mediums between said stubs averaged over the length of the stubs.

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