US3453483A - Microwave linear beam tube employing an extended interaction resonator operating on an odd pi mode - Google Patents

Description

GH 3,453,483 NG AN EXTENDED INTERACTION ODD PI MODE Sheet of 5 INVENTOR WILLIAM J.LE|D|GH ATTORNEY July 1, 1969 w. J. LEIDIGH 3,

I MICROWAVE LINEAR BEAM TUBE EMPLOYING AN EXTENDED INTERACTION RESONATOR OPERATING ON AN ODD PI MODE Filed Dec. 5, 1966 Sheet 4 of 5 INVENTOR BY WILLIAM J.LE|D|GH ATTORNEY July 1, 1969 w. J. LEIDIGH 3, 53,483

MICROWAVE LINEAR BEAM TUBE EMPLOYING AN EXTENDED INTERACTION RESONATOR OPERATING ON AN ODD PI MODE Filed Dec. 5. l966 Sheet 3 of 5 2OI 200 (N-S) Y I w --\I) fiN 2) 2 I\ I l 203 fii l i \1\(N3)V3 FUNDAMENTAL nd CE SPACESHARMONIC L {HARMONIC T T 5 SPACE} I I I I HARMONIC m4 m2 311/4 TI s'n/4 2111311114 2n 9TI/4 511/2 "TI/431T;

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C OMPE NSATED END SECTIONS \UNCOMPENSATED END SECTIONS ATTORNEY July 1, 1969 w. J. LEIDIGH 3,

MICROWAVE LINEAR BEAM TUBE EMPLOYING AN EXTENDED INTERACTION RESONATOR OPERATING ON AN ODD PI MODE Filed Dec. 5. 1966 Sheet Q of 5 I NVEN TOR 7 7 BYWILLIAM J. LEIDIGH wow/9J2 A T TORNE Y Sheet of 5 July 1, 1969 w. J. LEIDIGH MICROWAVE LINEAR BEAM TUBE EMPLOYING AN EXTENDED INTERACTION RESONATOR OPERATING ON AN ODD PI MODE 66 Filed Dec. 5. l9

ATTORNEY United States Patent U.S. Cl. 315-539 15 Claims ABSTRACT OF THE DISCLOSURE A microwave linear beam tube is provided with an extended interaction resonator for interaction with the beam. The extended interaction resonator comprises a plurality of coupled microwave circuit elements which are dimensioned for an odd 1r mode of operation as seen by the electrons of the beam at the microwave frequency of operation of the tube. In a preferred embodiment, the first and last microwave circuit element of the extended interaction resonator structure is dimensioned to have an unperturbed resonant frequency midway between the perturbed mode and the unperturbed mode frequencies for the intermediate resonant circuit elements, whereby more uniform interaction is obtained with the beam throughout the length of the extended interaction resonator.

Heretofore, klystron amplifier tubes have been built employing coupled cavity type extended interaction resonators for interaction with the beam. Generally, such coupled cavity extended interaction resonators have been employed as the buncher and/or output circuits for a klystron amplifier. This type of circuit, when properly designed, permits higher power handling capability than obtained by resonant ring and bar circuits of the type described and claimed in US. Patent 2,945,155, issued July 12, 1960 and assigned to the same assignee as the present invention. Tubes employing the improved coupled cavity type of extended interaction resonator are described and claimed in copending U.S. application Ser. No. 363,900, filed Apr. 30, 1964, and assigned to the same assignee as the present invention. Such a tube is also described in article entitled Experiments with High Power CW Klystron Using Extended Interaction Catchers in the May 1963, issue of IEEE Transactions on Electron Devices at pages l211 and in an article entitled, A Two-Cavity Extended Interaction Klystron Yielding 65 Percent Efliciency, IEEE Transactions on Electron Devices of August 1964, pages 369-373.

In these prior klystron tubes, employing the coupled cavity type of extended interaction resonator, the composite resonator is formed by a plurality, i.e., n number of separate cavity resonators coupled together by means of inductive coupling slots communicating through the common end walls of adjacent ones of the separate resonators. The end resonators, at opposite ends of the composite structure, are not terminated in a matched load such that they form end reflectors for the composite structure to form a composite coupled cavity resonator. A coupled circuit, When terminated in non reflective loads at 3,453,483 Patented July 1, 1969 its ends, is a well known slow wave circuit characterized by a lower frequency pass band having an upper cut off frequency and a lower cut off frequency. The upper cut off frequency of the pass band corresponds to a mode of resonance wherein the electric fields in the resonators are all of the same phase (even numbered 1r modes), i.e., directed in the same direction, and this frequency occurs approximately at the resonant frequency of the individual cavities, assuming the inductive coupling slots did not exist. Thus, this even 1r mode of resonance is the unperturbed resonant mode of the circuit.

On the other hand, the other out off frequency of the pass band corresponds to a mode of resonance of the coupled circuit wherein the resonant electric fields in the resonators are out of phase, i.e., oppositely directed, in adjacent resonators (odd 1r mode). The odd 1r mode is the perturbed resonant mode of the coupled circuit inasmuch as its resonant frequency corresponds in frequency to the resonant frequency of the individual cavities as inductively loaded or perturbed by the coupling slots. Thus, thi lower cut off frequency mode may be referred to as the perturbed resonant mode. The greater the inductance of the coupling slots communicating between adjacent cavities the greater the inductive loading of the individual cavities and, thus, the greater the difference in frequency between the upper and lower cut off frequencies.

When the slow wave circuit is provided with end reflectors to form the composite resonator (extended interaction cavity) the dispersion characteristic of the circuit is altefed from a continuously propagating structure between upper and lower cut off frequencies to a resonant structure having a number, n, of discrete resonant operating points for each space harmonic, of the resonant circuit. Each resonant operating point corresponds to a different fractional number of radians of phase shift per period of the circuit.

In such an inductively coupled multicavity circuit, assuming the individual cavities are all of about the same dimensions, the cavities at either end of the circuit are less perturbed for the perturbed odd 1r mode because they have coupling slots in only one end wall, whereas the intermediate cavities are more heavily inductively loaded (perturbed) due to having slots in both end walls. Thus, the perturbed mode (odd 1r mode) resonant frequencies of the end cavities are detuned from the resonant frequencies of the intermediate cavities. As a result, when operating the composite circuit at the frequency of the perturbed mode (odd 1r mode), the R.F. electric fields of the end cavities will be weaker than the electric fields of the intermediate cavities. This will lead to uneven power distribution in the composite resonator structure.

Thus, in the tube of the prior art, as described in the aforecited articles, the resonant coupled cavity resonator was operated in the unperturbed even 1r mode correspond ing 21r radians of phase shift between adjacent coupled resonators (second space harmonic). In this manner, the beam interacted with R.F. fields of uniform intensity in succeeding ones of the coupled resonators. However, there are certain disadvantages to operating on this 211', i.e., second space harmonic, mode. More particularly, the coupling slots have a pass band with a low frequency cut oil? somewhat above the 211' mode of the coupled cavities.

Such a slot mode can be excited by the bunched beam and, as a result, the slots store energy in the resonant slot mode in the composite resonant circuit, thereby producing a spurious output, possibly overheating the circuit, and reducing the efficiency of the tube on the desired mode.

The odd 1r mode is perturbed down in frequency from the even mode and, thus, operation on the odd 1r mode provides greater frequency separation from the slot mode, thereby making it less likely that the slot mode will be excited and further making it easier to discriminate against excitation of the slot mode, as by selective loading thereof.

Moreover, the interaction impedance of the second harmonic resonant mode of operation (21r mode) is not as great as the interaction impedance of the fundamental or 'n' mode of resonance and, therefore, operation on the second harmonic leads to a lower circuit efficiency and bandwidth than obtainable if the tube is properly designated for operation on the lower order fundamental space harmonic 1r mode.

In the present invention, an extended interaction cavity resonator is dimensioned for and operated for near synchronous interaction with the lowest odd numbered 11' resonant mode consistent with the power dissipation requirement of the resonator, whereby increased efiiciency and bandwidth are obtained. Near synchronous interaction, means that the beam velocity is within 30% to 15% of the phase velocity of the selected odd 1r resonant mode of the resonator. If the extended interaction cavity resonator is employed as a buncher cavity, the beam velocity may operate within 150% of the phase velocity of the resonator and is typically to 30% lower than the phase velocity of the cavity for bunching the beam, whereas if the extended interaction cavity is used as the catcher or output cavity, the beam velocity is preferably 15 to 30% higher than the phase velocity of the composite cavity such that the bunches deliver their energy to the resonator.

In a preferred embodiment of the present invention, the extended interaction cavity is formed by a plurality of coupled cavities with the end cavities dimensioned differently than the intermediate cavities of the coupled resonant circuit such that all of the coupled cavities have substantially the same resonant frequency as loaded by the perturbing coupling means and as excited for the odd 1r mode frequency. In this manner, the R.F. electric potentials interacting with the beam have the same amplitude in all of the coupled cavities, whereby enhanced electronic interaction is obtained with the beam. In addition, this end compensation yields equal power dissipation and distribution throughout the composite resonator, thereby substantially increasing the power handling capability and bandwidth of the circuit.

The principal object of the present invention is the provision of an improved microwave linear beam tube employing an extended interaction cavity resonator.

One feature of the present invention is the provision of a microwave linear beam tube employing an extended interaction resonator dimensioned for approximately synchronous interaction with the beam on an mr mode of the resonator where n is the smallest odd integer number consistent with the required thermal capacity of the resonator, whereby uniform power distribution, and maximum etficiency and bandwidth of the resonator is obtained.

Another feature of the present invention is the same as the preceding feature wherein n is one for operation on the fundamental space harmonic of the resonator, whereby increased efficiency and bandwidth are obtainable as compared with operation on a higher 1r mode such as the 211' mode.

Another feature of the present invention is the same as any one or more of the preceding features wherein the microwave tube is a klystron amplifier, whereby increased efficiency and bandwidth are obtained for the amplifier.

Another feature of the present invention is the same as any one or more of the preceding features wherein the extended interaction resonator is a coupled cavity structure containing a plurality of cavities each of which is electromagnetically coupled to the others and the end cavities are suitably dimensioned to have a resonant frequency in its uncoupled state that is different from the resonant frequency of the intermediate cavities in their uncoupled state so that an axial R.F. voltage and electric field can be established across each end cavity gap that is equal in magnitude to the RF. electric field and voltage across the interaction gaps of the cavities intermediate the end cavities.

Other objects of this invention will be more apparent from the following description taken in conjunction with the accompanying drawings.

In the drawings:

FIGURE 1 is an elevational view of a klystron with the output portion of the interaction section in cross section to show the extended interaction structure;

FIGURE 2A is a view taken on the line 2-2 of FIG- URE 1 showing one type of slow wave structure;

FIGURE 2B is another type of slow wave structure that could be used in place of the slow wave structure of FIGURE 2A;

FIGURE 3 is a cross-sectional view of another embodiment of an extended interaction output section;

FIGURE 3A is a view taken on the line 33 of FIG- URE 3 of the slow wave structure of FIGURE 3;

FIGURE 4 is an w-B diagram illustrating the relationship between the resonant frequency and the phase shift;

FIGURE 5 is a graph showing the magnitude and direction of the electric field across the interaction gap in each cavity of the coupled cavity structure of FIGURE 1 with compensated end sections versus the magnitude of the electric field across the interaction gap in each cavity of a coupled cavity structure without compensated end sections;

FIGURE 6 is a diagrammatical view of the cavities and their resonant frequencies before being coupled to each other to form the coupled cavity structure disclosed in FIGURE 1;

FIGURE 7 is an elevational view of a klystron having another extended interaction output section embodiment shown in cross section;

FIGURE 8 is a View of the slow wave structure of FIGURE 7 taken on the line 88 of FIGURE 7;

FIGURE 9 is an elevational view of a klystron having another extended interaction section embodiment shown in cross section;

FIGURE 10 is a sectional view of the slow wave structure of FIGURE 9 taken on the line 10-10 of FIGURE 9; and

FIGURE 11 is an w-B diagram for the extended interaction circuit of FIGURES 9 and 10.

Briefly described, this invention relates to a klystron having an electron beam forming means forming a beam of electrons, an interaction section containing input means for bunching portions of the beam of electrons and output means for extracting R.F. energy from the bunched portions of the beam of electrons, and collector means for collecting the spent beam of electrons after it has passed through the interaction section. The interaction section contains at least one plural gap, electromagnetically coupled, coupled cavity structure having axial RF. electric fields across each gap for interaction with the beam of electrons. The phase shift between electric fields in adjacent gaps is substantially equal to mr radians where n. is any odd integer and preferably one. The RF. electric fields across adjacent interaction gaps are opposite in sign and substantially equal in magnitude to each other and the RF. voltages appearing across all of the interaction gaps are substantially equal,

Referring to FIGURES l and 2A, a klystron has an electron beam forming gun section 12, radio-frequency interaction section 14, and a collector section 16. As is well known in the art, the electron beam forming gun section, the radio-frequency interaction section and the collector section are hermetically united in axial alignment to enable the projection of an electron beam through a series of drift tube sections 18, 20, and '22. The input cavity 24 is mounted between drift tube sections 18 and 20 which are spaced from each other within the input cavity 24 to provide an interaction gap therebetween as is well known in the art (not shown). The input cavity 24 preferably contains an input loop as part of an input coupler 26. If desired, an additional cavity 28 is mounted between the drift tube sections 20 and 22 to provide additional bunching of the electron beam in the klystron 10. Tuners 30 and 32 are provided for the input cavity 24 and the bunching cavity 28, respectively. It should be apparent to those skilled in the art that either the input cavity 24 or the additional bunching cavity 28, can be fixed tuned cavity structures which would do away with the need for tuners 30 and 32.

An output cavity 34 is mounted between the collector 16 and the additional bunching cavity 28. The output cavity 34 is of the extended interaction type an example of which is described in copending application Ser. No. 363,900 filed Apr. 30, 1964 and assigned to the assignee of this invention. The output extended interaction cavity 34 is a coupled cavity structure wherein at least two, and in this figure four, smaller cavities are located Within the preferably tubular electrically conductive wall 36 forming the outer boundary of the output extended interaction cavity 34. The four smaller cavities located within the output extended interaction coupled cavity 34 are formed by means of three apertured electrically conductive metal disks 38, 40 and 42 located intermediate a pair of end walls 35 defining thereflective end walls of the composite cavity resonator 34. A first interaction gap 44 is located between the apertured metal disk 38 and conically shaped drift tube end portion 46 of the drift tube 22. A second interaction gap 48 is located between the apertured metal disks 38 and 40. A third interaction gap 50 is formed between the apertured metal disks 40 and 42, and a fourth interaction gap 52 is formed between the apertured metal disk 42 and comically shaped drift tube end portion 54. The spent beam of electrons passes into the collector section 16 through drift tube 56 after giving up a large part of its energy in the output extended interaction coupled cavity structure 34.

R.F. energy is extracted from the output extended interaction coupled cavity structure 34 through the penultimate cavity, containing interaction gap 50, into the output waveguide 58 and then through a window assembly 60 to a load which is not shown. A suitable iris 57 in the wall 36 is located between apertured plates 40 and 42 so as to permit energy to be coupled from the penultimate cavity into the output waveguide 58. Compensation means are provided in the penultimate cavity to compensate for the electrical distortion caused by the iris 57. In this embodiment, the compensation means are three longitudinally extending rib members 59, two of which are shown in FIGURES 1 and 2A. The rib members 59 are made of electrically conductive material and are disposed between the apertured metal disks 40 and 42.

FIGURE 2A depicts one type of cavity end wall 62 that can be used for each of the apertured metal disks 38, 40 and 42. The cavity end wall structure 62 resembles a cart wheel and consists of an annular, longitudinally extending, metal portion 64 on which are mounted four metal spokes 66. The annular, longitudinally extending, metal portion 64 serves to focus or concentrate the axial R.F. electric field across the interaction gap of each cavity of the coupled cavity structure 34. The spokes 66 are 90 apart and are brazed to the outer conductive wall 36. With this type of coupled cavity structure, the R.F. magnetic field in each cavity of the coupled cavity structure 34 can be coupled into the adjacent coupled cavity through apertures 68 located between the spokes '66.

FIGURE 2B depicts another type of coupled cavity structure wherein an apertured metal disk assembly 70 is shown which can be utilized for each of the apertured metal disks 48, 50 and 52 of the output cavity 34. The apertured metal disk assembly 70 shown in FIGURE 2B contains an annular, longitudinally extending, metal portion 72 and an apertured disk-shaped metal portion 74 which contains at least one slot therein and preferably two substantially kidney-shaped slots 76 and 78 which are preferably located close to the peripheral region of the metal disk 74 in order to more easily couple the magnetic fields from one cavity to the adjacent cavity of the coupled cavity structure 34. In this manner, magnetic fields from one cavity in the output coupled cavity 34 can be coupled into the adjacent cavity and so on down the line for each of the four cavities of the output coupled cavity 34. The dotted lines shown in FIGURE 2B depict the substantially kidney-shaped slots in the other apertured metal disks located behind the apertured disk shown in this figure. While the kidney-shaped slots in adjacent apertured metal disks are shown to be out of alignment with respect to each other, it should be apparent to those skilled in the art that each pair of kidney-shaped slots in each apertured metal disk could be lined up with the other pairs of kidneyshaped slots, if desired. It may be desired to modify the magnetic field coupling between cavities which can be achieved by not aligning the slots with respect to each other as shown in FIGURE 2B.

Referring to FIGURE 3, another output extended interaction cavity structure 34 is shown wherein only two apertured metal disks 80 and 82 are mounted transversely across the tubular, electrically conductive wall 36 to form three cavities within the extended interaction output cavity 34. In this embodiment, the output waveguide 58 is connected to the end cavity of the coupled cavity output structure 34 that is located closest to the collector.

Referring to FIGURE 3A, the metal disk 82 is shown to contain three substantially kidney-shaped slots 84 which are located in the metal disk portion surrounding an annular, longitudinally extending, metal portion 86. The R.F. electric field between the apertured metal disks 80 and 82 can be concentrated substantially along the axis through the center holes in the apertured metal disks 80 and 82 where the electron beam passes so that good interaction between the electron beam and the R.F. electric field across the interaction gap of the intermediate cavity can be maintained. The tapered portion of the longitudinally extending annular metal member 86 provides an even better electric field focusing arrangement. Similarly, the R.F. electric fields across the interaction gaps of the end cavities are also concentrated in the region of the electron beam for increased interaction.

Referring now to FIGURE 4, there is illustrated an (ii-,8 diagram which shows the relationship between the resonant frequency and phase shift of the coupled cavities comprising the four coupled cavity extended interaction section as shown in FIGURES l-2. The ordinate of the dispersion diagram is frequency and the abscissa of the diagram is phase shift times the period P, where a period is the distance between the center of one interaction gap associated with one of the coupled cavities to the center of the interaction gap associated with an adjacent coupled cavity.

The coupled cavity extended interaction cavity resonator 34 has a dispersion curve 200 as shown in FIGURE 4 which is similar to that of a similar coupled cavity slow wave circuit except that the presence of the reflective end walls 35 of the composite cavity 34 causes the dispersion curve to be discontinuous. More particularly the curve 200 is caused to have discrete operating points, indicated by the vertical lines, corresponding to fre quencies which allow an integral number of half wavelengths of phase shift along the circuit in between the reflective walls 35. Thus, if there are n number of coupled cavities, there are 11 number of resonant modes per space harmonic of the circuit. These resonant modes are identified as the N, N-l, N2, N3, etc. modes, as indicated in FIGURE 4.

The even numbered upper cut off modes N N etc., corresponding to 0, 2w, 41r etc. phase shift per period, have resonant electric fields of equal amplitude and of the same phase, i.e., the electric field points in the same direction in all of the coupled cavities (periodic elements of the composite resonator). The N or 21r mode was employed in the klystron amplifiers described in the aforecited IEEE transaction articles. The 21r mode was arranged for near synchronous interaction with the beam velocity V of the electron beam. It turns out that this second space harmonic mode of operation has less interaction impedance than the lower order fundamental space harmonic 1r mode (N Hence, operation on the (N mode with a higher beam velocity V provides increased electronic interaction, thus, yielding improved etficiency and bandwidth for the tube. Generally, the higher the order of the space harmonic the lower the electronic interaction. Therefore, operation on the lowest order space harmonic, consistent with the power handling capacity of the circuit, is to be preferred.

In case a lower beam voltage such as V or lower is desired it may be necessary to operate on the N or 31r mode or even higher order modes, such as 41r, 51r, etc., but such higher order space harmonic operation will lead to reduced efiiciency and/ or bandwidth compared to operation on the fundamental space harmonic cut off" resonant mode N As regards operation on the lower cut off frequency u modes of electronic interaction N N etc. of the composite cavity, this mode corresponds to resonance of the individual cavities as perturbed or loaded by the coupling means communicating between adjacent cavities. However, it is seen that if all the cavities had equal dimensions then the end cavities of the composite cavity 34 have only one half the number of coupling means since only one end wall is coupled, whereas the intermediate cavities have coupling means in both of their end walls. As a consequence, the end cavities are perturbed or detuned, for the perturbed or lower cut off resonance mode, less than the intermediate cavities. Their resonant frequency would correspond to w on the w-B diagram of FIGURE 4 whereas the intermediate cavities are resonant at m Thus, when operating on the odd 1r modes (N N N etc.) the end cavities would be detuned from the frequency of the intermediate cavities, thereby having weaker R.F. electric fields as indicated by the dotted lines of FIGURE 5. This is undesired because the power distribution in the composite cavity would be non uniform and, moreover, less than optimum interaction cells would be obtained because of the the end cavities.

Therefore, the end cavities of the extended interaction section 34 of FIGURE 1 are dimensioned such that they have a resonant frequency w (point 202 of FIGURE 4), in an uncoupled condition, which is substantially the median value between the resonant frequencies corresponding to the 0 (point 201) and 1r (point 203) modes of the complete coupled cavity structure 34. The inner cavities of the extended interaction section of FIGURE 1 are dimensioned such that they have a resonant frequency w (point 201 of FIGURE 4), in an uncoupled condition, which corresponds to the 211- mode of electronic interaction (point 201) of the complete coupled cavity structure 34. By so dimensioning the cavities which comprise the extended interaction section 34, all the cavities of the complete coupled cavity section 34 are enabled to have a perturbed resonant frequency w weaker fields in impedance for a given number of corresponding to point 203. Thus, the RF. fields in each of the cavities are of equal amplitude for uniform power distribution and increased efiiciency and bandwidth.

When the extended interaction section 34, comprising the four coupled cavities, of FIGURE 1 operates in the odd 1r mode, the RF. electric field across each interaction gap 44, 48, 50 and 52 is equal in magnitude but opposite in direction to the RF. electric field in the adjacent gap as illustrated by the solid curve of FIGURE 5 which illustrates the RF. electric field across each interaction gap when the extended interaction section 34 is operated in the odd 1r mode. That is, the phase shift between each gap is 1r radians.

The operation of the three coupled cavity interaction section 34 illustrated in FIGURE 3 is such that the outer cavities, in an uncoupled state, have a resonant frequency w (point 202 of FIGURE 4) and the inner cavity has a resonant frequency o in the uncoupled state, corresponding to the 0 or mmode (point 201 of FIGURE 4). This enables the entire coupled three cavity extended interaction section to operate in the odd mode as discussed hereinabove in connection with FIGURE 1.

It is noted that there are a number of midband resonant modes for each space harmonic of the circuit, i.e., (N 1), (N -2), etc. and (N l), (N -2),

etc. It is possible to operate on these modes. However, these modes are characterized by a null in the RF. electric fields of at least one of the cavities and, therefore, non uniform power distribution with lower efficiency and bandwidth are to be expected from operation purely on one of these modes. Thus, insofar as possible, as ditated by beam voltage and power handling capacity of the composite resonator 34, it is preferred to operate on the lowest order band edge resonant mode.

FIGURE 6 illustrates some of the electrical characteristics that are present in the extended interaction output section 34 of FIGURE 1. However, as noted from FIGURE 5, the axial R.F. electric field across the interaction gaps of the end cavities are opposite in sign but equal in magnitude to the axial R.F. electric fields across the intreaction gaps of the adjacent cavities and in addition, the R/Qs for each cavity are the same.

Extended interaction output cavities, such as those described above, have an efficiency in excess of 60%. That is, they extract in excess of 60% of the electron beam power. Also, the bandwidth of such extended interaction sections is a factor of 2 to 5 greater than prior art devices. It becomes clear then, that the overall bandwidth of a klystron may be increased by a factor of 2 to 5 by making the input, buncher and output cavities extended interaction cavities. Further, the use of extended interaction output cavities substantially reduces the power dissipated in the output cavity due to circulating R.F. currents for the same amount of power output.

A figure of merit of klystron output cavities is given by the product of bandwidth times efficiency. Since extended interaction output cavities have higher efficiencies and a wider bandwidth, they have a higher figure of merit. Also, a figure of merit of klystron input cavities is the product of gain times bandwidth. Although extended interaction input cavities may not increase the gain of a klystron, they do increase the bandwidth to improve the figure of merit of these cavities.

Referring to FIGURE 7, wherein the numerals shown in FIGURE 1 apply to the klystron structure shown in FIGURE 7, there is illustrated another odd 'n' mode operated output extended interaction cavity 34 which contains three apertured metal disks 88, and 92 transversely mounted across the annular conductive wall 36 of the cavity 34. The output waveguide 58 extends from the intermediate cavity which is located between the apertured disk 90 and the apertured disk 92. With regard to FIGURE 8 which is a cross-sectional view taken on the line 88 of FIGURE 7, the apertured metal disk 92 is shown to contain a pair of slots 94 and 96 which are located substantially opposite each other substantially at the periphery of the disk 92. It should be noted that the metallic disk 92 is absolutely flat with no extensions therein or flange portions which extend in the direction of the electron beam. The apertured metal disks 90 and 88 also have a pair of slots located therein but are rotated 90 with respect to the slots located in the disk 92. In addition, the slots in disk 90 are rotated 90 with respect to the slots located in disk 88.

Referring to FIGURE 9 wherein like reference numerals refer to the same or corresponding parts illustrated in FIGURES 1 and 7, an odd 1r mode operated output extended interaction cavity 34 is shown which contains three three apertured metal disks 98, 100 and 102 each of which contains a coolant channel 104. The coolant fluid is caused to circulate in each of the apertured metal disks 98, 100 and 102 through the annular channel 104 so as to cool the apertured disks and prevent their deformation due to excess heating caused by electron bombardment and R.F. ohmic heating. FIGURE is a cross-sectional view of the disk 98 showing the coolant channel 104 with its entrant 106 and exit 108 passages.

The extended interaction section illustrated in FIG- URES 9 and 10 is capable of handling large amounts of power due to the liquid cooling feature. Each of the four cavities formed by the metal disks 98, 100, 102 and the annular wall 36 are capacitively intercoupled by way of the central aperture located on each disk 98, 100 and 102 through which the electron beam passes.

The w-fi diagram for the composite coupled cavity resonator 34 of FIGURES 9 and 10 is shown in FIGURE 11. In this case the perturbed odd 1r mode defines the upper cut off frequency u of the composite resonator. As before, the end cavities are dimensioned to have an intermediate unperturbed resonant frequency w to provide a uniform R.F. electric field distribution in all cavities for the odd 7r mode operation.

Although specific examples of inductive and capacitive coupled periodic elements (cavities) have been depicted and described, it is to be understood that other types of resonant sections of periodic slow wave circuits having high thermal capacity may be used to advantage in the present invention. For example, the extended interaction cavity 34 could comprise a plurality of cavities coupled together by means of negative mutual inductive coupling means in the manner as described and claimed in U.S. Patent 3,233,139, issued Feb. 1, 1966 and assigned to the same assignee as the present invention to provide a fundamental forward wave circuit as exemplified by the diagram of FIGURE 11. Also a shorted section of long slot coupled cavities as described in U.S. Patent 3,205,398, issued Sept. 7, 1965 may be employed for the extended interaction cavity resonator.

What has been described are odd 1r mode operated extended interaction sections comprising a plurality of coupled periodic elements that function to improve the operational characteristics of a linear beam tube such as a klystron.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of this invention and that numerous modifications or alterations may be made therein without departing from the spirit and scope of this invention as set forth in the appended claims.

What is claimed is:

1. A linear beam radio frequency tube comprising, electron beam forming means for producing a beam of electrons of a certain beam velocity; an interaction section for bunching portions of said electron beam and for extracting R.F. energy from said bunched portions of said electron beam at a certain radio frequency of operation; said interaction section including at least one resonant extended interaction portion having a plurality of coupled periodic elements for providing a plurality of gaps that interact with said beam, said coupled elements dimensioned to produce interaction with the beam at the certain radio frequency of operation with a phase shift between R.F. electric fields across adjacent gaps as seen by electrons of the beam traveling at the certain beam velocity that is substantially equal to mr radians wherein n is any odd integer; and collector means for collecting the beam of electrons after it has passed through said interaction section, whereby increased efficiency and bandwidth are obtainable.

2. The apparatus of claim 1 wherein said coupled elements are dimensioned to produce substantially equal magnitude R.F. electric fields across said purality of gaps, whereby uniform power distribution is obtained in said extended interaction portion.

3. The apparatus of claim 1 wherein said periodic elements are cavity resonators, whereby the thermal capacity of said extended interaction portion is enhanced.

4. The apparatus according to claim 3 wherein said extended interaction portion comprises the R.F. energy extracting portion of said interaction section.

5. The apparatus of claim 1 wherein the microwave tube is a klystron amplifier.

6. The apparatus of claim 1 wherein said resonant extended interaction portion is dimensioned for interaction with the electrons of the beam at the certain radio frequency of operation where n is one.

7. The apparatus according to claim 3 wherein said extended interaction coupled cavity portion includes at least one substantially flat electrically conductive member transversely mounted with respect to said electron beam and providing a common wall for two adjacent cavities in said intercoupled cavity portion.

8. The apparatus in accordance with claim 7, in which said substantially flat metal member transversely mounted with respect to said electron beam includes a central aperture to permit the electron beam to pass therethrough, and at least one slot spaced radially from said central aperture to enable electromagnetic coupling between said adjacent cavities.

9. The apparatus according to claim 8 wherein the central aperture includes a longitudinally extending drift tube portion that surrounds the electron beam path.

10. The apparatus according to claim 3 wherein adjacent cavities in said extended interaction portion are inductively coupled.

11. The apparatus according to claim 3 wherein adjacent cavities in said extended interaction portion are capacitively coupled.

12. The apparatus in accordance with claim 7, in which said substantially flat metal member transversely mounted with respect to said electron beam in said coupled cavity structure is provided with a single central aperture for permitting the passage of said electron beam and for coupling adjacent cavities.

13. The apparatus in accordance with claim 7, in which said substantially flat metal member is provided with a circular channel encircling said beam of electrons whereby a cooling fluid is permitted to run through said channel and cool said metal member.

14. The apparatus according to claim 3 wherein the coupled cavities of said resonant extended interaction portion which are disposed at the ends thereof are dimensioned differently than the intermediate coupled cavities to provide uncoupled resonant frequencies for said end coupled cavities which are intermediate the upper and lower cut olf frequencies of said extended interaction portion, whereby the magnitude of the R.F. electric fields across the gaps in said end cavities are equalized with the magnitude of the RF. fields across the gaps of said intermediate coupled cavities.

15. The apparatus of claim 14 wherein said coupled cavities are dimensioned to have substantially equal ratios of R/ Q, where R is the shunt resistance of the individual cavities and Q is the Q of the individual cavities.

(References on following page) 1 1 1 2 References Cited Preist-Eitel-McCullough, Inc., San Carlos, Ca1if., Extrait UNITED STATES PATENTS des Travaux du 5 Congres International Tubes Pour Hyperfrequences, Pans France, 1964. 2,647,219 7/1963 Touraton et a1. 315-5.39 X

i323 g fi g 5 ELI LIEBERMAN, Primary Examiner.

o orow 1 43 1 5 chodorow 315 5.39 X SAXFIELD CHATMON, JR. ASSZSMHI Exammer. 3,270,240 8/1966 Lavoo 3155.39 X US Cl XR 3,375,397 3/1968 Leidigh et a1 3155.51 X

315-543, 5.51, 5.52 OTHER REFERENCES 10 The Future of Extended Interaction Klystrons by

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