N-WAY COAXIAL SIGNAL INJECTOR WITH AXIAL FEEDS

Abstract

An axially-fed RF power combiner combines a plurality of input signals to generate a single fundamental-mode transverse electromagnetic (TEM) output. The combiner comprises a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive the input signals. The feedthroughs are arranged axially around the vacuum coaxial transmission line. An increasing gap is provided between the inner conductive surface and the outer conductive surface of the vacuum coaxial transmission line to gradually transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope of the vacuum coaxial transmission line. In some conductive-ridge embodiments, the inner conductive surface of the vacuum coaxial transmission line may comprise a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within a vacuum envelope of the vacuum coaxial transmission line. In some ridge-less embodiments, the inner conductive surface includes a tapered region within the vacuum envelope to provide an increasing gap between the inner conductive surface and the outer conductive surface.

Description

RELATED APPLICATION

This application is related to patent application entitled “RIDGED N-WAY COAXIAL SIGNAL INJECTOR WITH RADIAL FEEDS” (attorney docket no. 1547.A32US1) filed concurrently herewith.

TECHNICAL FIELD

Embodiments pertain to high-power RF signal combiners and signal injectors. Some embodiments relate to vacuum coaxial transmission lines. Some embodiments relate to coaxial signal injectors configured to deliver a drive signal to a coaxial traveling-wave tube (CoTWT).

BACKGROUND

Vacuum high-power amplifiers currently under development utilize highly over-moded coaxial structures as electron beam-wave interaction regions. That is, the beam and the wave interact between the center and outer conductors of a large evacuated coaxial structure. Their input is desirably driven by a large number of independent yet coherent RF sources that need to be combined to generate a high-purity transverse electromagnetic (TEM) wave that minimizes the presence of unwanted higher-order waveguide modes.

Thus, what is needed are apparatus that can combine signals from a number of RF sources to generate a high-purity transverse electromagnetic (TEM) wave. What is also needed are apparatus that can combine signals from a number of RF sources to generate a high-purity TEM wave for input to an amplifying coaxial vacuum-electron device (CoVED) such as a coaxial traveling wave tube (CoTWT).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an axially-fed signal injector with conductive ridges, in accordance with some embodiments.

FIG. 2 illustrates the input to the axially-fed signal injector of FIG. 1, in accordance with some embodiments.

FIG. 3 illustrates an annular slice of an axially-fed signal injector, in accordance with some embodiments.

FIG. 4 illustrates an axially-fed signal injector without conductive ridges, in accordance with some embodiments.

FIG. 5 illustrates the input to the axially-fed signal injector of FIG. 4, in accordance with some embodiments.

FIG. 6 illustrates an end-view of the axially-fed signal injector of FIG. 1, in accordance with some embodiments.

FIG. 7 illustrates a cutaway side view of a signal injector with a cathode, in accordance with some embodiments.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims.

Some embodiments disclosed herein are directed to power combiners that can combine signals from a number of RF sources to generate a high-purity transverse electromagnetic (TEM) wave. Some embodiments disclosed herein are directed to coaxial signal injectors configured to deliver a drive signal to an amplifying coaxial vacuum-electron device (CoVED), such as a coaxial traveling-wave tube (CoTWT). Some embodiments are directed to generation of a high-purity TEM signal for delivery to a beam-wave interaction region such as a slow-wave structure. These embodiments, as well as other are described in more detail below.

In some embodiments, an axially-fed RF power combiner combines a plurality of input signals to generate a single fundamental-mode transverse electromagnetic (TEM) output. The combiner comprises a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive the input signals. The feedthroughs are arranged axially around the vacuum coaxial transmission line.

In some conductive-ridge embodiments, the inner conductive surface of the vacuum coaxial transmission line may comprise a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within a vacuum envelope of the vacuum coaxial transmission line. Each of the conductive ridges may be coupled to a center conductor of a corresponding one of the coaxial vacuum feedthroughs. The conductive ridges may have a taper to provide an increasing gap between the top of the conductive ridges and an outer conductive surface of the vacuum coaxial transmission line. The increasing gap may gradually transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope allowing the quasi-TEM mode signals from each conductive ridge to spread azimuthally within the vacuum envelope and combine to generate a substantially pure TEM mode signal. These embodiments are discussed in more detail below.

In some ridge-less embodiments (i.e., embodiments without conductive ridges) the inner conductive surface includes a tapered region within the vacuum envelope of the vacuum coaxial transmission line. The tapered region provides an increasing gap between the inner conductive surface and the outer conductive surface to transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope. These embodiments are discussed in more detail below.

FIG. 1 illustrates an axially-fed signal injector with conductive ridges, in accordance with some embodiments. The axially-fed injector illustrated in FIG. 1 is equipped with input signal lines that are parallel with the longitudinal axis of the injector (i.e., the z-direction). This embodiment combines the outputs of a number of input signals delivered via coaxial cable to ridged transitions to generate a high-purity TEM input signal that is delivered to the input of an amplifying coaxial vacuum-electron device (CoVED), such as coaxial traveling-wave tube (CoTWT). The injector comprises two sections, a combiner and an injector (See FIG. 1). The combiner accepts a plurality of inputs 102 (e.g., 32 inputs are illustrated) in the form of vacuum coaxial transmission lines.

FIG. 1 also illustrates conductive ridges 120 of the axially-fed signal injector in accordance with some embodiments. As illustrated in FIG. 1, the coaxial lines are arranged axially around the input of the vacuum coaxial transmission line and the center conductor 122 of each joins a ridge 120 on the center conductor of the combiner itself. The injector begins at the end of the combiner section, where it transitions from the large diameter of the combiner to the smaller diameter of the coaxial output. The injector section may accommodate a plurality of electron beam apertures 132 through which electron beams pass from a cathode 134 (which may be housed inside the combiner section) (e.g., see also FIG. 7) into the interaction region of a slow-wave structure (e.g., a beam-wave interaction region). The injector section comprises a transition 112 from the large-diameter combiner to the input of the smaller-diameter coaxial portion 110. In some embodiments, the injector section 130 may include a beam-wave interaction region where an electron beam and the electromagnetic wave interact, although the scope of the embodiments is not limited in this respect. The smaller-diameter coaxial portion 110 may be an input to a beam-wave interaction region such as a slow-wave structure.

FIG. 2 illustrates the input to the axially-fed signal injector of FIG. 1, in accordance with some embodiments. As shown in FIG. 2, coaxial vacuum feedthroughs 104 deliver signals from the center conductor 122 to a corresponding ridge 120. A slot 142 is provided at each of the conductive ridges at a transition from a corresponding one of the coaxial vacuum feedthroughs. This is illustrated in more detail in FIG. 3.

As illustrated in FIG. 1, the illustrated volume 106 represents the vacuum envelope between the center conductor 116 and the outer conductor 114 of the coaxial transmission lines in which signals are free to propagate. Coaxial vacuum feedthroughs 104 (see FIG. 2) may deliver signals to the injector inputs and combine them in the large-diameter coaxial combiner section 109. The injector section 130 comprises a transition 112 from the large-diameter combiner to the input of the smaller-diameter coaxial output 110. Also included in the injector section 130 are the electron beam apertures 132 which allow passage of electron beams from the cathode 134 through the wall and into the beam-wave interaction region.

The vacuum feedthroughs 104 may be compatible with Type-N connectors for added power-handling capacity, although the scope of the embodiments is not limited in this respect. The center conductor 122 of the feedthrough may be molybdenum and may have a diameter of 0.104″ while the outer conductor is stainless steel and may have an inner diameter of 0.240″. The outer conductor of each feedthrough joins to the outer conductor of the large-diameter coaxial input structure, and each center conductor 122 penetrates the vacuum envelope and joins to the end of one of ridges 120 that are an integral part of the center conductor of the large-diameter coaxial input structure. Apart from the vacuum feedthroughs 104, all other conducting boundaries may be copper although this is not a requirement.

In some embodiments, the ridges 120 to which the vacuum feedthrough center conductors 122 join are tapered 123 to transform from the 50 ohm impedance of each individual coaxial input to the much lower 6.8 ohm impedance (e.g., a 10 cm inner conductor radius, 11.2 cm outer conductor radius) at the end of the tapered ridge transitions.

FIG. 3 illustrates an annular slice of an axially-fed signal injector, in accordance with some embodiments. The input transition from a single coaxial input line is illustrated in more detail in FIG. 2. In these example embodiments, the annular slice of a complete combiner has an angular width of 360 deg/32=11.25 deg. The center conductor crosses a vacuum gap before it joins with the end of a tapered ridge transition. When viewed end-on, each ridge transition has a trapezoidal cross section 126, wider at the top than at the bottom. In this way, the spacing between adjacent ridges is rectangular which is advantageous from a fabrication standpoint.

FIG. 4 illustrates an axially-fed signal injector without conductive ridges, in accordance with some embodiments. FIG. 5 illustrates the input to the axially-fed signal injector of FIG. 4, in accordance with some embodiments. In these embodiments, the inner conductive surface 116 includes a tapered region 150 (i.e., without conductive ridges) within the vacuum envelope 106 of the vacuum coaxial transmission line 108. In these embodiments, the tapered region 150 is configured to provide an increasing gap 124 (see FIG. 3) between the inner conductive surface 116 and the outer conductive surface 114. As illustrated in FIG. 5, a shorted annular slot 152 is provided at a base of the inner conductive surface 116 at a transition from a corresponding one of the coaxial vacuum feedthroughs 104. In these embodiments, the distance between a center of the center conductor 122 and end 146 of the annular slot 152 may be selected to be approximately a quarter-wavelength of an operating frequency.

FIG. 6 illustrates an end-view of the axially-fed signal injector of FIG. 1, in accordance with some embodiments. Details of the electron beam apertures 132 are shown in FIG. 6 which is a view of the injector looking from the output end towards the plurality of coaxial inputs. The beam apertures 132 form a circle around the axis of the injector; each being a rectangular port. While individual apertures act as waveguides in cutoff, their short length may allow excessive coupling of evanescent fields into the interior of the injector if the aperture dimensions are not properly chosen.

Some embodiments are directed to an axially-fed RF power combiner 100 configured to combine a plurality of input signals 102 and generate a single fundamental-mode transverse electromagnetic (TEM) output 103 (see FIG. 1). The combiner comprises a vacuum coaxial transmission line 108 having a plurality of coaxial vacuum feedthroughs 104 configured to receive the input signals 102. The feedthroughs 104 may be arranged axially at the input of the vacuum coaxial transmission line 108. In some embodiments, the vacuum coaxial transmission line 108 comprises a vacuum envelope 106 having an annular shape. The vacuum envelope 106 may be provided between an inner conductive surface 116 and an outer conductive surface 114 (see FIG. 4). In these embodiments, the inner conductive surface 116 may be an inner conductor of the vacuum coaxial transmission line 108. In these embodiments, the inner conductive surface may comprise a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within a vacuum envelope of the vacuum coaxial transmission line 108. In these embodiments, each of the conductive ridges 120 may be coupled to a center conductor 122 of a corresponding one of the coaxial vacuum feedthroughs 104 (See FIG. 2). In some embodiments, the conductive ridges 120 may have a taper 123 to provide an increasing gap 124 (see FIG. 3) between the top of the conductive ridges 120 and the outer conductive surface 114.

In some embodiments, the input signals 102 may be in the range of 4.5 GHz to 5.5 GHz, although this is not a requirement as other microwave frequency and millimeter-wave frequency ranges may also be used.

In some embodiments, the increasing gap 124 may be configured to gradually transition the input signals 102 from each coaxial vacuum feedthrough 104 to quasi-TEM mode signals within the vacuum envelope 106 of the vacuum coaxial transmission line 108. In these axial-feed embodiments, the coaxial vacuum feedthroughs 104 may be arranged axially around the vacuum coaxial transmission line 108 and the center conductors 122 of the coaxial vacuum feedthroughs 104 are parallel with the conductive ridges 120 providing a plurality of axial feeds.

In some embodiments, the conductive ridges 120 may be configured to allow the quasi-TEM mode signals from each conductive ridge 120 to spread azimuthally within the vacuum envelope 106 and combine to generate a composite TEM mode signal that propagates within a portion 125 of the vacuum envelope 106 without the conductive ridges 120, the composite TEM mode signal corresponding to the fundamental-mode TEM output 103. In these embodiments, the fundamental-mode TEM output 103 may be a substantially pure TEM mode signal. In these embodiments, the TEM mode signals that propagate along the conductive ridges 120 are referred to as quasi-TEM mode signals since the propagating electric field will have a small z-component due to the taper of the conductive ridges 120. A pure TEM mode signal, on the other hand, has no axial-field component (i.e., no component in the z-direction). The z-component/direction is parallel to the axis of the coaxial transmission line 108.

In some embodiments, when each of the input signals 102 received at the coaxial vacuum feedthroughs 104 have substantially the same frequency and substantially the same phase, the composite TEM mode signal may be substantially devoid of higher-order waveguide modes. In these embodiments, dimensions of conductive ridges 120 and the length of the vacuum coaxial transmission line 108, among other things, may be selected so that a high-purity TEM mode output may be produced. Accordingly, a high-power output signal may be generated by the coherent combining of many input signals 102. In these embodiments, the phase difference between the input signals 102 may be constrained to a value close to or near zero. In these embodiments, the tapered conductive ridges 120 provide a smooth transition for signals on the input coaxial lines coupled to the coaxial vacuum feedthroughs 104 (i.e., 50 ohm) to the vacuum coaxial transmission line 108 which has a larger diameter (and a much lower impedance 50 ohms).

In some embodiments, the conductive ridges 120 may have a trapezoidal cross section 126 (see FIG. 3) and may provide a rectangular gap between each conductive ridge 120 in the radial arrangement. In these embodiments, the machined gaps separating adjacent ridges 120 are rectangular for ease of fabrication, while the ridges themselves have a trapezoidal cross-sectional profile.

In some embodiments, each coaxial vacuum feedthrough 104 may be configured for receiving one of the input signals 102. In these embodiments, a number of the coaxial vacuum feedthroughs 104 comprises one or more of: an odd number, an even number, and an integer power of two. In these embodiments, unlike conventional N-Way power combiners, the power combiner 100 may have a non-power of two and/or an odd number of inputs. In these embodiments, the number of inputs that may be combined may be as few as 8 or 10 and may range up to 50 or more for embodiments that include conductive ridges 120 and may range up to 64 or more for ridge-less embodiments, although the scope of the embodiments is not limited in this respect.

In some embodiments, the vacuum coaxial transmission line 108 comprises a larger diameter portion 109, a smaller-diameter portion 110 and a transition 112 (see FIG. 4). In these embodiments, the transition 112 may be provided between the larger diameter portion 109 and the smaller-diameter portion 110. In these embodiments, the larger diameter portion 109 may operate as a combiner and includes the conductive ridges and the smaller-diameter portion 110 may provide the composite TEM mode signal as the fundamental-mode TEM output 103.

In some embodiments, the vacuum envelope 106 provides a region between the inner conductive surface 116 and the outer conductive surface 114 to maintain a vacuum therein. In some embodiments, the inner conductive surface 116, including the ridges 120, and the outer conductive surface 114 of the vacuum coaxial transmission line 108 comprise copper. In some embodiments, the coaxial vacuum feedthroughs 104 may comprise Type-N coaxial vacuum feedthroughs with molybdenum center conductors 122 and stainless steel outer conductors 118. In some embodiments, the copper may be Oxygen Free High Thermal Conductivity Copper (OFHC), although the scope of the embodiments is not limited in this respect.

In some embodiments, the combiner 100 may further include a plurality of electron-beam (E-beam) apertures 132 (see FIG. 4 and FIG. 6) within injector section 130 and may be arranged to allow passage of electrons emitted by a cathode 134 (FIG. 7) into a beam-wave interaction region between the center conductor and outer conductor of the vacuum coaxial transmission line 108. In these embodiments, the injector section 130 comprises a transition 112 from the large-diameter combiner to the smaller-diameter portion 110 (i.e., the input to the beam-wave interaction region).

In some of these embodiments, the cathode 134 may be housed within a hollow portion of the center conductor 122 of the larger diameter portion 109 of the vacuum transmission line 108. In some embodiments, the electron beam apertures 132 may comprise holes in the wall separating the cathode-housing interior of the center conductor 122 from a beam-wave interaction region. In these embodiments, dimensions of the apertures 132 may be selected to allow passage of electrons and inhibit passage of RF energy.

In some embodiments, the smaller-diameter portion 110, may operate as an injector and may be coupled to an amplifying coaxial vacuum-electron device (CoVED). In these embodiments, the fundamental-mode TEM output 103 is injected into an input of the amplifying CoVED. In these embodiments, the amplifying CoVED may comprise any type of amplifying coaxial vacuum-electron device including, for example, a coaxial klystron, a coaxial traveling-wave tube, etc.

Some embodiments are directed to a method of combining a plurality of input signals. In these embodiments, the method may comprise receiving the input signals 102 through a plurality of coaxial vacuum feedthroughs 104 arranged axially around a vacuum coaxial transmission line 108. The method may also comprise transitioning the input signals 102 within a vacuum envelope 106 of the vacuum coaxial transmission line 108 to quasi-TEM mode signals along a plurality of tapered conductive ridges 120 of an inner conductive surface 116 of the vacuum coaxial transmission line 108. In these embodiments, the method may also comprise azimuthally spreading and combining the quasi-TEM mode signals from each conductive ridge 120 within the vacuum envelope 106 to generate a composite TEM mode signal that propagates within a portion 125 of the vacuum envelope 106 without the conductive ridges 120. In these embodiments, the composite TEM mode signal may comprise a single fundamental-mode TEM output 103.

In these embodiments, the method may comprise gradually transitioning, with the increasing gap 124, the input signals from each coaxial vacuum feedthrough 104 to quasi-TEM mode signals within the vacuum envelope 106 of the vacuum coaxial transmission line 108. In these embodiments, the method may further comprise combining the input signals in the larger diameter portion and injecting fundamental-mode TEM output 103 from the smaller-diameter portion 110 into an input of an amplifying coaxial vacuum-electron device (CoVED), although the scope of the embodiments is not limited in this respect.

Some embodiments are directed to an axially-fed signal injector. In these embodiments, the axially-fed signal injector may comprise an RF power combiner 100 comprising a vacuum coaxial transmission line 108 having a plurality of coaxial vacuum feedthroughs 104 configured to receive the input signals 102. In these embodiments, the feedthroughs 104 may be arranged axially around the vacuum coaxial transmission line 108. The axially-fed signal injector may also comprise a cathode 134 housed within a hollow portion of a center conductor 122 of the larger diameter portion 109 of the vacuum transmission line 108. In these embodiments, the vacuum coaxial transmission line 108 may comprise a vacuum envelope 106 having an annular shape. The vacuum envelope 106 may be provided between an inner conductive surface 116 and an outer conductive surface 114. The inner conductive surface 116 may be an inner conductor of the vacuum coaxial transmission line 108. In these embodiments, the inner conductive surface may comprise a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within a vacuum envelope of the vacuum coaxial transmission line 108. Each conductive ridge 120 may be coupled to a center conductor 122 of a corresponding one of the coaxial vacuum feedthroughs 104. In these embodiments, the conductive ridges 120 have a taper 123 to provide an increasing gap 124 (see FIG. 3) between the top of the conductive ridges 120 and the outer conductive surface 114. In some of these embodiments, the coaxial signal injector may also include a plurality of electron-beam (E-beam) apertures 132 (see FIG. 6) within an injector section 130 arranged to allow passage of electrons emitted by the cathode into a beam-wave interaction region between the center conductor and an outer conductor of the vacuum coaxial transmission line 108.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.

Claims

1. An axially-fed RF power combiner configured to combine a plurality of input signals and generate a fundamental-mode transverse electromagnetic (TEM) output, the combiner comprising: a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive the input signals, the feedthroughs arranged axially around the vacuum coaxial transmission line;wherein the vacuum coaxial transmission line comprises a vacuum envelope having an annular shape, the vacuum envelope provided between an inner conductive surface and an outer conductive surface, the inner conductive surface being an inner conductor of the vacuum coaxial transmission line,wherein an increasing gap is provided between the inner conductive surface and the outer conductive surface of the vacuum coaxial transmission line to transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope of the vacuum coaxial transmission line.

2. The axially-fed RF power combiner of claim 1, wherein the inner conductive surface comprises a cylindrical conductive base and a plurality of radially aligned conductive ridges azimuthally distributed within the vacuum envelope of the vacuum coaxial transmission line, each conductive ridge coupled to a center conductor of a corresponding one of the coaxial vacuum feedthroughs, and wherein the conductive ridges have a taper to provide the increasing gap between the conductive ridges and the outer conductive surface.

3. The axially-fed RF power combiner of claim 2, wherein a slot is provided at each of the conductive ridges at a transition from a corresponding one of the coaxial vacuum feedthroughs.

4. The axially-fed RF power combiner of claim 3, wherein the conductive ridges are configured to allow the quasi-TEM mode signals from each conductive ridge to spread azimuthally within the vacuum envelope to generate a composite TEM mode signal that propagates within a portion of the vacuum envelope without the conductive ridges, the composite TEM mode signal corresponding to the fundamental-mode TEM output.

5. The axially-fed RF power combiner of claim 4, wherein the conductive ridges have a trapezoidal cross section to provide a rectangular gap between each conductive ridge in the radial arrangement.

6. The axially-fed RF power combiner of claim 1, wherein the inner conductive surface includes a tapered region without conductive ridges within the vacuum envelope of the vacuum coaxial transmission line, the tapered region to provide the increasing gap between the inner conductive surface and the outer conductive surface.

7. The axially-fed RF power combiner of claim 6, wherein an annular slot is provided at the inner conductive surface at a transition from a corresponding one of the coaxial vacuum feedthroughs.

8. The axially-fed RF power combiner of claim 1, wherein when each of the input signals received at the coaxial vacuum feedthroughs has substantially a same frequency and substantially a same phase, the composite TEM mode signal being substantially devoid of higher-order waveguide modes.

9. The axially-fed RF power combiner of claim 8, wherein each coaxial vacuum feedthrough is configured for receiving one of the input signals, and wherein a number of the coaxial vacuum feedthroughs comprises one or more of: an odd number, an even number, and an integer power of two.

10. The axially-fed RF power combiner of claim 9, wherein the vacuum coaxial transmission line comprises a larger diameter portion, a smaller-diameter portion and a transition portion, the transition portion between the larger diameter portion and the smaller-diameter portion, wherein the larger diameter portion is configured to operate as a combiner and includes the conductive ridges, andwherein the smaller-diameter portion is configured to provide the composite TEM mode signal as the fundamental-mode TEM output.

11. The axially-fed RF power combiner of claim 7, further comprising a plurality of electron-beam (E-beam) apertures within an injector section to allow passage of electrons emitted by a cathode into a beam-wave interaction region between the center conductor and outer conductor of the vacuum coaxial transmission line, wherein the injector section comprises a transition to the smaller-diameter portion of the vacuum transmission line, andwherein the cathode is housed within a hollow portion of the center conductor of the larger diameter portion of the vacuum transmission line.

12. The axially-fed RF power combiner of claim 11, wherein the smaller-diameter portion is coupled to an amplifying coaxial vacuum-electron device (CoVED), and wherein the fundamental-mode TEM output is injected into an input of the amplifying CoVED.

13. A method of combining a plurality of input signals in an axially-fed RF power combiner, the method comprising: receiving the input signals through a plurality of coaxial vacuum feedthroughs arranged axially around a vacuum coaxial transmission line;transitioning the input signals within a vacuum envelope of the vacuum coaxial transmission line to quasi-TEM mode signals along an inner conductive surface the vacuum coaxial transmission line, andazimuthally spreading and combining the quasi-TEM mode signals within the vacuum envelope to generate a composite TEM mode signal that propagates within the vacuum envelope, the composite TEM mode signal comprising a fundamental-mode TEM output,wherein an increasing gap is provided between the inner conductive surface and the outer conductive surface of the vacuum coaxial transmission line to transition the input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope of the vacuum coaxial transmission line.

14. The method of claim 13, wherein transitioning the input signals comprises transitioning the input signals within the vacuum envelope of the vacuum coaxial transmission line to the quasi-TEM mode signals along a plurality of tapered conductive ridges of the inner conductive surface the vacuum coaxial transmission line, and azimuthally spreading and combining the quasi-TEM mode signals comprises azimuthally spreading and combining the quasi-TEM mode signals from each conductive ridge within the vacuum envelope to generate the composite TEM mode signal that propagates within a portion of the vacuum envelope without the conductive ridges.

15. The method of claim 14, wherein the vacuum envelope has an annular shape, the vacuum envelope provided between the inner conductive surface and an outer conductive surface, the inner conductive surface being an inner conductor of the vacuum coaxial transmission line, wherein the conductive ridges are radially aligned and azimuthally distributed within the vacuum envelope of the vacuum coaxial transmission line, each conductive ridge coupled to a center conductor of a corresponding one of the coaxial vacuum feedthroughs, andwherein the conductive ridges have a taper to provide an increasing gap between the conductive ridges and the outer conductive surface.

16. The method of claim 15, wherein the vacuum coaxial transmission line comprises a larger diameter portion, a smaller-diameter portion and a transition portion, the transition portion between the larger diameter portion and the smaller-diameter portion, wherein the larger diameter portion is configured to operate as a combiner and includes the conductive ridges, andwherein the smaller-diameter portion is configured to provide the composite TEM mode signal as the fundamental-mode TEM output,wherein the method further comprises:combining the input signals in the larger diameter portion; andinjecting fundamental-mode TEM output from the smaller-diameter portion into an input of an amplifying coaxial vacuum-electron device (CoVED).

17. An axially-fed signal injector, comprising: an RF power combiner comprising a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive the input signals, the feedthroughs arranged axially around the vacuum coaxial transmission line, anda cathode housed within a hollow portion of a center conductor of the vacuum transmission line,wherein the vacuum coaxial transmission line comprises a vacuum envelope having an annular shape, the vacuum envelope provided between an inner conductive surface and an outer conductive surface, the inner conductive surface being an inner conductor of the vacuum coaxial transmission line,wherein the inner conductive surface of the vacuum coaxial transmission line comprises a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within the vacuum envelope of the vacuum coaxial transmission line, each conductive ridge coupled to a center conductor of a corresponding one of the coaxial vacuum feedthroughs, andwherein the conductive ridges have a taper to provide an increasing gap between the conductive ridges and the outer conductive surface.

18. The axially-fed signal injector of claim 17, wherein the increasing gap allows the input signals from each coaxial vacuum feedthrough to transition to quasi-TEM mode signals within the vacuum envelope of the vacuum coaxial transmission line.

19. The axially-fed signal injector of claim 18, wherein the conductive ridges are configured to allow the quasi-TEM mode signals from each conductive ridge to spread azimuthally within the vacuum envelope to generate a composite TEM mode signal that propagates within a portion of the vacuum envelope without the conductive ridges, the composite TEM mode signal corresponding to the fundamental-mode TEM output.

20. The axially-fed signal injector of claim 19, further comprising a plurality of electron-beam (E-beam) apertures within an injector section to allow passage of electrons emitted by the cathode into a beam-wave interaction region between the center conductor and an outer conductor of the vacuum coaxial transmission line.