Cooled coupled-actovity TWT circuit

Abstract: A PPM coupled-cavity travelling wave tube has an RF structure comprising several cavities separated by iron pole pieces (5), (6), (7), with penetrating slots (10), (11), (19) to provide RF coupling between the cavities. Re-entrant copper bars (8), (16), (17), (18) are attached to both sides of the pole pieces (5), (6) that define the boundaries between adjacent pairs of cavities. These bars (8), (16), (17), (18) extend diametrially across the cavity interior around beam drift tubes (9), (24). The re-entrant bars (8), (16), (17), (18) on adjacent pole pieces are rotated relative to each other by 90 degrees about the beam axis. The bars (8), (16), (17), (18) are hollow along their length, and thereby provide channels for coolant flow around the drift tubes (9), (24). These channels communicates through apertures (20), (21), (22), (23) with coolant distribution channels (12), (13), (14), (15) in the outer cavity walls (1), (2), (3), (4) extending along the length of the traveling-wave tube. In an alternative embodiment, the coolant is distributed through external piping to tubes (25), (26), (27), (28) extending outward from the cavity walls (1), (2), (3), (4), which tubes communicate with the channels in the re-entrant bars (8), (16), (17), (18). The direct cooling structure allows the traveling-wave tube to operate at higher power over a broad range of frequencies.

Description: Background of the Invention Field of the Invention This invention pertains generally to the field of slow-wave structures for a traveling-wave tube (TWT), and more particularly, to re-entrant ladder-type coupled-cavity circuits with periodic permanent magnet (PPM) focusing having direct liquid cooling of the beam tunnel. Description of the Background Art Coupled-cavity TWT structures are advantageously and widely utilized in the design of high-power wide-band amplifiers. In the "ladder-type" coupled-cavity circuits, the periodic interaction elements resemble the rungs of a ladder extending across a hollow tube. The spaces between the rungs constitute the cavities, and coupling apertures between adjacent cavities are defined by the spaces around the rungs. -The bandwidth of the structure increases with increasing intercavity coupling. By providing PPM focusing of the beam, it is possible to design a compact lightweight structure having the above advantages of high power and good bandwidth characteristics. Such TWT's combine the PPM periodicity with that of the RF circuit, and the magnetic circuit forms a part of the cavity structures. One typical ladder structure is disclosed in United States Patent No. 4,409,519 issued October 11, 1983 to Arthur Karp. This patent discloses a structure with wide rungs and coupling apertures staggered on alternating opposite sides of the rungs so that each cavity is coupled only to its immediately neighboring cavities. This staggered coupling increases the usable bandwidth of the structure. The wide rungs also allow heat conduction in two dimensions away from the beam tunnel, thereby improving the thermal properties of the tube. A double staggered ladder circuit is disclosed in United States Patent No. 4,586,009 issued April 29, 1986 to Bertram G. James, who is also the present inventor. This structure includes two coupling apertures between each pair of adjacent cavities. The relative locations of these apertures are rotated by 90 degrees about the beam axis in successive intercavity interfaces. This double coupling further increases the bandwidth of the system. Another type of double coupling is disclosed in GB-A-2 045 520 (United States Patent No. 4 237 402). This structure is the "comb-quad" circuit, which comprises two mutually orthogonal ladders with their rungs interleaved. There are construction difficulties in aligning the components of this structure. Further, the heat conduction away from the beam tunnel occurs essentially in one-dimension along the rungs or "teeth" of the comb, and this limits the average power at which the tube can operate. A further improvement on the double-staggering design is disclosed in United States Patent No. 4,866,343 issued September 12, 1989 to Bertram G.James, the present inventor. This improvement is termed the "Re-Entrant Double-Staggered Ladder Circuit", in which each plate or "wall" between adjacent cavities has a wide transverse ridge on either side enclosing the axial beam aperture. The ridges are orthogonal to the coupling slots in these walls, and the slots and ridges in neighboring plates are rotated by 90 degrees about the beam axis relative to each other. These re-entrant ridges increase the efficiency and bandwidth of the traveling wave tube. All of the foregoing cited United States patents are assigned to the assignee of the present invention. In all of the traveling-wave tubes discussed above, the average power capability is limited by the heat generated from the interception of the electron beam by portions of the RF structure. This heat must be conducted away from the beam by the structure, and therefore the structure must have good thermal conducting properties to maximize the operating power of the tube. Copper is often used in these structures because of its high thermal conductivity. In coupled-cavity PPM TWT's, heat is generated by the electron beam interception in the iron pole pieces, which have lower thermal conductivity than copper. In order to improve the thermal conduction path away from the electronbeam tunnel, a ferrule bar may be utilized, as described in the article by Alan Griggs entitled "A New Coupled-Cavity Circuit for High Mean Power Traveling-Wave Tubes", IEEE Transactions on Electron Devices , Vol. 38, No. 8, August 1991, pp. 1952 - 1957. This ferrule bar is essentially a high-conductivity copper bar extending from the iron ferrule around the beam to the outer copper cavity wall, which is in contact with coolant channels. The author states that this technique is useful at operating frequencies that exceed 4 GHz, the maximum frequency at which direct liquid cooling of the beam tunnel is feasible. For frequencies greater than 4 GHZ, the article reports that the ferrule bar technique improves the mean power capability of the tube by a factor ranging from 1.5 to 3, depending on the frequency. When the frequency substantially exceeds approximately 30 GHz, the intercavity walls become too thin to serve as magnetic pole pieces. The magnetic circuit is then made external to the RF structure, and the cavity walls are made of copper. In this high frequency region, the increase in available mean power from the ferrule bar technique would be less significant, but it is still useful. Other designs have been utilized to increase the thermal capacity of PPM coupled-cavity circuits. These designs include laminated plates of copper and iron serving as the pole pieces, and the use of water channels and heat pipes through the pole pieces. These techniques are useful to some degree, but they are limited either because of mechanical restrictions in that thick pole pieces are required, or because in tubes operating at high frequencies the structure must be made so small that the design is not practical. Disclosure of Invention An object of this invention is to provide a PPM coupled-cavity traveling wave tube that operates at higher power levels for all frequencies, compared to previous PPM-focused TWT's. An additional object is to provide the advantages of re-entrant double staggered ladder circuits of previous devices. According to the present invention there is provided a circuit for a PPM traveling-wave tube according to claim 1. The dependent claims describe particular embodiments of the invention. The present invention provides a re-entrant double staggered ladder circuit for PPM focused coupled-cavity traveling-wave tubes, with direct liquid cooling. The re-entrant bars (8), (16), (17), (18) are hollow, and provide channels for coolant flow. These bars (8), (16), (17), (18) extend diametrically across the cavity interior around the beam drift tubes (9), (24) to provide direct cooling. The channels of the bars (8), (16), (17), (18) communicate throuGH apertures (20), (21), (22), (23) with coolant channels (12), (13), (14), (15) in the cavity walls (1), (2), (3), (4), to provide a continuous flow of coolant through all of the bars (8), (16), (17), (18). These and other objects, advantages, characteristics and features of this invention may be better understood by examining the following drawings together with the detailed description of the preferred embodiments. Brief Description of the Drawings Figure 1 is a top view of the first two cavities, and a part of the third cavity, of a PPM coupled-cavity traveling-wave tube according to the present invention. Figure 2 is an end view of the TWT of Figure 1, viewed along the direction of the beam line. Figure 3 is an oblique sectional view of the first cavity of the TWT of Figures 1 and 2, where the section is taken along the sectional lines 3-3 shown in Figure 2. Figure 4 is an oblique view of an alternative version of a PPM coupled-cavity traveling-wave tube, showing the first two cavities and a part of the third cavity. Detailed Description of the Preferred Embodiments Figures 1, 2 and 3 show the re-entrant double-staggered ladder circuit structure for the first two cavities of the PPM coupled-cavity traveling-wave tube. For simplicity the electron gun, collector, ports, power supplies and other common components of a TWT are omitted. The circuit has cylindrical cavity sections defined by copper cylinder walls 1 , 2 , 3 , 4 , with cylinder axes coincident with the common beam axis. The cavities are separated by end walls constituting magnetic pole pieces 5, 6, 7, which are fabricated preferably from iron or other magnetic material and are spaced periodically to form the PPM focusing structure. For example, the first cavity section is defined by cylindrical wall 2 and the cylinder ends 5 , 6 . Drift tubes 9 , 24 are aligned along the beam axis and pass through the centres of the pole pieces 5 , 6 to provide beam apertures for passage of a charged particle beam through the end walls 5 , 6 of the cavities, as shown in Figure 3. RF coupling between cavities is provided by coupling slots 10 , 11 , 19 in the end wall pole pieces, 5 and 6. In the first pole piece 5, coupling slots 10 and 11 are both perpendicular to the beam line and mutually parallel and are located on opposite sides of the beam line. Coupling slot 19 in the next pole piece 6 is transverse to the beam line. Another coupling slot in this pole piece 6 , not shown in the drawings, is parallel to this slot 19 and is located on the opposite side of the beam line. The coupling slot 19 in the second pole piece 6 is rotated by 90 degrees about the beam axis relative to the coupling slots 10 , 11 in the first pole piece 5 . In a similar manner, the coupling slots in each successive pole piece are rotated by 90 degrees relative to the neighboring slots so that only adjacent cavities are coupled. The slots in alternate walls are axially aligned. Coolant channels 12 , 13 , 14 , 15, are provided inside the cylinder walls 1 , 2 , 3 , 4 , and through the pole pieces 5 , 6 , 7 , running parallel to the beam axis along the entire length of the tube through all the cavity sections. These channels 12 , 13 , 14 , 15 are azimuthally disposed about the beam axis at 90 degree intervals. Re-entrant bars 8 , 16 , are attached to opposite sides of the pole piece cavity wall 5 and extend across the cavity interior around the drift tube 9 , perpendicular to the beam axis. In a similar manner, re-entrant bars 17 , 18 , are attached to opposite sides of the adjacent pole piece 6 and extend diametrically across the cavity around the drift tube 24 . These re-entrant bars 8 , 16 , 17 , 18 are made of copper, and are attached at diametrically opposite positions on the interior cylinder walls 1 , 2 , 3 , 4 . The re-entrant bars 8 , 16 attached to the first pole piece 5 intersect the cavity walls 1 , 2 , respectively, at the azimuthal locations of the diametrically opposed coolant channels 13 and 15 . Similarly, the re-entrant bars 17 , 18 attached to the second pole piece 6 intersect the cavity walls 2 , 3 , respectively, at the azimuthal locations of the diametrically opposed coolant channels 12 and 14 . The re-entrant bars 8 , 16 , 17 , 18 are hollow, and the interiors of the bars 8 , 16 , 17 , 18 provide channels for coolant flow. At the locations where the bars 8 , 16 , 17 , 18 meet the cavity walls 1 , 2 , 3 , apertures in the walls 1 , 2, 3 are provided so that the coolant channels 12 , 13 , 14 , 15 communicate with these interior channels to supply a flow of coolant to the bars 8 , 16 , 17 , 18 . For example, as shown in Figure 3, apertures 20 , 23 in the interior cavity wall 2 lie at the points where the ends of the re-entrant bar 17 intersect this wall 2 , and allow coolant to flow between coolant channels 12 and 14 through this bar 17 . Similarly, the apertures 21 , 22 in the cavity wall 3 allow coolant to flow between coolant channels 12 and 14 through the re-entrant bar 18 . A corresponding set of apertures are provided in the cavity walls 1 , 2 at the locations of the re-entrant bars 8 , 16 to allow coolant flow between the coolant channels 13 and 15 through these bars 8 , 16 . These apertures are not shown in the drawings. Similar apertures are provided along the entire length of the tube. Coolant fluid is supplied to the coolant channels by the usual means and flows through all the re-entrant bars. This PPM ladder circuit allows the traveling-wave tube to operate at much higher levels of average power compared to previous devices, because fluid coolant flow is supplied directly to the drift tubes 9 , 24 , the components where the heat generation tends to be the largest. The thermal capacity is further increased by the high thermal conductivity of the copper re-entrant bars 8 , 16 , 17 , 18 themselves. This innovation can be implemented in circuits operating at high frequencies since the cooling structure does not depend on the thickness of the pole pieces 5 , 6 , or any of their other physical characteristics. In addition, the circuit retains all the advantages of previous re-entrant structures, including increased mutual capacitance between ladder rungs, greater bandwidths and improved efficiencies. An alternative version of the invention is shown in Figure 4, which shows an external oblique view of the first two cavity sections. This version has no coolant channels in the cavity walls 1 , 2 , 3 , 4 , and coolant is supplied to the re-entrant bars through tubes 25 , 26 , 27 , 28 passing through these walls. Tubes 26 and 27 supply coolant to the re-entrant bars 17 and 18, while tube 25 and another tube not seen from the view of Figure 4 supply coolant to the re-entrant bars 8 and 16 . Figure 4 does not show the internal passages through the pole pieces 5 , 6 , 7 that allow the coolant to flow from the re-entrant bar on one side of each pole piece to the re-entrant bar on the opposite side. Also omitted from the Figure for simplicity is the external piping necessary to distribute coolant to the various supply tubes 25 , 26 , 27 , 28 . The foregoing disclosure of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and, obviously, many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suitable to the particular use contemplated. The invention is limited only by the scope of the claims appended hereto.

Claims: A circuit for a PPM traveling-wave tube, said circuit comprising: a hollow enclosed channel extending along a central axis of the tube, said channel being comprised of a series of sections, each section being a conductive portion of said channel; an array of wall members transverse to said channel, each member being disposed between and connecting adjacent sections of said channel and extending across said channel to form a cavity in each of said sections, said wall members having beam apertures aligned with respect to said axis for passage of a beam of charged particles; a first set of said wall members having axially aligned first coupling apertures near a first side of said channel and axially raised bars extending across corresponding faces of said wall members, transverse to the orientation of said coupling apertures about said axis and enclosing said beam apertures; a second set of said wall members interleaved with said first set along said axis having coupling apertures and bars respectively transverse to those of said first set, said bars being axially raised and extending across corresponding faces of said wall members, transverse to the orientation of said coupling apertures about said axis and enclosing said beam apertures; each of said bars having a hollow interior forming a channel along the length of said bar to allow coolant to flow through said bar; and coolant supply means for supplying a flow of coolant through each of said bars. The circuit of claim 1, further comprising in each of said wall members a second coupling aperture opposite to said first coupling aperture from said axis. The circuit of claim 1, further comprising on each of said wall members a second bar parallel to said first bar on the opposite face of said wall member. The circuit of claim 2, further comprising on each of said wall members a second bar parallel to said first bar on the opposite face of said wall member. The circuit of claim 1; wherein each section of said hollow enclosed channel has a plurality of coolant channels parallel to said central axis and extending longitudinally through the walls of said section, the coolant channels in each section being aligned with the coolant channels in the neighboring sections; and wherein each of said wall members has a plurality of coolant apertures through said wall member, each of said coolant apertures corresponding to a coolant channel in each of the sections adjacent to said wall member and being aligned therewith; said coolant channels and said coolant apertures thereby forming a plurality of coolant distribution channels extending along the length of said hollow enclosed channel; each of said sections further having a plurality of wall apertures, each of said wall apertures being located at the end of one of said bars and connecting the channel in said bar to one of said coolant channels, such that coolant is allowed to flow between said coolant channel and the channel in said bar; and wherein said coolant supply means includes means for supplying a flow of coolant to said coolant distribution channels. The circuit of claim 1, further comprising a plurality of channels extending out from said hollow enclosed channel, each of said plurality of channels communicating with the channel in the interior of one of said bars, and wherein said coolant supply means includes means for supplying coolant to said plurality of channels. The circuit of claim 1, wherein each of said wall members comprises a magnetic pole piece for focusing said charged particle beam. A circuit of claim 7, wherein said bars are comprised of conducting material. The circuit of claim 1, wherein said bars are comprised of conducting material.

Inventors: JAMES BERTRAM G, JAMES, BERTRAM G.
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1992
Main classification: H01J25
IPC codes: H01J23, H01J25