US3904993A

US3904993A - High power solid microwave load

Info

Publication number: US3904993A
Application number: US438495A
Authority: US
Inventors: Bertram G James
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1974-01-31
Filing date: 1974-01-31
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09

Abstract

A high power solid microwave load, useful as a TWT sever load, employs a block of lossy material having a tapered lead-in section. The block has metallized surface strips which are brazed to the waveguide into which the block is fitted. The strips hold the load away from the waveguide walls so that included gases can be drawn off via the spaces between adjacent strips during vacuum pumping. The load is divided into sections by a plurality of conductive septa aligned in the direction of energy flow; these aid the thermal handling properties of the load by providing uniform temperature planes which reduce deleterious temperature variations and by dividing the waveguide into three propagating waveguides into which the power is more uniformly distributed.

Description

James [111 3,904,993 51 Sept. 9, 1975 HIGH POWER SOLID NIICROWAVE LOAD [75] Inventor: Bertram G. James, Redwood City,

Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif. [22] Filed: Jan. 31, 1974 I [21] Appl. No.: 438,495

[56] References Cited UNITED STATES PATENTS 6/1954 Sensiper 333/22 R Woodcock 333/22 R Primary Examiner-Paul L. Gensler Attorney, Agent, or FirmStanley Z. Cole; D. R. Pressman; R. B. Nelson [57] ABSTRACT A high power solid microwave load, useful as a TWT sever load, employs a block of lossy material having a tapered lead-in section. The block has metallized surface strips which are brazed to the waveguide into which the block is fitted. The strips hold the load away from the waveguide walls so that included gases can be drawn off via the spaces between adjacent strips during vacuum pumping. The load is divided into sections by a plurality of conductive septa aligned in the direction of energy flow; these aid the thermal handling properties of the load by providing uniform temperature planes which reduce deleterious temperature variations and by dividing the waveguide into three propagating waveguides into which the power is more uniformly distributed.

8 Claims, 5 Drawing Figures CONDUCTIVE 4co-nucnv .SEPTUM z co'unucnvrsmws 3109311 MATERlAL HIGH POWER SOLID MICROWAVE LOAD FIELD OF INVENTION AND DESCRIPTION OF PRIOR ART This invention relates to a high power microwave load and particularly to such a load which can be formed of a solid state material and has a compact design.

In the fabrication of microwave loads, it is generally required that the load be'able to absorb power without reflecting it back to the source, beable to be cooled ef-v ficiently, and be able to last for a relatively long period of time. Although these criteria are relatively easy to' achieve. by fabricating the load of a lossy material of sufficient volume to handle the power desired and provide sufficient area to enable an adequate coolant flow around the load, the task of providing a compact load with adequate power handling capacity is redoubtable.

.The need for a compact loadis present in many applications. For example in the design of high power traveling wave tubes it is generally required, in order to prevent oscillations due to reverse power flow, that the and Espinoza/Ruetz U.S. Pat. No. 3,335,314, granted Aug. 8, 1967, all of which are assigned to the present assignee. As will be apparent to those skilled in the art,

such TWTs may require a number, up to' eight or more,

sever loads to dissipate the power extracted at the terminations of each slow wave circuit and to dissipate reverse power encountered at the front end of the next slow wave circuit. Such loads must be relatively compact in size because they must be mounted relatively closely to the TWT in order to make the TWT compact and easily handleable. Prior art TWT sever loads, while able to handle the power required and provide low standing wave ratios (SWRs), were relatively large in size and therefore limited the compactness of the TWT. Accordingly several objects of the present invention are to provide a microwave load which is especially useful as a TWT sever load, has compact size,has low SWR, can be easily coole'd,.'and has good longevity. Further objects and advantagesof the present invention will become apparent from a consideration of the ensuing description thereof. 1

DRAWINGS:

FIG. 1 is a cutaway isometric view of a load-according to the present invention.

FIG. 2 is a cross-sectional view of the load of FIG. 1 taken along the lines 2--2.

FIG. 3 is a cross-sectional view of the load of the invention mounted on a TWT with the showing of the outer fluid coolant chamber used in association with said load.

FIG. 4 is a diagram of the voltage pattern associated with a prior art load.

FIG. 5 is a voltage diagram of the voltage pattern associated with the load of the invention.

DESCRIPTION Theinventive load, shown in FIGS. 1 to 3, comprises a three-section block or body of lossy material which is fitted into the end of a closed end waveguide 12. Block 10 isdivided into three sections by two conductive septa 14. The sections of block 10 and septa 14 are assembled together in a compact unit having a relatively thick body portion 16 and a tapered lead-in portion 18. The septa, which have edges in the surface of the tapered lead-in portion facing down the waveguide and surfaces in parallel with the narrow side of the waveguide, are shown for exemplary purposes as two in number, but one septum or three or more septa can also be employed. The septa are preferably spaced to divide block 10 into three equal-thickness sections as shown, but other spacings can be employed.

Although many materials are suitable for block 10 andsepta l4,"in one satisfactorily-operated embodiment of the invention, block 10 was composed of a sintered body of lossy (partially conductive) material consisting of 40 percent silicon carbide particles and percent beryllium oxide particles, such material being sold under the trademark CARBOLOX 40 dielectric by the National Berrylia Corporation. Septa 14 were made of copper sheets about 20 mils thick. The block was dimensioned to fit into a reduced-size x-band waveguide, i.e.,one which had dimensions of about 0.25 inch by about 0.7 inch, as contrasted with the standard size of 0.4 by 0.9 inch for an x-band waveguide. Although the dimensions of the load are not critical, in the aforementioned embodiment of the invention, the length of main portion 16 was about one-half inch and the length of tapered portion 18 was about 1 inch.

The load is mounted in waveguide 12 by means of conductivestrips 20 which are painted on the bottom and both sides of the load. These strips, of standard molybdenum-manganese metalizing, are about Vath inch wide and are spaced apart about U 16th inch. Although the thickness of the metalizing strips are only several mils, they are shown exaggered in FIG. 2 (sectional view) and FIG. 3 (thickness indicated by space I 21) for clarity. The load is attached to the waveguide by joining metalizing strips 20 to the interior surface of the waveguide using standard brazing techniques. I

. Although the inventive load itself will dissipate arela-' .coolant fluid to circulate around the load, thereby ab-.

sorbing power-therefrom. Because of its low cost and high heat absorbing capacity, water is preferred as a coolant but various other fluids, including air, maybe used.

OPERATION In operation, when microwave energy is applied to lead-in portion 18 of the load it will be absorbed by lossy material 10 of the load. As is well-known, tapered portion 18 is provided to aid in absorbing, rather than reflecting, microwave energy. The absorbed heat will be conducted, via strips 20, to the walls of waveguide 12, whereupon it will be removed by the flow of cooling fluid in outer jacket 24 (FIG. 3) or by radiation, conduction, and convection to the ambient itself if cooling jacket 24 is not used. In absence of conductive septa 14, the voltage waveform diagram taken along 'a cross section of the load will be as shown in FIG. 4, i.e., a large voltage will appear across the center of the load in the waveguide, as shown by arrow 28, while the voltage across the outer edges of the load in waveguide will be zero as indicated by the ends of voltage magnitude line 30. It will be apparent that a maximum amount of current will flow and hence a maximum amount of power will be dissipated at the center of the load, whereas at the edges of the load where the voltage is zero or close to it, a minimum current will flow and a minimum amount of power will be dissipated. Since the lossy material itself has relatively poor thermal conductivity, the maximum power handling capability of the load will be determined by the amount of power which the center portion can dissipate. The outer portions of the load will dissipate a relatively small amount of power and hence will not be used efficiently.

in the inventive load the presence of conductive septa 14 will effectively change the single waveguide into three separate propagating waveguides, causing the voltage level diagram to be as shown in FIG. 5. The voltage peaks tend to equalize in all three sections, whereby the waveguide load material is used far more efficiently, thereby substantially increasing its power handling capability.

In addition to dividing the waveguide into three different propagating waveguides and distributing the power far more uniformly in the load, due to their high thermal conductivity conductive septa 14 also provide uniform temperature planes at spaced locations in the waveguide. This aids in distributing temperature more uniformly throughout the volume of the lossy material, causing hot spots" to be substantially eliminated.

As a result, the power handling capability of the' load employing conductive septa 14 is substantially greater than that of a prior art load without the septa. A waveguide load as shown and described was able to handle and dissipate 2 kw of microwave energy.

While the above description contains many specificities, these should not be construed as limitations upon the scope of the invention, but merely as an exemplification of several preferred embodiments thereof. For example, instead of two septa, a single septum or more than two septa canbe used. The waveguide need not be rectangular but can have other shapes, such as circular, oval, or curved. Various other known lossy materials can be used in lieu of lossy material and the lossy material can be mounted in the waveguide by means other than the use of conductive strips 20. Other means of cooling the outside of the waveguide can beused. For example, a finned heat radiator can be provided. Accordingly, the scope of the invention should be determined only by the wording of the following claims and their legal equivalents.

What is claimed is:

1. A microwave load comprising: a waveguide having an input for microwave energy and a conductive wall having an internal cross section with a narrow dimension and a wide dimension, a block of lossy material mounted in said waveguide, said block being divided into a plurality of sections, adjacent sections being separated by a plurality of electrically conductive septa imbedded in said block in heat-conductive relationship therewith, one edge of each of said septa being in thermal contact with said wall of said waveguide, the plane of each septum being oriented parallel to said narrow dimension of said waveguide and parallel to the axis of said waveguide, the edge of each septum facing said input for microwave energy being substantially flush with the surface of said block facing said input.

2. The load of claim 1 wherein said block and said septum is tapered to a common edge which is spaced from the thickest part of said block toward said input.

3. The load of claim 1 further including a plurality of conductive strips on the surface of said block separating said block from the adjacent surface of said waveguide, whereby said waveguide can be exhausted of atmosphere more readily.

4. The load of claim 1 further including means for providing a flow of fluid around said waveguide for removing heat from the surface of said guide.

5. The load of claim 1 wherein said septa are formed of copper and said lossy material is' formed of a sintered mixture of silicon carbide and beryllium oxide.

6. The load of claim 1 wherein said block has a crosssectional dimension conforming to the internal shape of said waveguide.

7. The load of claim 6 further including a plurality of conductive portions on the surface of said body separating said body from said waveguide for facilitating vacuum pumping of said waveguide.

8. A microwave load comprising: a waveguide having an inputfor microwave energy and an axis in the direction of power flow and a conductive wall having an internal cross section perpendicular to said axis with a narrow dimension and a wide dimension, a block of lossy material mounted in said waveguide, said block being divided into a plurality of sections, a plurality of electrically conductive septa imbedded in said block in heat conductive relationship therewith, each of said wall of said waveguide, the planes of said septa being oriented parallel to said narrow dimension of said waveguide and parallel to said axis, the edges of said septa facing said input being substantially flush with the surface of said block facing said input.

Claims

5. The load of claim 1 wherein said septa are formed of copper and said lossy material is formed of a sintered mixture of silicon carbide and beryllium oxide.

6. The load of claim 1 wherein said block has a cross-sectional dimension conforming to the internal shape of said waveguide.

8. A microwave load comprising: a waveguide having an input for microwave energy and an axis in the direction of power flow and a conductive wall having an internal cross section perpendicular to said axis with a narrow dimension and a wide dimension, a block of lossy material mounted in said waveguide, said block being divided into a plurality of sections, a plurality of electrically conductive septa imbedded in said block in heat-conductive relationship therewith, each of said septa separating adjacent sections of said block, one edge of each septum being in thermal contact with said wall of said waveguide, the planes of said septa being oriented parallel to said narrow dimension of said waveguide and parallel to said axis, the edges of said septa facing said input being substantially flush with the surface of said block facing said input.