US3434076A

US3434076A - Waveguide window having circulating fluid of critical loss tangent for dampening unwanted mode

Info

Publication number: US3434076A
Application number: US316865A
Authority: US
Inventors: Floyd O Johnson
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1963-10-17
Filing date: 1963-10-17
Publication date: 1969-03-18
Anticipated expiration: 1986-03-18

Description

March 1969 F. o. JOHNSON 3 76 WAVEGUIDE WINDOW HAVING CIRCULATING FLUID 0F CRITICAL LOSS TANGENT FOR DAMPENING UNWANTED MODE .Filed Oct. 1?, 196a E INVENTOR.

r FLOYD O.JOHNSON '0' BY ATTORNEY United States Patent 4 Claims ABSTRACT OF THE DISCLOSURE A microwave waveguide window structure is disclosed. The window structure includes a dielectric Wave permeable structure hermetically sealed across the interior of a waveguide. The dielectric window structure includes or is constructed to define a fluid passageway therethrough. Means are provided for circulating a dielectric fluid througn the passageway, said fluid having a loss tangent between 0.001 and 0.009 for mode damping of the window structure and for cooling of the window structure in use. By providing the loss tangent between 0.001 and 0.009, the loss presented by the fluid to the transmission mode is substantially less than that which would be presented by water as the coolant. Therefore, a relatively small amount of the desired power to be transmitted through the window is absorbed in the dielectric fluid, thereby permitting relatively eflicient cooling of the window structure in use. However, this relatively low loss tangent provides suflicient damping for certain resonant ghost modes to prevent sustained excitation thereof, whereby overheating of the window structure in use is prevented since the ghost modes are effectively damped.

This invention relates, in general, to waveguide transmission, and more particularly to waveguide Windows, and to devices using same.

In many high frequency systems it is necessary to pass wave energy through a barrier which separates two regions in the system, one of which is normally evacuated. Ideally, such .a barrier, called a window, is vacuum tight so that the integrity of the vacuum region is maintained, and permits passage of a broadband of frequencies at high power, while presenting a minimum of interference so that energy will not be reflected or dissipated as it passes through.

One major limitation in waveguide transmission has been that certain resonant modes (hereinafter referred to as resonant ghost modes) exist at discrete frequency regions across the frequency band passed by windows constructed according to the teachings of the prior art. The half power points of the ghost mode bandwidth are usually 2 me. or less apart giving a mode Q of 1000 to 2000. Such a high Q circuit, when excited with high RF. fields dissipates a great deal of power in the window in the form of heat, frequently causing the window to rupture. In order to prevent window failure, the frequency range of such devices had to be limited to a range between resonant ghost modes, or the power to be transmitted had to be decreased. Thus, a severe limitation was placed on bandwidth and power handling capabilities of devices characteristic of the prior art.

In accordance with the teachings of the present invention, the mode Q of the resonant ghost modes is lowered by increasing losses within the circuit. A dielectric structure transparent to electromagnetic waves is sealed within and across a waveguide. The structure has a fluid passageway therethrough positioned in energy exchanging relationship with the waves that pass through. A moderately ice lossy fluid having a low dielectric constant relative to air, preferably within the range of 1.0 to 3.0 inclusive is caused to circulate through the passageway within the passageway through the dielectric structure. By a moderately lossy fluid is meant one which has a loss tangent high with respect to ceramic but low with respect to water, preferably within the range of 0.001 and 0.009 inclusive. The use of this fluid serves both to facilitate removal of extra heat and provide cooling of the dielectric structure, as well as to damp out and prevent excitation of the harmful resonant ghost modes. In this way, power handling capabilities and bandwidth are greatly increased.

It is the object of this invention, therefore, to provide a waveguide window, and devices using same, capable of passing high power over a broad band of frequencies.

One feature of the present invention is the provision of a microwave transmission device including a dielectric structure sealed within a waveguide and having a fluid passageway therethrough positioned in energy exchanging relationship with the Waves passing therethrough, and means for circulating a moderately lossy fluid having a low dielectric constant relative to air through the structure.

Another feature of the present invention is the provision of a microwave transmission device including a dielec tric structure sealed within a waveguide comprising a pair of thin spaced-apart solid dielectric members, the dielectric members forming with the internal walls of the waveguide a chamber for passage of fluid therethrough, for example, a moderately lossy fluid having a low dielectric constant relative to air.

Still another feature of the present invention is the provision of a microwave transmission device including a dielectric structure sealed within a waveguide comprising a half wavelength rectangular block of dielectric material containing spaced bores within said material for passage of a moderately lossy fluid with a low dielectric constant relative to air therethrough.

A further feature of the present invention is the provision of a microwave transmission device including a. dielectric structure sealed within a waveguide and a pair of members to act as corona shields and support means, disposed within the inner wall of said waveguide, one of said members being juxtapositioned to either side of the dielectric structure, the members having a curved outer surface, and preferably with a radius of curvature greater than 0.02".

A still further feature of the present invention is the provision of a microwave transmission device of any of the above types in an electron discharge device.

These and other objects and features of the present invention and a further understanding may be had by referring to the following description and claims, taken in conjunction with the following drawings in which:

FIG.1 is a plan view of an electron discharge device utilizing features of the present invention;

FIG. 2 is an enlarged cross-sectional view of a preferred embodiment of the microwave transmission device of the present invention, taken along the lines 22 of FIG. 1;

FIG. 3 is a cross-sectional view taken along the lines 3-3 of FIG. 2;

FIG. 4 is an enlarged cross-sectional view of another embodiment of the novel microwave transmission device of the present invention;

FIG. 5 is a cross-sectional view taken along the lines 55 of FIG. 4;

I IG. 6 is a plot of frequency vs. voltage standing wave ratio (VSWR) for a novel waveguide window constructed in accordance with the teachings of the present i tron; and

FIG. 7 1s an enlarged cross-sectional view of the area 3 delineated by the line 7-7 of FIG. 2.

Referring now to the drawings and in particular to FIG. 1, there is shown an electron discharge device ern ploying novel features of the present invention. A multicavity klystron amplifier tube 11 of the type shown and described in more detail in US. application Ser. No. 148,- 520, filed Oct. 30, 1961 and assigned to the same assignee as the present invention, comprises three main portions: a beam producing section 12 on one end which serves to form and project a beam of electrons over a predetermined path directed axially and longitudinally of the tube 11; a central beam interaction section 13 where interaction takes place between the projected electron beam and an applied electromagnetic wave to produce amplification of the wave; and, a collector structure 14 at the terminating end of the tube 11 where the electrons of the spent beam are collected. A coolant such as water is supplied to the collector structure 14 via fluid fittings 15 and circulates through ducts (not shown) in the collector structure 14.

The tube 11 is evacuated to a suitable low pressure, for example, 10- torr. Input wave energy to be amplified is coupled to the upstream end of the beam interaction section 13 via the intermediary of a rectangular waveguide 16 and through a vacuum tight Waveguide structure 17 which supports a window sealed therein (not shown) transparent to electromagnetic waves. Amplified output wave energy is extracted in the conventional manner at the downstream end of the beam interaction section 13 via the intermediary of a rectangular waveguide 18 and through an output waveguide window structure 19 to be described in more detail below.

Referring now to FIGS. 2 and 3, the waveguide 18, made of, for example, copper is brazed in vacuum tight communication to the cylindrical housing 20, as of copper, of waveguide window structure 19. The housing 20 carries transversely therein a section of circular waveguide 21 as of copper. A pair of circular, thin dielectric discs 22 are mounted within section 21 substantially normal to the direction of propagation of energy through section 21, being spaced from each other on either side of an annular rib portion 23 of section 21. The discs 22 are preferably made from an alumina type ceramic although any solid dielectric material which is both transparent to electromagnetic waves and capable of being sealed in vacuum tight communication to the inner wall of section 21 can be used. For example, A1 0 BeO, fused quartz, single crystal sapphire and boron nitride may be used to advantage. Sealing of the discs 22 to the inner wall of section 21 is by any of the well known sealing techniques, as for example, by brazing. The discs 22 together with rib portion 23 form a narrow chamber 24 substantially normal to the direction of energy propagation.

In a preferred embodiment, the abrupt transitions between rectangular 18 and circular 21 waveguides ar made electrically in the order of 11/2 wavelengths apart at the center frequency of the passband, where n can be any integer value with a net capacitive discontinuity existing at the junction of the waveguides. The discs 22 are centered at the midpoint between the two transitions, and the thickness of the discs plus the distance between the discs 22 is subsantially less than one-half an electrical wavelength at the center frequency of the passband.

The minimum thickness of the discs 22 is determined by mechanical strength and ability to maintain a vacuum. Where the discs 22 are particularly thin the problem becomes more acute. To circumvent this hoops 25 of round cross-section, as of copper wire are brazed to the discs 22 i and section 21 around the circumferenc of the discs 22.

It has been found that these hoops 25 increase the strength of the discs 22 by a factor of 1.5 and affect electrical characteristics only slightly. The discs 22 with hoops 25 are less likely to fracture during bakeout or during high power operation when overheating is experienced or when there is excessive waveguide pressurization.

A pair of bores 26, 27 in section 21 are aligned with a pair of openings 28, 29 in housing 20. A nozzle 30, as of Monel, mounted within opening 28 and connected to tubing 31, as of nickel or platinum, mounted within bore 26 provides an entry-way to chamber 24 from a schematically represented external fluid circulating means 32, while a similar nOZZle 33 and tubing 34 provide an exit Passageway. Of course, additional entry-ways and exit passageways could be provided.

External fluid circulating means 32 comprises: a fluid circulating pump 35 causing a fluid to flow; a heat exchanger 36, for example, a 5 kilowatt heat exchanger capable of maintaining the temperature of the fluid between 50-150" F.; and, conduits (not shown) providing connection between the pump 35, chamber 34 and heat exchanger 36.

In the present invention it has been discovered that when the external fluid circulating means 32 provides a flow, preferably turbulent, of a moderate lossy fluid having a low dielectric constant relative to air through chamber 24, the circulation of this fluid at an adequate flow rate serves both to facilitate removal of extra heat and provide cooling of the discs 22, as well as to damp out and prevent excitation of the harmful resonant ghost modes. For example, inert fluorocarbons such as FC- or FC-43 manufactured by the Minnesota Mining and Manufacturing Company may be used to advantage. These fluids have a dielectric constant of approximately 1.9 and are some twenty times as lossy as the ordinary solid dielectric materials used in waveguide windows. Water, however, which is some twenty three times more lossy than these fluorocarbons would not be suitable since too much power would be dissipated by the fluid, thus resulting in a substantial reduction in the power transmitted through the window.

Generally, th selection of the fluid represents a compromise between having a substance with a loss tangent high enough to damp out and prevent excitation of the harmful resonant ghost modes, yet low enough to keep the loss of power being transmitted to a minimum. At the same time, the dielectric constant of the fluid relative to air should be low enough to minimize impedance matching problems. For higher power operation, a useful range of loss tangent would be 0.001 to 0.009, and of dielectric constant relative to air, 1.0 to 3.0. Ideally, the loss tangent would be 0.001 and the dielectric constant 1.5. For this case the discs 22 could be spaced for most efiicient cooling, with less effect on the impedance match, and minimum power loss.

In a typical embodiment of the present invention, a waveguide window structure to as constructed using AL- 400 ceramic discs 1.4" in diameter, 0.03" thick, spaced 0.08" apart and supported by wire hoops of cross-sectional diameter 0.062". FC-75 was circulated through the chamber between the discs at the rate of 0.5 gallon per minute. With test frequencies centered at 7762 me. (see FIG. 6) the window was able to transmit kw. of power over a bandwidth of better than 33.3", and with a VSWR under 1.2. No resonant ghost modes were detectable. Approximately 1300 watts out of 180 kw., or 0.72% of the input power was dissipated in the window in the form of heat. The temperature difference across the window was 20 C. The embodiment described was found particularly suitable when transmitting the TE mode, and for damping out all resonant ghost modes including TE1111 211 221, sn, TMOIO: and ui- Referring now to FIGS. 4 and 5 disclosing another embodiment of the present invention, a rectangular half wavelength block 37 of dielectric material as, for example, alumina ceramic is mounted within a rectangular waveguide 38. A series of holes 39 are drilled through block 37 to form fluid ducts. Waveguide 38 also contains apertures 40 which are in alignment with holes 39. The space 41 between housing 42 and waveguide 38 is adapted to receive a moderately lossy dielectric coolant which flows through nozzle 43 in opening 44 through ducts 39 and blocks 37, and out through nozzle 45 and opening 46. The fluid is prevented from flowing just through the space 41 by means of a pair of diametrically opposed septums or fins 47.

Referring now to FIGS. 2 and 7, in sealing the discs 22 to the section 21, a metallized coating 48, such as, for example, a brazing alloy is disposed between disc 22 and section 21 thereby forming a vacuum tight seal. The thickness of the coating 48 about the periphery of disc 22 may be on the order of 0.005 to 0.020 and sometimes slightly wider.

In practice, a certain amount of the coating material 48 comes up to and beyond the edge of the disc 22. The coating material flows down the side of disc 22 to a small extent. Thus a fillet 49 is formed, but with a sharp edge 50. Because this sharp edge 50 is in a region of high electric field, it tends to cause corona discharge and eventually breakdown. It has been found that if a member having a curved outer surface is juxtapositioned, as by sealing, on both sides of the disc 22 and disposed within the inner walls of the waveguide, as by sealing, and the radius of curvature of the member is greater than the width of the sealing material, the members will act as corona shields. For most devices this would call for the members to have a radius of curvature greater than 0.02". Thus hoops 25 serve a second purpose, acting as corona shields.

While the invention has been described with respect to the output window of a klystron tube, it is obvious that the invention can be used anywhere in a microwave system where a window which must transmit high power over a broad band of frequencies is required.

Since many changes can be made in the above construction and many apparently widely dilferent embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A microwave transmission device for transmitting electromagnetic waves comprising, conductive wall means defining a waveguide for the electromagnetic waves, a dielectric structure transparent to said waves sealed across said waveguide, said dielectric structure having a fluid passageway therethrough positioned in energy exchanging relationship with said waves passing therethrough, means forming a dielectric fluid having a loss tangent between 0.001 and 0.009 inclusive, and means for circulating said dielectric fluid through said passageway for mode dampening said dielectric structure.

2. The device according to claim 1 wherein said fluid has a dielectric constant relative to air between 1.0 and 3.0 inclusive.

3. The apparatus of claim 1 wherein said dielectric structure transparent to said waves includes, a pair of spaced apart solid dielectric members transparent to said waves and sealed across said waveguide, and wherein said fluid passageway through said dielectric structure includes a fluid chamber defined between said dielectric members and by said waveguide.

4. The apparatus of claim 1 wherein said dielectric structure includes, a block of dielectric material transparent to said waves sealed across said waveguide, said block containing a plurality of spaced bores forming said fluid passageway.

References Cited UNITED STATES PATENTS 2,925,515 2/1960 Peter 31539.3 X 2,958,834 11/1960 Symons et a1 33398 2,990,526 6/1961 Shelton 333-98 3,101,461 8/1963 Henry-Bezy et a1 33398 3,110,000 11/1963 Churchill 33373 3,158,823 11/1964 Bird et al 17415 X FOREIGN PATENTS 669,250 4/ 1952 Great Britain.

ELI LIEBERMAN, Primary Examiner.

MARVIN NUSSBAUM, Assistant Examiner.