US3673370A

US3673370A - Microwave applicator system with cylindrical resonant cavity

Info

Publication number: US3673370A
Application number: US25402A
Authority: US
Inventors: Ray M Johnson
Original assignee: Cryodry Corp
Current assignee: MICRODRY Inc A CORP OF KENTUCKY; Cryodry Corp
Priority date: 1970-04-03
Filing date: 1970-04-03
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27

Abstract

A cylindrical microwave resonant cavity is excited in the TM010 mode to generate an electric field extending parallel to the axis of the cavity and having a maximum intensity along the axis. A filament or thread is conveyed axially through the cavity and heated. The cavity housing is formed into two sections along radial planes. The sections are hinged about a line parallel to the axis to facilitate opening the housing and cleaning its interior. Liners may be provided to cover the interior of each section. A slot is formed between the sections opposite the hinge line for threading the filament into its operative axial position. Clamps are provided to resiliently urge the sections together, yet the clamps permit adjustment of the angle of inclination between the sections by means of a screw mechanism to tune the resonant frequency of the cavity to the operating frequency of the microwave source. Air, preheated by cooling the source, is routed through the feed guide into the cavity to purge vapors from the cavity and prevent accumulation of residue. An electrical interlock is provided to inhibit operation of the microwave source when the sections are separated beyond a predetermined limit.

Description

United States Patent Johnson June 27, 1972 [54] MICROWAVE APPLICATOR SYSTEM Primary Examiner-R. F. Staubly WITH CYLINDRICAL RESONANT Assistant Examiner-Hugh D. Jaeger CAVITY Attorney-Carl C. Batz 57 ABSTRACT [72] Inventor: Ray M. Johnson, Danville, Calif. 1

A cylindrical microwave resonant cavity is excited in the Asslgnee: cryodl'y corpon'ilmi San Ramon: Callf- TM mode to generate an electric field extending parallel to 22 Fl (1: A l 3 1970 the axis of the cavity and having a maximum intensity along 1 l e the axis. A filament or thread is conveyed axially through the [21] Appl. No.: 25,402 cavity and heated. The cavity housing is formed into two sections along radial planes. The sections are hinged about a line parallel to the axis to facilitate opening the housing and clean- I52] U.b. 9| ..2l9/l0.55 ing its Marion Liners may be provided m cove, the interior of I "Hosb 9/06 each section. A slot is formed between the sections opposite [58] Field of Search l 9/l0.55 the hinge line for threading the filament into its operative axial position. Clamps are provided to resiliently urge the sections [56] Rekremes Cited together, yet the clamps permit adjustment of the angle of inclination between the sections by means of a screw UNITED STATES PATENTS mechanism to tune the resonant frequency of the cavity to the operating frequency of the microwave source. Air, preheated 3,339,054 8/1967 Deaton ..2l9/l0.55 by cooling the source, is routed through the feed guide into 3,407,279 10/1963 Greenberg the cavity to purge vapors from the cavity and prevent accu- 3,457,385 7/1969 Cumming ....2l9/10.6-1 mulation of residue. An electrical interlock is provided to in- 3,461,26l 8/1969 Lewis et al.... ....219/l0.6l hibit operation of the microwave source when the sections are 3,480,753 I l/ I969 Wilson et al. ..2l 9/ 10.55 separated beyond a predetermined limit.

17 Claims, 8 Drawing Figures i J J l l I l [Ilium 1 0 TERMINATION 7 STAB! LIZ/N6 NE T WORK I I IL J0 F'A'TENTEDJUNZ'I m2 SHEET 10F 2 STAB/L/ZING NETWORK TERMINATION STABILIZING NETWORK PATENTEDJum m2 SHEET 2 OF 2 MICROWAVE APPLICATOR SYSTEM WITH CYLINDRICAL RESONANT CAVITY BACKGROUND AND SUMMARY The present invention relates to microwave heating applicators; and more particularly, it relates to a system for applying microwave energy to a lossy dielectric object passing through a resonant cylindrical cavity.

Systems have been developed for applying microwave energy to objects for heating them. The application of microwave energy has been particularly useful in cooking food products. Among such developments have been batch type ovens in which the food material is placed in a multimode cavity, continuous type microwave ovens in which the food is passed through a multimode cavity, and serpentine configurations of waveguides of rectangular cross section in which the product (such as wallboard) is passed through aligned slots in opposing broadwalls of a number of waveguide sections folded so that all of the slots are aligned.

A copending, co-owned application of mine entitled Continuous Microwave Heating Or Cooking System, Ser. No. 816,722, filed Apr. 16, 1969, describes a rectangular waveguide applicator excited in the TE mode wherein the material being treated is conveyed through the applicator along the direction of power flow. The symbol TE refers to the transverse electric field vector; and TM" refers to the transverse magnetic field vector.

Another copending, co-owned application of mine entitled Resonant Cavity Microwave Applicator, Ser. No. 852,374, filed Aug. 22, 1969 discloses a system for heating a singleended filament including a resonant cavity having a cylindrical side wall and excited in the TM mode by a source of microwave energy. The filament is conveyed axially through the cavity and inthe location of maximum field intensity. The present invention relates to improvements in this latter type of microwave heating system.

The applicator of the present invention is a cylindrical resonant cavity which is excited in the TM mode preferably via a hollow waveguide to generate an electric field extending parallel to the axis of the cavity and having a maximum intensity along the axis. The cavity housing is formed into two sections divided by radial planes. The sections may, of course, be half-sections; and they are hinged about a line parallel to the axis of the cavity of facilitate opening and cleaning the interior of the cavity. Removable liners of low dielectric loss factor may be provided to cover the interior of each section. Thus, for example, when the applicator is being used to dry a filament that is impregnated with latex, the movement of the filament as it is transported through the applicator splashes the latex against the walls of the cavity. With the present invention, the down (i.e. inoperative) time of the applicator is greatly minimized because the liners may simply be replaced after shutting the microwave power off and opening the cavity about its hinges. After they are removed, the liners may be cleaned and re-used. Even if liners are not used, the ability to open the cavity about a hinge line greatly facilitates access to the interior of the cavity for cleaning.

An electric interlock is provided for preventing coupling of microwave power to the cavity when the sections of the cavity housing are separated beyond a predetermined limit.

At the input and exit apertures in the transverse plates of the cavity there are formed outwardly extending semi-cylindrical neck members; and spring clamps are fitted about associated pairs of these neck members for holding the cavity sections resiliently together. A screw mechanism is provided external of the cavity for adjusting the separation of the halfsections against the resilient urging of the spring clamps; and this provides a convenient mechanism for fine tuning of the resonant frequency of the cavity. The slot formed between half sections and communicating with the axis of the cavity is enlarged along the half-sections of the cavity opposite the hinge to facilitate threading of the filament into the cavity.

A probe is provided in one of the transverse end plates of the cavity at a peripheral location for monitoring the power coupled to the cavity to assist in matching the resonant frequency of the cavity to that of the source, which may be a conventional magnetron oscillator.

The air in the cavity is purged by a source of air which is derived from the cooling air for the magnetron; and this aid is fed through the feed waveguide to the cavity and exits through the apertures formed in the transverse end plates of the cavity as well as the threading slot; Thus, the air fed to purge the cavity is preheated and by forcing it through the feed waveguide, the accumulation. of material at the excitation aperture is prevented.

. Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like elements in thevarious views.

THE DRAWING FIG. 1 is a side elevational view of a microwave applicator constructed according to the present invention;

FIGS. 2 and 3 are respectively left and right end views of the applicator of FIG. 1,

FIGS. 4 and 5 are perspective views of the applicator of FIG. 1 showing it closed and opened respectively;

FIGS. 6 and 7 are views of the applicator diagrammatically illustrating the orientation andintensity of the electric field vector within the cavity; and

FIG. 8 is a perspective view of liners that may be used in the cavity of the present invention.

DETAILED DESCRIPTION As used herein, the word cavity" refers to the conductive wall members of the applicator as well as the volume defined thereby, and the volume alone is sometimes referred to as the heating chamber. The entire metallic structure surrounding the heating chamber is sometimes called the cavity housing. The applicator includes the cavity as well as the other elements attached to the housing and the microwave feed and excitation structure.

In the drawing, the applicator is generally designated by reference numeral 10 and, in the illustrated embodiment, the applicator takes the form of a right circular cylinder divided into

semi-cylindrical sections

11 and 12. Each of the

sections

11 and 12 is integrally formed of a conductive metal such as aluminum, and each preferably has an interior coating of material of still lower resistivity, such as silver. if a silver plating is used, a thickness of about 0.2 mils is sufficient to exceed the skin depth of the currents in the cavity walls at 2.45 GHz and thereby to provide the necessary shielding from the bulk portion of the metal comprising the housing of the resonator. The use of metals with an even greater conductivity will result in a corresponding increase in efiiciency.

The housing section 1 l is provided with a pair of apertured ears 13,14; and the other housing section 12 is provided with a corresponding set of apertured ears 15,16. A hinge pin 17 is fitted through the apertures in the cars 13 and 15, and a similar pin 18 is fitted through the apertures in the ears l4 and 16 so that the

sections

11,12 may be rotated apart about these hinge pins as shown in FIG. 5 to pemiit access to the interior of the cavity. Although in the illustrated embodiment the sections are separated along a common plane, persons skilled in the art will appreciate from the entire disclosure that these sections need be separated only along two radially extending planes passing through the axis, or simply radial" planes.

As seen best in FIG. 5, the section 11 includes asidewall 11a having a semi-cylindrical shape and transverse end plate sections 1 lb and 1 1c. Similarly, the section 12 includes a semicylindrical sidewall 12a and transverse end plates 12b and 126.

About the center of each of the semi-cylindrical transverse end sections or

plates

11b, 11c, 12b, and 121:, there if formed an aperture, designated respectively 20, 21, 22 and 23. About each of the apertures 23 and extending outwardly from its associated transverse end section, there are attached semicylindrical neck members designated respectively by reference numerals 24-27. Thus, when the housing sections of the resonant cavity are closed as illustrated in FIG. 4, the

side neck members

25 and 27 form an entrance 28 for a filament, designated 29 and passing along the axis of the cavity. Similarly, the

side neck members

24 and 26 form an exit aperture generally designated by reference numeral 30 in FIGS. 1 and 2.

Turning now to FIG. 4, portions of the adjacent edges of the

sections

11 and 12 are cut away to form a widened slot generally designated 32 which extends in a radial plane passing through the axis of the cavity (along which the string 29 extends) and communicates the axis of the cavity with the exterior thereof. The slot 32 also extends through the

neck members

24, 25, 26 and 27; and it permits threading of the filament 29 into its operative position without having to open the two sections of the cavity housing.

First and second spring clips 33 (see FIG. 5) and 34 of similar shape are placed about the respective pairs of

neck members

25, 27 and 24, 26 to resiliently bias or hold the

halfsections

11 and 12 in a closed position.

As best seen in FIGS. 1, 2 and 5, a pair of

opposing flanges

35, 36 are welded respectively to the outer surfaces of the

transverse end sections

11b and 12b and extend radially outwardly from the

respective neck members

24 and 26. The flange 36 defines an internally threaded aperture 37 for receiving an externally threaded mating stud or bolt 38. By screwing the bolt 38 through the aperture 37 of flange 36, it engages the underside of flange to force the half-sections ll, 12 apart against the action of the spring clamps 33, 34 to tune the resonant cavity to the operating frequency of the microwave source, as will be more fully explained within.

Turning now to FIGS. 3,4 and 5,

similar flanges

40, 41 are welded respectively to the exterior surfaces of the transverse end sections 11c and 12c; and these flanges also extend radially outwardly from the

neck members

25 and 27 respectively. A limit switch 43 is mounted on the transverse end section 110, and it includes a plunger 44 which extends through an aperture 45 in the flange 40. The plunger 44 engages the inner surface of the opposing flange 41 when the

housing sections

11, 12 are closed, as seen by the dashed lines at 44a in FIGS. 5. When the

housing sections

11, 12 are opened beyond a predetermined limit the plunger 44 disengages the flange 41 and opens the switch 43 to shut off the source of microwave energy, which preferably includes a magnetron oscillator tube. The operation of switch 43 does permit sufiicient movement between the housing sections to tune the resonant frequency of the cavity over the desired range before shutting off the microwave source.

Turning now to FIGS. 1 and 2, a probe generally designated by reference numeral 47 is attached to the transverse end section 11b, and it includes a center conductor or lead 48 (FIG. 5) which extends into the interior of the cavity for coupling a small amount of microwave energy from the cavity to monitor the power within the cavity. As will be made clear from subsequent discussion, the probe 47 is located toward the periphery of the transverse end plate 11b and at a location of minimum electric field intensity so as to couple a relatively small amount of power from the cavity for monitoring purposes. The probe is useful, in combination with an oscilloscope (not shown) to determine the resonant frequency of the cavity by adjusting the tuning screw 38 to vary the opening between the half suctions 11, 12 in tuning the frequency of the cavity to the frequency of the microwave source. That is, by observing the power level on an oscilloscope coupled to the cavity by the probe 47, one knows that the resonant frequency of the cavity is tuned to that of the source when the power level is at a maximum.

Turning now to the structure which couples the microwave energy to excite the cavity, the partially illustrated block 50 is a diagrammatic representation of the housing of a source of microwave energy which preferably includes a magnetron oscillator tube; and the output microwave energy is coupled by means of a flanged rectangular waveguide 51 to a similarly flanged end termination section 52 which, as seen in FIG. 4, includes a peripheral flange 53, a side wall 54 of rectangular cross section and a transverse end plate 55. An aperture providing a coupling iris 57 is formed in the transverse end plate 55 in line with a corresponding aperture 58 formed in the semi-cylindrical side wall 1 1a of the housing section 11.

As seen in FIG. 1, a second waveguide 59 is connected to a broadwall of the feed guide 51 and couples the feed guide to a terminating load which is schematically designated in the drawing by the E-plane Tee" formed by the

guides

51 and 59 for a stabilizing network, block 59a. The dimension of the side wall of the coupling wave guide 59 may be one-half the dimension of the side wall of the feed wave guide 51. The stabilizing network may be constructed. according to the manner disclosed in Microwave Electronics by .l. C. Slater (Van Nostrand Co. 1950) at about p. 200 or according to the disclosure of the article of H. F. Huang entitled Microwave Apparatus For Rapid Heating of Threadlines" and published in the Fourth Symposium of International Microwave Power Institute (I969). The function of the stabilizing network is to insure the generation of power by the magnetron at the resonant frequency of the cavity. It has been found that, because of the impedance of the cavity is so highly frequency dependent, power can be generated at frequencies other than the resonant frequency of the cavity and, therefore, never accepted by it.

Turning now to FIG. 8, there are shown first and second

semi-cylindrical liners

60 and 61 made of a material having a low dielectric loss factor such as rigid polyethylene. Since there is some wobble in transporting the filament through the cavity at fairly high speeds (of the order of 50-100 feet per section) in those applications wherein the filament is im' pregnated with a liquid such as latex and it is desired to dry the filament, there will be some splashing of the impregnated material on to the side walls of the cavity where the material will collect. For these applications and for other applications in which the interior of the cavity is likely to be coated during operation, the

plastic liners

60 and 61 have been found to be advantageous in that they can be removed from the cavity and replaced by clean liners to minimize the down time of the equipment.

Turning back to FIGS. 1 and 2, air is passed by a fan (not shown) over the magnetron for cooling the tube, and this heated air, the flow of which is represented by the arrows 63, is passed from the housing 50 of the microwave source through the feed waveguide 51, the termination section 52, the aperture 57, and into the interior of the heating chamber from which it is forced either through the slot 32, or the entrance or exit apertures 28, 30. As mentioned in the aboveidentified copending application, Ser. No. 852,374, the combination of forced air into the cavity and the microwave heating achieves much better results than the heating alone, not only to carry moisture away, but to cause thermal equilibrium within the cavity. That is, if the cavity walls change in temperature, the resulting expansion will cause a shift in the resonant frequency of the cavity. It has been found that more effective drying results are achieved by preheating the air, for example as disclosed herein, by passing it over the operating magnetron oscillator tube. In the particular arrangement illustrated, namely wherein the air passes through the coupling aperture between the feed waveguide and the cavity sidewall, the forced air also serves to purge the resonator and feed guide of vapors and thus inhibit the collection of material about the coupling aperture and the interior of the feed waveguide 51.

Before discussing the nature of the electromagnetic field within the cavity, the features of the above-described structure will be summarized. The cavity is fabricated in two separate sections which are hinged about an axial edge opposite the threading slot 32 for opening. This arrangement allows the cavity to be conveniently opened for cleaning or removing of liners and, secondly, it permits the tuning of the resonant frequency of the cavity. Tuning is accomplished by means of the thumb screw 38 threadably received in the flange 36 and acting against the flange 35. The sections of the cavity housing are biased in a closed position by means of the clamping springs 33 and 34.

An electrical switch is provided to sense when the cavity is opened to provide an interlock so that microwave power cannot be generated with the cavity opened. However, there is no hazard if the interlock is not provided because if either the filament 29 is not present or the cavity is opened, the mismatch between the cavity and the source is so great that very little energy is coupled to the cavity. Under normal operating circumstances with about 1,000 watts of microwave power generated by the magnetron and the interlock system defeated, radiation of microwave energy at the plane of the slot with the cavity open was greater than db below the acceptable density of 10 mw/cm.

By routing the air used in cooling the magnetron through the feed waveguide and exhausting it into the applicator at the coupling position, the incoming air is preheated and serves to purge the resonator vapors, to inhibit buildup about the input coupling iris and feed waveguide, and create conditions of thermal equilibrium within the cavity.

Turning now to the electromagnetic field within the cavity, if one selects the dimensions of a conducting right cylindrical resonator with regard to the wavelength of the excitation frequency, the resonant frequency of a specific electric field pattern within the resonator can be made to match the output frequency of the magnetron that excites the cavity. The mode used in the hinged applicator of the present invention is the lowest order mode, referred to as the TM mode. As schematically illustrated in FIG. 7, the electric field vector (designated by the vertical arrows 65) extends parallel to the axis of the cylindrical side wall; and this is referred to as the z direction. The magnitude or intensity of the electric field varies with the radius, r, or distance from the axis according to the following equations:

2 0( c wherein E the electric field intensity at r= 0;

k 2.405/a (where a is the radius of the cavity);

r= the radial position from the axis; and

J the Bessel function of first kind, order 0.

Ez is independent of the longitudinal position along the axis,

2, and the angular position, D. If the filament is run parallel to the z direction at the position r O (or a web fed transversely through the axis), the material will be located at the position of maximum electric field intensity and, hence, it will experience the maximum heating rate which is proportional to the square of the electric field intensity. Moreover, in the unperturbed case, this intensity is substantially uniform in the z direction, and the actual heating rate as a function of z will not vary appreciably due to large changes in the electric field intensity. That is, the heating rate will be primarily a function of the dielectric loss properties and/or the moisture present as a function of axial distance, z.

In this particular mode, the magnetic field lines (represented schematically by the dashed circular lines in FIG. 6) extend about the axis of the cavity, and they increase in intensity in proceeding from the axis to the sidewall, pictorially demonstrated by the closeness of the dashed circular lines. It can also be seen diagrammatically from FIG. 6 that the intensity of the electric field is at a maximum at the center of the cavity. In FIG. 6, the electric field vectors are represented by the small circles. The currents flow along the inner surface of the cavity as schematically illustrated by the lines 70, and it will be appreciated that the currents are substantially equal for all angular positions about the interior surface of the cavity.

In addition to allowing the slot opening for threading the strand with minimum radiation during operation, the current orientation of this mode permits the applicator to be split completely apart as illustrated in FIG. 5 at the plane of the slot (although as already mentioned, the radial planes of the two slots need not be coextensive).

In a preferred embodiment a JC 300 magnetron tube manufactured by the General Electric Co. is used as the power source. It operates at a frequency of 915 MHz. and a power level of 1.0 Kw. The feed waveguide is of the type known in the art as WR 975. About one-third of the power is consumed in the stabilizing network load and the rest absorbed in the cavity. The cavity is about 10in. in diameter and 8 in. in axial length. The inlet and outlet apertures for the filament are 1.5 in. in diameter. The width of the threading slot is variable from an almost closed position to about one-fourth in. at the edge. The amount of air may range from to cubic feet per minute.

It will be appreciated by persons skilled in the art that the cavities of the type disclosed herein may be used in tandem so that the filament passes through a first cavity and then directly through a second, following cavity in the same manner as illustrated above. In a situation such as this, the second cavity could be made somewhat longer to more evenly divide the power consumption because the filament will be more dry upon entering the second cavity.

Persons skilled in the art will appreciate that the inventive principle may also be employed in an applicator of the type described above but adapted to receive a web to be dried or a conveyor supporting article to be dried wherein the web is moved in a plane passing through the axis i.e., in the area of maximum electric field intensity in the neighborhood of the axis. Such a system is disclosed in my copending, co-owned application for Microwave Applicator For Heating Continuous Web, Ser. No. 860,657, Filed Sept. 29, I969. Further, although the TM mode of excitation is preferred, other higher order modes may as well be used wherein the maximum field intensity extends along the axis of the cavity.

Having thus described in detail a preferred embodiment of the inventive principle, it will be apparent to persons skilled in the art that certain modifications may be made to the structure illustrated and that elements may be substituted for those disclosed; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims.

I claim:

1. In a system for applying microwave energy to material, the combination comprising resonant cavity means including a cylindrical side wall and formed by first and second separatable housing sections, hinge means pivotally securing said first and second sections about a hinge line extending generally parallel to the axis of said side wall whereby said sections may be moved to a closed position in which the sides of said sections form the general shape of a cylinder or split apart to gain access to the interior of said cavity, microwave energy source means for exciting said cavity to generate an electric field within said cavity extending generally parallel to the axis of said side wall when closed and having a maximum intensity in the neighborhood of said axis, said housing sections defining aperture means for transporting said material through the axis of said cavity.

2. The system of claim 1 further comprising means for continuously forcing air into the interior of said cavity to purge the same of moisture vapor.

3. The system of claim 2 wherein said last-named means includes means receiving heated air passed over the source of microwave energy and forcing the same through said excitation means and into said cavity to be exhausted through said aperture means at a rate in the range of 80-120 cubic feet per minute.

4. The system of claim 1 wherein said housing sections are semi-cylindrical in form and are separated along a plane passing through said axis and wherein said cavity further includes transverse end plates at each end of each of said sections, said end plates defining inlet and outlet apertures for permitting passage of a resonent along the axis of said cavity.

5. The system of claim 1 further comprising means for resiliently urging said housing sections in a closed position, and further comprising adjusting mechanism including screw thread means secured to one of said housing sections and engaging the other for selectively moving said sections relative to each other about said hinge means to tune said resonant frequency of said cavity.

6. The system of claim 1 further comprising first and second neck members secured respectively to said first and second housing sections adjacent said aperture means through which said material passes, spring means engaging said neck members for urging the same together to bias said housing sections in closed position, and adjustable means secured to one of said housing sections and engaging the other section to force said sections apart against the action of said spring means to adjust the angle between said sections in tuning the resonant frequency of said cavity.

7. The system of claim 1 further comprising electrical interlock means responsive to the separation between said housing sections for preventing the excitation of said cavity by said microwave source when said sections are open beyond a predetermined limit.

8. The system of claim 7 further comprising probe means mounted on said cavity housing and including a probe extending into the cavity adjacent the cylindrical side wall for monitoring the energy therein in tuning the resonant frequency of said cavity to the frequency of the microwave source.

9. The system of claim 1 wherein said excitation means includes a rectangular waveguide defining an iris in register with a corresponding aperture in the side wall of said cavity for exciting the same in the TM mode.

10. The system of claim 1 wherein said housing sections are separated along one plane extending through said axis and wherein said material is a filament and said aperture means includes an inlet and an outlet aperture for passing said filament along said axis.

1 1. The system of claim 10 wherein the separation between said cavity sections from the axis to the side opposite said hinge means permits threading of said filament.

12. in a system for applying microwave energy to a filament,

the combination comprising resonate cylindrical cavity means excited by microwave source means to generate an electric field having maximum intensity along the axis of said cavity and provided with transverse end plates each defining an axial aperture for permitting passage of said thread through the axis of said cavity and along the region of maximum field intensity of said cavity when excited, said cavity being divided into first and second separatable housing sections, and tuning means for predetermining the separation between said halves about a hinge line to tune the resonant frequency of said cavity to the frequency of a source of microwave energy exciting the same.

13. The system of claim 12 wherein said tuning means further comprising resilient means urging said housing sections together and rigid means for setting the minimum distance at which said sections may close.

14. In a system for applying microwave energy the combination comprising resonant cavity means including a cylindrical side wall and provided with aperture means to pass the material being treated through the axis thereof, microwave source means including feed means connected to said cavity for exciting said cavity to generate an electric field having a maximum intensity along said axis, and means for passing air through said feed means and into said cavity to purge vapors from said cavity at least partially through said aperture means and to minimize the collection of material about the connection between said feed means and said cavity means.

15. The system of claim 14 wherein said feed means includes hollow waveguide means exciting said cavity through a coupling aperture and wherein said air is preheated and passed through said waveguide and said coupling aperture.

16. The system of claim 14 wherein said cavity is characterized by having first and second semi-cylindrical sections separatable about a hinge line along the side wall thereof to facilitate access to the interior of said cavity.

17. The system of claim 14 wherein said source means includes a magnetron oscillator, and further comprising stabilizing network means connected in circuit with said cavity and said source means for causing said magnetron to oscillate at the resonant frequency of said cavity and thereby establish the coupling of power to said cavity from said source.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3.673.370 Dated June 2?.i972

Patent No.

Inventor s) RAY M. JOHNS ON It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 75, "there if formed" should read --there is formed-. Column 3', lines 44 and 45, "Figs. 5" should read --Fig. 5--. Column 5, line 23, "resonator vapors" should read --resonator of va ors--. Column 6, Claim 4, line 74, "passage of a resonent should read --passage of a filament--.

Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER JR A'ttesting Officer ROBERT GOTTSCHALK Commissioner of Patents FORM 5 0-1050 (10-69) USCOMM-DC 603764 69 U.S, GOVERNMENT PRINTING OFFICE: I969 O-366-334 UNITED STATES PATENT OFFICE CE 5 THECATE OF 0" ECT-lQ-N' Patent No. 3 ,673 ,370 I Jated June 27. 1972 RAY M.. JOHNSON Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 75, "there if formed" should read -there is Column 3, lines 44 and 45, "Figs. 5" should read --Fig. 5- Column 5, line 23, "resonator vapors" should read --resonator of va ors--. Column 6, Claim 4, line 74, "passage of a resonent should read --passage of a filament--.

formed---o Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GO'I'TSCHALK Attesting Officer Commissionerof Patents USCOMM-DC 60376-P69 FORM PO-1050 (10-69) u.s. covsmmem PRINTING OFFICE: was oasa-aa4

Claims

9. The system of claim 1 wherein said excitation means includes a rectangular waveguide defining an iris in register with a corresponding aperture in the side wall of said cavity for exciting the same in the TM010 mode.

11. The system of claim 10 wherein the separation between said cavity sections from the axis to the side opposite said hinge means permits threading of said filament.

12. In a system for applying microwave energy to a filament, the combination comprising resonate cylindrical cavity means excited by microwave source means to generate an electric field having maximum intensity along the axis of said cavity and provided with transverse end plates each defining an axial aperture for permitting passage of said thread through the axis of said cavity and along the region of maximum field intensity of said cavity when excited, said cavity being divided into first and second separatable housing sections, and tuning means for predetermining the separation between said halves about a hinge line to tune the resonant frequency of said cavity to the frequency of a source of microwave energy exciting the same.