US3559043A

US3559043A - Bimodal cavity resonator and microwave spectrometers using same

Info

Publication number: US3559043A
Application number: US650891A
Authority: US
Inventors: James S Hyde
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1967-07-03
Filing date: 1967-07-03
Publication date: 1971-01-26
Anticipated expiration: 1988-01-26
Also published as: DE1773746A1; FR1571862A; GB1228419A; DE1773746B2; DE1773746C3

Abstract

A CENTRAL PORTION OF THE CAVITY IS CAPABLE OF SUPPORTING TWO RESONANT MODES WHILE ADJOINING EACH SECTIONS EACH SUPPORT ONLY ONE OF THE MODES. WAVEGUIDES CONNECTED TO EACH OF THE END SECTIONS COUPLE MICROWAVE ENERGY TO THE CAVITY TO EXCITE THE TWO MODES THEREIN. A SAMPLE OF MATTER IS INSERTED INTO THE CENTRAL PORTION SO AS TO BE IRRADIATED BY BOTH MODES. A TUNING SCREW IN EACH OF THE END SECTIONS PERMITS INDEPENDENTLY TUNING THE CAVITY TO THE RESONDANT FREQUENCY OF ONE OF THE RESONANT MODES. ADJUSTING SCREWS IN THE CENTRAL PORTION OF THE CAVITY STRUCTURE PERMIT ADJUSTMENT OF THE CROSS COUPLING BETWEEN THE TWO MODES. THE MICROWAVE SPECTROMETER DESIGNS TAKE ADVANTAGE OF THE ABILITY OF THE CAVITY TO BE INDEPENDENTLY TUNED TO THE RESONANT FREQUENCIES OF EITHER MODE. THE SEVERAL DESIGNS PROVIDE FOR AMPLITUDE MODULATION OF THE FREQUENCY OF THE PUMPING MODE OR SCANNING OF THE FREQUENCY OF THE PUMPING MODE OR MODULATION OF AN APPLIED UNIDIRECTIONAL MAGNETIC FIELD EITHER WITH OR WITHOUT MODULLATION OF THE FREQUENCY OF THE PUMPING MODE.

Description

Jan. 26, 1971 J s Hm: 3,559,043

BIMODAL CAVITY 'REoNAToR AND MICROWAVE SPECTROMETERS USING SAME Filed July 3. 1967 5 Sheets-Sheet 1 FIG. I

. INVENTOR JAMES S. HYDE BY a6...

Jan. 26, 1971 J. 5, HYDE 3,559,043

BIMODAL CAVITY RESONATOR AND MICROWAVE SPECTROMETERS USING SAME 2 3 Sheets-Sheet 2 Filed July 3. 1967 |00KHz- 63 v INVFNTOR. OSCILLATOR AME .HY E

V ORNEY Jan. 26, 1971 J. s. HYDE 3,559,043

BIMODAL CAVITY RESONATOR AND MICROWAVE SPECTROMETERS USING SAME Filed July 5. 1967 3 Sheets-Sheet 8 FIG. 5

56 A 5? M 2011 IOKH Z Z AMPL'F'ER MODULATOR A AMPLIFIER & 1 2 5 PHASE PHASE PHASE 3EEk8 3E TWR v'fi'AX I e4 65 7' 59 I00 KHZ E msconnm A OSCILLATOR v62 INVENTOR.

J MES HYDE BY ORNEY United States Patent O 3,559,043 BIMODAL CAVITY RESONATOR AND MICRO- WAVE SPECTROMETERS USING SAME James S. Hyde, Menlo Park, Calif., assiguor to Varian Associates, Palo Alto, Calif., a corporation of California Filed July 3, 1967, Ser. No. 650,891 Int. Cl. G01n 27/78 US. Cl. 324-5 Claims ABSTRACT OF THE DISCLOSURE A central portion of the cavity is capable of supporting two resonant modes while adjoining end sections each support only one of the modes. Waveguides connected to each of the end sections couple microwave energy to the cavity to excite the two modes therein. A sample of matter is inserted into the central portion so as to be irradiated by both modes. A tuning screw in each of the end sections permits independently tuning the cavity to the resonant frequency of one of the resonant modes. Adjusting screws in the central portion of the cavity structure permit adjustment of the cross couplingbetween the two modes. The microwave spectrometer designs take advantage of the ability of the cavity to be independently tuned to the resonant frequencies of either mode. The several designs provide for amplitude modulation of the frequency of the pumping mode or scanning of the frequency of the pumping mode or modulation of an applied unidirectional magnetic field either with or without modulation of the frequency of the pumping mode.

DESCRIPTION OF THE PRIOR ART Heretofore, bimodal cavities have been employed for observing electron paramagnetic resonance of samples at microwave frequencies. In one such prior art arrangement, the apparatus was equivalent to the Bloch and Hansen crossed-coil nuclear resonance apparatus operating at microwave frequencies. Such a device included a bimodal cavity excited in cylindrical TE modes. Both orthogonal TE modes within the cavity occupied the same region of space. Four capacitive plugs and two resistive plugs projected through the side walls of the cavity for balancing the coupling between the input and output waveguides. A sample was disposed in the center of the cavity and, at resonance of the sample, the balanced coupling was disturbed between the input and output waveguides such that a resonance signal was coupled through the sample to the receiver of the spectrometer. Such an apparatus is described in an article entitled Microwave Farraday Rotation: Design and Analysis of a Bimodal Cavity, Journal of Applied Physics, vol. 29, No. 12, December 1958, pp. 1692-1697. I

In another example of the prior art, electron paramagnetic resonance was "observed in a bimodal microwave cavity excited with two orthogonal degenerate cylindrical TM modes which could be tuned with respect to each other. Such an apparatus is described in an article entitled Cross Relaxation Studies in Diamond, Physical Review, vol. 118, No. 4, May 15, 1960, pp. 939-945, As in the previous example of the prior art, both modes occupied the same region of the cavity. Capacitive tuning screws were inserted into the cylindrical cavity for tuning one mode of the cavity relative to the other mode of the cavity. One of the modes was excited'from a paramagnetic resonance spectrometer for observing resonance of one part of the EPR spectrum of the sample at an observing frequency determined by the observing mode of the cavity. While this resonance was being observed, other lines of the spectrum of the sample were successively ex-v cited into resonance by a pumping microwave frequency 3,559,043 Patented Jan. 26, 1971 applied to the other resonant mode of the cavity. Approximately 40 db of isolation was obtained between the pump mode and the observing mode of the cavity. This technology is useful in the investigation of paramagnetic samples and has been called by various authors electronelectron double resonance, double electron resonance, and double electron spin resonance, all of which are synonyms. It appears useful also in microwave spectroscopy and in other forms of microwave spectroscopic analysis.

While these prior art bimodal cavities were adequate for certain experiments, it is desirable, for electronelectron double resonance experiments, to provide a bimodal cavity which is easier to tune without introducing cross-coupling between the two resonant modes and one in which the coupling to the cavity is relatively noncritical to facilitate manufacture of such cavities without undue adjustment thereof.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved bimodal microwave cavity resonator and microwave spectrometers using same.

One feature of the present invention is the provision, in a microwave spectrometer, of a bimodal cavity structure having a portion shared by both resonant modes and a portion unshared by the resonant modes to facilitate tuning and coupling to the structure in the unshared regions while permitting the sample to be disposed in the shared portion and exposed to the fields of the two resonant modes.

Another feature of the present invention is the same as the preceding feature wherein one or both of the resonant modes are tunable by providing tuning means coupled to the fields in the unshared regions of the cavity structure, whereby each of the separate modes may be tuned as desired.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of adjusting members protruding into the shared region of the cavity for cancelling undesired cross coupling between the first and second resonant modes.

Another feature of the present invention is the same as any one or more of the preceding features wherein resonance of the sample is excited by one of the resonant modes, identified as an observing mode, while the sample is being irradiated with a second microwave frequency of the second resonant mode of the cavity, identified as the pumping mode, and wherein the pumping energy is modulated and that modulation detected which is carried over into the observed resonance of the sample.

Another feature of the present invention is the same as the preceding feature wherein the pumping mode of the cavity resonator is tuned relative to the observing mode and the frequency of the pumping microwave energy is caused to track the frequency changes of the pumping mode of the cavity.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective, schematic, line diagram of a bimodal cavity resonator incorporating features of the present invention,

'FIG. 2 is a schematic, perspective, line diagram of an I alternative bimodal cavity embodiment of the present resonance spectrometer incorporating features of the present invention, and

FIG. is a schematic block diagram of an alternative spectrometer embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a bimodal cavity structure 1 of the present invention. The cavity resonator structure 1 includes a central shared portion 2 capable of supporting two resonant modes of oscillation. A pair of rectangular waveguide sections 3 and 4 are coupled to the central shared region 2 at opposite ends thereof. The rectangular waveguide sections 3 and 4 of the cavity resonator structure 1 are each capable of supporting only one of the resonant modes and are oriented at 90 with respect to each other to form the unshared portions of the cavity for each of the resonant modes. The end walls of the composite cavity resonator structure 1 are defined by transverse walls 5 and 6 closing off the outer ends of the waveguide sections 3 and 4, respectively. Coupling irises 7 and 8 are disposed in the transverse walls 5 and 6, respectively, for coupling wave energy from the cavity 1 to the waveguide circuits external of the composite bimodal resonator 1.

Central section 2 of the composite bimodal cavity structure 1 is approximately square in cross-section for supporting two dominant TE modes in orthogonal relation to each other between the orthogonally disposed pairs of mutually opposed side walls. The central section 2 has end walls 9 and 9' which are an integral number of half wavelengths apart and are defined by the wall structure connecting the broad walls of waveguide section 3 and 4 to the side walls of the central section 2. The rectangular waveguides 3 and 4, as oriented, are beyond cutoff for the mode which has its electric field parallel to the broad walls of the waveguides 3 and 4.

Top and bottom walls of the central section 2 are centrally apertured and provided with a pair of

conductive tubes

10 and 10 extending away from the top and bottom walls in alignment with the apertures therein. The tubular members 10 and 10' define a pair of circular waveguides beyond cutoff for the microwave energy within the cavity structure 1. The sample of matter to be investigated is disposed within the cavity by being inserted through the central bore of the

tubular members

10 and 10. The region of the sample within the central portion 2 of the resonator is subjected to the orthogonal microwave magnetic fields H and H For electron paramagnetic resonance studies, the cavity structure 1 is preferably disposed in a DC. polarizing magnetic field H in such a manner that H and H both have components normal to the direction of the polarizing magnetic field H In one embodiment of the cavity 1, the shared and unshared regions taken together for each mode are dimensioned to have a length to support the TE mode, i.e., they have an overall length of 3 halves of a guide wavelength. Tuning screws 11 and 12 are disposed in each of the unshared portions of the cavity 3 and 4, respectively, for tuning each of the resonant modes. The tuning screws 11 and 12 may be conductive or of a dielectric material for tuning the cavity structure in a conventional manner.

Two sets of adjusting screws 13 and 14 are provided in the shared portion 2 for cancelling the cross coupling between the two resonant modes within the bimodal cavity 1. Screws 13 are disposed in positions of maximum electric field of the two modes and are made of a resistive material, whereas screws 14 are disposed in the regions of maximum magnetic field for the two modes and are made of conductive material for balancing the reactiv components of the waveguide structure. In a typical example of the cavity of FIG. 1 as dimensioned for operation at X-band, the composite cavity 1 had a Q of 8000 for each of the two modes and the cross-coupling between the two modes was adjusted to provide minus db crosscoupling.

Referring now to FIG. 2 there is shown an alternative bimodal cavity similar to the structure of FIG. 1. The apparatus is essentially the same as that of FIG. 1 with the exception that it is necessary, as designed, that only one of the resonant modes be tunable. Thus, the unshared portion for the fixedly tuned mode is removed, namely, waveguide section 4, and replaced by a conductive wall 15 with a central iris 16 for coupling to an E-bend section of waveguide 17. The unshared portion of the tunable resonant mode, formed by the rectangular waveguide section 3, is coupled to an E-bend waveguide section 18 by means of a short section of coaxial line 19 communicating through the broad wall of the E-bcnd waveguide 18 and the narrow wall of the waveguide section 3. An antenna 21, formed by an extension of the center conductor of the coaxial line 19, couples signals from the E-bend guide 18 into the rectangular waveguide section 3 via an inductive coupling loop 22. This coupling arrangement forms the subject matter of US. Pat. 3,214,684 issued Oct. 26, 1965 and assigned to the same assignee as the present invention. The tuning screw 11, for tuning the tunable resonant mode, projects into the waveguide section 3 through a central opening in its endclosing wall 5. Thus, the composite cavity structure 1 includes a first tunable resonant mode section formed by waveguide section 3 and central section 2 and is resonant in the TE mode. The fixedly tuned resonant mode occupies the central region 2 and is resonant in the TE mode.

Referring now to FIG. 3 there is shown an alternative embodiment of the present invention. More specifically, the bimodal cavity resonator structure 1 includes a central cylindrical section 2 capable of supporting two resonant orthogonal modes such as for example two cylindrical TE modes. Two cylindrical sections of waveguide 3 and 4 are joined to the opposite ends of the central Waveguide section 2. Each of the waveguide sections 3 and 4 includes a conductive septum 25 and 26, respectively, extending diametrically across the guide, dividing the guide into two parallel semi-cylindrical sections. The septums 25 and 26 are oriented within planes in orthogonal relation such that each of the cylindrical waveguide sections 3 and 4 is beyond cutoff for the other mode and is capable of supporting only one of the two resonant modes to the exclusion of the other resonant mode.

The waveguide sections 3 and 4 are closed off at their outer ends by

conductive walls

27 and 28, respectively. Coupling irises 29 and 31 are centerally disposed of the

conductive walls

27 and 28.

Rectangular waveguide sections

32 and 33 are coupled to the cylindrical waveguide sections 3 and 4, respectively, via the coupling irises 29 and 31.

Waveguide sections

32 and 33 are orthogonally oriented to each other with the broad walls thereof parallel to the septums 25 and 26, respectively. Thus, the composite bimodal resonator structure 1 includes two resonant mode structures having a common shared region 2 with their separate unshared regions 3 and 4, respectively. In a preferred embodiment, the two resonant mode structures are dimensioned for resonance in the cylindrical TE113 mode.

Tuning screws 11 and 12 are provided for tuning each of the separate resonant modes by varying the fields in the unshared regions 3 and 4, respectively. The sample to be analyzed is disposed in the center of the shared region by passing coaxially through tubular sections 10 and 10' and the central section 2.

Referring now to FIG. 4 there is shown a microwave paramagnetic resonance spectrometer incorporating features of the present invention. The bimodal sample cavity 1 is coupled to one arm of a microwave bridge 35. A reflex klystron oscillator 36 is coupled to another arm of the bridge and feeds power to the bridge via isolator 37 and variable attenuator 38. The arm of the bridge opposite the arm containing the sample cavity 1 is terminated in a resistive load 39 and the other arm of the bridge includes a diode detector 41. A variable slide screw tuner 42 is included in the sample cavity arm for adjusting the coupling to the cavity 1.

The other mode of the bimodal cavity 1 is coupled to a second bridge 44 for applying the pumping microwave excitation to the sample. The bridge 44 includes a threeport circulator 45 having the sample cavity 1 connected to one port. The pumping microwave energy is supplied to the bridge 44 by a reflex klystron oscillator 46 connected to another port of the circulator 45' via a waveguide including a pair of

isolators

47 and 48, a variable attenuator 49 and a diode switch 51. A diode detector 52 is disposed in another arm of the circulator 45.

The

klystron oscillators

36 and 46 are each locked to the resonant frequency of their respective coupled resonant modes of the bimodal cavity 1 by means of an automatic frequency control channel. The automatic frequency control channel includes an A.F.C. modulator which modulates the reflector voltage of the

klystrons

36 and 46 at a suitable audio frequency such as for example kH If the center frequency of the

klystron oscillator

36 or 46 departs from the resonant frequency of the respective orthogonal mode in the bimodal cavity 1, a component at the A.F.C. modulation frequency will be detected in

detectors

41 and 52, respectively. These A.F.C. modulation frequencies are amplified by

amplifiers

56 and 57, respectively, and the output fed to one input terminal of phase

sensitive detectors

58 and 59, respectively, wherein it is compared with a sample of the A.F.C. modulation signal derived from the A.F.C. modulator 55. The output of the phase

sensitive detectors

58 and 59 comprise a pair of DC. error signals which are applied to the

respective klystron oscillators

36 and 46 for tuning their center frequencies to the resonant frequencies of the respective resonant modes in the binmodal cavity 1. The tuning range required of the

reflex klystron oscillators

36 and 46 typically exceeds the reflector mode tuning range for the cavity mode of the reflex klystrons. Accordingly, mechanical tuners, not shown, are provided for tuning the klystron cavities, not shown. The mechanical tuners cause the cavity modes of the klystron to be tuned to the frequency of the reflector mode such that the reflector mode may track the respective mode of the bimodal cavity 1.

The spectrometer FIG. 4 also includes an observation channel for observing electron paramagnetic resonance of the sample via the other resonant modes of the cavity 1. More specifically, a microwave signal derived from klystron 36 excites the observing resonant mode of the cavity 1 which is coupled to the sample to excite a resonance thereof. At resonance, the observing cavity mode, which is coupled to the sample, reflects both a reactive and an absorption component to the bridge 35.

In one mode of operation of the spectrometer FIG. 4, the DC. polarizing magnetic field H is modulated by means of a modulating coil 61 excited with a high audio frequency current, for example 100 kHz derived from an oscillator 62 via switch 63. Modulation of the polarizing field produces a modulation component on the paramagnetic resonant signal at the 100 kHz. modulation frequency. This modulation component is amplified by amplifier 56 and fed to one terminal of 'the phase sensitive detector 64 wherein it is compared with a sample of the field modulation signal derived from oscillator 62 to produce an output paramagnetic resonant signal. The output resonance signal is fed to one input of a recorder 65 for recording as a function of a scan of the DC. polarizing magnetic field H as produced by a current derived from a field scan generator 66 and fed into a field scan coil 67. The output of the recorder 65 is an electron paramagnetic resonance spectrum of the sample under analysis. When field modulation is utilized, the recorded spectrum will be the first derivative of the paramagnetic resonance signal.v

Most electron paramagnetic resonance samples produce a relatively complicated spectrum having many overlapping lines. It has been found that such spectra can be often simplified by performing a double resonance of the sample. More specifically, a pumping microwave energy is applied to the sample via the pumping resonant mode of the cavity which is decoupled from the observation mode of the cavity. The pumping energy is amplitude modulated with a certain modulation frequency and the observed paramagnetic resonance signal is monitored and detected for any component of the modulation frequency carried over into the observed resonance. The spectrum is then recorded only for those resonance lines which include the pump modulation frequency.

The pumping mode of the cavity is set to a frequency which is separated from the frequency of the observing mode by some predetermined amount and the polarizing field is scanned to obtain a spectrum. Thus, it is often possible to greatly simplify the output spectrum of the sample.

Spectrometer FIG. 4 may be operated in the electronelectron double resonance mode by switching switch 63 to connect the kHz. oscillator 62 to the diode 51 and by tuning the pumping mode of the cavity 1 via screw 12 to some predetermined frequency difference compared to the frequency of the observation mode. The pumping level is then amplitude modulated at the 100 kHz. frequency and when the observation frequency corresponds to a resonance line the energy level of which are coupled to another line being irradiated by the pumping source, the modulation of the pumping source is coupled through the sample to the observed resonance signal. The detected resonance signal is displayed in the output of the recorder 65, thereby producing an output spectrum.

In the spectrometer FIG. 4 isolator 37, in the klystron arm of the observing frequency bridge 35, serves to prevent energy reflected from the bimodal cavity from pulling the frequency of the klystron 36. Likewise,

isolators

48 and 47 prevent microwave reflections from the cavity 1 from pulling the frequency of the pumping klystron oscillator 46. In addition, the second isolator 47 is provided between the diode switch 51 and the cavity 1 to absorb any energy at the observing frequency which is coupled through the bimodal cavity toward the diode 51 and which would otherwise be reflected therefrom back to the cavity 1. If this diode reflected signal were present it would produce a modulation in the observing resonance mode to be detected as a resonance signal when operating in the pumping mode.

A double electron resonance spectrometer of the type shown in FIG. 4 is especially useful, for observing relaxation rates of certain groups of electrons within the sample, for separating overlapping spectra from different paramagnetic species, for studying interactions of paramagnetic species in different sites to separate overlapping spectra from species in different physical environments, for example, in different orientations with respect to the polarizing magnetic field, and for obtaining hyperfine couplings in complex paramagnetic species.

As an alternative to scanning the magnetzic field H to scan both the observation microwave frequency and the pumping microwave frequency through the spectrum, the observation resonance line may be continuously observed while the pumping frequency is scanned through the remaming portion of the spectrum and double resonance signals observed via the modulation coupled from the pumping microwave level into the resonance line being observed. This is accomplished in spectrometer FIG. 4 by switching the output scan generator 66 via switch 68 to motor 69 which is coupled to the tuning screw 12. Switch 63 is also switched to apply the modulation frequency at 100 kHz. from oscillator 62 to the diode switch 51 for modulating the amplitude of the pumping energy. The pump energy is tuned by tuning the pumping mode of the cavity 1 since the A.F.C. causes the pump frequency to track the pump mode of the cavity 1. The output spectrum, at recorder 65, is then greatly simplified.

Referring now to FIG. there is shown an alternative double resonance spectrometer of the present invention. The spectrometer is essentially the same as that of FIG. 4 with the exception that the output of the 100 kHz. oscillator 62 is fed to the field modulating coils 61 for modulating the polarizing field at the 100 kHz. frequency. A low frequency signal derived from the 20 Hz. modulator 71 is fed to the diode switch '51 for modulating the amplitude of the pumping energy at a 20 Hz. frequency. The polarizing magnetic field is set for observing resonance of a given line of the sample and the frequency of the pumping mode and, thus, the frequency of the pumping excitation is scanned through the remainder of the spectrum.

When the frequency of the pumping excitation corresponds to an energy level which is coupled to the resonance line being observed by the observation channel, the 20 Hz. modulation is coupled through from the pumping source to the resonance signal of the line being observed. More particularly, the 20 Hz. modulation is coupled through to the output of phase sensitive detector 64 which is amplified by amplifier 72 and fed to one input of a second phase sensitive detector 73 wherein the 20 Hz. resonance modulation is compared with a sample of the 20 Hz. modulation signal derived from the modulator 71 to produce an output resonance signal recorded by recorder 65 as a function of the scan of the pumping frequency. The output signal will be a first derivative resonance signal of a simplified spectrum of only those lines which have a coupling therebetween.

Sometimes it is desirable to record the absorption spectrum rather than the first derivative of the absorption spectrum and in such case the spectrometer of FIG. 4 is switched such that the 100 kHz. oscillator applies its signal via switch 63 to the diode switch 51 and the scan generator 66 has its output switched via switch 68 to the motor '69. The absorption spectrum recorded without field modulation is especially desirable for analyzing anisotropic line broadening as obtained in powders, glasses, and large protein molecules.

Although the bimodal cavity of the present invention has been described primarily as utilized for electron paramagnetic resonance spectrometers, it is also useful in general for microwave spectroscopy. More particularly, it may be utilized to advantage in microwave absorption spectroscopy wherein resonance of a sample Within the common two resonant mode section of the cavity produces cross-coupling between the input and output modes.

Typically, electron paramagnetic resonance spectra are a few hundred megahertz in width, and for the observation of electron-electron double resonance signals both pump and observing microwave modes would be resonant at X-band and separated in frequency by from one to five hundred megahertz.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention 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. In a microwave spectrometer, means defining a bimodal cavity resonator, said cavity resonator including a shared portion capable of supporting first and second resonant modes of oscillation, means for supporting a sample of matter to be investigated in said shared portion of said cavity resonator, means applying microwave energy to said cavity resonator to produce said resonant modes of oscillation and means connected to said cavity resonator for detecting resonance of the sample, the improvement wherein said cavity resonator includes an unshared portion forming a resonant cavity with said shared portion and capable of supporting said first resonant mode to the exclusion of said second resonant mode.

2. The apparatus of claim 1 wherein said means apply- 8 ing microwave energy to said cavity resonator includes means for coupling a microwave energy source to said unshared portion to excite said first resonant mode therein, said coupling means being disposed in said unshared portion of said cavity.

3. The apparatus of claim 1 including, means for tuning said first resonant mode of said cavity resonator, said tuning means being disposed in said unshared portion of. said cavity resonator.

4. The apparatus of claim 1 wherein, said cavity resonator structure includes a second unshared portion capable of supporting said second resonant mode to the exclusion of said first resonant mode.

5. The apparatus of claim 4 including, means for tuning said second resonant mode, said tuning means for said second mode being disposed in said second unshared portion of said cavity resonator.

6. The apparatus of claim 1 including, means disposed in said shared portion of said cavity resonator for adjusting the coupling between said first and second resonant modes.

7. The apparatus of claim 1 wherein said means applying microwave energy to said cavity resonator includes, means coupled to said cavity resonator structure for exciting said first resonant mode of said cavity resonator for exciting the sample of matter at the microwave frequency of said first resonant mode, means coupled to said cavity resonator structure for exciting the second resonant mode of the cavity for exciting resonance of the sample at the microwave frequency of the second resonant mode, one of said means for exciting being coupled to said unshared portion of said cavity resonator structure, means for modulating the microwave excitation of said sample and said cavity in said first resonant mode at a certain modulation frequency, and wherein said means for detecting resonance of the sample includes means for detecting modulation of the resonance of the sample at the certain modulation frequency.

8. The apparatus of claim 7 including, means coupled to said unshared portion of said cavity resonator for tuning the resonant frequency of said first resonant mode of said cavity without changing the frequency of said second resonant mode of said cavity, and means sensitive to the tuned frequency of said first resonant mode for causing the frequency of the microwave excitation coupled to said first resonant mode to track the tuning frequency changes in the resonant frequency of said first resonant mode of said cavity.

9. The apparatus of claim 7 wherein, the sample and said cavity resonator are immersed in a polarizing magnetic field and including means for modulating the intensity of the polarizing magnetic field at a certain field modulation frequency, and said means for detecting resonance of the sample including, means for detecting modulation of the detected resonance of the sample at the certain field modulation frequency.

10. The apparatus of claim 8 wherein, said tuning means for tuning said first resonant mode of said cavity automatically scans the resonant frequency of said first resonant mode of said cavity resonator.

Microwave Faraday RotationPortis et al.-Jour. of

Appl. Physics29 12)-pp. 169-2-1698-December 8.

Cross Relaxation Studies In DiamondSorokin et al. Physical Review118(4)-pp. 939-944-May 15, 1960.

MICHAEL T. LYNCH, Primary Examiner US. Cl. X.R. 32458.5; 333-83