US3525953A - Plasma tuning means wherein the resonant frequency of a cavity resonator tracks the frequency of an ionizing control frequency - Google Patents

Description

3,525,953 FREQUENCY OF S. L. H ALVERSON PLASMA TUNING MEANS WHEREIN THE RESCNAN'? A CAVITY RESONATOR TRACKS THE FREQUENCY OF AN IONIZING CONTROL FREQUENCY 2 Sheets-Sheet l 7 0 6 9 1 7 2 N or d u w. A m

R %N wfibwmmzmw A N\ m m mmim l u N QM. u u Wm x KW \11 -\N k3 8c mm akmxi I; n F X X r @w mmQkQumk kwzak @w \T H I my 1Q mmmwduwk k2 Pfimm \sQmRmvfiE Suki Inventor jtemen L. Hair/e750 fitter/19v 5, 1970 s. l HALVERSON PLASMA TUNING MEANS WHEREIN THE RESONANT FREQUENCY OF A CAVITY RESONATOR TRACKS THE FREQUENCY OF AN IONIZING CONTROL FREQUENCY 2 Sheets-8heet 2 Filed Nov. 7, 196'? 698 DETECTED FREQUENCY IN MHZ, Im MODE 5 xutwbommhb mtbo mwkmxdx ms [f2 uezziaf fez/e12 L. Ha [Verso/z Zr United States Patent US. Cl. 33373 Claims ABSTRACT OF THE DISCLOSURE A variable-frequency signal generator is electrically coupled to a resonant cavity containing an ionized gas. The resonant frequency of the cavity may be changed by varying the frequency of the electrically coupled signal.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomlc Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to resonant cavities and more particularly to means for varying the resonant frequency of a cavity.

The resonant cavity is widely used in the electrical art as a resonant circuit. The characteristics of a resonant cavity are similar to those of a conventional LC resonant circuit, with the resonant cavity having a much higher efficiency than conventional circuits in the microwave region. It is often desirable to vary the frequency at which resonance occurs within a cavity. Present cavities accomplish this by changing the physical size or shape of the cavity. It will be appreciated that, where a cavity is in an inaccessible location, it is diflicult to change the cavity resonance by changing the physical shape of the cavity. Further, changing of the physical dimensions of a cavity is slow and often difficult even when accessible.

Accordingly, it is one object of the present invention to provide an improved means for changing the resonant frequency of a cavity.

It is another object of the present invention to provide means whereby the resonant frequency of a cavity may be changed remote from the cavity.

Other objects of the present invention will become more apparent as the detailed description proceeds.

In general, the present invention comprises a cavity in combination with an ionizing gas within the cavity. Means are provided for generating a frequency-variable signal which is electrically coupled to the cavity to effect changes in the resonant frequency thereof.

BRIEF DESCRIPTION OF THE DRAWINGS Further understanding of the present invention may best be obtained from consideration of the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an apparatus for the practice of the present invention.

FIG. 2 is a graphical representation of detected frequency vs. excitation frequency for a cavity operated according to the present invention.

FIG. 3 is a schematic diagram of an apparatus used to obtain the values plotted in FIG. 3.

In FIG. 1, a cavity has mounted therein cavity loop couplers 12 and 14 which are respectively connected to a receiving antenna 16 and a detector 18. This is a typical structure for a conventional cavity in a signal-receiving system. For the present invention, to this structure is added a cavity loop coupler 20 mounted within the cavity 10. A klystron oscillator 22 has its output connected via coaxial cable 24 to the loop coupler 20 and hence the cavity 10. A wide-band double-stub tuner 26 is connected to the coaxial cable 24 as shown. A vacuum pump 28 is coupled to the cavity 10 to permit partial evacuation of the interior thereof.

The apparatus of FIG. 1 operates to change the resonant frequency of the cavity 10 as follows. With the cavity partially evacuated by the vacuum pump 28, the klystron 22 is energized to deliver a signal of predetermined frequency to the cavity 10. The double-stub tuner 26 is adjusted to match the impedance of the klystron 22 and the impedance of the cavity 10 so that minimum power loss of signal is insured. The power output of the klystron 22 is set so that it is suflicient to ionize the remaining gas within the interior of the cavity 10. With the remaining gas within the interior of the cavity 10 ionized, a change in the frequency of the output signal from klystron 22 results in a change in the resonant frequency of the cavity 10. Thus, the resonant frequency of the cavity 10 may be changed, whereby detection of selected frequencies received by antenna 16 may be effected with detector 18.

It is to be noted that for the practice of the present invention the klystron 22 may be operated in one mode to tune the cavity 10 and effect detection of frequencies in another mode by the detector 18. For example, the klystron 22 may be operated in the TM mode to tune the cavity 10, while signals in the TB mode are detected by detector 18. Using this mode of operation, electrical isolation between the cavity loop coupler 20 and the signalreceiving and detecting loop couplers 12 and 14 is 40 db or better. Enhanced electrical isolation may be achieved by using selective band-pass filters in the receiving stage.

To further illustrate the resonant frequency tuning of the cavity 10 by changing the output frequency of the klystron 22, reference is made to FIG. 2 wherein are shown graphical plots of excitation frequency output from klystron 22 vs. detected frequencies by detector 18. To obtain the data illustrated in FIG. 2, the apparatus of FIG. 3 was used. It will be noted that the apparatus of FIG. 3 is the same as in FIG. 1 except that the conventiona receiving antenna 16 has been replaced by a sweep signal generator 30. Further, the cavity 10 is shown lined with a bell jar 32 which extends the length of the cavity 10 but is spaced with respect to the sides thereof and is sealed to the cavity at one end as shown. The bell jar was used to facilitate partial evacuation of the cavity 10 and is not a limitation on the present invention. The present invention will work on cavities Which do not contain a bell jar.

The cavity 10 in FIG. 3 had a diameter of 33 cm. and a length of 42 cm. with a characteristic resonance of 686.234 megahertz. As stated, the bell jar extended the length of the cavity 10 and was spaced approximately /2 inch from the interior walls thereof. The partial vacuum within the bell jar 32 was varied from .010 to .025 torr for different measurements. The output power of the klystron 22 was maintained at a level of 24 watts and was sufficient to ionize the remaining air within the bell jar 32. The klystron 22 was operated in the TE mode and the sweep generator 30 was operated in the TMOIO mode.

The resonant frequency of the cavity 10 was varied as hereinbefore described for the apparatus of FIG. 1. To detect the resonant frequency of the cavity, the sweep generator 30 was used. The curves 34, 36, 38 and 40 of FIG. 2, respectively, represent results obtained at partial bell-jar vacuum values of .010, .015, .020 and .025 torr, respectively. Curves 34, 36, 38 and 40 of FIG. 3 clearly 3 show that, if the excitation frequency from klystron 22 is changed, the resonant frequency of the cavity changes. For example, when operating with a pressure of .025 torr, a change in the frequency of klystron 22 from 625 megahertz to 627 megahertz in the TE mode causes the resonant frequency of the cavity 10 to change from 696 megahertz to 698 megahertz in the TM mode.

It is to be understood that gases other than air may be used within the interior of the cavity 10 to elfect the present invention, the only limitation on such gases being that they be capable of being ionized. Further, such gases may be at pressures other than those described. The re duced pressures set forth were used to facilitate ionization of air and are not intended to form a limitation on the invention. It will be appreciated that the present invention readily lends itself to remote control of the resonant frequency of a cavity and permits the adjustment of the resonant frequency of a cavity more rapidly than accomplished by changing the physical dimensions thereof, as heretofore taught in the art.

Persons skilled in the art will, of course, readily adapt the general teachings of the invention to embodiments far different from the embodiments illustrated. Accordingly, the scope of the protection atforded the invention should not be limited to the particular embodiment illustrated in the drawings and described above, but should be determined only in accordance with the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for varying the resonant frequency of a cavity comprising an ionized gas within said cavity, means for generating a frequency-variable signal, and means for electrically coupling said generated signal to said cavity, the frequency of said generated signal being 3 determinative of the resonant frequency of said cavity.

2. The apparatus according to claim 1 wherein said electrical coupling means comprise a coaxial line connected to said signal-generating means, loop coupling means terminating said coaxial line to said cavity, and means for matching the impedance of said cavity to the impedance of said signal-generating means.

3. The apparatus according to claim 2 wherein said impedance-matching means comprise a double-stub tuner electrically connected to said coaxial line to vary the impedance thereof.

4. In combination with a cavity containing a gas, means for partially evacuating said cavity, means for generating a frequency-variable signal at a power level to ionize gas remaining within said. partially evacuated cavity, and means for electrically coupling said frequency-variable signal to said partially evacuated cavity to vary the resonant frequency thereof responsive to the frequency'of said generated signal.

5. The apparatus of claim 4 wherein said coupling means comprise coaxial means connected to said signalgenerating means and loop-coupled to the interior of said cavity, and means interconnected of said coaxial line means to match the impedance of said cavity and said signal-generating means.

References Cited UNITED STATES PATENTS 2,659,028 11/1953 Kyhl 315-39 2,660,711 11/1953 Garbuny 333-17 3,348,169 10/1967 Tomeyasu 315-39 OTHER REFERENCES HERMAN KARL SAALBACH, Primary Examiner W. N. PUNTER, Assistant Examiner US. Cl. X.R.

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