US2997675A - Apparatus for electromagnetic wave guidance and control by electrical discharge plasmas - Google Patents

Description

Aug. 22, 1961 .1. M. ANDERSON 2,

APPARATUS FOR ELECTROMAGNETIC WAVE GUIDANCE AND CONTROL BY ELECTRICAL DISCHARGE PLASMAS Filed Jan. 2, 1959 in Va nt-or': John M A nder-son,

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2,997,675 APPARATUS FOR ELECTROMAGNETIC WAVE GUIDANCE AND CONTROL BY ELECTRICAL DISCHARGE PLASMAS John M. Anderson, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 2, 1959, Ser. No. 784,659 8 Claims. (Cl. 333-97) This invention relates to an electromagnetic wave guide structure for controlling propagation of electromagnetic waves.

In microwave transmission it is known to employ strip lines wherein a conductive strip is disposed in spaced relation to a conductive ground plane or base of considerably greater width than the strip. In such a structure, electromagnetic waves are readily propagated along the space generally between the strip and base and the same possess certain inherent advantages over other wave propagating structures such as conventional wave guides of the rectangular hollow or coaxial type.

It is fiequently desirable or imperative, however, to alter the dimensions or spacing of a strip in such a strip line for accommodating waves of diiferent frequencies or having other properties or for controlling the presence or absence of propagation along different paths. In the conventional strip line arrangements, the components are relatively fixed and adjusting or altering a wave guide structure to new and difierent wave requirements is frequently either impossible or involves major changes.

It is accordingly a primary object of my invention to facilitate creation or disintegration of an effective strip line structure capable of supporting propagation of microwaves of widely dilferent frequencies and capable of being established or disintegrated quickly and easily.

It is another object of my invention to facilitate quick and effective alteration of properties of an effective strip line tor accommodating waves of different characteristics.

It is another object of my invention to facilitate electronic switching of an electromagnetic wave along alternate paths by a quick and simple establishment or removal of an effective strip line wave guide Pursuant to the aforementioned objects and in accordance with my invention, an effective or pseudo strip is established above a ground plane in a chamber of low pressure gas by ionization of the gas to a negative glow wherein the electron density and the plasma frequency of electrons in the glow is so high as to take on a character of a conductive strip with respect to the electro magnetic waves. A cathode strip extending along but insulated from the ground plane, and a spaced anode comprising a wall of an envelop containing an ionizable gas comprise the electrodes across which a direct potential is applied. The potential gradient in the interelectrode space may be selected by selecting the appropriate applied potential to establish any predetermined extent of ionization. The extent of the negative glow may thus be controlled to produce an effective strip above the ground plane of predetermined dimensions. The eifective strip produced by the negative glow, together with the ground plane, accommodates the propagation of electromagnetic waves. Such an effective strip when produced may be controlled in size or removed altogether by mere application of the appropriate potential difierence across electrodes. Accordingly, switching maybe thus effectively accomplished.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawings in which:

Zfihidi Patented Aug. 22, 1961 FIG. 1 is a perspective view of an effective line producing structure of my invention;

FIG. 2 is a view taken along section 2-2 of FIG. 1;

FIG. 3 is an enlarged view of the cathode fastening structure;

FIG. 4 shows another embodiment of my invention; and

FIG. 5 shows still another embodiment of my invention.

Referring now to the drawings for a more detailed description of the invention in FIGS. 1 and 2, 10 represents generally the entire wave guide producing structure including a conductive and substantially planar ground plane 12 of any suitable metal, a conductive anode 14 having the shape of an inverted trough of uniform rectangular cross section with flanges 16 and 18 on respective sides thereof. Anode 14 extends in spaced parallel relation to the ground plane 12 and the flanges 16 and 18 terminate at the sides of the ground plane. Anode 14 is insulated at low frequencies from the ground plane 12 by pieces of insulation 20 and 22 which may be mica, disposed between the ground plane and respective flanges 16 and 18. For securing the anode and ground plane together, the mica pieces 2!) and 22 may be bonded to the respective flanges 20 and 22 and to the ground plane.

In accordance with a feature of the invention, a cathode 24 is mounted along a portion of the ground plane intermediate to its sides and is insulated from the ground plane at low frequencies. To this end, a plurality of fastening devices, one of which is shown enlarged at 26 in FIG. 3 are provided for securing the cathode. Such fastening device includes a bolt 28 extending through bores 30 and 32 in the cathode and ground plane, respectively, an insulating sleeve 34 disposed between the bolt 28 and the ground plane and having a flange 36 against which a nut 38' threaded on the bolt 28 abuts. Bore 30 is countersunk to snugly accommodate a flared head 40 of bolt 30 whereby tightening of nut 38 is effective to draw the cathode tightly against the ground plane. For insulating the cathode from the ground plane, a strip of insulation 42, which may be mica or any other suitable insulator is interposed between the cathode and ground plane and has suitable openings therein to accommodate bolts such as bolt 28.

As seen more clearly in FIG. 1, the ends of cathode 24 are preferably tapered to a point and the cathode extends along a substantial proportion of the length of the ground plane. The spacing between cathode and ground plane is so small that at microwave frequencies, the cathode is connected to the ground plane so as to appear as part of it to electromagnetic microwaves.

For introducing electromagnetic waves to and extracting the same from the wave guide structure 1-0, a pair of strip lines 48 and 5d are supported at each end thereof. Either strip line may be used for introducing the wave while the other is used for extracting the wave but for simplicity, herein, line 48 will be considered the input line and line 50 the output line. The strip lines are disposed parallel to the ground plane and for aiding in the transfer of energy to or from the strip lines, each terminates in a plurality of finger-like probes as shown at 52 and 54 disposed just above the respective ends of the cathode 24. Energy may be fed or taken from the strip lines in any suitable manner such as through coaxial cables 56 and 58, the respective center conductors 60 and 62, of which are connected to and provide the support for respective strip lines 48 and 50.

Anode 14 forms a closed structure and is filled with a suitable ionizable gas such as one of the noble gases, at a pressure of approximately 1 mm. of mercury. It is to be understood, however, that the gas pressure may be selected at different values as from 1 to l0 mm. of mercury, to vary the plasma spacing and dimensions depending upon the particular result to be accomplished as made clear hereinbelow.

A direct potential for application across cathode 24 and anode 14 is derivable from a direct potential source 64. A potentiometer with a resistor element 66 is connected across the source 64 and a movable arm 68 is connected to one terminal of a switch 69 and is engageable with the resistor 66. Conductor 44 is connected to the other terminal of switch 69 and a wire conductor 70 connects the positive terminal of source 64 to the anode. Thus, by the application of a suitable direct potential across cathode 24 and anode 14 by the closure of switch 69, a potential gradient between the cathode and anode sufiiciently great to create ionization of the enclosure gas, is established. Such ionization is represented by a visible discharge cloud 71 in FIG. 2. Gaseous discharges possess a property of electron resonance which has a characteristic frequency at which a group of electrons displaced in a gaseous discharge plasma will oscillate about a neutral position. Such plasma frequency depends upon the electron density. Gaseous discharge plasmas having plasma frequenciesgreater than the frequency of a microwave appear as a conductor to the microwave whereby the microwave cannot penetrate the plasma but rather is reflected from it. The spacing between the cathode and anode is interrelated with the enclosed gas pressure to limit the ionization to a region of negative glow approximately coincident with the cloud 71 in which there are a relatively high density of free electrons and in which the plasma frequency of electrons is greater than the frequency of the wave to be transmitted by structure 10.

Accordingly, the cloud 71, under such conditions takes on the character of a metallic conductor with respect to such electromagnetic waves and comprises a strip along the interior of anode 14 spaced above the cathode 24 and is relatively sharply delineated along its boundaries. Additionally, the cross-sectional dimensions of the cloud 71 are dependent upon the potential gradient between cathode and anode whereby the thickness of the cloud and therefore the spacing of the cloud from the cathode may be varied by potential variation since an increase in thickness of the cloud brings the lower edge thereof closer to the cathode. It is apparent that the cloud 71 together with ground plane 12 may be advantageously employed as a strip line wave guide structure and that by control over the gas pressure and ionizing potential, the same may readily be modified within rather wide limits to accommodate different electromagnetic waves since such control is effective to control the position of cloud 71 with respect to the cathode. waves may be introduced by the strip line 48, for example, and extracted by the strip line 50 and the finger-like probes 52 and 54 are helpful to effect a transfer of microwave energy from the strip line to 48 and 50 tothe plasma conductor 71.

By opening switch 69 to remove the potential gradient, the cloud quickly disintegrates whereby the electromagnetic wave is no longer capable of being transmitted through the device 10. Thus, the effective strip line produced by cloud 71 may be created or disintegrated quickly and easily to support or block the passage of electromagnetic waves as desired.

As seen more clearly in FIG. 4 of the drawings, according to another feature of my invention, switching from one strip line wave guide to any one or combination of other wave guides produced as hereinabove described is readily eifected in the device represented generally at 72. To this end a plurality of cathodes as, for example, cathodes 73, 74 and 76 are mounted on a ground plane 78 and radiating from a strip line 80 of structure similar to the strip lines 48 and 50 in FIG. 1. An anode 82 forming an enclosure is mounted in spaced insulated relationship from ground plane 78 and means including a direct potential source 84, switch 86 and selector switch 88 are provided for selectively establish- Such ing a potential gradient between any one of the cathodes or any combination thereof and the anode, of sufficient magnitude to ionize gas under low pressure in the interelectrode space, to produce an effective strip line above the cathode as hereinabove described. A potentiometer 90 is connected across source 84 for potential selection and selector switch 88 is provided with an arm 91 connectable with any one of contacts 92, 94 or 96 leading to respective cathodes 73, 74 and i6 and is further connectable jointly with contacts 98 and 100, 102 and 104, and 106 and 108 leading to combinations of cathodes 7'3 and 74, 73 and '76 and 74 and 76, or with contacts 110, 112 and 114, leading to all of the respective cathodes. The anode, cathode and strip line structure is sealed in the anode enclosure containing low pressure, ionizable gas, such as one of the noble gases. Thus, according to my invention, switching from strip line to any one or combination of a group of conventional strip lines may be accomplished through the effective strip line wave guide that may be established in device 72.

In accordance with a feature of another embodiment of my invention shown in FIG. 5 of the drawings, wherein elements similar to elements shown in FIGS. 13 are designated by the same numbers primed, the wave guide structure may be entirely similar to that shown in FIGS. 1 and 2 of the drawings with the exception that ground plane 12' and anode 14' are each directly made positive with respect to cathode 24' to constitute a composite anode electrode cooperable with cathode 24' to produce ionization. According to this embodiment, the pseudo strip line or ionized region is formed above the cathode as dscribed with respect to other embodiments of invention and electromagnetic wave propagation is sustained as hereinabove also described.

Whilethe present invention has beendescribed by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. 1, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An apparatus comprising a conductive ground plane, a cathode extending along a portion of said ground plane and being insulated therefrom, an anode insulated from said ground plane and having portions spaced from said ground plane to provide an interelectrode space therebetween, saidcathode being in saidiuterelectrode space, an ionizable gas in said interelectrode space and means establishing a potential gradient be tween said anode and said cathode of sufficient intensity to ionize a localized region of said gas to form a plasma of negative glow in said interelectrode space and from said cathode, the frequency of said electrons in said plasma having a predetermined value and means for in troducing an electromagnetic wave of frequency lower than said predetermined frequency into said interelectrode space whereby said electromagnetic wave is conducted into said interelectrode space between said plasma acting as an effective stripline and said ground plane.

2. An apparatus comprising a conductive ground plane, an anode insulated from said ground plane and having portions spaced from said ground plane to provide an interelectrode space, a cathode extending along said ground plane and being in said interelectrode space, an ionizable gas in said interelectrode space and means for establishing a potential gradient between said anode and said cathode of suflicient intensity to ionize a localized region ofsaidgas to form a plasma of negative glow in said interelectrode region and spaced from said cathode and said anode to form an effective strip line for the conduction of electromagnetic wavesbetween said plasma and said ground plane.

3. An apparatus comprising a conductive ground plane, an anode insulated from said ground plane and having portions spaced from said ground plane to provide an interelectrode space, a cathode extending along a portion of said ground plane and being insulated therefrom, said cathode being in said interelectrode space, an ionizable gas at a pressure of the order of 1 mm. of mercury in said interelectrode space and means establishing a potential gradient between said anode and said cathode of sufficient intensity to ionize a localized region of said gas to form a plasma of negative glow in said interelectrode space, said plasma being spaced from said cathode and anode to form an elfective strip line spaced from said ground plane and anode to support the propagation of said waves therebetween.

4. An apparatus comprising a conductive ground plane, an anode insulated from said ground plane and having portions spaced from said ground plane to provide an interelectrode space, a plurality of cathodes insulated from each other and from said ground plane and extending therealong from a location thereon, an ionizable gas at low pressure in the interelectrode space between said cathode and said anodes and means selectively establishing a potential gradient between said anode and said cathode of sufficient intensity to ionize a localized region of said gas to form a plasma of negative glow in the interelectrode regions between said cathode and said anode, and means for introducing an electromagnetic wave at said location of frequency less than the plasma frequency of the electrons in said plasma.

5. An apparatus comprising a conductive ground plane, an anode insulated from said ground plane at low frequencies and having portions spaced from said ground plane to provide an interelectrode space, a cathode narrow relative to said anode extending along a portion of said ground plane and being insulated therefrom, said cathode being in said interelectrode space, an ionizable gas at low pressure in said interelectrode space and means establishing a potential gradient between said anode and said cathode of suflicient intensity to ionize a localized region of said gas to form a plasma of negative glow in said interelectrode region and spaced from said cathode, the plasma frequency of electrons in said plasma having a predetermined value and means for introducing an electromagnetic wave of frequency lower than said predetermined value into said interelectrode space along said strip line whereby said electromagnetic wave is propagated in said interelectrode region between said plasma acting as an eifective strip and said ground plane.

6. An apparatus comprising a conductive ground plane, an anode insulated from said ground plane and having portions spaced from said ground plane to provide an interelectrode space, a cathode in said interelectrode space 5 extending along a portion of said ground plane and being insulated therefrom, strip lines extending into said interelectrode space and terminating, respectively, in the region of the different ends of said cathode, an ionizable gas in said interelectrode space and means establishing a potential gradient between said anode and said cathode of sufi'icient intensity to ionize a localized region of said gas to form a plasma of negative glow in said interelectrode region and spaced from said cathode, the electrons in said plasma having a predetermined plasma frequency and means for introducing an electromagnetic wave along said strip lines of frequency lower than said predetermined frequency whereby said electromagnetic wave is conducted in said interelectrode region between said plasma acting as an etfective strip line and said ground plane from one of said strip lines to the other thereof.

7. An apparatus comprising a conductive ground plane, a cathode mounted in close proximity to said ground plane and being insulated therefrom, means for enclosing a region adjacent to said cathode, said region being filled with an ionizable gas at low pressure, means for establishing a potential gradient between said ground plane and said cathode to ionize "a localized region of said gas at a location spaced from said cathode and means for introducing an electromagnetic wave to the region of said ionization whereby said ionized region and cathode provide an effective strip line to support propagation of electromagnetic wave energy therebetween.

8. An apparatus comprising a conductive ground plane and a cover member having spaced portions in contact with said ground plane and an intermediate portion spaced therefrom to provide an enclosure, an ionizable gas in said enclosure, a cathode in said enclosure and being closely spaced from said ground plane, means for applying a positive direct potential to said ground plane with respect to said cathode for ionizing a portion of the gas in said enclosure, means for introducing an electromagnetic wave in the enclosure between said ionized gas portion and said cathode whereby said ionized gas portion and said cathode provide an eifective strip line to support propagation of electromagnetic wave energy therebetween.

References Cited in the file of this patent UNITED STATES PATENTS 2,693,583 Rigrod Nov. 2, 1954 2,760,163 Arditi Aug. 21, 1956 2,866,164 Steele Dec. 23, 1958 FOREIGN PATENTS 592,401 Great Britain Sept, 17, 1947 OTHER REFERENCES Arams Electronics, November 1954, vol. 27, No. 11, pages 168-172.

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