US 3155924 A
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Nov. 3, 1964 l. KAUFMAN ETAL 3,155,924
PLASMA GUIDE MICROWAVE SELECTIVE COUPLER Filed April 20. 1961 2 Sheets-Sheet 1 1/4] 0 1/12 ouTPuT & euma E FiELD I IIIIIIII COUPLlNG I/III/Il/I/IIII/I III I D SC HARGE i7- 4 INVENTORS IRV/NG KAUFMAN I 111111 1/ 1/ 1/ 1\ H BY 1 .5 MAW Nov. 3, 1964 I. KAUFMAN ETAL 3,155,924
PLASMA GUIDE MICROWAVE SELECTIVE COUPLER Filed April 20. 1961 2 Sheets-Sheet 2 RESULTANT COUPLED MrcRo-WAvE. POWER m OUTPUT cmcun' APDLEED VOLTAGE Puma D\-SC.HARGE CURRENT A E; Y c. D COUPLING a 3 Q g A 6/ v 3,.l5,92d Patented Nov. 3, 1964 3,155,924 E LASB/llfih GUifill TidI CRQWAVE SELECTIVE (IQUPLER Irving Kaufman, Woodland Hills, (lalifl, and William H. Steier, Urhana, 151., assignors to Thompson Rama Wooldridge Ina, tllanoga Park, tlalif, a corporation of @liio Filed Apr. 2%), 1951, Ser. No. 115,961 3 Claims. (Cl. 333-=-7) This invention relates generally to microwave equipment and, more particularly, to a plasma guide microwave selective coupler which permits the selective transfer of electromagnetic energy between a pair of hollow waveguides.
Several switching arrangements are known in the prior art which permit selective energy transfer between waveguides. In many instances, the arrangements employ moving mechanical elements and accordingly are relative- 1y slow acting. Further, most of the mechanical switching arrangements are either unwieldy or they introduce, at the moment of switching, serious discontinuities which, in aggravated cases may lead to inefficiencies and flashover. Some switching arrangements employ frequency sensitive elements and therefore are limited in operation to particular frequency bands. Gas type switches which have been used employ a cavity which is resonant when the switch is in a quiescent condition but whose resonance is destroyed upon firing. Ferrite switch arrangements have also been used but inasmuch as they require the use of a magnetic biasing field, they cannot be satis factorily employed in applications where stray magnetic fields cannot be tolerated.
Inasmuch as most of the prior art devices have proved to be unsatisfactory for one reason or another, it is the principal object of this invention to provide an improved switching scheme for permitting selective energy coupling between a pair of waveguides.
More particularly, it is an object of this invention to provide improved switching means which are capable of satisfactorily operating over the entire waveguide band at extremely rapid speeds. Inasmuch as no frequency sensitive elements are employed, there is nothing to restrict the frequency band over which the invention is operable. Moreover, switching times of the order of microseconds are easily attainable.
It is a still further object of this invention to provide improved means for selectively switching microwave energy with means which require very small amounts of control switching power. Short voltage pulses are employed to provide momentary coupling between microwave circuits.
It is a still further object of this invention to provide novel and improved switching means for selectively controlling energy transfer between a pair of waveguides including no moving mechanical elements and not requiring the use of a magnetic field.
The wave propagating properties of a stationary plasma column have been discussed in the literature (A. W. Trivelpiece and R. W. Gould, Space charge waves in cylindrical plasma columns, J. Appl. Phys, vol. 30, pp. 1784-1793; November 1959) which shows that a plasma column surrounded by concentric dielectric and metal sleeves can propagate electromagnetic energy provided the plasma density is sufiiciently high. The modes of propagation are considered to be electromechanical in nature and are quite different from the modes of a hol low circular waveguide perturbed by a plasma column. These plasma guide modes are space-charge waves, in which the electron density within the plasma column or on its surface is modulated as the wave passes. In the absence of any D.C. magnetic fields, the plasma guide modes are essentially surface waves and can exhibit slow wave properties.
The invention herein employs these known properties of stationary plasma columns to affect the improved switching function above mentioned. As stated, a coaxial plasma waveguide will conduct electromagnetic energy in a plasma guide mode if the density of the plasma is sufiiciently high. With no axial D.C. magnetic field, this limiting density (low density cut-off) is given y a a:w, 1+1r, 2 where Ai -plasma electron density lel=magnitude of the electron charge m==electron mass a =permittivity of free space K relative dielectric constant of dielectric in the guide w=angular frequency u denotes the plasma frequency which it will be appreciated is dependent on the plasma electron density. It should be noted from the equations that w, the operating angular frequency at which energy is propagated in the plasma guide mode, is dependent on a Of course, the operating frequency is therefore also dependent upon the plasma electron density. It follows then that by properly modulating the plasma density, transfer of electromagnetic energy in the plasma guide mode can be controlled. The electron density indicated by the above equations represents the mode density cut-off. For densities greater than that given, energy can propagate in the plasma guide mode.
Plasma guides can be operated with or without an axial D.C. magnetic field. Although an applied magnetic field is not necessary for operation, if a magnetic field be applied, the low density cut-01f is altered, depending on the value of H and the mode excited.
Reference is now made to the accompanying drawings forming a part hereof wherein like numerals refer to like parts throughout, and in which:
FIGURE 1 (a) is a fragmentary perspective view illustrating the manner in which a plasma guide is physically coupled between a pair of hollow waveguides using a one angular variation mode and (b) is a fragmentary perspective view illustrating the manner in which a plasma guide is physically coupled between a pair of hollow waveguides using a symmetric mode;
FIGURE 2 is an enlarged vertical sectional view taken substantially along the plane 22 of FIGURE 1 ([1) showing the manner in which the plasma guide penetrates through a pair of spaced hollow waveguides and showing representative means for modulating the electron density of the plasma;
FIGURE 3 is an enlarged vertical sectional view taken substantially along the plane 3-3 of FIGURE 1 (12);
FIGURE 4 represents a graph upon which is plotted the percentage coupling between a pair of waveguides as a function of the discharge current in the plasma guide;
FIGURE 5 shows the relationship in time of (l) a voltage pulse applied between the anode and cathode of the plasma tube, (2) the resulting discharge current in the plasma tube, and (3) the resultant coupled microwave power in the output waveguide;
FIGURE 6 is a fragmentary perspective view of an alternate form of the invention illustrating the manner in which the conductive plasma tube shield may be enlarged so as to function as a cylindrical waveguide at extremely low plasma densities;
FIGURE 7 is an enlarged vertical sectional view taken substantially along the plane 7-7 of FZGURE 6; and
FTGURE S is a graph upon a blob is plotted the relationship of the percentage coupling between a pair of Waveguides, as shown in FIGURE 6, as a function of the discharge current, it being noted that there exists two distinct regions of coupling.
With continuing reference to the drawings, initial at tention is called to FiGURE 1(a) and (b). In (a) a numeral 10 generally represents a portion of a hollow rectangular waveguide which for convenience will be termed the input guide. Numeral 12 generally represents a portion of a second hollow rectangular waveguide which for convenience will be termed the output guide. The vertical arrow positioned at the right ends of the guides is labelled E field and indicates the guide mode. In (b) a pair of guide portions 14 and 16 are shown with the arrow at the right ends of the guides 14 and 16 indicating the electric field and the mode in that instance. In each of (a) and (b) the hollow rectangular waveguide portions are spaced from each other and extend parallel to each other. The only characteristic distinguishing (a) from (b) is the guide mode. Inasmuch as the operation of the invention can be readily explained with reference to either one of the figures since the operation is conceptually identical, further reference will be restricted to guides 14 and 16 in (1')).
Assume microwave energy is being propagated along input guide 14 and it is desired to selectively couple that energy to output guide 16. In or er to do this, a plasma guide generally designated by the numeral 13 is provided. The plasma guide it; includes a dielectric tube preferably formed of glass. The tube 29 extends between input guide 14 and output guide 16 and penetrates each of them in a direction transverse to the direction of energy propagation therein. Tightly fitted around the dielectric tube 20 is a conductive sleeve 22. The conductive sleeve 22 is fitted around the tube 29 on the portion of the tube between the guides 14 and 16.
As pointed out above, propagation in a plasma guide mode only takes place if the plasma density is at a sufficiently high level. In order to control coupling between the waveguides 14 and 16, it is necessary to be able to control the plasma density in dielectric tube 20. The tube 20 is closed and contains a plasma therein which may be formed of, for example, neon, argon, mercury, etc.
In FXGURE 2, it will be noted that the closed tube 20 has a bulbous portion 24 formed at one end thereof. A mercury pool 26 is disposed therein with an electrode 28 being supported in the pool. An electrode 30 is disposed in the tube 28 spaced from the electrode 28. Connected between electrodes 28 and 36 is a source of electrical energy in the form of a battery 32 serially connected to a toggle switch 34-. Disposed in the tube fit) at an end thereof remote from the bulbous portion 24 is an electrode 36 which is connected to the electrode 28 by a series circuit including electrical energy source 38, toggle switch 4%), and a variable resistor 42.
Consistent with the theory mentioned above, when the electron density of the plasma in the tube 20 is raised to a sufiicient level, electromagnetic energy may be coupled through the plasma guide 18 between the input guide 14 and output guide 16. The plasma density is of course determined by the gas used, the temperature, the gas pressure, and the voltage applied to the discharge, between electrodes 28 and 36. The most practical way of varying the plasma density and thus the conductivity of the plasma guide is to vary the discharge voltage applied between the electrodes, as 28 and 36. Of course, in the case where mercury is employed, as in FIGURE 2, the voltage applied between electrodes 23 and 39 will affect the number of free electrons available and accordingly will also atlect the discharge current. If neon, argon, etc. were used instead of mercury, there would of course be d no need to liberate electrons, as between 23 and 30 in FIGURE 2.
Referring to FIGURE 4, some significant results are illustrated of experiments conducted with the above type equipment. The graph represents a plot of percent energy coupling between input guide 14 and output guide 16 versus the plasma discharge current between electrodes 28 and 36. It should be noted that for low levels of discharge current there is no coupling between the input and output guides. However, as the discharge current increases, a plasma density is reached, as expressed by the aforementioned equations, which causes the coupling indicated between the guides 14 and 16.
F1 URE 2 shows the glass tube 29 simply penetrating through opposed walls of each of the waveguides. Such an arrangement represents all that is actually necessary but however, of course, rubber cushioning means or such may be employed between the metallic waveguides and the glass tube to prevent breakage. It will be readily appreciated by those skilled in the art that impedance matching means which, for instance, may be in the form of well-known wedges can be employed to match the plasma guide impedance to the impedance of the guides 14 and 16. As a practical matter, impedance matching means must be included if low insertion loss is to be assured.
Initial experimental results using a mercury plasma guide coupler as shown, having one angular variation, (FIGURE la), no D.-C. magnetic field and with operation being in the X-band showed that a greater than 40 decibel isolation between guides 14 and 16 is established at low plasma densities while an insertion loss of only 8.5 decibels is experienced at sufficiently high plasma densities. Moreover, the experiments indicated that the 8.5 decibel insertion loss probably could be considerably reduced by improved matching.
Attention is now called to FIGURE 5 wherein a plot of oscilloscope signals as related in time, is illustrated. FIGURE 5 shows the results of experiments in which the plasma coupler was pulsed on with a voltage pulse applied between electrodes 23 and 36 of the plasma guide 18. The applied voltage pulse 50 was 10 microseconds in duration and the operating frequency of the coupled energy was in the X-band. Trace 52 represents the discharge current resulting in tube 20 and trace 54 shows the resultant coupled microwave power in the output guide 16. Operating conditions for FIGURE 5 are:
V on the plasma tube=57 v. I through the plasma-:17 amp. V =432 v.
f of coupled power=8.778 kmc.
A switching time of S sec. was observed. Similar results were also observed at 8.369 and 9.320 ltmc.
In the use of the coupler described above, quiescent operation would preferably be at Q in FIGURE 4. If this value of discharge current represents the quiescent condition, a minimum voltage pulse is required to increase the discharge current to a sutlicient level so as to etfect coupling.
Attention is now drawn to a further embodiment of the invention as particularly set forth in FIGURES 6 through 8. In FIGURE 6, numeral 60 represents a portion of a hollow rectangular waveguide which will be termed the input guide and numeral 62 represents a portion of a hollow rectangular waveguide which will be termed the output guide. The guide portions 69 and 62 may be identical to guide portions 14 and 16. In an identical manner as is shown in FIGURE 2, a glass tube 64 having a plasma therein penetrates the opposite walls of the input and output guides. The feature distinguishing the coupler 66 of FIGURE 6 from the coupler 13 of FIG- URE 1 is that instead of the ti htly fitting conductive cylindrical shield 22 shown in FIGURE 2, a conductive cylinder 68 is used around the tube 64 between the guide tit and 62 which is of a size large enough in diameter to permit it to propagate energy as a hollow Waveguide at extremely low discharge currents corresponding efiectively to the absence of the plasma tube.
Referring to FIGURE 8, a plot is presented of percentage coupling of energy between guides 66 and 62 versus the discharge current in tube 64. In region A, the discharge is either off or very weak. Propagation through the cylinder 68 is thus in a hollow waveguide mode. Operation in region A is often desirable inasmuch as the noise often caused by higher discharge currents is not present. In region B, enough plasma density exists to extinguish the hollow Waveguide mode but not enough for propagation in 'a plasma mode. In region C, a plasma mode exists, and at still higher densities, coupling again falls 011, as in region D.
Quiescent operation can be at Q where there is considerable coupling and where coupling may be easily pulsed oii by slightly increasing the discharge current. Quiescent operation can also be at Q if it is desirable to rapidly pulse on. Of course quiescent operation may also conveniently be at Q From the above, it will be appreciated that the plasma guide coupler can be designed to operate over the entire waveguide band. With reasonable voltage pulses, the plasma density can be modulated strongly enough to selectively couple over a wide band with only the input and output matching limiting the device. The switching is accomplished in times of the order of 5 ,asec. using relatively small amounts of power. Switching is achieved without requiring the use of undesirable magnetic fields.
The foregoing is considered as illustrative only of the principles of the invention. Since numerous modifications will readily occur to persons skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described and accordingly all suitable modifications and equivalents are intended to fall within the scope of the invention as claimed.
The following is claimed as new:
1. In combination with first and second spaced waveguides, means for selectively coupling said first and second Waveguides to one another for controlling energy transfer therebetween comprising a closed dielectric tube penetrating through said first and second waveguides, a third waveguide supported around said tube between said first and second waveguides, said tube enclosing a plasma therein, and means for selectively varying the electron density of said plasma between a low density at which energy is coupled through said third waveguide, an intermediate density at which no energy is coupled, a high density at which energy is coupled through said plasma, and a still higher density at which no energy is coupled.
2. The combination of claim 1 wherein said means for varying said electron density includes an anode in said tube, a cathode in said tube spaced from said anode, and an external source of electrical energy connected between said anode and said cathode for establishing a discharge current therebetween.
3. The combination of claim 1 wherein said first and second waveguides extend parallel to each other with said tube being disposed transverse to the direction of energy propagation therein.
References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Trivelpiece and Gould: Space Charge Waves in Cylindrioal Plasma Columns, Journal Applied Physics 30: 1784-1793, November 1959.
Sterer and Kaufman: A Plasma Guide Micrqwave' Selective Coupler Inst. of Radio Engineers Transactions on Microwave Theory and Technique MTT-499-506, November 1961.