US3439225A - Electron injection plasma variable reactance device with perforated anode in the electron path - Google Patents

Description

April 15, 1969 I 3,439,225

ELECTRON INJECTION PLASMA VARIABLE REAC'IANCE DEVICE WITH PERFORATED ANODE IN'THE ELECTRON PATH Filed Oct. 24, 1966 Sheet of 3 April 15, 1969 KNECHTL' 3,439,225 ELECTRON INJECTION PLASMA VARIABLE REACI'ANCE DEVICE WITH PERFORATED ANODE IN THE ELECTRON PATH Filed Oct. 24, 1966 Sheet 3 Of 3 I I I 9/ a (5 I T 57481.5 W727 774/. sauna-5 .aza

April 15, 1969 c- K E I 3,439,225

ELECTRON INJECTION PLASMA VARIABLE REACTANCE DEVICE WITH PERFORATED ANODE IN THE ELECTRON PATH Filed Oct. 24, 1966 Sheet 5 Dr s Para/rm;

United States Patent Office 3,439,225 Patented Apr. 15, 1969 3,439,225 ELECTRON INJECTION PLASMA VARIABLE RE- ACTANCE DEVICE WITH PERFORATED ANODE IN THE ELECTRON PATH Ronald C. Knechtli, Woodland Hills, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Oct. 24, 1966, Ser. No. 589,119 Int. Cl. HOlj 19/80, 7/46; H01p 1/00 US. Cl. 315-39 10 Claims ABSTRACT OF THE DISCLOSURE The electron injection plasma device injects electrons into a gas in a waveguide so that plasma is created, with the plasma density determining the reactance of the plasma with respect to radio frequency electromagnetic energy transmitted through the waveguide. The device includes a perforated anode arranged so that the electrons are injected through the perforated anode, reflect off the far wall of the waveguide and return to the anode.

This invention relates to a variable reactance device and more particularly to a plasma variable reactance device providing electronically controllable variable reactance to radio frequency electromagnetic energy.

The use of variable reactance devices as either fixed or controllable reactance devices is well established in the art. For example, variable reactance devices have been used for electronic control of antenna arrays, especially slot antenna arrays; electronic switching of moderate to high power microwave energy; frequency multiplication; parametric amplification; electronically controlled phase shifters, to name just a few.

Variable reactance devices are generally classified into three categories-solid-state or semiconductor varactors, ferrite variable reactance devices and plasma variable reactance devices.

For low radio frequency (RF) power levels, solid-state variable reactance devices provide useful electronically controlled RF electromagnetic energy reactances, especially at microwave frequencies. Typical characteristics of a good solid-state variable reactance devices are a quality factor or Q of the order of 10 at X-band, but a microwave power handling capability of the order of only one watt.

Ferrite devices have been commonly used for electronic switching and/or phase control of microwaves but are somewhat limited in the amount of average RF power they can handle. Also, these devices introduce significant RF losses and are temperature sensitive to a substantial degree.

Variable reactance devices utilizing the plasma of a gas discharge are called plasma variable reactance devices and also are capable of providing an electronically controllable variable reactance. This type of variable reactance device is generally less efficient than solid-state variable reactance devices, introduce a considerable amount of noise to the system and have a relatively low quality factor or Q.

The electron injection plasma variable reactance devices of the invention on the other hand have the advantage of being able to effectively operate at relatively high average RF power levels and introduce lower RF losses than ferrite devices, for example, and also are less temperature sensitive. Variable reactance devices constructed according to the invention furthermore have a relatively lower equivalent noise temperature, a much higher Q and lower insertion loss than conventional plasma variable reactance devices.

Accordingly, it is an object of this invention to provide an improved electronically controllable radio frequency electromagnetic energy reactance device.

It is another object of the invention to provide a more eflicient plasma variable reactance device than heretofore obtainable.

It is still another object of the invention to provide a plasma variable reactance device wherein the plasma is maintained substantially free from unwanted oscillations and has a low equivalent radio frequency noise temperature.

It is yet another object of this invention to provide a variable reactance device with a very low RF insertion loss.

It is a further object of the invention to provide a variable reactance device having a relatively high Q.

It is still a further object of the invention to provide a variable reactance device having a high RF power handling capability. The above mentioned and other objects of the invention are achieved in an electron injection plasma variable reactance device adapted to interact with radio frequency electromagnetic energy. According to one embodiment of this invention, the variable reactance device comprises an electron emitter (cathode), an electron repeller, a gridanode element disposed between the emitter and repeller, and a gaseous medium maintained in. the region between the emitter and repeller. The emitter, repeller and gridanode elements are connected to an appropriate adjustable potential source to create a plasma sheath having an injection boundary from which electrons emitted by the emitter are injected into the gas and. repelled by the repeller and collected by the grid-anode element. The injected electrons travel a mean distance L from the injection boundary to the collector surface whereby the gas is ionized to form a plasma of a density dependent upon the magnitude of the potential difference between the plasma sheath and the collector and upon the discharge current. The gas is maintained at a pressure such that the ionization mean free path for the injected electrons is at least of the order of the distance L. Also, the device is arranged so that there is an interaction between the radio frequency electromagnetic energy to be influenced and the plasma.

The invention and specific embodiments thereof will be described hereinafter by way of example and with reference to the accompanying drawings wherein like reference numerals refer to like elements and parts, and in which:

FIG. 1 is a schematic diagram and associated potential distribution curve of a reflex electron injection plasma triode variable reactance device according to the invention;

FIG. 2 illustrates a reflex variable reactance device of the type shown in FIG. 1 that is integrated in a waveguide;

FIG. 3 is a schematic diagram and associated potential distribution curve of a reflex electron injection plasma tetrode variable reactance device according to the invention;

FIG. 4 is a cross sectional view of a tetrode reflex variable reactance device of the type shown in FIG. 3 integrated in a waveguide; and

FIG. 5 is a cross-sectional view of a plasma variable reactance device having a tetrode configuration.

In order to facilitate in the description of the invention, the following material relating to plasma variable reactance devices in general should be noted.

A plasma variable reactance device is a device producing a plasma of con-trolled density over an appropriate volume. The plasma acts as a dielectric Whose dielectric constant e is determined by the electron density n of the plasma and is given by Heald & Wharton at p. 6 in where:

n Electron density m =Electron mass e=Electrn charge 'y Electron collision frequency w=21rf (RF field frequency) e =Dielectric constant of free space It is seen that an increase in electron density n results in a reduction of dielectric constant e, which means essentially that the plasma behaves as an inductive medium, the inductive effect of the plasma being controlled by controlling n As an example, at X-band (f= c.p.s.), in order to make 6 0, it can be shown that a plasma density n =l0 electrons/0111. will be required. This value can readily be obtained by means of gas discharges.

The RF losses of a plasma variable reactance device can be calculated by means of Equation 1 and are best expressed in terms of the Q of the reactance presented by the plasma to the RF fields. Q is defined as:

where W=Kinetic RF energy stored in plasma P=RF power dissipated in plasma.

Power dissipation in the plasma is caused by electronion and electron-neutral collisions, which appear through the factor 7 in Equation 1. To evaluate Q from Equations 1 and 2, it may be assumed to the first approximation that the plasma is uniform and occupies a volume V .(constant dielectric constant 6 within the volume V). Then:

where:

E=RMS RF electric field.

From the above relationship and Equation 1:

Equation 3 shows that in order to make a high Q (low RF loss) plasma variable reactance device, it is necessary to use a gas discharge in which the collision frequency v is low. In the type of discharge devices to be considered, electron-neutral collisions predominate over electron-ion collisions and therefore the problem is to minimize the electron-neutral collision frequency. To obtain this goal, it is necessary to minimize the gas pressure for a given electron density n In a conventional positive column discharge, this can only be done at the cost of a prohibitive discharge sustaining power. The practical limit for the Q of a plasma variable reactance device using a positive column discharge has been found to be of the order of 3 at X-band, which is too low to be of practical interest. The following described device utilizing the principle of electron inject-ion according to the invention operates at a lower gas pressure for a given electron density n and for a given discharge power than positive column and other conventional discharge devices.

The gas discharge to be used in electron injection plasma variable reactance devices is characterized by the injection of electrons into the gas to be ionized, at an energy substantially higher than the ionization potential of the gas. This electron injection takes place according to this invention through a cathode sheath as can be seen from the potential distribution curve of FIG. 1. The electron injection energy is approximately equal to the applied discharge voltage and can therefore be externally controlled. By adjusting the electron injection energy to a value equal to or larger than about 1.5 times the ionization potential of the gas used, the following advantages are obtainable: (l) the ionization cross section is close to its maximum value; this permits operation at relatively low gas pressures, much lower than conventional discharges; and (2) most of the energy imparted to the electrons is effectively used for ionization; this results in a high ionization etficiency. Operation at low gas pressure means low electronneutral collision frequency and high Q. High ionization efliciency means low discharge power.

An electron injection plasma variable reactance device constructed according to the invention and integrated in a waveguide is illustrated in FIG. 2. Here, the inner surface 19 of a waveguide structure 21 also acts as a reflector electrode to which electrons injected by an emitting surface 23 and a cathode 25 are reflected. Supported adjacent the emitting surface 23 by a support structure 27 is an anode-grid mesh 29. The support structure 27 is of a cylindrical configuration and is uniformly spaced a short distance from a flange 31 connected to the waveguide 21 and surrounding an aperture 33 in the waveguide 21. The spacing between and the lengths of the grid support structure 27 and the flange 31 are chosen to act as a microwave choke at the frequency of the electromagnetic energy propagating through the waveguide 21.

The eathode 25 is shown as an indirectly heated type utilizing a heater element shown schematically as heater element 35. In order to emit electrons to form a plasma column 37, the heater element 35 is connected by suitable means to a source of heater current, not shown. Other potentials, as will be described, are provided by appropriate leads connected to potential sources, not shown.

In this invention, the reflector surface 19 is held at a potential equal to or slightly less than the cathode potential, for best effectiveness. As shown in FIG. 2, an axial magnetic field H is provided by any convenient means such as permanent magnets, for example. This magnetic field is adjusted to a value such that the electron cyclotron radius for electrons at an energy corresponding to the discharge voltage be between approximately half the grid opening of the mesh of the grid 29, and the radius of the plasma column 37. For a discharge voltage (V between 10 and 25 volts, a grid opening of 1 mm., and a plasma column diameter of 3 mm., a magnetic field of the order of 30 to 300 Oersted should be provided. Fields much lower than the minimum magnetic field defined above will fail to radially confine the plasma column; fields substantially higher than the maximum magnetic field defined above will lead to radial potential gradients, plasma instabilities and high RF noise.

The advantage of the reflex discharge configuration is an increase in the mean distance travelled by an electron before being collected by the anode-grid 29. This distance becomes several times the distance between the anode-grid and the reflector. This permits a corresponding reduction in gas pressure without loss in ionization efliciency. The result is a reduction in electron-neutral collision frequency approximately proportional to the pressure reduction. Reducing electron collision frequency means a reduced RF loss, i.e., increased quality factor Q.

The reactance of the invention of this reflex discharge type plasma variable reactance device can be controlled by controlling either the reflector voltage, or the anodegrid voltage.

In the triode configuration described above, electron injection at the discharge voltage takes place at the cathode, as indicated in FIG. 1. This implies operation close to temperature limited cathode emission (saturation). Space charge limited operation substantially below cathode saturation current density is possible by modifying the triode configuration of FIG. 1 into a tetrode configuration by addition of a second grid between the anodegrid and the cathode. This leads to the configuration of FIGS. 3 and 4.

In this embodiment, as may be the case in any of the embodiments of this invention, the waveguide section such as waveguide 71, here, comprises permanent magnet side walls 73 and a pole piece 75 acting as a repeller electrode and a pole piece 77 opposite pole piece 75 and in which is disposed a sealed plug assembly 79 having pins 81 insulatively mounted on an insulated member 83.

As described in connection with the embodiment of FIG. 2, the use of an axial magnetic field H as provided by pole pieces 75 and 77 helps to localize the plasma in a well-defined column such as column 85 in order to reduce the required discharge power for a given plasma density n An indirectly heated cathode 87 is shown supported by wire leads connected to two of the pins 81 which also supply heater current to the filament element within the cathode 87. In order to stabilize and support the cathode 87 in this position, glass or ceramic insulators 89 are connected to an outer grid shell 91 mechanically but insulatively connected to the cathode 87 through an annulus insulator 93. The outer grid shell 91 includes a control mesh or grid area 95 through which electrons emitted by the cathode and later injected into the plasma must pass.

Adjacent the control grid 95 on the opposite side thereof from the cathode 87 is mounted an anode-grid 97 that is supported in this position by a partition 99 attach ed to the inner walls of the side walls 73 by means of insulative support projections 100. The partition 99, of course, includes an aperture 101 across which the grid 97 is stretched. The aperture opening is tapered on the side adjacent the cathode 87 to facilitate the positioning of the control-grid 95 close to the anode-grid 97. The other of the pins 81 as shown in FIG. 4 are connected to appropriate sources of potential and to the various elements of this device in the same manner as described in FIG. 3, where (in the case of Xenon as the gas) Vgi has a negative potential very close to that of the cathode, where V has a positive potential of approximately 15 to 20 volts (variable from zero), and where V may have a slightly negative potential very close to the potential of the cath-' ode.

The spacing between the two grids is not critical but will preferably be smaller than the spacing between the anodegrid and the repeller. The grid mesh openings of the anode- -grid are best chosen to be equal to the size of the openings of the control grid, or coarser if RF leakage does not become excessive with larger openings. To maintain good discharge efiiciency, it is advantageous to have the wires of both grids approximately registered. The control of the reactance of a tetrode device of the type of FIG. 4 is similar to that of a triode device, the first or control grid of the tetrode device having the same function as the single grid of the triode device.

From the foregoing it should be evident that the plasma variable reactance devices described may be enclosed in a sealed-off container that is transparent to the electromagnetic energy into which the variable reactance is to be introduced. This has the advantages of having the gas restricted to the volume within the sealed-off container thereby obviating the necessity of providing a waveguide structure that is sealed to prevent the escape of gas. The

.gas container material can be ceramic, for example, which has a low loss dielectric characteristic. Such a device is shown in FIG. for tetrode device but it applies equally to triode type reflex variable reactance devices as well. The variable reactance device 101 that is inserted into a waveguide section 103 through an aperture 105 in one of the walls thereof comprises a repeller 107 fitted into a recessed portion 109 in the inner wall, opposite the aperture 105, of the waveguide 103. The anode 107 has a reduced diameter flange portion 111 upon the outer circumference of which is sealed a low loss dielectric envelope such as ceramic cylinder 113. At the other end of the cylinder 113 is fitted another metallic flange 115 having a centrally located aperture and to which is attached by any convenient means, such as welding, a cathode housing structure 117, extending away from the repeller 107 and adapted to fit in good electrical and mechanical contact, against the outer periphery of the aperture 105 in the waveguide section 103 and effectively close this opening in the wall of the waveguide.

Within the cathode housing structure 157 is mounted by conventional insulative means an indirectly heated cathode 119 having an emitting surface 121. The cathode 119 includes a heater element, not shown, that is provided current through leads 123 connected to pins 125 passing through a sealed lead-through header 127 of an insulative material such as ceramic and the like. The header 127 is sealed at its periphery to an annular curved surface flange 129 that is Welded or otherwise firmly attached to the end of the housing structure 117 farthest from the flange 115 which is attached to the other end of this housing. The cathode 119 is provided with a lead 131 that connects to pin 133 passing through the header 127 in order to allow for the proper potential being placed on this element. Also, a pin 135 is connected by soldering or spot welding techniques to a lead 137 that is attached by one of these techniques, for example, to the inner wall of the metallic housing structure 117 to provide the repeller potential to the repeller 107 through the waveguide 103.

In the case of the tetrode configuration as shown in FIG. 5, a metallic mesh or anode-grid 139 connected through a lead 140 to a pin 141 is stretched across an aperture 142 in an insulator ring 143 fitted to the cathode housing structure 117 adjacent the flange 115. This grid is then the anode-grid and is at a potential difference with respect to the repeller 107. A control grid 144 is supported in a position between the cathode emitting surface 121 and the anode-grid 139 by a support structure 145 that has a cylindrical configuration supported symmetrically with respect to the cathode 119 by insulating rings 147. Wire 149 connects the control grid support structure 145 to a pin 151 that passes through the lead-through header 127 for connection to a proper potential, as described previously with respect to the embodiment shown in FIG. 4. As was the case with previous embodiments described, an axial magnetic field may also be employed for the same reasons given.

Furthermore, it should be noted that the magnetic field shown for the purposes of plasma confinement in FIG. 4, for example, may be adjusted to a value such that the electron cyclotron frequency corresponds approximately to the frequency of the RF wave or fields to be affected by the plasma varactor. Taking advantage of the cyclotron resonance permits further reductions in discharge power, gas pressure, and RF losses, and leads to a further increase in variable reactance device Q. The magnetic field required in this configuration, however, will be greater than that required for confinement of the plasma column only.

Still further, it should be understood that the designation of the length L as shown in the drawings is only used as an aid to indicate between which points or places the injected electrons travel and is not shown to indicate an exact path. It should further be understood in viewing the figures that the plasma sheath thicknesses 6 are much smaller than the distance L.

From the foregoing, it will be evident that the invention provides an improved and more eflicient plasma variable reactance device having a relatively high Q, a very low RF insertion loss, a high RF power handling capability, and in which the plasma is maintained substantially free from unwanted oscillations.

Although specific embodiments of the invention have been described in detail, other organizations of the embodiments shown may be made within the spirit and scope of the invention. For example, as a modification of the tetrode devices shown, the plasma between the two grids may be used as the electronically controlled reactance instead of the plasma between the second grid and the reflector.

Accordingly, it is intended that the foregoing disclosure and drawings shall be considered only as illustrations of the principles of this invention and are not to be construed in a limiting sense.

What is claimed is:

1. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including an electron emitter for emitting electrons;

means including a repeller surface spaced from said emitter for repelling the electrons emitted by said emitter;

a substantially flat perforated anode-grid structure disposed in line with and between said emitter and repeller;

a gaseous medium maintained in the region between said emitter and said repeller;

means connected to respective ones of said emitter,

anode-grid, and repeller for connection to an adjustable source of potential to create a plasma sheath having an injection boundary for injecting said electrons into said gaseous medium, said electrons traveling a mean distance L from said injection boundary, through said anode-grid, toward and away from said repeller and back to said anode-grid, said gaseous medium being ionized by said electrons injected by said plasma sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said plasma sheath and said anodegrid and upon the discharge current, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L; and

means associated with the plasma for producing an interaction between said radio frequency electromag netic energy and said plasma.

2. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including a cathode surface for emitting electrons;

means including a repeller surface spaced from said cathode for repelling the electrons emitted by said cathode;

a substantially flat perforated anode-grid structure dis posed in line between said cathode and repeller;

a gaseous medium maintained in the region between said cathode and said repeller;

means connected to respective ones of said cathode,

anode-grid, and repeller for connection to an adjustable source of potential to create a cathode sheath having an injection boundary adjacent said cathode for injecting said electrons into said gaseous medium, said electrons traveling a mean distance L from said injection boundary, through said anodegrid, toward and away from said repeller and back to said anodegrid, said gaseous medium being ionized by said electrons injected by said cathode sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheath and said anode-grid and upon the discharge current, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L; and

means associated with the plasma for producing an interaction between said radio frequency electromagnetic energy and said plasma.

3. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising means including a waveguide structure and a cathode surface disposed within said waveguide structure for emitting electrons;

means including a repeller surface disposed within said waveguide structure, said repeller surface being spaced from said cathode for repelling the electrons emitted by said cathode;

a substantially fiat perforated anode-grid structure disposed within said waveguide in line with and between said cathode and repeller;

a gaseous medium maintained in the region between said cathode and repeller;

means connected to respective ones of said cathode,

anode-grid and repeller for connection to an adjustable source of potential to create a cathode sheath having an injection boundary adjacent said cathode for injecting said electrons into said gaseous medium, said electrons traveling a mean distance L from said injection boundary, through said anode-grid, toward and away from said repeller and back to said anodegrid, said gaseous medium being ionized by said electrons injected by said cathode sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said cathode sheath and said anode-grid and upon the discharge current, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L; and

means including windows disposed at the ends of said waveguide for allowing radio frequency electromagnetic energy to propagate therethrough but restraining said gaseous medium within said waveguide.

4. An electron injection plasma variable reactance device for presenting an electronically controlled variable reactance to radio frequency electromagnetic energy, comprising:

means including a waveguide structure and a cathode surface disposed within said waveguide structure for emitting electrons;

means including a repeller surface disposed within said waveguide structure, said repeller surface being spaced from said cathode for repelling the electrons emitted by said cathode;

a control grid structure disposed between said cathode and repeller;

a substantially flat perforated anode-grid structure disposed in line between said control grid and said repeller;

a gaseous medium maintained in the region between said cathode and said repeller;

means connected to respective ones of said cathode,

control grid, anode-grid, and repeller for connection to an adjustable source of potential to create a grid sheath having an injection boundary adjacent said control grid for injecting said electrons into said gaseous medium, said electrons traveling a mean distance L from said injection boundary, through said anode-grid, toward and away from said repeller and back to said anode-grid, said gaseous medium being ionized by said electrons injected by said grid sheath to form a plasma of a density dependent upon the magnitude of the potential difference between said grid sheath and said anode-grid and upon the discharge current, said gaseous medium having a pressure such that the ionization mean free path for said injected electrons is at least of the order of the distance L; and

means including windows disposed at the ends of said waveguide for allowing radio frequency electromagnetic energy to propagate therethrough but restraining said gaseous medium within said waveguide.

5. An electron injection plasma variable reactance device according to claim 3, wherein said variable reactance device also comprises magnetic field means for localizing said plasma in a well-defined column between said cathode and said repeller.

6. An electron injection plasma variable reactance device according to claim 4, wherein said variable reactance device also comprises magnetic field means for localizing said plasma in a well-defined column between said cathode and said repeller.

7. An electron injection plasma variable reactance device according to claim 1, wherein said gaseous medium is Xenon.

8. An electron injection plasma variable reactance device according to claim 1, wherein said gaseous medium is neon.

10 9. An electron injection plasma variable reactance device according to claim 3, wherein said gaseous medium is xenon.

10. An electron injection plasma variable reactance device according to claim 4, wherein said gaseous medium is Xenon.

References Cited UNITED STATES PATENTS 2,813,999 11/1957 Foin 315-39 2,817,045 12/1957 Goldstein et all. 315-39 2,837,693 6/1958 Norton 315-39 2,848,649 8/1958 Bryant 315-39 HERMAN KARL SAALBACK, Primary Examiner.

LOUIS ALLAHUT, Assistant Examiner.

US. Cl. X.R. 333--98, 99

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