US3439224A - Electron injection plasma variable reactance device - Google Patents

Description

A ril 15, 1969 R. c. KNECHTLI ET AL 3,439,224

ELECTRON INJECTION PLASMA VARIABLE REACTANCE DEVICE Filed 0ot.'24, 1966 Sheet of 4 WilMO/V/C J1! 56 14! 44 004 2444/; 6474005 EMA/s 144.900 M 4, [04/440 (3 (wax/7Z drratuy April 1969 R. c. KNECHTLI ET AL 3,439,224

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' ELECTRON INJECTION PLAS Filed 001;. 24, 1966 MA VARIABLE REAGTANCE DEVICE Sheet of 4 POTENTIAL United States Patent 3,439,224 ELECTRON INJECTIGN PLASMA VARIABLE REACTANCE DEVICE Ronald C. Kuechtli, Woodland Hills, and Yasuo Wada,

Canoga Park, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Oct. 24, 1966, Ser. No. 589,102 Int. Cl. H01j 7/06, 7/46, 19/80 US. Cl. 315-39 Claims ABSTRACT OF THE DISCLOSURE Plasma is controlled in a Waveguide to present variable reactance to radio frequency energy in the guide. Plasma is provided by containing a gaseous medium in the Waveguide and injecting electrons into the gaseous medium so that the gas becomes an ionized plasma. Control of plasma density is exercised by means of a grid control, to control reactance.

This invention relates to a variable reactance device and more particularly to a plasma variable recatcance device providing electronically controllable variable react- Variable reactance devices are generally classified into three categoriessolid-state or semiconductor varactors,

ferrite variable reactance devices and plasma variable reactance devices.

For low radio frequency (RF) power levels, solid-state varactors provide useful electronically controlled RF electromagnetic energy reactances, especially at microwave frequencies. Typical characteristics of a good solid-state varactor are a quality factor or Q of the order of 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/0r 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 varactors and also are capable of providing an electronically controllable variable reactance. This type of variable reactance devices is generally less etficient than sold-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.

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It is another object of the invention to provide a more efficient 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 yariable reactance device with a very low RF insertion oss.

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.

SUMMARY 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 collector (anode), a grid, mesh or other constriction disposed between the emitter and collector, and a gaseous medium maintained in the region between the emitter and the collector. The emitter, collector and grid structures are connected to an appropriate adjustable potential source to create a plasma (grid) sheath having an injection boundary from which electrons are injected into the gas. 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.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram and associated potential distribution curve showing how, according to the invention, electron injection takes place through a grid sheath;

FIG. 2 illustrates a low driving power embodiment of a triode type electron injection plasma variable reactance device constructed according to the invention;

FIG. 3 illustrates a low noise embodiment of a triode type variable reactance device according to the invention;

FIG. 4 is an embodiment of the invention shown integrated with an X-band waveguide;

FIG. 5 is a tetrode configuration of the invention;

FIG. 6 is a potential distribution of a tetrode type electron injection plasma variable reactance device according to the invention; and

FIG. 7 illustrates a further embodiment of the invention wherein a variable reactance device of the invention is disposed in a sealed-off container that is mounted in a waveguide.

DESCRIPTION A plasma variable reactance device is a device producing a plasma of controlled 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 Plasma-Diagnostics with Microwaves, John Wiley & Son, 1965, New York, as:

where n =electron density m =electron mass e=electron 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/cm. 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 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 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 injection 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 from an injection boundary in a plasma sheath 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 grid (plasma) sheath as can be seen from the potential distribution curve of FIG. 1, that is locked to a mesh or constriction. 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 efiiciency. Operation at low gas pressure means low electron-neutral collision frequency and high Q. High ionization elficiency means low discharge power.

An electron injection plasma variable reactance device constructed according to the invention and incorporated into a waveguide is illustrated in FIG. 2. Here, the inner surface of a waveguide structure 11 also acts as an anode to which electrons injected by a cathode 13 are collected to form a plasma column 14. The cathode may be supported by any convenient insulative means or by heavy lead wires such as wires 15 connected to two terminals of the terminal board 17. Another of these terminals is connected internally to the waveguide structure itself so that a potential may be seen between the anode and cathode by appropriate connection of the terminals to an adjustable DC voltage source, not shown. Also, there is mounted within the waveguide structure 11 a partition 19 provided with an aperture 21 disposed opposite an emitting surface 23 of the cathode 13. Across this aperture is disposed a conducting grid or mesh 25 member of materials such as nickel, copper or molybdenum, for example, that is DC insulated from the waveguide structure by means of an annular insulating member 27 fabricated from a suitable dielectric material such as, for example, ceramic. The grid 25 is connected to a terminal of the terminal board 17 by means of wire 29. As noted above, the grid 25 is insulated from the waveguide, but by means of the insulating member 27, it is capacitively coupled to the waveguide wall so as to prevent RF leakage out of the waveguide through the aperture 21. An ionizable gas such as neon, xenon or krypton, to name just a few, is confined by conventional means such as quartz windows, not shown, at the ends of the waveguide in order to allow the propagation of microwave energy through the volume wherein the gas is maintained.

In order to reduce the discharge power required for a given plasma density n and to help localize the plasma in a well-defined column, such as the column 14, a moderate axial magnetic field H can be applied. Typically, a field between and 500 oersted is adequate for this purpose. By confining the plasma column, this field reduces substantially the ion loss by radial diffusion, thus reducing the power required to sustain the discharge. The confining magnetic field can be provided by a permanent magnet circuit. Because of the relatively low value of the field required, the magnetic material and the pole pieces can be incorporated in the waveguide 11, making up the walls of the waveguide in the vicinity of the plasma variable reactance device.

The basic potential distribution of the above-described configuration is shown in FIG. 1. As noted before, the electron acceleration and injection into the active part of the discharge takes place through a plasma sheath locked to a mesh or constriction. The higher plasma density is found in the region between the mesh or grid 25 and the anode which in this case is the waveguide structure 11. The lowest mean electron energy is found in the region between the cathode 13 and the grid 25 where the electrons have a substantially Maxwellian energy distribution at a temperature of the order of the cathode temperature. The plasma in either region can be used as the electronically controlled reactive element. For minimum discharge power, the region between the grid and the anode may be used. This is the configuration of FIG. 2. On the other hand, for minimum noise radiation, the region between the cathode and the grid may be used. This is the configuration of FIG. 3 wherein, as compared with FIG. 2, like reference numerals denote like elements. The cathode 13, however, is supported by conventional insulative means, not shown, in a conventional microwave choke configuration 31 to prevent RF losses from the cathode area. The specific potential distribution curves of each of the devices shown in FIGS. 2 and 3 are set forth alongside these individual illustrations.

From FIG. 1, it can be seen that the electron injection plasma sheath is sufficiently thin that practically no electron-neutral collisions take place during the electron acceleration through this sheath. The gas pressure is adjusted such that the ionization mean free path for electrons at the injection energy (corresponding to the discharge voltage) is at least of the order of the distance L, which is the mean distance travelled by the electrons between the injection boundary, here located adjacent the grid, and the anode surface. Gas pressure lower than that described will result in a lower electron-neutral collision frequency and higher Q, however, with little improvement in ionization emciency.

The plasma density and reactance (dielectric constant) may be conveniently controlled either by means of the grid potential or by means of the anode potential. The grid potental controls the discharge current, reducing discharge current and plasma density as the grid is made more negative. For operation in the mode shown in FIG. 1, the grid potential is kept close to the cathode potential by appropriate connection to a potential source (not shown) and the mesh size of the grid is chosen such that the largest grid hole dimension d is smaller than the electron mean free path 1.. For a gas such as neon, for example, at a pressure of 0.3 torr., this means that d 0.3 cm. The anode potential controls the energy of the injected electrons and the plasma density by controlling the discharge voltage V in the range (above the ionization potential of the gas used) where the ionization cross section increases substantially with increasing voltage. In fact, the larger the cross section for a given discharge current, the larger the rate of the ion generation and the larger the resultant electron density n By using the configuration of FIG. 2, the following data was obtained:

Waveguide.Standard X-band size Gas.Xenon Gas pressure-20 millitorr.

Discharge conditions for a plasma density such that 8E0 at a frequency of 9.87 kmc. were:

Discharge voltage.-20 volts Discharge currentma.

Discharge power.0.3 watt Magnetic field-J50 oersted.

RF performance was characterized by quality factor.

FIG. 4 illustrates another embodiment of the invention integrated with an Xband waveguide. Here, a waveguide section 41 includes an aperture 43 about which is disposed a variable reactance device housing structure 45. Within the structure 45 is mounted an indirectly heated type cathode 47 having an emitting surface 49 and an inner heater element 51 for connection to a source of heater supply voltage, not shown. In order to substantially reduce any loss of microwave energy in the cathode area, the cathode 47 is partially enclosed in a microwave choke 53 of conventional design having an end portion 55 in which is an aperture 57. The cathode 47 is supported in a coaxial position with respect to the choke 53 and the aperture 57 by means of an annular insulator 59 of any insulative material adapted for this service such as ceramic or glass, for example. This same type of material may be used to insulate the choke 53 from the housing structure in the form of annular insulators 61. A mesh or grid element 63 is supported opposite the emitting surface 49 of the cathode 47 by a nonconductive partition member 65 which allows the grid to be placed at a desired potential by a conductive lead, not shown. The partition member 65 is supported in its position roughly midway between the cathode emitting surface 49 and the aperture 43 in the waveguide 41 by means of one of the annular insulators 61 and another annular insulator 67. This device is operated in the same manner as the embodiment of FIG. 2 previously described. In a particular device constructed according to this embodiment of the invention, the cathode emitting surface had an OD. of 0.45 cm.; the grid was supported on a partition having a 0.5 cm. I.D. aperture; and the aperture in the waveguide was 0.6 cm. in diameter.

In order to simplify the suppression of RF leakage through the gridded hole or aperture, the triode type structures can be modified into tetrode structures as shown in FIG. 5. In this embodiment, as may be the case in any of the preceding embodiments or those to be described later, the waveguide section such as waveguide 71, here, comprises permanent magnet side walls 73 and a pole piece 75 acting as an anode 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 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 37 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 by 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 metallic conductive partition 99 attached conductively to the inner walls of the side walls 73 by conventional means. 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 are connected to appropriate sources of potential and to the various elements of this device in the same manner as described in connection with the embodiment shown in FIG. 2.

In this tetrode structure, the anode mesh or grid that is mounted on a conductive partition is at the same potential as the waveguide and can therefore be in direct contact with it, thus most effectively suppressing RF leakage through the aperture in the partition irrespectively of the location of the aperture in the waveguide. The potential distribution can be seen for tetrode configurations in FIG. 6. The spacing between thetwo grids is not critical but will preferably be smaller than the spac ing between the anode grid and the anode. 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 efliciency, 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. 5 is similar to that of a triode device, the first of 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 sealedoff 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. 7 for tetrode device but it applies equally to triode type 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 an anode 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 anode 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 .117 is mounted by conventional insulative means an indirectly heated cathode .119 having an emitting surface 1'21. 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 12.7 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 anode potential to the anode 107 through the waveguide 103.

In the case of a tetrode configuration as shown in FIG. 7, a metallic mesh or grid 130 is stretched across an aperture 141 in the cathode housing structure 117 adjacent the flange 115. This grid is then the anode grid and is at the same potential as the anode 107. A control grid 143 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 1 19 by insulating rings 147. Wire 1149 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. 5. 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. 5, 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 variable reactance device. 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 efiicient 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 anode.

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 a cathode surface for emitting electrons;

means including an anode surface spaced from said cathode for collecting the electrons emitted by said cathode;

a control-grid structure disposed in the path of said electrons between said cathode and anode;

an anode-grid structure disposed in the path of said electrons adjacent said control grid and between said control grid and said anode, said control-grid and said anode-grid being connected to be maintainable at different potentials, said anode grid being electrically connected to said anode;

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

means connected to respective ones of said cathode,

control grid, anode grid, and anode 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 to said anode, 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 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 coupled to the plasma for producing an interaction between said radio frequency electromagnetic 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 waveguide structure and a cathode surface disposed within said waveguide structure for emitting electrons;

means including an anode surface disposed within said waveguide structure, said anode surface being spaced from said cathode for collecting the electrons emitted by said cathode;

a control-grid structure disposed within said waveguide in the path of said electrons between said cathode and anode;

an anode-grid structure disposed within said waveguide and electrically connected to said anode and in the path of said electrons adjacent said control grid and between said control grid and said anode;

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

means connected to respective ones of said cathode, grid and anode for connection to an adjustable source of potential to create a grid sheath having an injection boundary adjacent said grid for injecting said electrons into said gaseous medium, said electrons traveling a mean distance L from said injection boundary to said anode, 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 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.

3. An electron injection plasma variable reactance device according to claim 2, wherein said variable reactance device also comprises magnetic field means for localizing said plasma in a well-defined column between said cathode and anode.

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

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

References Cited UNITED STATES PATENTS 3,023,380 2/1962 11111 333-7 2,813,999 11/1957 Foin 315- 39 2,817,045 12/1957 Goldstein 315-39 2,837,693 6/1958 Norton 315-39 HERMAN KARL SAALBACH, Primary Examiner.

LOUIS ALLAHUT, Assistant Examiner.

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

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