US3309561A - Coaxial multipactor switch utilizing magnetic field to control impedance - Google Patents

Description

March 14, 1967 J. F. KANE 3,309,561 COAXIAL MULTIPACTOR SWITCH UTILIZING MAGNETIC FIELD TO CONTROL IMPEDANCE Filed Jan. 31, 1963 UTILIZATICN DEVICE DIELECTRIC T? INVENTOR 'JOHN E KANE ATTORNEY United States Patent 3,309,561 COAXIAL MULTIPACTQR SWITCH UTILIZING MAGNETIC FIELD TO CONTROL IMPEDANCE John Frederick Kane, Mountain View, Calif., assignor to General Electric Company, a corporation of New York Filerl Jan. 31, 1963, Ser. No. 255,362 5 Claims. (Cl. 315-39) prohibit the passage of microwaves through a particular waveguide is known as a waveguide switch.

Prior art variable impedances and switches for microwave transmission and control include mechanical, gaseous, and ferrite devices. In the mechanical devices shutters, plungers, or vanes are selectively positioned within the waveguide to control microwave transmission; i.e., one form of waveguide switch comprises a plate rotatable for closing the waveguide passage to prohibit transmission.

In the gaseous devices the microwaves are transmitted through gas-enclosing cells disposed within the waveguide, and a high voltage is applied across the gas cell to produce ionization of the gas molecules and thereby inhibit trans mission of microwaves through the cell. In the ferrite devices, various configurations of ferrite material are disposed within the waveguide, and controllable magnetic fields are applied to the ferrite material to thereby prohibit, permit, or restrictively permit the transmission of microwaves through the waveguide.

A common disadvantage of the prior art mechanical and gaseous variable impedances is their relative slowness of operation. In modern microwave systems, it is common to transmit microwaves in bursts, known as pulses, spaced but a few millionths of a sec-0nd (microseconds) apart, the duration of the bursts frequently being substantially less than one-millionth of a second. In such systems it is desirable to be able to alter the value of a microwave impedance in the interval between two successive pulses. However, in the mechanical type variable impedances and switches the inertia of the structure normally prohibits effective control, or change in operation, in less than one- -thousandth of a second. In the gaseous variable impedanoes, once the gas has become ionized many microseconds are required for the gas to deionize so as to once again permit unimpeded passage of the microwaves therethrough. This gaseous type of variable impedance is further characterized by a relatively short'life and by the generation of undesirable noise when microwaves are transmitted therethrough in the presence of residual gas ions. A disadvantages of the ferrite-type variable impedances is the presence of a permanent obstacle of ferrite material within the hollow waveguide. The ferrite obstacle induces reflections in the waveguide when microwaves are transmitted therethrough and also limits the power handling capabilities of the waveguide. For example, in one form of ferrite variable impedance relatively large microwave powers would tend to destroy the ferrite material. i p

Therefore, in modern microwave technology, it is desirable to provide a rapidly variable impedance or switch that functions as an unimpeded transmission path in the ice intervals between two successive pulses, which may be spaced but a few microseconds apart. Additionally, it is desirable that such a variable impedance or switch control the highest possible levels of microwave power.

Accordingly, it is the principal object of this invention to provide an improved microwave impedance.

Another object of this invention is to provide animproved microwave switch.

Another object of this invention is to provide a microwave device whose impedance may be varied at rapid rates.

Another object of this invention is to provide a microwave device whose impedance may be varied at rapid rates and which is adapted to operate at very high levels of microwave power.

The foregoing objects are achieved, according to one form of the invention, by providing a hollow coaxial transmission line having concentrically disposed inner and outer conductors for transmitting microwaves therethrough and by providing a variable impedance in the form of a con trollable multipactor discharge between such conductors. A section of the transmission line is evacuated and sealed to maintain a region between the conductors evacuated. The opposing surfaces of the two conductors comprise a material characterized by a secondary emission ratio greater than unity for the energy level of the electromagnetic waves supplied. The relative diameters of the two conductors are chosen to support a multipactor discharge therebetween. A multipactor discharge is a sustained secondary emission discharge between opposing surfaces as a result of the motion of secondary electrons produced at the surfaces in synchronism with the alternating field of electromagnetic energy existing between the surfaces. A controlling magnetic field of variable strength is applied to the evacuated section of the transmission line, the field being oriented in a direction parallel to the axis of the coaxial line. In operation, when the magnetic field strength has a minimum value the multipactor discharge has substantially maximum strength in the presence of the electromagnetic energy, the discharge thereby simulating a short circuit or relatively low impedance between the conductors. When the magnetic field strength is increased to a predetermined value, the multipactor discharge is rendered less intense or is extinguished, thereby permitting the passage of electromagnetic waves. A rapid application of the magnetic field provides an equally rapid extinction of the multipactor discharge, whereas a sudden reduction of the magnetic. field provides an equally rapid initiation of the multipactor discharge. Thus, a rapidly responding controllable impedance is provided by the unimpeded coaxial line with controllable multipactor discharge, which also provides for transmission of maximum levels of microwave power.

The invention will be described with reference to the accompanying drawing wherein:

FIGURE 1 is a perspective view, partly in cross-section, of one form of the invention; and

FIGURES 2a and 2b are partial cross-sectional views of the device of FIGURE 1, illustrating the principle of opi eration thereof.

The variable impedance cell 10 of FIG. 1 is illustrated as providing a variable impedance or switching function between a transmitter 11 of microwaves and a utilization devicp 12. Transmitter 11 is of the type provided for delivering electromagnetic waves with high levels of energy during very short periods, these periods of transmission being known as pulses. However, it is within the scope of the instant invention for transmitter 11 to deliver electromagnetic waves continuously. Transmitter 11 is coupled to a microwave transmission line 13, shown sche- 3 matically, for transmitting electromagnetic energy into one end of cell 10.

A utilization device 12, such as a microwave receiver or an antenna, utilizes the microwave energy delivered by cell 10. Utilization device 12 is coupled to a transmission line 14, shown schematically, for receiving the energy transmitted from the other end of cell 10.

Variable impedance cell comprises hollow coaxial transmission line 16 having concentrically disposed inner conductor 17 and outer conductor 18. A pair of annular dielectric windows 20 and 21 are each afiixed and sealed to the outer surface of inner conductor 17 and tothe opposing inner surface of outer conductor 18. Windows 20 and 21 function as gas seals, transparent to the passage of electromagnetic energy, but impervious to the passage of gas molecules. The interior of the section of coaxial line 16 between windows 20 and 21 is evacuated. Thus windows 20 and 21 provide a seal to maintain an evacuated region in the section of coaxial line 16 between the two windows, while permitting passage of electromagnetic waves through such section.

The evacuated section of coaxial line 16 is adapted for immersion in a magnetic field, B, provided by a source of magnetomotive force, such magnetic field being directed parallel to the axis of the coaxial line. This source of magnetomotive force, in one form, comprises an elongated hollow cylindrical solenoid 23. Solenoid 23 is energized by a controllable current source 24. Current source 24 is shown schematically as being electrically connected to solenoid 23 for providing electric current through the turns of the solenoid to produce the requisite magnetic field in coaxial line 16.

The manner of operation of the variable impedance cell, as presently understood, will now be described. The operation will first be described in the absence of any magnetic field provided by solenoid 23-. In FIG. 2a, the dashed radial lines represent, at a given instant, the distribution and direction of the electric field component of the electromagnetic energy between the opposed surfaces of conductors 17 and 18 at a particular cross-sectional plane. For electromagnetic energy of microwave frequencies, the electric field illustrated reverses its direction at thousands of megacycles each second. Thus, at one moment the radial electric field will be directed outwardly, as illustrated in 'FIG. 2a and one half cycle later it will be directed inwardly.

This electric field represents the force on positive electrical charges, and the direction of the electric field repre sents the direction of such force. Thus, at the location and at the moment illustrated in FIG. 2a positive charges will experience an outward radial force and, consequently, will be accelerated toward outer conductor 18. Conversely, negative charges at the same moment and location will experience an inward radial force and will be accelerated toward inner conductor 17.

It is well known that the entire region immediately above the earths surface is continually subjected to radioactive emissions from materials in the earths surface and from cosmic rays. A significant portion of the matter receiving these radioactive emissions and cosmic rays will be ionized thereby; in other words, such matter will be broken into positive and negative charges, wherein the negative charges are electrons. Therefore, the interior of the evacuated region of coaxial line 16 is always subjected to these radioactive emissions and cosmic rays. Since a perfect vacuum can never be obtained, this evacuated region always contains a small number of residual gas molecules. Consequently, ionization of a small fraction of these residual gas molecules will continually be occurring, so that a number of electrons are always present in the evacuated region of coaxial line 16. These electrons will be accelerated by the electric field of the microwave energy when transmitter 11 generates pulses and many electrons will strike one of conductors 17 and 18.

When electrons strike a surface, other electrons are driven from the surface by the energy of impact. The electrons striking the surface are termed primary electrons and the electrons driven from the surface are termed secondary electrons. The ratio of the number of secondary electrons created to the number of primary electrons striking a surface is defined as the secondary-emission ratio. The secondary-emission ratio varies with the velocity of the primary electrons and the material of the surface. For many types of surface materials secondaryemission ratios greater than unity are provided for a Wide range of electron velocities; for example, see K. R. Spangen=berg, Vacuum Tubes, pages 4857, McGraW-Hill Book Company, Inc., New York, 1948. However, these surface materials do not provide a secondary-emission ratio greater than unity for very slow or very fast primary electrons. The outer surface of inner conductor 17 and the inner surface of outer conductor 17 in the evacuated region are provided with such a surface material adapted to yield a secondary-emission ratio greater than unity for primary electrons having a wide range of velocities. One such surface material particularly suitable for employment for this purpose is an alloy of silver and magnesium, such as that described by V. K. Zworykin, J. E. Ruedy, and E. W. Pike, Silver-Magnesium Alloy as a Secondary Electron Emitting Material, J. Appl. Phys., vol. 12, pages 696698, September 1941.

Consequently, if the strength of the electric field component of the electromagnetic energy in the evacuated regions between conductors 17 and 18 is sufliciently great, a number of the electrons formed in the evacuated portion will be accelerated to strike the opposing surfaces of conductors 17 and 18 with sufficient energy to create a greater number of secondary electrons. Providing that the electric field reverses direction immediately after creation of the secondary electrons they will be accelerated in a direction away from the surface at which they were created and toward the opposing conductor surface. Thus, consider the instant depicted in FIG. 2a. Electrons created in the evacuated region of coaxial line 16 will be accelerated toward inner conductor 17. If the electric field persists in the direction shown sufficiently long for these accelerated electrons to strike conductor 17, secondary electrons will be emitted therefrom greater in number than the primary electrons, provided the electric field strength is sufficiently great. If, now, the electric field reverses its direction immediately after creation of the secondary electrons, they will be accelerated toward outer conductor 18. Once again,if the electric field pro vided in this reverse direction persists sufificiently long for this group of secondary electrons to travel from inner conductor 17 to outer conductor 18, a new group of sec ondary electrons will be impelled from the inner surface of outer conductor 18, this new group being greater in number than the original group of secondary electrons striking conductor 18. Again, if the electric field reverses immediately after formation of this new group of sec ondary electrons, the new group will be accelerated to- Ward inner conductor 17. Thus, with a sufficient elec trical field strength and an appropriate frequency of the electromagnetic energy, secondary electron groups travel back-and-forth between the opposed surfaces of conductors 17 and 18. With such favorable conditions the size of each secondary electron group grows after each reversal until a sheet of electron current is created across the evacuated regions between the opposed surfaces of conductors 17 and 18.

The current flowing in the manner described is termed a multipactor discharge and, therefore, is defined as a sustained secondary-emission discharge existing between the conductors as a result of the motion of secondary electrons in synchronism with a strong rapidly alternating electric field applied to the region. Accordingly, the relative diameters of coaxial line 16 are adjusted by design or experiment so that synchronism of the secondary electrons with electromagnetic energy of appropriate strength and frequency provides a multipactor discharge. Tr-ajectory 28 illustrates a path followed by the secondary electron groups in the multipactor discharge when no magnetic field is provided by solenoid 23. Such paths generally will occur along radial trajectories at all angles about the coaxial line axis in FIG. 2a.

As described thus far, a multipactor discharge occurs between conductors 17 and 18 in" the evacuated region of coaxial line 16 when electromagn'eticenergy of sufficient intensity is received therein, provided that source 24 is adjusted so that no magnetic field is supplied by solenoid 23. This multipactor discharge willfunction as an equivalent short circuit between conductors 17 and 18, thereby attenuating, or preventing, transmission of electromagnetic energy between transmitter 11 and utilization device 12. If electromagnetic energy is provided as pulses by transmitter 11, the multipactor discharge will take place in coaxial line 16 only during the occurrence of these pulses.

Consider, now, the operation of the 'cell of FIG. 1 when the value of current delivered by source 24 is increased to increase the strength of the axial magnetic field, B, applied to the evacuated region of coaxial line 16. In FIG. 2b this field is directed perpendicularly to the surface of and enters the cross-section shown. Whena charged particle moves through a magnetic field, it is subjected to a force oriented perpendicularly to both the direction of particle motion and the direction of the magnetic field vector. Therefore, an electron traveling from outer conductor 18 toward inner conductor 17 experiences a force tending to bend the path of the electron toward the right of the direction of motion, as shown in FIG. 2b. If the intensity of the magnetic field is increased to a sufficiently large value, the preponderance of the secondary electrons emitted from the interior surface of outer conductor 18 will be bent sufiiciently, as along trajectory 29, so as to miss inner conductor .17. With the application of a magnetic field of such intensity a group of secondary electrons emitted from outer conductor 18 will not generate a new and larger group of secondary electrons at inner conductor 17, so that the conditions for a multipactor discharge are not present.

Therefore, as shown in FIG. 2b, in the presence of an axial magnetic field of substantial intensity a multipactor discharge cannot be initiated. On the other hand, if a multipactor discharge has been induced in the absence of a magnetic field and the magnetic field is subsequently applied, the multipactor discharge will be quenched. Conversely, in the presence of electromagnetic fields of sufficient intensity, the removal of a strong axial magnetic field will permit the multipactor discharge to be initiated in the evacuated region. Accordingly, a controllable multipactor discharge is provided by applying a controllable magnetic field to the evacuated region of the cell of FIG. 1. By rapidly varying the intensity of the magnetic field the multipactor discharge may be rapidly initiated or quenched. Additionally, the intensity of the multipactor discharge may be varied by gradually increasing or decreasing the magnetic field so that a variable number of the secondary electrons created at outer conductor 18 strike inner conductor 17.

Although the simple trajectory 29 of FIG. 2b may be modified somewhat by the presence of the alternating electric field of the electromagnetic waves, nevertheless, the trajectory will be essentially as described and the theory of operation described above is essentially unchanged. The controllable magnetic field applied to the cell of FIG/1 will provide a multipactor discharge of variable intensity and impedance.

While the principles of the invention have now been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications in structure, arrangement, proportions, the elements, materials, and components used in the practice 6. of the invent-ion, and otherwise, which are particularly adapted for specific environments and operating requirements, without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

What is claimed is: 1. A microwave transmission line system comprising a variable impedance cell located between a source and a load, said cell for providing a variable impedance comprisingi a conductive wall partially bounding a region, means for maintaining said region substantially evacuated, an electromagnetic wave generator coupled to supply electromagnetic energy to said region, the level of said energy being sufficient to induce a multipactor discharge in said region along a path having at least one end terminating at said wall, and a magnetic field source for applying to said region a steady magnetic field, said magnetic field being oriented perpendicularly to said path, said multipactor discharge having a maximum value when the magnetic field is zero. 2. The cell of claim 1 further including means for varying the strength of said magnetic field.

3. A microwave transmission line system comprising a variable impedance cell located between a source and a load, said cell for providing a variable impedance comprising:

a conductive wall partially bounding a region,

means for maintaining said region substantially evacuated,

a controllable magnetic field source for applying to said region a steady magnetic field oriented in a predetermined direction,

an electromagnetic wave generator coupled to supply to said region electromagnetic energy having the electric field components thereof oriented perpendicularly to said direction, the level of said energy being suflicient to induce a multipactor discharge in said region in the absence of said steady magnetic field, and

means for controlling said field source to apply a magnetic field having sufficient intensity to quench said multipactor discharge.

4. A Wave transmission device for supporting a controllable multipactor discharge comprising: a coaxial transmission line having concentrically disposed inner and outer conductors, said conductors comprising a material characterized by a secondary-emission ratio than unity for electromagnetic waves in said section having said energy level, means for maintaining a section of said transmission line substantially evacuated, an electromagnetic wave generator coupled to transmit electromagnetic waves to said transmission line at an energy level suflicient to induce a multipactor discharge in said section, and a magnetic field source for applying to said section a magnetic field oriented parallel to the axis of said transmission line.

5. A Wave transmission device for supporting a controllable multipactor discharge comprising:

a coaxial transmission line having concentrically disposed inner and outer conductors,

means for maintaining a section of said transmission line substantially evacuated,

a controllable magnetic field source for applying to said section a steady magnetic field oriented parallel to the axis of said transmission line,

an electromagnetic wave generator coupled to transmit electromagnetic waves to said transmission line at an energy level sufficient to induce a multipactor discharge in said section between said conductors in the absence of said steady magnetic field, and

means for controlling said field source to apply a magnetic field having sufiicient intensity to quench said multipactor discharge.

References Cited by the Examiner 5 UNITED STATES PATENTS 8/1940 Keyston 313-104 X 10/1940 George et al 313104 X 2/1951 Linder 313103 X 7/1953 Varela 333-451 8 OTHER REFERENCES D. H. Preist and R. C. Talcott: On the Heating of Output Windows of Microwave Tubes by Electron Bombardment; IRE Transactions on Electron Devices, July, 1961 (pp. 243251 relied on).

HERMAN KARL SAALBACH, Primary Examiner.

R. D. COHN, Assistant Examiner.

Claims (1)

1. A MICROWAVE TRANSMISSION LINE SYSTEM COMPRISING A VARIABLE IMPEDANCE CELL LOCATED BETWEEN A SOURCE AND A LOAD, SAID CELL FOR PROVIDING A VARIABLE IMPEDANCE COMPRISING: A CONDUCTIVE WALL PARTIALLY BOUNDING A REGION, MEANS FOR MAINTAINING SAID REGION SUBSTANTIALLY EVACUATED, AN ELECTROMAGNETIC WAVE GENERATOR COUPLED TO SUPPLY ELECTROMAGNETIC ENERGY TO SAID REGION, THE LEVEL OF SAID ENERGY BEING SUFFICIENT TO INDUCE A MULTIPACTOR DISCHARGE IN SAID REGION ALONG A PATH HAVING AT LEAST ONE END TERMINATING AT SAID WALL, AND A MAGNETIC FIELD SOURCE FOR APPLYING TO SAID REGION A STEADY MAGNETIC FIELD, SAID MAGNETIC FIELD BEING ORIENTED PERPENDICULARLY TO SAID PATH, SAID MULTIPACTOR DISCHARGE HAVING A MAXIMUM VALUE WHEN THE MAGNETIC FIELD IS ZERO.

Images