US3619709A - Gridded crossed field traveling wave device - Google Patents

Abstract

A high-power high-frequency device is disclosed having a thermionically actuated cold cathode secondary electron source and means for controlling electron beam current interacting with energy on either a reentrant or nonreentrant propagating structure. In an exemplary embodiment a grid-controlled primary electron source is coupled to an array of individual secondary electron emissive members biased at sequentially varying voltages. The primary and secondary electron emitters are spaced apart a distance of approximately one electron cycloid path. Such emitters are disposed within a section of a cold cathode source of substantial size. Impingement of electrons upon each successive secondary electron emitting surface results in an electron emission multiplication process heretofore unattainable with cold cathode structures actuated simply by pulses of the input RF energy to be amplified. Operation of the device may be rapidly terminated by biasing of the grid-controlled electrode having very low voltage requirements with the current in the electron beam and power output decaying to zero in a relatively few revolutions of the electron spoke around the cathode structure.

Description

United States Patent [72] Inventors Kenneth W. Dudley Sudbury; George H. MacMaster, Waltham, both of Mass.

[21] Appl. No. 52,321

[22] Filed July 6, 1970 [45] Patented Nov. 9, 1971 [73] Assignee Ratheon Company Lexington, Mass.

[54] GRIDDED CROSSED FIELD TRAVELING WAVE GRID CONTROL SUPPLY CATHODE VOLTAGE SUPPLY 3,255,422 6/1966 Feinstein etal ABSTRACT: A high-power high-frequency device is disclosed having a thermionically actuated cold cathode secondary electron source and means for controlling electron beam current interacting with energy on either a reentrant or nonreentrant propagating structure. in an exemplary embodiment a gridcontrolled primary electron source is coupled to an array of individual secondary electron emissive members biased at sequentially varying voltages. The primary and secondary electron emitters are spaced apart a distance of approximately one electron cycloid path. Such emitters are disposed within a section of a cold cathode source of substantial size. impingement of electrons upon each successive secondary electron emitting surface results in an electron emission multiplication process heretofore unattainable with cold cathode structures actuated simply by pulses of the input RF energy to be amplified. Operation of the device may be rapidly terminated by biasing of the grid-controlled electrode having very low voltage requirements with the current in the electron beam and power output decaying to zero in a relatively few revolutions of the electron spoke around the cathode structure.

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sum 3 OF 3 BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to cathodes for traveling-wave-type electron discharge devices.

2. Description of the Prior Art Devices of the type under consideration involve a net exchange of energy through interaction between an electron beam launched from a suitable cathode source with the highfrequency electric fields of traveling waves of electromagnetic energy propagated by suitable means. The combined fields of the propagated energy may be resolved into space harmonic waves having varying phase velocities. To result in amplification and/or generation of high-frequency electromagnetic energy a synchronous relationship is established between the electron beam velocity and the phase velocity of a desired space harmonic component.

Oscillators and/or amplifiers of the foregoing class have mutually perpendicular unidirectional electric and magnetic fields along the interaction path defined between the energy propagating structure and the cathode source. Substantially high-power RF energy has been generated in structures referred to as the Amplitron. This device utilizes a nonreentrant traveling wave st propagating structure comprising the anode circuit along with a reentrant electron beam from a continuous cathode electrode member spaced from and concentrically disposed from the wave structure. A very high DC potential is established between the cathode electrode and anode circuit. A magnetic field is applied parallel to the axis of the cathode member and transverse to the electric field. In operation the Amplitron provides properties substantially similar to conventional forward and backward wave oscillators where the electron beam interacts with a predetermined spatial harmonic. Such devices conventionally employ coldcathode-emitting structures activated by input electromagnetic energy drive pulses propagated along the wave structure. An advantage of such devices compared with tubes of the magnetron-type is that a change of applied voltage to the anode structure primarily increases the power output instead of changing the velocity of the adjacent electron beam. Efficiencies in the order of 70-75 percent are attained together with average output powers of many kilowatts and peak powers of megawatts. Further particulars regarding such prior art devices may be had by referring to our copending application, for U.S. Letters pat. Ser. No. 42,180, filed on June 1, 1970, and assigned to the assignee of the present invention.

Present day requirements in radar systems has resulted in the use of new techniques requiring very high pulse repetition rates. Such pulse applications require fast rise time and shutoff, with accompanying complex and costly electronic circuits for the modulators. In addition, multitube chains such as those employed in phased array radar systems have pulsing requirements for precise, simultaneous switching of many amplifiers. The Amplitron traveling wave device due to its high efficiency and high-power capabilities can provide some of the characteristics of the new pulse requirements if rapid switching between the operative and nonoperative states can be achieved with a minimum of jitter and noise. A prior art solution commonly employed involves the use of a single bulky control segment over a substantial portion of a large circular cold cathode electrically insulated from the remaining structure and controlled by a separate supply voltage. A sample of such a structure is disclosed in US. Letters Pat. Nos. 3,503,001, issued Mar. 24, 1970 to George K. Famey. in accordance with such structure the control electrode is rendered positive with respect to the remaining emissive portion in order that the electrons in the interaction region will be removed by brute force" upon reaching the area of the control electrode. Since the applicable traveling wave devices are conventionally of circular configuration and have a substantial radius to achieve the high power levels desired, electrons in the interaction region will be required to traverse many RF cycles before being removed from the beam or the phase coherence is destroyed. As a result, such prior art turnoff control electrodes for rapid switching of high-power amplifiers and/or oscillators have been unsatisfactory. A need arises, therefore, for a traveling wave device which will not only provide rapid switching with relatively low control voltage requirements to thereby eliminate the need for expensive and complex high-power modulator supplies.

SUMMARY OF THE INVENTION In accordance with the teachings of the present invention a cathode structure is provided having primary and secondary electron sources to generate a reentrant electron beam along an interaction path adjacent to an energy propagating structure of the slow-wave-type. The cathode incorporates plural individualized members spaced apart a distance substantially equal to one electron cycloid path. A low-voltage control elec trode such as a grid encompasses the primary emitter. By suitable means each succeeding secondary electron emitter is biased at a different voltage potential to result in electron multiplication of a magnitude unattainable with a unitary secondary electron source driven by input RF pulses. The voltage differential is selected in accordance with the type of material employed for secondary electron emission. By controlling the grid electrode member voltage a traveling wave electron discharge tube having normal cathode voltages of 40 kilovolts and higher may be turned on. The electron emission current upon a change in the grid control electrode current is rapidly reduced to zero after cessation of the input pulse of the energy to be amplified. Electrons from the primary emitter upon being turned on strike the first secondary electron member with a substantial force to provide a maximum yield of electrons from a material such as, for example, platinum, by reason of the spacing and voltages. Sequentially, electrons bombard successive secondary electron members with a greater force to provide a profusion of electrons by the process of multiplication with resultant higher electron beam current. If an initial primary electron current of, for example, 0.1 amperes/cm. were applied, the first secondary electron emitter would yield approximately 0.18 ampereslcmf. The remaining successive sequentially electrically variable secondary electron sources would then yield an initial beam current along the beginning of the electromagnetic-wave-propagating circuit of approximately 1.05 amperes/cm,. This beam current impinges on the main cold cathode source which has a substantial curvature to yield very high electron beam currents. The tube power output is correspondingly higher than conventional devices operating with only the input RF signals on the wave-propagating structure as the driving pulse.

Upon cessation of the input pulse on the wave structure a very low power DC voltage is applied to the grid control electrode to quench further primary electron emission. Subsequently, the electron beam current already in the interaction path will continue until contacting the primary electron emitter again. By this time, however, the electrons in the rotating beam spoke having traveled this distance would have an equipotential value below that of the main cathode and further secondary emission ceases. After only a few revolutions the beam current will decay to zero amplitude and power output will terminate.

Embodiments of the invention can effectively handle 1.25 megawatts of peak power and kilowatts average power. Such embodiments can be employed in large numbers in linear accelerator system applications.

Numerous methods for control of the gridded control member to activate the device are envisaged. The relatively low-power requirements for such control will eliminate the need for expensive and complex modulators. Many combinations of cathode primary and secondary electron sources will also be evident.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, as well as the details for the provision of illustrative embodiments, will be readily understood after consideration of the following detailed description and reference to the accompanying drawings wherein:

FIG. 1 is a schematic representation of a traveling wave device embodiment of the invention;

FIG. 2 is a similar representation as FIG. 1 with the electron trajectories plotted upon application of a small biasing voltage to the control member to operate the device;

FIG. 3 is a schematic representation of the electron beam spokes in the final operative condition of the embodiment;

FIG. 4 is a schematic representation of the electron beam current spoke upon the application of the turnoff voltage to the control member;

FIG. 5 is a schematic representation of the orientation of the electron spokes upon termination of operation; and

FIG. 6 is a schematic representation of an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 represents schematically any traveling wave device such as the type disclosed in the aforereferenced patent application. The device includes an envelope 2 having an energy propagating structure 10 comprising a cylindrical array of elements 12 to form an anode circuit in the manner well known in this art. The invention is equally applicable to crossed field devices having reentrant, as well as nonreentrant propagating structures. A magnetron oscillator, for example, has an reentrant array of resonant cavities concentrically disposed about a central cathode member. Crossed field amplifiers such as those of the Amplitron-type conventionally have a nonreentrant propagating structure of the slow wave type with input and output connector means coupled to the ends. An exemplary connection is realized with terminal 14 for the input energy and terminal 16 for the output energy. In this configuration a space or septum is disposed at a point along the interaction region in place, for example, a member 18 to prevent the high-frequency energy from recirculating. The illustrated connections would be util ized for backward wave operation with the electron beam traveling clockwise in the direction of arrow 10 in the interac tion path 22 adjacent to the energy propagating structure. For forward wave interaction the terminals 14 and 16 would be reversed. I

Cathode member 24 of the invention is concentrically disposed with respect to propagating structure 10. An electric field is applied by conventional means between the cathode and energy propagating structure as indicated by the arrow 26 across the interaction path 22. A magnetic field indicated by circle 28 extends parallel to the axis of cathode 24 and is mutually perpendicular to the electric field.

The cathode structure is a combination of a source of primary electrons such as a thermionically actuated emitter 30. Additionally, sequentially disposed individual secondary electron emitters 32, 34 and 36 occupy a sector 38 of approximately 50 of main cathode body member 40 of a secondary electron-emissive material. The secondary electron structure may be realized by the use of platinum for the rod-shaped segments, 32, 34 and 36, as well as body member 40. Such material has a secondary electron-emission ratio value of 1.8 which is well in excess of unity for effective secondary electron emission. Thennionic primary electron emitter 30 is provided from any of the well-known earth metal oxide-coated or impregnated materials and is directly or indirectly heated. Control of the emission of primary electrons is provided by a grid electrode member 42 which is biased by any suitable means 4 to render this member either positive or negative with respect to the primary emitter.

In the operation of the disclosed device as a traveling wave amplifier the primary and secondary electron cathode members are biased by supply 6 at a voltage differential optimized for maximum electron emission with the material selected. Hence, for platinum a high secondary emission ration value is yielded at a biasing potential of approximately 800 volts. In the exemplary embodiment for a very high output power a normal cathode voltage relative to the anode is 40 kv. A sequentially varying differential of 800 volts for the material platinum results in secondary electron member 36 being biased at 40.8 kv., for example. Similarly, secondary electron emitter 34 has a biasing value of 41.6 kv. and emitter 32 has a value of 42.4 kv. The primary emitter in this example has an initial biasing voltage, then, of 43.2 kv.

The tube is rendered operative upon receipt of an input RF pulse on the wave structure of the energy to be amplified by means of a positive bias on the control grid electrode 42 of a value of, illustratively, 10 volts. The primary emitter 30 is thereby driven less negative and with the electron sources spaced a distance of approximately one cycloid a trajectory path 44 as shown in FIG. 2 will result, Impingement of primary electrons from emitter 30 at substantially all the emission current of 800 electron volts will release a profusion of secondary electrons depicted by paths 46 and 48 for the selected material.

This release of secondary electrons sequentially by successively larger numbers of impinging electrons all at an optimum energy level of 800 electron volts is repeated throughout the cathode structure. Secondary electron emitter 34, therefore, releases electrons along plural cycloidal trajectories 50 and emitter 36 a greater number indicated by trajectory paths 52. There is a resultant multiplication process at each cycloidal point so that the energy of the bombarding electrons on the main secondary electron emitter body 40 is far in excess of the energy released solely from a separate thermionic emitter or the input RF signal pulse alone.

The phase coherence of the electrons in the beam with respect to the RF electromagnetic fields is indicated by the path 54 directed towards the energy propagating structure 10. The potential energy of such in-phase electrons interacts in a net energy transfer relationship with the waves propagating along the structure 10. The out-of-phase electrons indicated by paths 56 extract energy from the electromagnetic waves and return to bombard the the main cathode 40 again approximately'one-half an RF cycle later to generate more secondary electrons depicted by arrows 58. Some of these released electrons are emitted in-phase to continue towards the anode circuit while the out-of-phase electrons back bombard the main cathode emitter 40. All electrons released by the foregoing process have much higher potential energy levels to provide a higher overall electron beam current during operation.

In FIG. 3 the favorable orientation of the bunched electron spokes along the interaction path is illustrated. For a 22-element anode circuit approximately 10 equally spaced electron spokes will evolve in the 1r mode of operation. The overall energy system is indicated generally by numeral 60 and spokes 62 interact with energy on the elements 12. The electron trajectory paths within the sector 38 are intended to be essentially as shown in FIG. 2. The adjacent electron spokes in the formative stages before complete phase coherence is attained are indicated by numeral 64.

Referring next to FIG. 4 the condition of the operation of the device is disclosed between pulses of the RF energy to be amplified when a sharp reduction in electron beam current is desired. To achieve this condition the grid-controlled electrode 42 is driven more negative than the primary electron emitter by a negative value of, for example, 10 volts. As a result, primary emission ceases and the trajectory paths 44, 46 and 48 have been eliminated to indicate the commencement of complete tube shutoff. The only electron source emitting then, after three or four RF cycles would be emitter 36 which is biased at a value of 40.8 kv. Since the main cathode body member is biased at only 40 kv., relatively few high-energy electrons will be emitted by the multiplication process. The electron current spoke already in the interaction path 22 will continue around the cathode member for a few revolutions and the equipotential plane of the cathode energy level is indicated by the dotted line 68. In the area adjacent the primary electron source relatively few electrons remaining in the electron spoke have sufficient energy to per it further emission or primary electrons since in the illustrative example the primary emitter is biased 3.23 kv. more negative than the main cathode body member. Further travel around the interaction region by the electrons will result in movement away from the equipotential plane with the result that considerable numbers of the electrons are absorbed by the anode elements 12. The electron beam current, therefore, falls below the value necessary to interact with the waves on the propagating structure. Power output thereby decays to zero amplitude in the order of a few tens of nanoseconds.

In FIG. 5 the absence of phase coherence is noted and the electron bunches have disintegrated into relatively small isolated groups or islands indicated by the numerals 70. Between pulses of RF energy to be amplified, therefore, the grid control electrode 42 is maintained negative with respect to the primary electron emitter to keep the device in the nonoperative condition. As soon as the next input pulse signal is received the device is cycled to operate in accordance with the method described and illustrated with respect to FIGS. 2 and 3.

In FIG. 6 an alternative embodiment of the invention is illustrated. Again, as before, a grid control electrode 72 provides the means for the regulation of the operation of the device. In this embodiment primary electron emitter 74 is provided with a substantially flattened surface 76 to contribute measurably to the desired electron trajectories. Similarly, secondary electron emitters 78, 80 and 82 are provided with emission-assisting surfaces.

In this embodiment still another possible combination is illustrated in that the main cathode member may consist of a plurality of individual rodlike segments 84 arranged in a circular array. In accordance with the teachings of the aforereferenced patent application alternate secondary emission cathode members 86 may be provided of a predetermined lower secondary emission ratio material to evolve an interdigital arrangement which will measurably assist in the shutting off of the operation of the device after transmission of the RF pulses. The interdigital structure, then, could be fabricated of a material such as titanium for the members 86 while members 84 would be fabricated of platinum. The illustrated alternative embodiment has certain advantages in that the alternate secondary electron rods permit the reduction in beam current at an earlier point along the interaction path compared to the solid main cathode body member of relatively large curvature shown in FIGS. 1-5.

It is evident that many other combinations in the materials,

as well as orientation of the cathode structures will be evident to those skilled in the art. In addition, certain circuits may be evolved for varying the voltages applied to the grid control electrode to thereby control the amplitude of the power output and not merely switch the device from the on to the off state. It is intended, therefore, that the embodiments shown and described herein be considered as exemplary only and not in a limiting sense in the application of the appended claims to define the spirit and scope of the invention.

What is claimed is:

l. A traveling wave device comprising:

a nonreentrant slow wave structure having input and output energy terminal ends for propagating electromagnetic wave energy;

cathode structure concentrically disposed with respect to said propagating means and defining therebetween an interaction path;

said cathode structure including a primary electron and plural secondary electron sources spaced apart a distance substantially equal to one electron cycloid path;

a grid electrode disposed adjacent to said primary source;

means for electrically biasing said grid electrode to control the emission of the primary electrons;

and voltage-biasing means applied to each of said secondary electron sources to provide a successively decreasing negative voltage differential relative to said primary source;

said voltage differential being determined by the maximum voltage potential sufficient to optimize the emission of secondary electrons from each secondary electron source.

2. A traveling wave device according to claim I wherein said cathode structure is substantially circular and said spaced primary and secondary electron sources are disposed within a portion of said cathode structure.

3. A traveling wave device according to claim I wherein said cathode structure is substantially circular and said spaced primary and secondary electron sources are disposed within a portion of said cathode structure located within approximately one-half of the interaction path adjacent the output energy terminal.

4.'A traveling wave device according to claim 4 wherein said secondary electron source material is platinum and said voltage differential is in the order of 800 volts between each secondary electron source.

5. A traveling wave device according to claim 1 and means for producing mutually perpendicular electric and magnetic fields in said interaction path.

t i I Patent No.

Dated ljovember 9, 1971 Inventor(s) Kenneth W. Dudley and George H. MacMaster Column 1,

Column Column 4,

Column insert Column 4,

Column Column 6,

(SEAL) Attest:

At [73] change line line

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EDWARD M.FLETCHER,JR.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

"Ratheon" to Raytheon 26, after "wave" delete "st" 44, change "arrow 10" to arrow 20 2, change "ration" to ratio 18, after "result" delete and 41, delete "the" second occurrence 4, change "per it" to permit 42, claim 4, change "claim 4" to --claim l- Signed and sealed this 27th day of June 1 972.

ROBERT GOTTSCHALK M PO-IOSO (10-69) Intesting Gfficcr Commissioner of Patents USCOMM-DC 50375-P59 V U,S GOVERNMENT PRINYING OFFICE IBQ 0-365-334

Claims (5)

1. A traveling wave device comprising: a nonreentrant slow wave structure having input and output energy terminal ends for propagating electromagnetic wave energy; cathode structure concentrically disposed with respect to said propagating means and defining therebetween an interaction path; said cathode structure including a primary electron and plural secondary electron sources spaced apart a distance substantially equal to one electron cycloid path; a grid electrode disposed adjacent to said primary source; means for electrically biasing said grid electrode to control the emission of the primary electrons; and voltage-biasing means applied to each of said secondary electron sources to provide a successively decreasing negative voltage differential relative to said primary source; said voltage differential being determined by the maximum voltage potential sufficient to optimize the emission of secondary electrons from each secondary electron source.

2. A traveling wave device according to claim 1 wherein said cathode structure is substantially circular and said spaced primary and secondary electron sources are disposed within a portion of said cathode structure.

3. A traveling wave device according to claim 1 wherein said cathode structure is substantially circular and said spaced primary and secondary electron sources are disposed within a portion of said cathode structure located within approximately one-half of the interaction path adjacent the output energy terminal.

4. A traveling wave device according to claim 4 wherein said secondary electron source material is platinum and said voltage differential is in the order of 800 volts between each secondary electron source.

5. A traveling wave device according to claim 1 and means for producing mutually perpendicular electric and magnetic fiElds in said interaction path.

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