US3221207A - Microwave power generating by periodic sweep of electron beam along length of resonant waveguide - Google Patents

Description

Nov. 30, 1965 l. KAUFMAN ETAL 3,221,207

MICROWAVE POWER GENERATING BY PERIODIC SWEEP OF ELECTRON BEAM ALONG LENGTH OF RESONANT WAVEGUIDE Filed June 5, 1965 2 Sheets-Sheet 1 MES WAVE \NPuT FlELD D\STR\BUT\ON 23 2 9 y ym 5m wt \8 E smBy 5m nwt 35 y 26' '1 W m 56 Yo E cos y cos nwt F 5k /2 cAvlTy A /2CA\/\TY 46 flQV/N KAUFMAN ROBE/P7 V. LANG/MUM? 44 INVENTORS I I l o .2 .4 .6 .25 BY ym/Ag 59 5 Filed June 5, 1965 Nov. 30, 1965 KAUFMAN ETAL 3,221,207

MICROWAVE POWER GENERATING BY PERIODIC SWEEP OF ELECTRON BEAM ALONG LENGTH OF RESONANT WAVEGUIDE 2 Sheets-Sheet 2 INVENTORS //2V/N6 A L/FMAA/ BY ROBE/e7 l/ ZANGML/l/Q United States Patent Oil-ice 3,221,207 MICROWAVE POWER GENERATING BY PERIODIC SWEEP F ELECTRON BEAM ALONG LENGTH OF RESONANT WAVEGUIDE Irving Kaufman, Woodland Hills, and Robert V. Langmuir, Altadena, Calif., assignors, by mesne assignments to TRW Inc., a corporation of Ohio Filed June 5, 1963, Ser. No. 285,781 4 Claims. (Cl. 315-5.25)

The present invention relates generally to the art of generating, amplifying and frequency multiplying microwave signals and is especially useful for converting direct current energy to radio frequency energy in the millimeter and submillimeter wavelength regions.

More particularly, this invention has reference to frequency multipliers, oscillators, and amplifiers employing microwave or ultramicrowave cavity resonators in which the propagation of electromagnetic fields is excited and sustained by the use of controlled electron beams which traverse portions of the cavity resonator. One apparatus of the general class to which the present invention belongs is disclosed in copending patent application Ser. No. 135,- 640, filed September l 1961, by Irving Kaufman, which application is assigned to the same assignee as that of the present invention. In that application, there is disclosed a microwave generating system in which an electron beam is circularly deflected and is directed to pass through a toroidol waveguide resonator which is circularly slotted at its front and back surfaces to permit ingress and egress required for causing the circularly deflected electron beam scribe a circular Lissajous pattern. Further, such apparatus has the disadvantages that it requires precise control of the input deflection signal voltage amplitude and, as an oscillatory system, it is not self-starting.

Accordingly, it is a general object of the present invention to provide improved apparatus for generating electromagnetic waves in the millimeter and submillimeter wavelength ranges.

It is another object of the present invention to provide an apparatus for generating alternating current power in the frequency range from about 3.0 to about 3000 kilo-. megacycles in which such power is derived from a direct current energy input rather than solely from high frequency energy sources.

It is a further object of our invention to provide for generation of ultramicrowave power at frequencies above the operative limits of practical prior art generating devices.

It is a different object ofourinvention to provide for generation of ultramicrowave or submillimeter wavelength signals at power levels substantially exceeding the output capacity of previously available ultramicrowave oscillators and amplifiers.

It is a still further object of our invention to provide improved apparatus for generation of electromagnetic oscillations within a resonating chamber having dimensions of the order of millimeters by means of a cont-rolled cathode ray which is deflected at relatively low frequencies compared to that of the oscillations within the resonating chamber.

It is still another object of our invention to provide apparatus of the type described for frequency multiplication and/ or power amplification of ultramicrowave signals.

It is another general object of our invention of our invention to provide improved apparatus for generating electromagnetic waves having wavelengths in the IOU-10,000 micron range of the electromagnetic spectrum.

Various devices are known for generating radio frequency power at wavelengths of a few millimeters. Such devices which have utilized linear electron beams are divisible operationally into two general classes: (1) devices employing longitudinal compression or bunching of the electron beam, and (2) devices using radio frequency deflection of an electron beam in conjunction with a slotted or apertured target for chopping the beam. In systems of the first above-mentioned type, such as klystr-ons and traveling wave tubes employing beam density modulation or beam velocity modulation, the electron bunches created must be considerably shorter than the wavelength which is to be generated. It has been extremely diflicult to attain that requirement, at millimeter wavelengths and with appreciable electron densities. Generally, extremely low efliciencies result and even if sufliciently short electron bunches are created in such devices, the bunches are quickly stretched by longitudinal space charge forces within the beam. Thus devices of the electron bunching type have not been particularly satisfactory for generation of sub-' millimeter wavelength electromagnetic energy.

Referring to the second class of device mentioned above, it is a relatively simple matter to create electron bunches by the beam chopping technique. However, such sys-. terns have the extreme disadvantage that as the frequency is increased, the electron bunch is shortened without be,-

ing increased in density and therefore contains propor-,

tionately fewer electrons. Accordingly, the amount of high frequency power which can be generated becomes prohibitively small as the frequency is increased toward the submillimeter wavelength regions.

One example of a prior art b'eam-chopping system is disclosed in United States Patent No. 2,408,437, issued October 1, 1946, wherein an electron beam is deflected in a sinusoidal manner at a high frequency and periodically.

enters a single small aperture in the wall of a microwave cavity resonator. That prior art apparatus periodically shoots a pulse or short durationgroup of electrons through the cavity resonator to shock excite electromagnetic oscillation therewithin. When properly timed, such pulses would be effective to regenerate or sustain microwave oscillations within the cavity. The difliculty with such apparatus is that for proper timing the electron beam can be permitted to enter the cavity aperture only for a very small fraction of each beam deflection cycle. Thus only a very small percentage of the beam electrons pass through the cavity and most of the kinetic energy of the beam is wasted by heat dissipation at the exterior walls of the cavity. That Patented Nov. 30, 1965 is, only those electrons which pass through the cavity aperture can contribute energy to the microwave oscillations therein. All the kinetic energy represented by other beam electrons is dissipated without doing any useful work. The consequent low efliciency has prohibited .eractical utilization of such systems.

The present invention overcomes the foregoing handicaps of previous systems by utilizing a major portion of the beam current produced by the electron gun and by recovering most of the kinetic energy of the beam electrons to reduce by an order of magnitude the load imposed on the high voltage direct current source used for accelerating the beam. In addition, the present invention deflects the electron beam in a single direction or single plane only, thereby reducing by a factor of two the input microwave power required for deflection of the electron beam. Further, the present invention, by the provision of means for post-deflection acceleration, very substantially reduces the deleterious eflects of aberration in the longitudinal velocity of the cathode ray beam electrons.

In accordance with a preferred embodiment of the present invention, the foregoing results are accomplished by an arrangement in which the electron beam is deflected in a single plane under the control of and in synchronism with an input microwave signal. The successive electrons of the beam traverse successive portions of an elongated microwave resonator device which is adapted to derive energy from the beam by deceleration of the beam electrons. Preferably, the microwave resonator comprises an elongated waveguide member having its longitudinal axis positioned in a plane substantially perpendicular to the normal axis of the electron beam and arranged to internally receive the electrons of the beam through a longitudinally extending slot or aperture which coincides with the plane of deflection of the electron beam.

The microwave resonator is constructed and arranged to provide a decelerating field in the region where each electron is internally traversing the structure at the time of such traversal. By decelerating the beam electrons, the electromagnetic fields within the microwave resonator absorb kinetic energy from the moving electrons and thereby regenerate the oscillations within the resonator. Further, in accordance with preferred embodiments of the invention, the electron beam is deflected by the input microwave signal at a point in the system where the beam electrons have a relatively low velocity. The beam is thereafter increased in energy by a direct current energized accelerating system so that the total ultrarnicrowave energy may be derived from the resonator can be greater than the microwave input power required for deflecting the beam.

More specifically, the apparatus of the present invention diflers from that of the above-mentioned Patent No. 2,408,- 437 in that our ultrarnicrowave resonator is not a simple resonant cavity but rather is a length of waveguide adapted to support one or more standing waves and having at least one wall which is slotted or longitudinally apertured over a length such that the beam electrons are internally received by the resonator during a major portion of each beam deflection cycle. Because of such increased utilization of the beam electrons, it is possible to convert a much larger fraction of the kinetic energy of the beam to sustenance of the electromagnetic fields Within the resonator. Accordingly, the present invention provides a practical apparatus for generation of several watts of power at any selected frequency in the millimeter and submillimeter wavelength ranges of the spectrum.

The present invention, together with further objects and advantages thereof may be best understood by reference to the following description taken in accordance with the accompanying drawings in which like reference characters indicate like parts and in which:

FIGURE 1 diagrammatically illustrates a microwave power generating system of the type to which the present invention relates;

FIGURE 2 is a perspective view of a ridged waveguide cavity structure which is used for deflection of the electron beam in a preferred embodiment of the invention;

FIGURES 3 and 4 are diagrammatic representations useful in considering the operation and certain inherent characteristics of the apparatus of FIGURE 1;

FIGURE 5 is a graphical representation showing the optimum dimensional parameters of one embodiment in accordance with the present invention for generation of maximum ultrarnicrowave power;

FIGURE 6 is a diagrammatic illustration of the beam deflection geometry of the apparatus of FIGURE 1 which is useful for analysis of the power output characteristics of such apparatus;

FIGURES 7a and 7b schematically illustrate a further embodiment of apparatus in accordance with the present invention; and

FIGURE 8 is a fragmentary perspective view representing a structural modification of the post-deflection acceleration means which forms a part of the apparatus of FIGURE 7a.

In FIGURE 1 of the accompanying drawings, there is shown, diagrammatically, a system in accordance with the present invention which utilizes linear or planar der flection of an electron beam 36. The electron beam is derived from a conventional electron gun 12 which is positioned to normally direct a pencil beam of electrons along a central axis 10. The electron gun 12 may take any one of various forms known in the art. Along the electron beam axis 10 immediately subsequent to the beam forming gun 12 is positioned a beam deflection system 14 preferably comprising a generally rectangular resonant chamber or microwave cavity having top and bottom walls 28 and 29 and front and rear walls 25 and 27. In describing the various components of the present invention, the term front is used as meaning that wall or surface of the component which faces toward the source of the electron beam. Thus the front wall 25 is that wall of the deflection cavity 14 which is first traversed by the beam electrons. The rear wall 27 is that wall through which the beam electrons exit from the deflection cavity. As more clearly shown in FIGURE 2, the top wall 28 of the deflection cavity is provided with an inwardly projecting rectangular member 21 and the bottom wall 29 is provided with a similar inwardly projecting ridge member 23 with the spacing between the inner surfaces of the ridges 21 and 23 being slightly larger than twice the electron beam diameter. This structural arrangement provides for high intensity electric fields in the small space or gap between the ridges 21 and 23 so that a high velocity electron passmg through the deflection cavity is subjected to maximum deflection during the relatively small time interval required for the electron to pass between the ridges 21 and 23. To permit ingress of the electron beam to the deflection cavity, the front wall 25 is provided with a circular aperture 17 having a diameter slightly larger than that of the electron beam. After deflection by the electric field between ridges 21 and 23 the beam electrons pass out of the deflection cavity through a substantially rectangular aperture 19 which has a width slightly larger than the maximum beam width and which has a height dependent on the beam deflection angle which is to be used.

The ridged waveguide or ridged cavity structure of the deflection cavity 14 enables cflicient deflection of the electron beam at microwave frequencies without wasteful radiation of microwave energy into the immediate environment. The ridged or reentrant structure may be similar to that of known waveguide structure of the so-called ridged type. In accordance with one embodiment of the invention which has been used for harmonic frequency multiplication of microwave signals the deflection cavity 14 has been designed to receive and utilize deflection power of 8.5 kilomegacycles. By deflection of the beam at that frequency, generation of 34 kilomegacycles electromagnetic fields in the output resonator 20 has been accomplished. In that particular apparatus, the H-shaped cavity provided between the walls 25, 27 and the ridges 21, 23 had a maximum height of 0.658", a gap height of 0.90" With the cavity being 0.658" between the front and rear walls 25, 27 and being 1.50" deep along the length of the ridges 21 and 23.

The cavity 14 is preferably formed by constructing a mandrel having the above dimensions. The cavity itself is then electro-formed by electrolytic deposition of copper or the like on all of the exterior surfaces of the mandrel, after which the mandrel is dissolved by means of an etching solution which etches the mandrel material without etching 'away the electro-deposited walls of the cavity. As shown in FIGURE 2, the cavity structure further comprises an exterior flange 33 to which a microwave waveguide fitting may be bolted for feeding the 8.5 kilomegacycles input deflection energy to the deflection cavity 14. Further, in accordance with a preferred embodiment of the invention, the wall of the cavity opposite the flange 33 is provided with a cylindrical boss 35 for supporting an axially extending turning screw (not shown) which extends into gap between the ridges 21 and 23 for adjustment of the deflection cavity to precise resonance at the frequency of the input 8.5 kilomegacycle deflection power.

After exiting from the deflection cavity through the exit aperture 19, the deflected electron beam 36 passes through a slotted plate 16 which constitutes a means for post-deflection acceleration of the beam electrons. To that end, the plate 16, as shown in FIGURE 1, is connected to ground or a point of reference potential as indicated by the numeral 39, and the electron gun 12 is connected to a l5 kv. direct current source as indicated at 32. The deflection cavity 14 is likewise connected to the direct current source 32 through a small resistance 30. By this arrangement, the electron gun 12 forms and projects a relatively low voltage electron beam which may be deflected with a relatively small amount of input radio frequency energy. After deflection, the beam electrons are post-deflection accelerated by the high potential gradient (15 kv.) which exists between the deflection cavity 14 and the acceleration plate 16. Thus the deflected electrons which comprise the beam 36 have a kinetic energy corresponding to the potential of the direct current source. These deflected electrons travel outwardly from the acceleration plate 16 to traverse an output resonant cavity 18.

Preferably, the output cavity 18 takes the form of elongated waveguide structure which has its longitudinal axis 19 positioned in a plane substantially perpendicular to the axis of the electron gun 12. The front wall 20 of the output cavity is provided with a longitudinally extending slot or aperture which extends substantially the entire length of the output cavity and which has a width substantially equal to or slightly larger than the diameter of theelectron beam. The longitudinal aperture is electrically closed by a screening 24 which is welded to the wall 20 at the edges of the cavity. and which is preferably formed of gold-plated tungsten wire screen having 100 x 100 wires per inch. 'In a similar manner the rear wall of the output cavity is provided with a longitudinally extending aperture which is electrically closed by a similar tungsten screening 22 and which permits the high velocity electrons to exit from the output cavity to thereby avoid generation of deleterious secondary electrons within the resonator itself. Additionally, it is frequently desirable to provide aFaraday cup collector (not shown) adjacent the rear wall of the output cavity for collection of the beam electrons. If the beam electrons, after traversing the output cavity 18, were allowed to strike a target or anode having the same direct current potential as the output cavity 18, a considerable amount of heat loss would occur at the target. Also, and more importantly, that dissipation of the kinetic energy of the beam electrons would impose a substantial load on the high voltage direct current source 32 which supplies the system. By the use of a Faraday collector connected to a point of potential equal to that of the electron gun, the electrons which exit from the output cavity 18 are decelerated before collection thereby conserving the energy represented by the velocity of the electrons and thereby substantially reducing the load which otherwise would be imposed on the direct current source 32.

In accordance with one particular embodiment of the present invention, which has been constructed for the generation of 34 kilomegacycle energy in the output cavity 18, the output cavity has been constructed to have a length from top to bottom of 0.664, a width from front to back of 0.123", a depth of 0.28" and a slot width of 0.110". It will be appreciated, of course, that the slot width is not particularly critical but may be as wide as is necessary to accommodate the particular electron beam which is being utilized. In the particular embodiment above mentioned, a 2.8 millimeter beam diameter has been used.

To clarify the operation of the system of FIGURE 1, it is advantageous to consider the waveguide device 18 as having a physical length along the longitudinal axis 19 of exactly 3 half wavelengths at the ultramicrowave frequency which is to be generated. This characteristic is illustrated by the curve 26 of FIGURE 1b which represents three half-waves of a standing wave within the resonator 18. In this particular embodiment, the cavity is designed to resonate in the TE 'mode. This means, of course, that the electric field vector of the waves within the cavity extend between the front and rear walls of the cavity and thus substantially parallel to the direction of travel of the beam electrons. When a high velocity beam electron passes through such an electric field parallel to the electric field vector, the beam electron is decelerated in proportion to the intensity of the electric field through which it passes. Thus, if the electron beam 36 is synchronously deflected from top to bottom along the resonator 18, in step with an electromagnetic field which travels along the resonator, the beam electrons traversing the resonator will be, at all times, coincident with the maximum decelerating portion of the electric field in the resonator.

In accordance with a preferred form of the invention, the resonator 18 supports a standing wave rather than a traveling wave. In a standing wave resonator, as is well understood in the art, a wave arriving at one end of the resonator is reflected with a phase reversal between the impinging wave and the reflected wave. To maintain synchronism between the deflecting electron beam and the wave in the resonator it is therefore necessary to sweep the electron beam sinusoidally up and down along the resonator with the sweep amplitude being somewhat larger than the length of the resonator. That is, the beam sweeps beyond the top end of the resonator and dwells there for a time period equal to one half cycle of the harmonic frequency which is being generated within the resonator. Then, when the electron beam starts the return sweep downwardly along the resonator, it arrives at the top end of the resonator and begins to pass through the upper portion of the resonator precisely in synchronism of the decelerating electric field maximum of the reflected wave within the resonator.

More precisely, the vertical motion of the electron beam may be considered in terms of a cycle of the input deflection frequency f. The beam sweeps from the beam axis 10 to the top end of the resonator in a time K fK wherein is the period of the input deflection signal. It then leaves the resonator. After a dead time or dwell time outside the resonator, it returns at a time If n is equal to the number of waveguide wavelengths at the harmonic frequency mf between the axis 10 and the top end of the resonator 18 it can be shown that the frequency mf generated in the resonator is a harmonic of 1 We have found that the output frequency of the linear waveguide structure 18 of the apparatus of FIGURE 1 can be determined from equations governing the beam position and field distribution with the equations being integrated to determine the available power that can be generated for various possible generated harmonics. FIG- URE 3 illustrates the position of the electron beam 36 and the field distribution 26 for a cavity 18 which has a length equal to one waveguide wavelength at the frequency to be generated. By consideration of the relationship arising from the configuration of FIGURE 3, the power generated by a cavity having a length equal to even multiples of one-half wavelength may be theoretically determined. FIGURE 4 shows the beam position and field distribution which exists in a cavity whose length is an odd multiple of one half-wavelength at the frequency to be generated. From the relationships arising out of FIGURE 4, the power generated by cavities having lengths equal to various odd multiples of one half- Wavelength may be calculated.

Considering an even multiple cavity 18 as per FIG- URE 3, if during operation the beam is not deflected past the ends of the cavity the average power generated is:

m l AEbI Sin[ Wherein A is a beam-to-field coupling factor dependent on electron transit time through the cavity, P is the generated power, T is the radio frequency period of the generated harmonic, k is the waveguide wavelength of the generated harmonic signal, y is the y coordinate of the maximum beam deflection at the resonator 18, b is the cavity length or height along the longitudinal axis 18 and n is the harmonic number of the generated harmonic sin wt] sin nwtdt (2) By integration of Equation 2, the power output can be shown to be:

P =AEbIJ,,( 1 for n odd;

number of half wave- }cavity length an even lengths P,;=O; for n even Thus, it is demonstrated that a cavity which is an even number of half wavelengths long will generate power only at odd harmonics. The generated power for all of the even harmonics is zero so long as the cavity or resonator 18 has a length corresponding to an even number of half wavelengths. It can be shown that solution of the same equation for the configuration of FIGURE 4 in which the cavity has a length corresponding to an odd number of wavelengths gives the opposite results. That is, if the cavity has a length equal to an odd number of half wavelengths, the power generated at all of the odd harmonics is zero and the power at all of the even harmonies is the same as given by the first alternative of Equation 3. Thus, a cavity which is an even number of half wavelengths long can generate any odd harmonic to which it is tuned, and a cavity which is an odd number of half wavelengths in length can generate any even harmonic and no odd harmonics.

The even-odd relationship illustrated above arises from the symmetry of the electric field within the cavity 18 in which the undefiected beam is located. Thus, if the electron beam axis 10 passes through a point of pure even symmetry of the fields within the standing Wave resonator 18 no odd harmonics can be generated, and conversely for the case where the beam axis 10 passes through a point of pure odd symmetry. This characteristic holds true even if the electron beam axis 10 is not located centrally between the ends of the output cavity 18 or even if the beam is deflected past the top and bottom ends of the output cavity 18.

In FIGURE 5 is plotted the 4th harmonic power which may be generated by an output cavity 18 which is a single half wavelength in length (curve 44) and for an alternative cavity having a length of three half wavelengths (curve 46). It can be shown that when the electron beam 38 (FIGURE 6) is deflected beyond the ends of the resonator 18', the output power may decrease or may continue to increase. For the cavity (FIGURE 4) the power decreases sharply because of the decrease in the current which is interacting with the electromagnetic fields in the cavity. For the cavity (curve 44) the power continues to increase in spite of loss of interaction current because the beam position and the position of the electric fields within the cavity are caused to have better synchronism throughout the deflection cycle. As shown in FIGURE 5, the ordinate represents the ultramicrowave power developed in the output waveguide 18' of FIGURE 7 and the abscissa represents the ratio being the maximum beam deflection as shown by the electron beam position 38 and A being the waveguide wavelength within the cavity.

In FIGURES 7 and 8, there is illustrated a further embodiment of the present invention which embodies a very substantial increase in the power generating capacity of an apparatus of the type described. Specifically, the apparatus of FIGURES 7 and 8 difiers from the embodiment of FIGURE 1 primarily in that a sheet-like electron beam rather than a cylindrical electron beam is used. FIGURE 7a is a cross section view taken in the plane of beam deflection of the apparatus and FIGURE 7b is a front view of the output resonator 62 of FIGURE 7a. As shown in FIGURE 8, the electron gun utilized in this embodiment is a substantially rectangular structure adapted to produce an electron beam 76 having a substantially rectangular beam cross section, with the beam width being several times greater than the beam thickness. To that end, the electron gun 52 is provided with an elongated beam forming aperture 74 in the wall 72 through which the beam electrons exit from the electron gun. The rectangular cross section electron beam 76 passes from the electron gun through a resonant cavity deflection means 54 (FIGURE 7a) and through a post-deflection acceleration means 60 so that the deflected beam sweeps up and down in the plane of the paper along the output resonator 62.

The system of FIGURES 7 and 8 is generally similar to that of FIGURE 1 with the exception that the electron beam thickness is several times smaller than the beam 9. width and the various structural elements of the system have a correspondingly increased dimension normal to the plane of FIGURE 7a. That is, as shown in FIGURE 7b, the output resonator 62 has a width somewhat larger than its height from top to bottom and several times greater than its thickness from the front screen 66 to the back screen 64. In a preferred form of the embodiment of FIGURES 7 and 8, substantially the entire front wall of the resonator 62 is comprisedof tungsten screen having 100 x 100 wires per inch with this screen being gold plated to provide for maximum conductivity of the high frequency currents in the resonator walls. As shown in FIGURE 7a, the system is provided with a Faraday cup collector 70 which preferably is connected to the same potential level as the electron gun 52 so that the beam electrons are decelerated prior to collection.

In FIGURE 8 there is shown a further variant of the apparatus of FIGURE 7 in which the post-deflection acceleration means takes the form of two spaced apart cylindrical metal plates 86 and 88 each having a major area 90 formed of small apertured metal screening such as the 100 X 100 tungsten mesh mentioned heretofore. The sheetlike electron beam 76 passes through the deflection cavity 54 through an elongated entrance aperture (not shown) in the front wall of the deflection cavity with the entrance aperture having substantially the same dimensions as the cross-sectional dimensions of the electron beam 76. As shown in FIGURE -8, the top wall 78 and the bottom wall of the deflection cavity have a width in the direction perpendicular to the electron beam direction substantially greater than the front to back dimensions of the deflection cavity. The rear or beam exit wall 82 of the deflection cavity is provided with a substantially rectangular aperture 84 which preferably has a height slightly greater than twice the thickness of the electron beam 76and which has a length substantially corresponding to the Width of the electron beam. Thus the exit aperture .84 permits deflection of the beam within the cavity and exit of the deflected electrons from the cavity along the maximum deflection lines 96.

The top to bottom dimension of the screened apertures in the post-deflection acceleration plates 86 and 88 is, of course, designed to accommodate the maximum deflection of the electron beam. Post-deflection acceleration is accomplished by biasing the first acceleration screen 86. to 15 kv. as indicated at 94 and by connecting the second acceleration plate 88 to ground potential as indicated at 92. Accordingly, since the first acceleration plate 86 is connected to the same potential as the electron gun 52, the electron beam 76 has a relatively low velocity during its passage from the gun 52 through the deflection cavity 54 and to the first deflection plate 86. Between the plates 86 and 88 there is extremely high potential gradient for abruptly accelerating the deflected beam electrons to a velocity corresponding to a kinetic energy of 15 kv. Accordingly, by deflection of the beam while the beam electrons have a relatively low velocity and thereafter accelerating the beam electrons to a high velocity the radio frequency power input required for deflection of the beam is minimized while, at the same-time, the kinetic energy of the beam electrons traversing the output resonator 62 is maintained at a high level.

Thefabrication of complex structures for the low millimeter and submillimeter wavelength regions is a difiicult problembecause of minute dimensions and closetolerances involved. Thus, for the output resonator of the exemplary apparatus of FIGUREI, we have, in practice, used rectangular TE mode waveguide configurations which have the advantage of structural simplicity. For this type of waveguide it can be shown that the output power of the apparatus of FIGUREljisdependent upon the front-to-back height of the resinator 18 between the screen walls .24 and 22. This dependence is, in part, a function of the electron transit time coupling factor A of the foregoing Equation 2. We have determined that,

10 in a system using a TE mode resonant waveguide structure, the maximum output power is obtained when the electron transit time through resonator 18 is an odd multiple of a half cycle of a rad-i0 frequency cycle within the waveguide, i.e., =1r, 31r, 51r Accordingly, where a rectangular cross section output resonator 18 is to be employed, it is definitely not advantageous to mini mize the electron transit angle or the front-to-back waveguide dimension. Rather, best results can be obtained by constructing the resonator to have a height dimension dependent upon the beam particle velocity. This means that the useful structures are not limited to small heights. The front-to-back dimension may be relatively quite large providing that it is related to the electron transit velocity in a manner to make the transit time an odd multiple of a half-cycle at the generated frequency. Additionally, we contemplate that the use of ridged waveguide structures for the output resonator 18 can appreciably increase the power generated by systems such as those of FIGURES 1 and 7.

While the principles of our invention have been described in the foregoing in connection with specific exemplary apparatus, it is to be clearly understood. that such exemplary apparatus, in which the harmonic signal generating structure is a waveguide having a resonant length, is merely a special case of apparatus in accordance with broader principles. We have determined that it is not essential to utilize an output resonator 18 having a critical length equal to an integral number of half wavelengths at the harmonic frequency. Rather, it is also within the scope of the invention to use as the output member a slotted waveguide of unspecified length with the length being uncritical and variable at will to meet the contingencies of the particular application. With such an arrangement, the output member is not tuned to a specific frequency or to a specific harmonic and consequently is adaptable for broad band operations. Likewise, the untuned output waveguide does not require tun ing or adjustment or critical dimensioning of its length. When such an elongated section of waveguide is used as the output member, it will produce equal amounts of harmonic output power at each end, and the two ends may be coupled together to a common load by any of various well-known microwave power coupling arrangements. The power output at each end of the untuned waveguide member at any selected harmonic frequency is given by the following expression:

Where P is the nth harmonic output power at each end of the slotted waveguide member;

Where a and b are respectively the width and height of a rectangular Waveguide operating in the lowest normal mode; in this normal mode b is the waveguide dimension parallel to the electric field vector;

Where ,u. is. the permeability of the dielectric (in MKS units) in the waveguide (for air ,u. is equal to 4m; 10" henries per meter);

e is the permittivity of the same dielectric. material (for air 6 is equal to 8.85 x l() farads per meter);

f is the cutoff frequency of the waveguide;

n is the harmonic number; i

I is the bessel function of the first type and the nth order;

K is equal to 21r/k where A is the waveguide wavelength;

S is the total length of deflection or sweep of the electron beam along the waveguide structure;

to is equal to Zn); and

T is the electron transit time through the waveguide structure.

By critical inspection of the foregoing equation, it will be observed that the power output P is dependent upon the dielectric characteristics of the material which fills the waveguide structure. The power output can be very appreciably increased by utilizing a waveguide filling material which has a relatively high permeability and a relatively low permittivity. An ionized gas plasma can be used as the filling material for the untuned or unresonant output waveguide structure. A plasma can have a permitivity 6 much smaller than that of air while having a permeability equal to that of air. Thus, filling the waveguide with plasma gives an increased value of p/e and, therefore, increased power output from an apparatus in which all other parameters and characteristics remain the same. The foregoing equation, and the above described structure to which it relates, defines in general terms a class of ultramicrowave signal generating devices within the scope of the invention. The equation defines the power output for apparatus in which the length of the output waveguide is immaterial provided that the ends are coupled to an appropriate microwave load or otherwise nonreflectively terminated. It will be understood that the specific exemplary systems of FIGURES 1 through 7 are special cases in which the ends of the output resonator 18 are reflectively terminated by conductive end plates. It is to be noted that Equations 1, 2, and 3 are applicable to only the special cases of FIGURES 1 to 7 in which the output resonator 18 is tuned, and are not applicable to the general case in which the output waveguide member is a broad band structure.

Thus, while the present invention has been described with reference to certain specific embodiments only, it is intended that it is not so limited but is susceptible of the above mentioned changes and modifications as well as various others which will appear to those skilled in the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an ultramicrowave power generating system:

electron beam forming means to project a beam of electrons along a first axis; deflection means for sinusoidally deflecting said beam to and fro in a single beam deflection plane;

microwave resonant means for receiving the deflected beam electrons and periodically extracting kinetic energy from the same by virtue of electromagnetic interaction between the beam electrons and the electric fields propagated within said resonant means to thereby generate ultramicrowave power; said resonant means comprising an elongated waveguide device having a longitudinal axis substantially perpendicular to said first axis and coincident with said beam deflection plane, with the front and rear walls of said device longitudinally apertured along the regions where said walls intercept said beam deflection plane and with said device being internally dimensioned so that the electric field vector of the electromagnetc fields therein is oriented substantially parallel to the direction of travel of the beam electrons; v

and said deflection means comprising a double-ridged waveguide structure having a pair of inwardly extending ridge members positioned on opposite sides of said beam axis in a manner to provide a gap therebetween, said gap having a length along said first axis approximately equal to the product of the beam velocity and the beam transit angle, and a gap width between said ridge members approximately corresponding to twice the beam deflection lateral displacement within the gap region plus the maximum beam diameter in the gap region.

2. A harmonic generating system comprising, in combination:

a fundamental frequency energy source;

a hollow conductive waveguide having a substantially rectilinear longitudinal axis and having an active length equal to an integral number of half-wavelengths at a harmonic frequency which is an integral multiple of said fundamental frequency, said waveguide being resonant at said harmonic frequency;

means to project a beam of electrons along a beam axis substantially perpendicular to the longitudinal axis of said waveguide;

said waveguide having an elongated beam entrance aperture extending rectilinearly along said waveguide in a direction parallel to the direction of wave propagation therewithin, said aperture having a length corresponding to at least about one wavelength of the harmonic frequency waves supported within said waveguide;

deflection means coupled to and driven by said fundamental frequency source for sweeping said electron beam to and fro with a substantially sinusoidal deflection motion in a single deflection plane which includes said beam axis and is aligned with said aperture;

said beam being deflected through a distance at said waveguide substantially greater than the length of said aperture so that during the time intervals when the beam enters said waveguide the beam has a scanning velocity substantially equal to the phase velocity of the harmonic frequency waves propagated in said waveguide and energy is transferred from said beam electrons to the harmonic frequency electromagnetic fields sustained within said waveguide;

and output coupling means for deriving harmonic frequency energy from said waveguide and applying the same to a load.

3. In a microwave harmonic generator, the combination of:

cathode means for providing a substantially rectangular-in-cross section sheet-like electron beam with the cross-sectional beam width being at least several times the beam thickness, said beam being normally directed along a predetermined beam axis;

a source of input energy of a fundamental frequency;

deflection means coupled to said source for sinusoidally deflecting said beam to and fro in a deflection plane which extends substantially normal to the cross-sectional width of said beam;

a hollow conductive waveguide constructed and positioned to propagate waves of a harmonic frequency linearly in a direction perpendicular to said beam axis, said waveguide having a substantially rectilinear longitudinal axis which angularly coincides with the direction of harmonic Wave propagation;

'said waveguide having at least one elongated beam entrance aperture having a width substantially equal to the cross-sectional width of said sheet-like beam, said aperture extending rectilinearly along the region where said waveguide intercepts said deflection plane and. having a length corresponding to at least onehalf waveguide wavelength of the harmonic frequency waves propagated within said waveguide;

said beam being deflected along said aperture in synchronism with the propagation of harmonic frequency waves within said waveguide, and through a deflection amplitude sufficiently greater than the length of said aperture so that said beam enters said aperture only during portions of the sinusoidal deflection period when the beam scanning velocity is approximately constant and substantially equal to the propagation velocity of the harmonic waves in said waveguide and so that said beam is swept past the ends of said aperture and thereby decoupled from said harmonic waves during those portions of the 13 sinusoidal deflection period when the beam scanning velocity is minimal.

4. Apparatus in accordance with claim 3 and further characterized in that the deflection means therein recited comprises a microwave cavity resonator tuned to said fundamental frequency and including a pair of inwardly extending ridge members for providing a narrow gap therebetween, said gap having a width between said ridge members slightly exceeding twice the beam displacement Within the gap region and said ridge members having lengths at least exceeding the cross-sectional width of said sheet-like beam.

References Cited by the Examiner UNITED STATES PATENTS McRae 315-525 Strutt et al. 3155.25 Hartley 3155.24 Haelf 315-527 Ramo 3155.38 Bramley 31382 10 HERMAN KARL SAALBACH, Primary Examiner.

GEORGE N. WESTBY, Examiner.

Claims (1)

1. IN AN ULTRAMICROWAVE POWER GENERATING SYSTEM: ELECTRON BEAM FORMING MEANS TO PROJECT A BEAM OF ELECTRONS ALONG A FIRST AXIS; DEFLECTION MEANS FOR SINUSOIDALLY DEFLECTING SAID BEAM TO AND FRO IN A SINGLE BEAM DEFLECTION PLANE; MICROWAVE RESONANT MEANS FOR RECEIVING THE DEFLECTED BEAM ELECTRONS AND PERIODICALLY EXTRACTING KINETIC ENERGY FROM THE SAME BY VIRTUE OF ELECTROMAGNETIC INTERACTION BETWEEN THE BEAM ELECTRONS AND THE ELECTRIC FIELDS PROPAGATED WITHIN SAID RESONANT MEANS TO THEREBY GENERATE ULTRAMICROWAVE POWER; SAID RESONANT MEANS COMPRISING AN ELONGATED WAVEGUIDE DEVICE HAVING A LONGITUDINAL AXIS SUSTANTIALLY PERPENDICULAR TO SAID FIRST AXIS AND COINCIDENT WITH SAID BEAM DEFLECTION PLANE, WITH THE FRONT AND REAR WALLS OF SAID DEVICE LONGITUDINALLY APERTURED ALONG THE REGIONS WHERE SAID WALLS INTERCEPT SAID BEAM DEFLECTION PLANE AND WITH SAID DEVICE BEING INTERNALLY DIMENSIONED SO THAT THE ELECTRIC FIELD VECTOR OF THE ELECTROMAGNETIC FIELDS THEREIN IS ORIENTED SUBSTANTIALLY PARALLEL TO THE DIRECTION OF TRAVEL OF THE BEAM ELECTRONS; AND SAID DEFLECTION MEANS COMPRISING A DOUBLE-RIDGED WAVEGUIDE STRUCTURE HAVING A PAIR OF INWARDLY EXTENDING RIDGE MEMBERS POSITIONED ON OPPOSITE SIDES OF SAID BEAM AXIS IN A MANNER TO PROVIDE A GAP THEREBETWEEN, SAID GAP HAVING A LENGTH ALONG SAID FIRST AXIS APPROXIMATELY EQUAL TO THE PRODUCT OF THE BEAM VELOCITY AND THE BEAM TRANSIT ANGLE, AND A GAP WIDTH BETWEEN SAID RIDGE MEMBERS APPROXIMATELY CORRESPONDING TO TWICE THE BEAM DEFLECTION LATERAL DISPLACEMENT WITHIN THE GAP PLUS THE MAXIMUM BEAM DIAMETER IN THE GAP REGION.

Images