US3369191A

US3369191A - High power microwave noise generator employing traveling-wave tube type device with reflected electron beam

Info

Publication number: US3369191A
Application number: US425833A
Authority: US
Inventors: Andrew C Schram; Kenneth P Grabowski
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1965-01-15
Filing date: 1965-01-15
Publication date: 1968-02-13
Anticipated expiration: 1985-02-13

Description

Feb. 13, 1968 A. c. SCHRAM ETAL 3,369,191

HIGH POWER MICROWAVE NOISE GENERATOR EMPLOYING TRAVELING-WAVE TUBE TYPE DEVICE WITH REFLECTED ELECTRON BEAM Filed Jan. 15, 1965 4 Sheets-Sheet l A/o/sz our M /Z /i E Z 5 iv jfl Z 4 C l 24 /T a 25 1- 4 za- T 6 AX/AA fl/sr/m/ci lira Z.

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V Kw! 9%M/ A. c. SCHRAM ETAL 3,369,191 HIGH POWER MICROWAVE NOISE GENERATOR EMPLOYING TRAVELING-WAVE Feb. 13, 1968 TUBE TYPE DEVICE WITH REFLECTED ELECTRON BEAM Filed Jan. 15, 1965 4 Sheets-Sheet a /mm Feb. 13, 1968 A. c. SCHRAM ETAL 3,369,191

HIGH POWER MICROWAVE NOISE GENERATOR EMFLOYING TRAVELING-WAVE TUBE TYPE DEVICE WITH REFLECTED ELECTRON BEAM Filed Jan. 15, 1965 4 Sheets-Sheet 5 l 1 I 1 z -.05 0 05 5 2 .25 .3

4mm warm ca {wax/5) A. c. SCHRAM ETAL 3,369,191 HIGH POWER MICROWAVE NOISE GENERATOR EMPLOYING TRAVELING-WAVE Feb. 13, 1968 TUBE TYPE DEVICE WITH REFLECTED ELECTRON BEAM 4 Sheets-Sheet 4 Filed Jan. 15, 1965 EH8 4 a w w w Q \R QXNQ 38 mm mw/mc 7 w 4 M 1; ww 2 I W/U a; 9% Z J J M 3 a a -a United States Patent 3,369,191 HIGH POWER MICROWAVE NOISE GENERATOR EMPLOYING TRAVELING-WAVE TUBE TYPE DEVICE WITH REFLECTED ELECTRON BEAM Andrew C. Schram, Torrance, and Kenneth P. Grahowski, La Mirada, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Jan. 15, 1965, Ser. No. 425,833 11 Claims. (Cl. 331-78) ABSTRACT OF THE DISCLOSURE This invention relates to noise generation, and more particularly relates to an efiicient device for generating high power microwave over a wide frequency spectrum.

Noise generators have found increasing use in simulating broadband signals, providing calibrated standards for measuring other noise, and for jamming radar and other communications and tracking systems. Some of the devices commonly employd for the generation of noise include hot-cathode arcs, photoelectron multipliers, noise diodes and gas discharge tubes. While such devices are satisfactory for generating relatively low power noise, they are not practical for generating microwave frequency noise at high power density levels of the order of one watt per megacycle, which is necessary for jamming applications.

Since some noise is inherently produced in electron beam-microwave amplifying devices such as klystrons and traveling-wave tubes, a technique which has been employed to generate high power noise at microwave frequencies involves connecting three traveling-Wave tubes in cascade. No input signal is applied to the first tube of the cascaded chain; and shot, thermal, and other noise generated near the input end of the first tube, after passing through the three stages of amplification, reaches the desired power level at the output end of the third tube in the cascaded chain. Since three tubes are employed, and isolators are required between successive tubes in order to prevent regenerative feedback which could otherwise cause discrete oscillations, a noise generator of this type is excessively large, complex and expensive. In addition, the efficiency of operation of such a device is very low.

Accordingly, it is an object of the present invention to provide a device for generating high power noise over a wide range of microwave frequencies, and which device is considerably smaller, simpler, and less expensive than comparable noise generators of the prior art.

It is a further object of the present invention to provide a high power microwave noise generator Which operates with greater efficiency than has been possible with 65 prior art noise generators.

It is a still further object of the present invention to provide a device for generating high power microwave noise having a more nearly constant power vs. frequency characteristic than in the past.

It is still another object of the present invention to 3,369,191 Patented Feb. 13., 1968 provide a device which may be employed as either a traveling-wave amplifier or as a microwave noise generator, depending upon the selection of bias potentials.

In accordance with the foregoing objects, the noise generator of the present invention includes an electron gun for generating a stream of electrons, means for focus ing the electrons into a well-collimated beam along a predetermined path, a slow-wave structure disposed along and about the electron beam path for propagating electromagnetic wave energy with a phase velocity substantially less than the velocity of light, and a collector electrode disposed at the end of the electron beam path remote from the electron gun for collecting a substantial portion of the electrons in the beam. The collector electrode is biased relative to the slow-wave structure to establish a region of electrical potential capable of reflecting a substantial portion of the beam electrons back toward the electron gun.

Electromagnetic wave noise energy propagating along the slow-wave structure toward the collector interacts with the electron beam to produce amplification of the noise energy and velocity and charge density modulation of electrons in the beam. Reflected electrons traveling toward the electron gun interact similarly with electromagnetic wave noise energy propagating along the slow- Wave structure toward the electron gun to produce amplification of this noise energy. Upon arrival at the electron gun end of the slow-wave structure this noise energy is reflected, and during its propagation along the slowwave structure toward the collector, is further amplified by interaction with electrons traveling toward the collector. An output arrangement coupled to the collector end of the slow-wave structure obtains amplified electromagnetic wave noise energy from the slow-wave structure.

Additional objects, advantages and characteristic features of the present invention will become readily apparent from the following detailed description of preferred embodiments of the invention when considered in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating a noise generator according to the present invention;

FIG. 2 is a graph illustrating the potential along the axis of the noise generator of FIG. 1 in the absence of an electron beam as a function of axial distance;

FIG. 3 is a longitudinal sectional view of a noise generator constructed according to one embodiment of the present invention;

FIG. 4 is a cross-sectional view of a portion of the noise generator of FIG. 3 as taken along line 4-4;

FIG. 5 is a longitudinal sectional View of a portion of a noise generator constructed according to another embodiment of the present invention;

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 5;

FIG. 7 is a graph illustrating the ratio of the potential on the noise generator axis to the helix potential as a function of axial distance in the vicinity of the collector region of a noise generator according to FIG. 3 in the absence of an electron beam;

FIG. 8 is a graph illustrating the axial magnetic field as a function of axial distance in the vicinity of the collector region of a noise generator according to FIG. 3;

FIG. 9 is a graph illustrating the percentage of beam current intercepted by the helix and the anode as a function of the percentage of collector depression for a noise generator in accordance with FIG. 3; and

FIG. 10 is a graph showing the noise power density output as a function of frequency for a noise generator in accordance with FIG. 3 having an exemplary combination of anode, helix, and collector potentials.

Referring now to the drawings, and in particular to FIG. 1, a noise generator 11 according to the present invention may be seen to include an enclosed housing, or envelope, 12 of a substantially tubular configuration. Disposed at one end of the envelope 12 is a cathode 14 for emitting a stream of electrons along the longitudinal axis of the envelope 12. In order to collect electrons in the stream, while at the same time providing a potential profile which reflects a substantial portion of the stream electrons, a collector electrode 16 is disposed at the end of the envelope 12 remote from the cathode 14. Disposed along the longitudinal axis of the envelope 12 between the cathode 14 and collector 16 is a slow-wave structure, illustrated as a helix 18, for propagating electromagnetic waves along and about the electron stream path with a phase velocity substantially less than the velocity of light so that interaction between the electron stream and the electromagnetic waves may occur. A tubular anode 20 is interposed between the cathode 14 and the adjacent end of the slow wave structure 18, while a conductor 22 is connected to the opposite end of the slow-wave structure 18 for obtaining the output noise energy from the device.

In order for the device 11 to generate microwave noise with high efficiency, the cathode 14, the anode 20, the helix 18 and the collector 16 should be maintained at predetermined potentials with respect to one another. In the illustrative biasing arrangement of FIG. 1, a voltage source 24 is provided to maintain the anode 20 at a positive potential of V relative to the cathode 14, a voltage source 25 is provided to maintain the helix 18 at a positive potential of V with respect to the cathode 14, and a voltage source 26 maintains the collector 16 at a positive potential of V with respect to the cathode. The portion of the total electron beam current I which is intercepted by the anode 20 is designated as l the portion of the beam current l intercepted by the helix 18 is denoted i and the electron current reaching the collector 16 is designated as I An exemplary potential profile for the noise generator of FIG. 1 is given in FIG. 2 in the form of a curve 27 illustrating the voltage V along the longitudinal axis of the envelope 12 as a function of distance along the axis and in the absence of an electron beam. Considering the potential of the cathode 14 as the zero, or reference level, as the longitudinal axis of the envelope 12 is traversed from the cathode 14 to the collector 16-, the axial potential V increases rapidly to the anode potential V,, which may be of the order of +1000 to +1500 volts. The potential V, exists throughout the relatively short length of the anode 20, after which the axial potential is reduced slightly to the helix potential V which extends substantially throughout the length of the helix 18. The helix potential V may be in a range essentially between 80% to 98% of the anode voltage V,,. In the region at the collector end of the slow-wave structure 18, the axial potential V decreases rapidly from the helix potential V to the collector potential V along the solid line portion 28 of the potential profile curve 27. The collector potential V is substantially lower than the helix potential V and may be in a range varying from essentially to essentially 30% of the helix voltage V,,. For purpose of comparison a dashed curve 29 is included in FIG. 2 to depict the potential in the collector region of a conventional traveling-wave amplifier operating with a depressed collector.

A particular noise generator constructed in accordance with one embodiment of the present invention is illustrated in more detail in FIGS. 3 and 4. As is shown in these figures, the electron emissive cathode 14 is mounted in a cup-like ceramic housing 30 having its open end facing the helix 18. A frustoconical beam focusing electrode 32, maintained at the cathode potential, is disposed about the electron beam in front of the cathode 14. The tubular anode 20 defines a radially outwardly extending flange 22 which is hermetically sealed to the open end of the member 30 to complete the housing for the electron gun portion of the noise generator. The anode 20 functions to accelerate the emitted electrons toward the helix 18, and since it is biased positively with respect to the helix, the anode 20 also serves to prevent positive ions which might be produced in the interaction region of the device from reaching the cathode 14 and thereby damaging the cathode emissive surface.

The helix 1%, which may be of molybdenum for example, is coaxially mounted within a tubular barrel 34, of stainless steel for example, on a plurality of longitudinally extending support rods 36 of a ceramic material such as alumina or beryllia. Although three support rods 36 are shown equally circumferentially spaced about the helix 18, other numbers and orientations of helix support rods may alternatively be employed. The end of the barrel 34 adjacent the electron gun portion of the noise generator is hermetically sealed to a planar ceramic insulating ring 38 which abuts against and is hermetically sealed to the radially extending flange 22 of the anode 20. Thus, the ring 38 enables the helix 18 to be maintained at a different potential than the anode 20, and the hermetic seals between the

elements

30, 22, 38 and 34 allow these elements to constitute a vacuumtight envelope (designated 12 in FIG. 1) so that the interior portion of the noise generator may be maintained at a reduced pressure such as 10 mm. Hg.

In an illustrative embodiment of the present invention the helix 18 was designed to have an outer diameter of .118 inch, an inner diameter of .098 inch, and a pitch (i.e., the axial distance in which the helical conductor makes one complete revolution about its axis) of .020 inch, with the inner diameter of the barrel 34 being 0.242 inch.

An attenuating coating 40, such as a pyrolytically deposited carbon film, may be provided on one or more of the support rods 36 at a predetermined axial location therealong to afford a dissipative termination which severs the noise generator into a pair of circuit wave isolated sections in order to prevent undesired oscillations from developing. The coating 40 also affords a resistive connection between the barrel 34 and the helix 18 so that the helix biasing source may be electrically connected to the barrel 34. The attenuating coating 40 preferably extends completely around the circumference of the rod 36 and may be axially tapered in order to minimize the reflection of electromagnetic wave energy. It should be apparent that although only one attenuating coating is shown in FIG. 3, several such terminations may be afforded at different axial locations along the noise generator to sever the noise generator into more than two wave isolated sections. Moreover, a slightly different helix pitch may be afforded in each section in order to increase the bandwidth of the noise generator.

For focusing the emitted electrons and constraining them to flow in the desired axial path a periodic permanent magnet focusing arrangement may be provided, as shown in FIG. 3. The focusing arrangement includes a plurality of ring-shaped permanent magnets 42 respectively interposed between a plurality of annular substantially planar pole pieces 44 of ferromagnetic material. Each pole piece 44 extends radially inwardly of the magnets 42 and defines a ferrule 46 at its inner extremity. The ferrules 45 protrude outwardly from the planes of the pole pieces 44 along directions parallel to the longitudinal axis of the noise generator, with the inner lateral surfaces of the ferrules 46 abutting against the barrel 34. Axially adjacent ones of the magnets 42 are stacked with opposite polarity, thus causing a reversal of the magnetic field at each pole piece 44 and thereby providing a periodically focused device. It is pointed out that although a periodic permanent magnet focusing arrangement is shown, other focusing schemes, for example,

nonperiodic permanent magnet focusing, solenoid focusing, or electrostatic focusing, may be employed instead. Regardless of the particular focusing scheme selected, it should be appreciated that in order to generate noise at maximum efficiency levels, the focusing arrangement should be able to maintain an accurately focused, wellcollimated electron beam.

The collector electrode 16, which is disposed at the end of the helix 18 remote from the cathode 14, not only functions to collect a portion of the stream electrons, but also provides a potential profile which reflects a substantial portion of the electrons in the stream to enable noise to be produced in the manner discussed in more detail below. As may be seen from FIG. 3, the collector electrode 16 is constructed with a substantially cup-like portion 47 having an open end facing the helix 18 and the lateral surface of which has a diameter the same as that of the barrel 34. In addition, the collector 16 defines in the vicinity of its open end a radially inwardly projecting portion 48 which terminates in tubular lip portion 50. The portion 50 is coaxially aligned with and has a diameter essentially the same as the helical diameter of the helix 18. The axial extent of the tubular portion 50- and its location relative to the end of the helix 18 for a noise generator having the particular illustrative dimensions set forth above is indicated by the x-scale along the axis of the noise generator of FIG. 3, the dimensions on this scale being given in inches.

In order to remove the generated microwave noise energy from the device, a coaxial output arrangement 51 is provided having a tubular outer conductor 52 and a coaxially disposed inner conductor 54. The inner conductor 54 extends through a radial bore in a ceramic ring 56 and is attached to the end of the helix 18 at 58. The ring 56, which has an outer diameter essentially equal to that of the barrel 34 and the collector 16 and a radial extent greater than that of the barrel 34, is hermetically sealed to both the barrel 34 and the collector 16 to complete the vacuum envelope 12 for the device. Thus, the ring 56 functions as a vacuum window which enables the desired evacuated pressure to be maintained within the noise generator, while allowing output microwave noise energy to readily pass through the window 56. The outer conductor 52 of the output arrangement 51 is attached to the outer lateral surface of the ring 56 and may be threaded, as shown at 60, to facilitate attachment to external \vaveguiding circuitry (not shown).

It should be apparent that the noise generator of FIGS. 3 and 4 is similar in many respects to a conventional traveling-wave amplifier tube. Accordingly, by providing a coaxial input arrangement similar to the output arrangement 51 at the cathode end of the helix 18, a dual purpose device may be afiorded which, by appropriate selection of electrode potentials, can be used either as a high efiiciency microwave noise generator or as a traveling-wave amplifier. In the event such a tube is to be employed as a noise generator, a smooth broadband short circuit should be provided between the inner and outer conductors of the coaxial input arrangement in order to maximize the efiiciency of the device.

A voltage source 62, tapped at the appropriate poten tials as shown, provides the desired operating potentials V V and V relative to the cathode 14 for the anode 20, the helix 18, and the collector 16, respectively, to operate the device in its high efiiciency noise generating mode.

In order to improve the impedance match between the helix 18 and the output waveguiding arrangement 51, the pitch of the helix 18 may be varied as a function of axial distance in the vicinity of its end connected to the output conductor 54. However, such a variation may reduce the efiiciency of the device due to the elimination of a wavebeam interaction region.

An alternate arrangement for improving the output impedance match without sacrificing efficiency is illustrated in FIGS. 5 and 6, certain portions of the device which are identical to those of the embodiment of FIGS. 3 and 4 being omitted from FIGS. 5 and 6 for simplicity. Respective components in the embodiment of FIGS. 5 and 6 which are similar to those of the embodiment of FIGS. 3 and 4 are designated by the same reference numerals as their counterpart components in FIGS. 3 and 4 except for the addition of the suflix a. Also, in the embodiment of FIGS. 5 and 6 a tape-like helical conductor 18a replaces the circular helix 18 of FIGS. 3 and 4. Moreover, a plurality of electrically conductive impedance matching elements 62 in the form of axially tapered tubular sectors are equally circumferentially disposed on the inner lateral surface of the barrel 34a between the respective support rods 36a in the vicinity of the collector end of the helix 18a. The window element for the embodiment of FIGS. 5 and 6 takes the form of a cylindrical ceramic button 64 disposed in a radial bore through one of the impedance matching elements 62, with the inner conductor 54a of the coaxial output arrangement extending through a hole in the button 64 and being electrically connected to the end of the helix 18a at 58a. A ceramic ring 66 is hermetically sealed between the radially extending portion 48a of the collector 16a and the ends of the impedance matching elements 62 to electrically insulate the elements 62 from the collector 16a, while completing the vacuum envelope 12 for the device. As may be seen from FIG. 5, the tapered portions of the respective impedance matching elements 62 terminate slightly before the end of the helix 18a so that their inner radius remains constant in their end regions adjacent the ring 66.

As has been mentioned above, in order to generate high efficiency microwave noise the cathode 14, the anode 20, the helix 18, and the collector 16 should be maintained at predetermined potentials with respect to one another, and as will be explained more fully below, a carefully selected potential profile should exist in the region of the collector end of the helix 18 and the tubular lip portion 50 of the collector 16. The ratio of the potential V on the noise generator axis to the helix potential V as a function of the axial distance x (corresponding to the scale on FIG. 3) in the aforementioned region is illustrated by the curve 70 of FIG. 7. This curve was made from a noise generator having the configuration illustrated in FIGS. 3 and 4 with the aforementioned illustrative dimensions and in the absence of an electron beam. The collector 16 was grounded, while a positive potential was applied to the helix. As may be seen from the curve 70, the axial potential begins to decrease from essentially the helix potential slightly before the end of the helix 18 is reached and becomes essentially equal to the collector potential slightly beyond the radial extending portion 48 of the collector 16. It should be pointed out that by minimizing the distance between the tubular lip portion 50 and the end of the helix 18, a maximum number of electrons may be reflected back toward the cathode 14; however, too great a percentage of reflected electrons may result in inefiicient operation.

As has been mentioned above, for high efiiciency noise generation it is necessary to provide highly accurate focusing of the electron stream. A typical axial magnetic field variation as a function of the axial distance x in the vicinity of collector region of a noise generator according to FIGS. 3 and 4 having the aforementioned illustrative dimensions is illustrated by the curve 72 of FIG. 8.

The effect of collector depression on the helix current I and on the anode current I is illustrated in FIG. 9 at (a) and (b), respectively. As used in FIG. 9, the percent collector depression is defined as respectively, as shown in FIGS. 1 and 3. The curves of FIG. 9 were made from a noise generator having the configuration illustrated in FIGS. 3 and 4 with the afore- 'mentioned illustrative dimensions and with a helix potential V of 1120 volts, an anode potential V of 1170 volts, and an initial electron beam current 1,, of 45.5 ma. As may be seen from the curve portion 74 of FIG. 9, as the percent collector depression is increased from 50% to 85% essentially no current is intercepted by the helix. However, when the collector depression becomes slightly greater than 85%, helix current commences [FIG 9 at (a)] and increases essentially instantaneously along the curve portion 76 to around 64% of the total beam current at the point 78. For the particular noise generator from which the curves of FIG. 9 were plotted, at the point 78 the collector potential V was 160 volts, the helix current I was 29 ma., the anode current I,, was 3 ma., and the collector current 1,, was 13.5 ma. A noise power output of 2.75 watts was obtained at an efficiency of 7.2%.

For further increases in collector depression up to slightly greater than 90%, the percentage of beam current intercepted by the helix increases gradually as shown by the curve portion 80. When the percentage of collector depression is then decreased, the percentage of beam current intercepted by the helix decreases gradually along the curve portion 82 until the collector is depressed by slightly less than 70%, after which the helix current 1 drops to essentially zero along curve portion 86. For the aforementioned exemplary noise generator, at point 84 along the curve portion 82, the collector voltage V was 340 volts, the helix current I was 23 ma., the anode current I,, was 2. ma., and the collector I,, was 20.5 ma. A noise power output of 2.75 watts was obtained at an efficiency of 7.4%. It should be apparent from FIG. 9 at (a) that a substantial hysteresis effect is present for the helix current as a function of the percentage of collector depression.

As is illustrated in FIG. 9 at .(b), as the percentage of collector depression is varied in the aforementioned manner, the percentage of the beam current intercepted by the anode traverses the

curve portions

74, 88, 90, 92 and 94 to produce a hysteresis effect similar to that for helix current I The noise power density output from a noise generator according to FIGS. 3 and 4 as a function of frequency for a typical combination of anode, helix, and collector potentials is illustrated by the curve 100 of FIG. 10. This curve was obtained from a noise generator having the aforementioned illustrative dimensions and for an anode voltage V of 1370 volts, a helix voltage V of 1220 volts, a collector voltage V of 245 volts, a helix current I of ma., an anode current I, of 1.2 ma., and a collector current I of 43 ma. The DC. power input was 30.5 watts.

A brief discussion of the operation of the noise generator of the present invention will now be given. As in a conventional traveling-wave tube, noise is inherently produced from three sources. These are: (1) shot noise resulting from the emission of electrons from the cathode; (2) thermal, or Johnson, noise caused by molecular motion of electrons in the conductive elements in the tube; and (3) additional electron beam noise resulting from such effects as the secondary emission of electrons when primary electrons strike the helix or the anode, ionization, etc. When the aforementioned noise is of frequencies capable of propagating as electromagnetic waves along the helix it is amplified by interaction between the traveling waves and the electron beam. Thus, at the collector end of the helix a noise power output is produced given approximately by P =kTBG+N where k is Boltzmanns constant, T is the absolute temperature of the tube, B is the bandwidth of the tube, G is the average gain of the tube over the bandwidth B, and N represents the total electron beam noise generated by shot and other elfects.

As a result of interaction with the waves traveling along the helix, when the electron beam reaches the collector end of the helix the electrons have been velocity and charge density modulated, with the average electron velocity corresponding to the voltage on the helix. However, since the collector potential is substantially less than the helix potential, as the electrons become influenced by the field from the collector they experience a strong decelerating force and are rapidly slowed down.

It is known from Brillouin flow studies that for an electron beam of a given charge density confined to a cylindrical path, the voltage is a minimum along the beam axis and increases as a function of the square of the radial distance from the axis. Thus, when the electron beam reaches the region within the tubular portion of the collector 16, the electrons along the periphery of the beam are at a significantly higher potential than the electrons along the beam axis. In the annular space between the periphery of the electron beam and the collector tubular portion 50 the potential increases as a function of the natural logarithm of the radial distance from the axis.

When the beam passes beyond the radially extending portion 48 at the inner end of the collector tubular portion 50, the radial distance between the periphery of the beam and the collector wall increases abruptly. Since the entire collector is at a uniform potential, and since the aforementioned logarithmic potential distribution still exists in the annular space between the lateral wall of the cuplike portion 47 and the beam periphery, the potential at the periphery of the beam is lowered after the beam has passed the inner end of the tubular portion 50, which, in turn, lowers the potential along the beam axis. If the collector potential is selected relative tothe helix potential so that the potential along the beam axis approaches zero while the beam is traversing the tubular portion 50, when the beam enters the cup-like portion 47 the axial potential is driven negative by the aforementioned potential changes, producing an unstable condition in the vicinity of the axis of the beam. This negative potential condition creates a virtual cathode in a region encompassing the beam center which oscillates in its axial position in accordance with the velocity and charge density modulation on the beam. The net effect is that electrons generally along the peripheral regions of the beam are spread radially outwardly as they continue to travel in an axial direction away from the helix 18, and these electrons eventually are intercepted by either the lateral or the end surfaces of the cup-like collector portion 47. At the same time, on account of the virtual cathode condition, electrons generally near the central regions of the beam are reflected back toward the helix 18. These reflected electrons are attracted by the positive potential applied to helix 18 causin g them to diverge radially outwardly by a small amount as they approach the helix. However, the magnetic focusing field constrains the backwardly traveling electrons into a path substantially radially coincident with the electrons in the beam traveling toward the collector 16.

Electrons in the backwardly traveling beam substantially retain the velocity and charge density modulation imposed on electrons in the forwardly traveling beam.

These modulations on the backwardly traveling beam induce backwardly traveling waves along the helix 18 which are amplified by interaction with the backwardly traveling electron beam. Backwardly traveling waves reaching the end of the helix 18 adjacent the anode 20 are reflected into forwardly traveling waves which, due to the aforementioned forward and backward wave amplification, are much larger in amplitude than the originally described noise-induced waves. These forwardly traveling waves are then further amplified by interaction with the forwardly traveling electron beam, producing additional velocity and charge density modulation on the forwardly traveling beam, and the aforedescribed process is repeated until a saturated condition of beam modulation is achieved. The net result is that a noise power output is produced at the collector end ofthe helix 18 which is considerably greater wave tube, in fact, being comparable to the noise achievable with three such traveling-wave tubes coupled in cascade.

It should be mentioned that electrons in the backwardly traveling beam gradually diverge radially outwardly as they travel toward the electron gun. This divergence results primarily from space charge forces in the two beams together with the initial divergent tendencies of the reflected electrons which leave the collector tubular portion 50. The backwardly traveling electrons eventually are intercepted by either the helix 18 (or perhaps the barrel 34) or the anode 20.

In order to insure stable operation and prevent spurious oscillations at discrete frequencies, the slow-wave circuit 18 should be carefully constructed to possess highly uniform characteristics and have a relatively fiat frequency response over its entire bandwith. Moreover, in order to prevent excessive gain at any particular frequency which might result in a discrete oscillation, the active circuit amplification length should be limited by affording one or more terminating regions such as that of the type provided by the attenuating coating 40.

It is further pointed out that for a particular slowwave circuit, as long as an appropriate collector potential and geometry is employed, the device of the present invention will operate in a noise-generating mode over a wide range of slow-wave circuit voltages without degrading the relatively fiat power spectrum and high efficiency achievable with the illustrated embodiments.

Thus, it should be understood that although the present invention has been shown and described with respect to particular embodiments, various changes and modifications obvious to a person skilled in the art to which the invention pertains are deemed to be within the spirit, scope and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. A noise generator comprising: cathode means for emitting a stream of electrons, means for focusing electrons in said stream into a well-collimated beam along a predetermined path, slow-wave structure means disposed along and about said path for propagating electromagnetic wave energy with a'phase velocity substantially less than the velocity of light, a first substantially tubular electrode disposed about said path between said cathode means and one end of said slow-Wave structure means, a second substantially tubular electrode disposed about said path adjacent the other end of said slow-wave structure means, means for maintaining said first electrode at a first predetermined positive potential relative to said cathode means,

means for maintaining said slow-wave structure means at a second predetermined positive potential relative to said cathode means, said second predetermined positive potential being in a range essentially between 80% and 98% of said first predetermined potential, means for maintaining said second electrode at a third predetermined positive potential relative to said cathode means, said third predetermined potential being in a range essentially between 5% and 30% of said second predetermined potential, and output means coupled to said slow-wave structure means in the vicinity of its said other end for obtaining electromagnetic wave noise energy therefrom.

2. A noise generator comprising: cathode means for emitting a stream of electrons, means for focusing electrons in said stream into a well-collimated beam along a predetermined path, slow-wave structure means disposed along and about said path for propagating electromagnetic wave energy with a phase velocity substantially less than the velocity of light, a first electrode disposed about said path between said cathode means and one end of said slow-wave structure means, a second electrode disposed about said path adjacent the other end of said slow-wave structure means, said second electrode defining a substantially cup-like portion having an open end facing said slow-wave structure means and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with respect to said slow-Wave structure means with said tubular lip portion being axially spaced from said other end of said slowwave structure means by a preselected small distance, means for maintaining said first electrode at a first predetermined positive potential relative to said cathode means, means for maintaining said slow-wave'structure means at a second predetermined positive potential relative to said cathode means slightly less than said first predetermined potential, means for maintaining said second electrode at a third predetermined positive potential relative to said cathode means substantially less than said second predetermined potential, and output means coupled to said slow-wave structure means in the vicinity of its said other end for obtaining electromagnetic wave noise energy therefrom.

3. A device for generating microwave noise energy with high efliciency comprising: electron gun means including a cathode for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined well-collimated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, an electrically conductive helix mounted within said envelope and coaxially disposed about said electron stream path, a substantially tubular anode disposed within said envelope about said electron stream path between said cathode and one end of said helix, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with a respect to said helix with said tubular lip portion being axially spaced from the other end of said helix by a preselected small distance, means for maintaining said anode at a first predetermined posi tive potential relative to said cathode, means for maintaining said helix at a second predetermined positive potential relative to said cathode slightly less than said first predetermined potential, means for maintaining said collector means at a third predetermined positive potential relative to said cathode substantially less than said second predetermined potential, and output means coupled to said other end of said helix for obtaining electromagnetic wave noise energy therefrom.

4. A device for generating microwave noise energy with high efficiency comprising: electron gun means for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined Well-colliminated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, an electrically conductive helix mounted within said envelope and coaxially disposed about said electron stream path, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at'said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with respect to said helix with said tubular lip portion having a diameter substantially the same as the diameter of said helix l l and being axially spaced from an end of said helix by a preselected small distance, a ceramic ring disposed about said electron stream path at said end of said helix. between said envelope and said collector means, said ring defining a radial bore hole, a tubular conductor attached to and extending radially outwardly from said ring in a region surrounding said bore hole, an electrically conductive rod coaxially disposed Within said tubular conductor and extending through said bore hole and being electrically connected to said end of said helix, and means for applying an electrical potential to said collector means relative to said helix to establish in the vicinity of said tubular lip portion and said end of said helix a region of electrical potential capable of reflecting a substantial portion of the electrons in said stream toward said electron gun means.

5. A device for generating microwave noise energy with high efliciency comprising: electron gun means including a cathode for lauching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined well-collimated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, an electrically conductive helix mounted within said envelope and coaxially disposed about said electron stream path, a substantially tubular anode disposed within said envelope about said electron stream path between said cathode and one end of said helix, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cuplike portion and said tubular lip portion being coaxially disposed with respect to said helix with said tubular lip portion having a diameter substantially the same as the diameter of said helix and being axially spaced from the other end of said helix by a preselected small distance, a ceramic ring disposed about said electron stream path at said other end of said helix between said envelope and said collector means, said ring defining a radial bore hole, a tubular conductor attached to and extending radially outwardly from said ring in a region surrounding said bore hole, an electrically conductive rod coaxially disposed within said tubular conductor and extending through said bore hole and being electrically connected to said other end of said helix, means for maintaining said anode at a first predetermined positive potential relative to said cathode, means for maintaining said helix at a second predetermined positive potential relative to said cathode slightly less than said first predetermined potential, and means for maintaining said collector means at a third predetermined positive potential relative to said cathode substantially less than said second predetermined potential.

6. A device for generating microwave noise energy with high efficiency comprising: electron gun means for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined Well-collimated path toward saidcollector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, a plurality of ceramic support rods mounted within said envelope and disposed parallel to said electron stream path, a coating of an attenuating material disposed on a portion of at least one of said rods at a predetermined axial location therealong, said portion extending at least 180 circumferentially around said rod and extending for a predetermined distance axially along said rod, an electrically conductive helix mounted on said support rods and coaxially disposed about said electron stream path, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with respect to said helix with said tubular lip portion being axially spaced from an end of said helix by a preselected small distance, means for applying an electrical potential to said collector means relative to said helix to establish in the vicinity of said tubular lip portion and said end of said helix a region of electrical potential capable of reflecting a substantial portion of the electrons in said stream toward said electron gun means, and output means coupled to said end of said helix for obtaining electromagnetic wave noise energy therefrom.

7. A device for generating microwave noise energy with high efiiciency comprising: electron gun means including a cathode for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined well-collimated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, a plurality of ceramic support rods mounted within said envelope and disposed parallel to said electron stream path, a coating of an attenuating material disposed on a portion of at least one of said rods at a predetermined axial location therealong, said portion extending at least circumferentially around said rod and extending for a predetermined distance axially along said rod, an electrically conductive helix mounted on said support rods and coaxially disposed about said electron stream path, a substantially tubular anode disposed within said envelope about said electron stream path between said cathode and one end of said helix, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with respect to said helix with said tubular lip portion being axially spaced from the other end of said helix by a preselected small distance, means for maintaining said anode at a first predetermined positive potential relative to said cathode, means for maintaining said helix at a second predetermined positive potential relative to said cathode slightly less than said first predetermined potential, means for ma ntaining said collector means at a third predetermined positive potential relative to said cathode substantially less than said second predetermined potential, and output means coupled to said other end of said helix for obtaining electromagnetic wave noise energy therefrom.

8. A device for generating microwave noise energy With high efficiency comprising: electron gun means for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined wellcollimated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, a plurality of ceramic support rods equally circumferentially mounted within said envelope and disposed parallel to said electron stream path, an electrically con ductive tape-like helix mounted on said support rods and coaxially disposed about said electron stream path, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly f the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with respect to said helix with said tubular lip portion having a diameter substantially the same as the diameter of said helix and being axially spaced from an end of said helix by a preselected small distance, a plurality of tubular sector-like electrically conductive elements equally circumferentially mounted within said envelope between respective pairs of circumferentially adjacent support rods in the vicinity of said end of said helix, at least a portion of each of said elements being axially tapered such that its portion of smallest radial thickness is axially remote from said collector means, means for applying an electrical potential to said collector means relative to said helix to establish in the vicinity of said tubular lip portion and said end of said helix a region of electrical potential capable of reflecting a substantial portion of the electrons in said stream toward said electron gun means, and output means coupled to said end of said helix for obtaining electromagnetic wave noise energy therefrom.

9. A device for generating microwave noise energy with high efficiency comprising: electron gun means including a cathode for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined well-collimated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, a plurality of ceramic support rods equally circumferentially mounted within said envelope and disposed parallel to said electron stream path, an electrically conductive tape-like helix mounted on said support rods and coaxially disposed about said electron stream path, a substantially tubular anode disposed within said envelope about said electron stream path between said cathode and one end of said helix, said collector means defining a substantially cuplike portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with respect to said hel x with said tubular lip portion having a diameter substantially the same as the diameter of said helix and being axially spaced from the other end of said helix by a preselected small distance, a plurality of tubular sectorlike electrically conductive elements equally circumferentially mounted within said envelope between respective pairs of circumferentially adjacent support rods in the vicinity of said other end of said helix, at least a portion of each of said elements being axially tapered such that its portion of smallest radial thickness is axially remote from said collector means,.means for maintaining said anode at a first predetermined positive potential relative to said cathode, means for maintaining said helix at a second predetermined positive potential relative to said cathode slightly less than said first predetermined potential, means for maintaining said collector means at a third predetermined positive potential relative to said cathode substantially less than said second predetermined potential, and output means coupled to said other end of said helix for obtaining electromagnetic wave noise energy therefrom.

10. A device for generating microwave noise energy with high efficiency comprising: electron gun means for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined wellcollimated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, a plurality of ceramic support rods equally circumferentially mounted within said envelope and disposed parallel to said electron stream path, an electrically conductive tape-like helix mounted on said support rods and coaxially disposed about said electron stream path, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cup-like portion and said tubular lip portion being coaxially disposed with respect to said helix with said tubular lip portion having .a diameter substantially the same as the diameter of said helix and being axially spaced from an end of said helix by a preselected small distance, a plurality of tubular sector-like electrically conductive elements equally circumferentially mounted within said envelope between respective pairs of circumferentially adjacent support rods in the vicinity of said end of said helix, at least a portion of each of said elements being axially tapered such that its portion of smallest radial thickness is axially remote from said collector means, one of said elements and an adjacent portion of said envelope defining aligned first radial bore holes, a cylindrical ceramic member disposed in said first radial bore holes, said member defining .a second radial bore hole, a tubular conductor attached to and extending radially outwardly from said ceramic member in a region surrounding said second bore hole, an electrically conductive rod coaxially disposed within said tubular conductor and extending through said second bore hole and being electrically connected to said end of said helix, and means for applying an electrical potential to said collector means relative to said helix to establish in the vicinity of said tubular lip portion and said end of said helix a region of electrical potential capable of reflecting a substantial portion of the electrons in said stream toward said electron gun means.

11. A device for generating microwave noise energy with high etficiency comprising: electron gun means including a cathode for launching a stream of electrons in a predetermined direction, collector means spaced from said electron gun means for intercepting a substantial portion of the electrons in said stream, focusing means for constraining said stream of electrons to flow in a predetermined well-collimated path toward said collector means, a tubular electrically conductive envelope coaxially disposed within said focusing means and about said electron stream path between said electron gun means and said collector means, a plurality of ceramic support rods mounted within said envelope and disposed parallel to said electron stream path, an electrically conductive tape-like helix mounted on said support rods and coaxially disposed about said electron stream path, a substantially tubular anode disposed Within said envelope about said electron stream path between said cathode and one end of said helix, said collector means defining a substantially cup-like portion having an open end facing said helix and further defining at said open end a tubular lip portion disposed radially inwardly of the lateral surface of said cup-like portion, said cuplike portion and said tubular lip portion being coaxially disposed with respect to said helix with said tubular lip portion having a diameter substantially the same as the diameter of said helix and being axially spaced from the other end of said helix by a preselected small distance, a plurality of tubular sector-like electrically conductive elements equally circumferentially mounted within said envelope between respective pairs of circumferentially adjacent support rods in the vicinity of said other end of said helix, at least a portion of each of said elements being axially tapered such that its portion of smallest radial thickness is axially remote from said collector means, one of said elements and an adjacent portion of said envelope defining aligned first radial bore holes, a cylindrical ceramic member disposed in said first radial bore holes, said member defining a second radial bore hole, a tubular conductor attached to and extending radially outwardly from said ceramic member in a region surrounding said second bore hole, an electrically conductive rod coaxially disposed within said tubular conductor and extending through said second bore hole and being electrically connected to said other end of said helix, means for maintaining said anode at a first predetermined positive potential relative to said cathode, means for maintaining said helix at a second predetermined positive potential relative to said cathode slightly less than said first predetermined potential, and means for maintaining said collector means at a third predetermined positive potential relative to said cathode substantially less than said second predetermined potential.

References (Iited UNITED STATES PATENTS FOREIGN PATENTS 1/ 1957 Great Britain.

ROY LAKE, Primary Examiner.

I. B. MULLINS, Assistant Examiner.