US 3171055 A
Description (OCR text may contain errors)
"WI-a Feb. Z3, 1965 R. HARRISON ETAI. 3,171,055
RIPPLE VELOCITY MxcgowAvE TUBE Filed sept. 9, 19Go O wel /9 INVENTORS RICHARD I. #ARR/50N GE/g/RD E. WEIBEL ATTORNEY United States Patent Oilce 3,171,055 Patented Feb. 23, 1965 Richard H. Harrison, Mineola, and Gerhard E. Weibei,
Manhasset, NFL, assigner-s to General Telephone and Electronics Laboratories, lne., a corporation of Delaware Filed Sept. 9, i960, Ser. N 551,949 3 Claims. (Cl. 315-541) This invention relates to microwave tubes and in particular to microwave tubes ofthe beam type.
Beam-type microwave tubes may be divided into two broad categories; those having an extended or slow-wave, microwave interacting structure and those not provided with such a structure. Slow-wave structures commonly used in beam type magnetron tubes include helices, folded lines, and periodically loaded wave guides. When attempts are made to design slow-wave structures for operation at wavelengths of a millimeter or less, serious mechanical and electrical diiiiculties arise. ln particular, the small dimensions of the slow-wave structure and the close mechanical tolerances required make fabrication dificult and expensive.
Microwave tubes which do not use a slow-wave structure in the amplifying section hereinafter described as circuitless tubes) have also been developed. These tubes include the ldystron, velocity-jump and resistance wall amplifiers. It should be noted that these circuitless tubes actually require input and output coupling structures which become increasingly inelhcient at higher frequencies. However, if the electronic amplification obtained in the circuitless section is large enough, the overall gain obtained is still substantial. Accordingly, we have invented an improved tube of the circuitless beam type having electrical and mechanical characteristics at high frequencies which are superior to those found in known tubes. It is an object of our invention to provide an irnproved microwave tube of the beam type having high gain at wavelengths of a millimeter or less.
Another object is to provide at microwave tube which may be employed either as an ampliiier or as an attenuator without decreasing beam current.
Still another object is to provide a microwave tube which may be adapted for use as an oscillator.
A further object is to provide a microwave tube which is relatively inexpensive to manufacture, simple in design and easy to fabricate.
In the present invention, there is provided a beam type microwave tube (termed a ripple velocity tube) in which the spatial velocity distribution of the electrons in the interaction region is caused to vary substantially sinusoidally along the length of the beam. In one embodiment of the ripple velocity tube, an electron source is located at one end of an evacuated envelope and a collector, maintained at a positive potential with respect to the electron source, s located at the other end. An input coupler mounted adjacent the electron source permits Velocity modulation of the beam in accordance with an applied input signal. An output coupler located adjacent the collector extracts energy from the beam and delivers it to an external circuit. In the interaction region between the input and output couplers, a plurality of Zn-i-l annular conductive electrodes are uniformly spaced along the beam, n being equal to any integer greater than zero. These electrodes comprise a iirst set of n+1 electrodes (including those immediately adjacent the input and output couplers) connected to a first direct voltage source and a second set of n electrodes interleaved between the iirst set connected to a second direct voltage source. If the tube is to function as an amplifier, the lrst set of electrodes is -made negative with respect to the collector While the second set of electrodes is made positive with respect to the collector. Alternatively, if the tube is used as an attenuator, the voltage applied to the rst set of electrodes is positive and the voltage applied to the second set of electrodes negative with respect to the collector. A magnetic eld is applied along the longitudinal axis of the envelope to focus the electron stream.
The operation of the ripple velocity tube may best be understood by rst considering the behavior of space charge waves in the Well known two-cavity klystron. The two-cavity klystron, like the present amplilier, includes an electron gun, a collector, and input and output couplers in the form of resonant cavities. However, in the klystron the beam is surrounded by a continuous metal cylindrical shield extending the length of the interaction region while in the ripple velocity tube the beam is surrounded by a plurality of annular electrodes.
In the klystron, A.C. velocity modulation is supplied to the electron beam at the signal frequency by means f the input cavity. As a result some of the electrons leaving the input cavity are moving faster than they would in the absence of an applied R-F field, while others are moving more slowly. As the electrons move down the drift region, their position and velocity are iniluenced by two space-charge waves traveling in the beam. One of those waves travels faster than the beam while the other travels more slowly. They are deiined as fast and slow waves although their velocities differ only slightly.
The magnitude of the current density of the beam at any point in the interaction region is determined by the sum of the magnitudes of the fast and slow waves at that point. Similarly, the amplitude of the velocity modulation of the beam at any point in the interaction region is determined by the difference between the magnitudes of the fast and slow Waves at that point. The magnitude of the current density is zero at the output ofthe input coupler, increases sinusoidally to a maximum value, and i'alls again to Zero a half plasma wavelength down the tube. This cycle is repeated each half Wavelength, the magnitude of the current density at corresponding points in each spatial cycle remaining the same. Similarly, the velocity density varies along a sinusoidally shaped curve of constant amplitude, the velocity density being a maximum at the input coupler and essentially zero at the output coupler. Since the peak amplitude of the current density is the same in every plasma wavelength of the interaction region, the gain is not increased in this portion of the tube.
In the vripple-velocity amplier, on the other hand, the peak spatial current density and velocity modulation increases exponentially along the length of the interaction region providing greatly increased gain. This added gain is obtained by replacing the cylindrical shield found in the two-cavity klystron by the series of energized conductive annular electrodes. In the ripple-velocity tube the drift velocity of the electrons is first made to decrease slightly by the negative (with respect to the collector) voltage on the rst annular electrode. The drift or DC. velocity is then increased by the positive voltage (with respect to the collector) on the next electrode to a speed somewhat greater than the uniform drift velocity which would be experienced in the absence of the ripple structure. By properly selecting the ratio of the electrode width to the electrode spacing, the velocity of the electrons may be caused -to vary sinusoidally as they proceed down the interaction region.
If the electron beam is A.C. velocity-modulated at the signal frequency by an input coupler, and the electron drift velocity is made to vary sinusoidally along the beam by a series of D.C. energized electrodes as just described, the current and velocity modulation in the drift region can no longer be described as a superposition of fast and slow waves. Instead it is found that exciting a fast wave also gives rise to a slow wave of smaller amplitude. Similarly, exciting a slow wave produces a secondary fast wave of smaller magnitude, Thus, as a result of the non-uniform drift velocity, there is a coupling between the fast and slow space-charge Waves creating other Wave components. rThese waves combine so as to cause the current and velocity modulation to increase exponentially from relatively low values at the input coupler to high values at the output coupler.
The increase in the current and velocity modulation produced by the ripple structure described becomes somewhat more evident when the equations governing the interaction of an electron beam and an R-F field are examined. It is shown in chapter 8 of the book Beam and Wave Electronics in Microwave Tubes by R. G. Hutter, published by D. Van Nostrand Company, lne., that space charge waves on an electron beam with an arbitrarily varying drift velocity can be described by the following equation:
J,=current density modulation u=velocity of the electrons in the interaction region w=A.-C. signal frequency in radians per second ^r=D.-C. transit time in the interaction region l/m=ratio of charge to mass of an electron J0=D.C. current density e0=the dielectric constant of the medium Jm :the current in the circuit (this value is zero in the case ofthe circuitless tubes under consideration) 1n this equation x a-2-7-rz Ap 7tp=the plasma wavelength z=axial distance along the interaction region measured from the input coupler =the ratio of the amplitude of the spatial sinusoidal component of the DC. electron velocity in the interaction region to its average velocity As is well known this equation is obtained in the analysis of many electrical and mechanical systems and is extensively treated in the literature. The most remarkable feature of the solutions to this differential equation is that, under certain circumstances, they can exhibit eX- ponential growth or decay. This physically means that a system with periodic perturbances of the kind mentioned can exhibit growing or decaying oscillations. For the ripple velocity tube it can be shown that exponentially growing solutions, which are necessary for high electronic gain, are obtained if the ripple velocity period is half a plasma wavelength. Assuming that the input cavity gives the beam a non-zero initial A C. velocity modulation and a zero initial A.C. current or charge density modulation, the solution of Equation 2 yields a current density modulation at any point along the interaction region and a velocity density modulation (2) jo)Joe cos M 4 e where K is a constant.
Thus, as shown by Equations 3 and 4, the successive peaks of current and velocity modulation grow as the tube is lengthened. With the output cavity located at a point of peak A.C. current modulation, the ratio 0f the gain of the ripple amplifierl to that of the two cavity klystron is From Equation 5 it is clear that the gain of the ripple velocity amplifier may be increased by increasing the tube length (by using larger values of n) or by increasing the amplitude of the ripple voltage (resulting in larger values of The above objects of and the brief introduction to the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following description in connection with the drawings, wherein FIG. 1 is a schematic cross sectional diagram of the present invention;
FlG. 2 is a graph depicting the D.C. velocity of the electrons in the interaction region; and
FIG. 3 is a plot of the current and velocity density modulation in the interaction region.
Referring to FIG. l, there is shown a ripple velocity microwave amplifier structure enclosed within an evacuated envelope tl. An electron beam generated by an electron gun` 11 travels through a reentrant cavity 12, an interaction region 13, a reentrant cavity 14, and nally impinges upon a collector 15 maintained at a positive potential with respectto the electron gun by a voltage source 16. Reentrant cavities 12 and 14 act as input and output couplers respectively and are maintained at the collector potential. It should be noted that cavity couplers are shown for purposes of illustration only and could be replaced by any one of a number of suitable coupling structures. A longitudinal magnetic focussing field is provided by a magnet 19 surrounding the tube.
ln the interaction region 13, a plurality of annular conductive electrodes are uniformly spaced along the longitudinal axis 25 of the tube. A total of 2n+1 annular electrodes are provided, Where n equals 1, 2, 3, A first set, consisting of 1z+l electrodes, is maintained at a negative potential with respect to the collector by voltage source 26 while a second set, comprising the remaming n electrodes, is held at a positive potential with respect to the collector by a voltage source 27. As
shown, each of the electrodes in the second set is interposed between two of the electrodes comprising the first set. When the ripple tube is used as an amplilier (as in FIG. l), the electrodes 28 and 29 immediately adjacent input and output couplers 12 and 14 respectively are kept at a negative potential relative to the collector. When the tube is employed as an attenuator, the polarity of voltage sources 25 and 27 is reversed and electrodes 23 and 29 are made positive with respect to the collector.
The electron beam emitted by gun 1l is velocity modulated by a signal applied to the input cavity 12. As a result of this modulation some of the electrons in the beam are caused to speed up while others are retarded. Thus, bunches of electrons are formed which drift through the interaction region at an average D.C. velocity um). When these bunches enter the interaction region, they are initially slowed as a result of the lower potential on electrode 28, then accelerated by the higher potential on electrode 30. As shown in FG. 2, the velocity or" the electrons have a sinusoidal spatial variation about the average value un., over the entire length of the interaction region 13. In order to obtain a sinusoidal electron velocity distribution, the static potential along longitudinal axis 25 should also be substantially sinusoidal for moderate velocity variations. This distribution is obtained by selecting values for the width of the electrodes and the spacing between them which minimize the harmonic content in the axial potential. For example, if the width of each electrode is equal to one-half the distance between it and the adjacent electrode, all harmonics up to the rth will be zero. The period ofthe axial potential (and therefore the period of the electron velocity) as measured along the z axis is equal to one-half the plasma wavelength.
In order to maximize the gain, the output coupler 14 is located at a current modulation peak and couplers l2 and 14 are separated by an odd number of quarter plasma wavelengths. When reentrant cavities are used, the output of the input coupler 12 and the input of the output coupler 14 are each spaced one eighth of a plasma wavelength from the center of the adjacent electrodes 2S and 29 respectively. When other types of couplers are employed, this spacing will vary in accordance with the characteristics of the particular coupler. Energy from the beam is extracted by coupler 14 and applied to an external load (not shown).
The magnitudes of the current and velocity modulation are depicted in FIG. 3. As shown, the current modulation Jo is zero at the input coupler 12, increases along a sinusoidally shaped curve, and is zero a half plasma wavelength further along the tube. This cycle is repeated along the length or the tube, the amplitude of the current modulation increasing exponentially every half wavelength. ln the same way, the velocity modulation Vw increases exponentially every half wavelength falling to zero at the current modulation peaks.
A significant feature of this invention is that the input and output couplers may be arranged relative to the ripple velocity structure so that either a growing or a decaying wave may be excited. Specifically, if the electrode potentials are arranged so that the DC. electron velocity in the half ripple period just after the input coupler (in the direction of electron travel) and just before the output coupler is lower than the average DC. velocity in the interaction region, the tube will exhibit electronic gain. On the other hand, if the D.C. electron velocity in the half ripple period just after the input coupler and just before the output coupler is higher than the average D.C. electron velocity in the ripple velocity region, the tube will function as an electronic attenuator. Thus, by merely reversing the polarities of voltage sources 26 and 27 the tube can be made to operate either as an amplifier or as an attenuator. The tube may also be employed as an Oscillator by connecting voltage sources 25 and 27 for operation as an ampliiier and coupling an external feedback connection between the input and output cavities.
It should alsobe noted that, in the above explanation, it has been assumed that the ripple velocities are small when compared with the D.C. average velocity. For large ripple velocities the ripple velocity structure must be modified since, in high velocity portions of the interaction region, the plasma wavelength becomes longer than in the low velocity portions of this region. Thus, for large ripple velocities, the successive annular electrodes in the interaction region must be unequally spaced in such a manner as to avoid distorting the sinusoidal velocity variations.
As many changes could be made in the above construction and many different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
l. A microwave tube comprising an evacuated envelope having a longitudinal axis, means for producing a magnetic iield along said axis, an electron source mounted at one end of said envelope, said electron source projecting an electron beam along said longitudinal axis, a collector mounted at the other end of said envelope, input coupling means mounted adjacent said electron source, output coupling means mounted adjacent said collector, a first set of n+1 electrodes spaced along said electron beam in the interaction region between said input and output couplers, n being Iany integer except zero, a second set of n electrodes spaced along said electron beam in the interaction region between said input and output couplers, each of the electrodes in said second set being located between two electrodes of said first set, the ratio of the width of each of said electrodes to the distance between adjacent electrodes being substantially equal to one half, means for coupling iirst and second voltage sources between said iirst and second sets of electrodes respectively and said electron source, the spacing of said electrodes and the magnitudes of said iirst and second voltage sources being adapted to cause the velocity of the electrons in the electron beam to vary substantially sinusoidally along the portion of said beam situated between said input and output coupling means.
2. A microwave tube comprising an electron source for generating an electron beam; means for producing a magnetic elld along said beam; input coupling means located adjacent said electron source; output coupling means; a irst set of conductive electrodes surrounding said beam in the region between said input and output coupling means; means connecting the electrodes in said irst set electrically together, said first set of electrodes being maintained at a rst common positive voltage with respect to said electron source; a second set of conductive electrodes surrounding said beam in the region between said input and output coupling, each of the electrodes in said second set being located between two electrodes of said rst set, the ratio of the width of each of said electrodes to the distance between adjacent electrodes being substantially equal to one half; and means connecting the electrodes in said second set electrically together, said second set of electrodes being maintained at a second common positive voltage with respect to said electron source, the electric iield produced by said iirst and second sets ot electrodes causing ,the velocity of the electrons in said beam to vary substantially sinusoidally along the portion of ,the beam between said input and output coupling means.
3. A microwave tube comprising yan evacuated envelope having a longitudinal axis; means for producing a magnetic iield along said axis, an electron source mounted at one end of said envelope; input coupling means mounted adjacent said electron source; output coupling means located at the other end of said envelope; rst and second sets of conductive rings situated along said longitudi nal axis between said input and output coupling means,
each conductive ring of said second set being positioned between two conductive rings of said iirst set, the conductive rings of said rst and second sets being spaced from the inner Wall of said envelope and having equal inner diameters and equal lengths, the ratio of the width of each of said electrodes ,to the distance between adjacent electrodes being substantially equal to one half; first voltage means coupled between said rst set of conductive rings and said electron source for maintaining said first set of conductive rings at a rst common positive potenltial with respect to said electron source; and second voltage means coupled between said `second set of conductive rings and said electron source for maintaining said second set of conductive rings at a second common positive References Cited by the Examiner UNITED STATES PATENTS 2,190,511 2/40 Cage 315-551 2,888,596 5/59 Rudenberg 315-35 3,012,170 12/61 Heil S15-5.51 X
GEORGE N, WESTBY, Primary Examiner.
RALPH G. NILSON, ROY LAKE, Examiners.