Abstract:
Some electrons reflected from the collector of a klystron form a beam current flowing back toward the input end of the tube. This beam is modulated and can carry a regenerate signal which distorts the tube&#39;s performance when amplifying a television signal. The reflected electrons are removed by a spiralling transverse magnetic field having a pitch equal to the cyclotron wavelength in the axial magnetic field used to focus the beam. The rotative sense of the spiral is such that forward-going beam electrons are not affected but returning electrons are accelerated in their cyclotron orbits until they are driven outside the beam and are collected.

Description:
FIELD OF THE INVENTION 
     The invention pertains to linear beam electron tubes used to amplify microwaves, particularly waves having amplitude-modulated signals such as television video signals. Klystrons are widely used for this purpose. The invention may also be incorporated in traveling-wave tubes. 
     A problem which has long bothered television transmitter klystrons has been identified as caused by electrons returning from the collector backward along the beam path toward the electron gun. The harmful electrons travel with approximately the velocity of the original beam. They are called either &#34;reflected electrons&#34; or &#34;high speed secondary electrons&#34;. 
     In passing through the klystron cavities, the stream of returning electrons is velocity modulated by the cavity voltages and thereby bunched by the klystron mechanism to form a beam with modulated current density. This secondary radio-frequency current passing through the input (or other upstream) cavity induces voltage in the cavity exactly the same as modulated primary beam current, since the klystron cavity is completely bi-directional. The final effect is signal regeneration--highly non-linear in amplitude and phase. 
     Two undesirable effects are produced by such regeneration: 
     (1) Wiggles in the amplitude transfer characteristic which are manifested as brightness discontinuities in the picture; 
     (2) A phenomenon known as &#34;sync pulse ringing&#34;. 
     The latter phenomenon may be explained as follows. At the end of each scan line (and frame), a sharp synchronizing pulse is transmitted at an amplitude near the peak saturation output of the transmitter. This pulse has very fast rise and fall time, limited only by the transmitter bandwidth. The gain of the klystron varies during the rise and fall due to the delay in build-up or falloff of voltages in the cavity as a result of their high Qs. When regeneration is added, the voltages can overshoot their equilibrium values, creating a ringing after the rise or fall of the pulse. 
     PRIOR ART 
     Several schemes have been tried to prevent such signal regeneration by reducing the number of backstreaming electrons. One scheme depends on the fact that the percentage yield of high speed secondary electrons from a bombarded surface is an increasing function of atomic number. Thus the collector surface is coated with a material of low atomic number. Carbon is effective, but greatly increases the time required to de-gas the tube. U.S. Pat. No. 4,233,539 issued Nov. 11, 1980 to Louis R. Falce and assigned to the assignee of this application, describes an improved aluminum boride coating which is much easier to outgas. 
     Another prior-art scheme is to modify the geometry of the collector to reduce the probability of secondary electrons re-entering the drift tube. U.S. Pat. No. 3,936,695 issued Apr. 26, 1974 to Robert C. Schmidt and assigned to the assignee of this application, describes a series of baffles inside the collector designed to permit passage of the entering beam, but intercept some of the secondaries. 
     Still another scheme is described in U.S. Pat. No. 3,806,755 issued Apr. 23, 1974 to E. L. Lien and M. E. Levin and also assigned to the assignee of this application. Its purpose is to statistically reduce the fraction of reflected electrons re-entering the collector entrance aperture by removing the bombarded surface as far as possible from the aperture. 
     All of the above-mentioned schemes have proven to help reduce regeneration. Each of them, however, only reduces the number of backstreaming electrons, and does not eliminate them. 
     Several attempts have been made to eliminate backstreaming electrons by magnetic fields transverse to the beam axis. Because magnetic fields deflect moving charges in accord with the &#34;handedness&#34; rule, returning electrons would be deflected in a direction opposite to that direction in which the forward beam would be deflected. Therefore, in principle the returning electrons could be separated from the forward beam and collected. None of these schemes has had any commercial success, due to high cost and to difficulties associated with the asymetric geometry and non-uniform collector dissipation characteristic of these schemes. 
     Of course, many other examples of more sophisticated schemes utilizing the interaction of magnetic field with an electron beam can be found in the prior art, but they have been directed to other purposes, and have not been of any help regarding the backstreaming electron problem. For example, U.S. Pat. No. 3,398,376 to Hirshfield describes an electron cyclotron maser which generates and amplifies electromagnetic radiation in the microwave and millimeter wave bands. Such generation and amplification is achieved by subjecting a beam of electrons immersed in a longitudinal magnetic field to the action of a corkscrew magnetic or electric field to impart a spiral trajectory, and with the spiralling beam then passing through a cavity having a mode frequency equal to the cyclotron frequency of the spiralling electrons. The action of corkscrew field increases the transverse velocity of the electron beam at the expense of its axial velocity, making possible interaction with the transverse fields in the cavity. Again, however, such schemes have not provided a solution for the backstreaming electron problem. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a linear-beam tube having negligible regeneration. 
     A further object is to provide a tube having uniform collector dissipation. 
     A further object is to provide a tube which is cheap to manufacture. 
     These objects are achieved by incorporating along the beam path a direction-sorting trap for electrons. A periodic transverse magnetic field rotates with distance opposite to the sense in which the forward-traveling beam electrons rotate in the axial uniform field used for focusing the beam. The time average of the periodic forces on forward electrons is zero. The period of the transverse field is about equal to the cyclotron wavelength. Returning electrons see the sense of rotation of the transverse field to be the same as their cyclotron rotation, so they are accelerated to larger cyclotron orbits and eventually strike the drift tube and are collected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic partial section of a klystron embodying the invention. 
     FIG. 2A is a diagram of the magnetic deflection of an electron in the primary beam. 
     FIG. 2B is a diagram of the magnetic deflection of a reflected electron. 
     FIG. 3 is a section of an alternative embodiment. 
     FIG. 4a and FIG. 4b are a side view and a section view of another embodiment of opposed pairs of discrete magnets arrayed along drift tube 20. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a klystron embodying the invention. Klystrons are widely used as amplifiers in UHF television transmitters. The invention should find its greatest utility in klystrons which suffer from regeneration by backstreaming electrons. Backstreaming also occurs in traveling-wave tubes. The regeneration is less in TWTs because the reflected beam, traveling opposite to the primary rf circuit wave, is not synchronous with it and hence, will be modulated to a much lesser extent than is the case in klystrons. Nevertheless, the invention can produce some improvement in TWT performance. 
     In FIG. 1 a beam of electrons 10 is drawn from a thermionic cathode 12 by a positive potential on a hollow anode 14. Cathode 12 is heated by radiation from a resistive heater 16. Beam 10 is focused by a focusing electrode 18 to a small diameter to pass thru a long, hollow drift tube 20. Along the length of drift tube 20, beam 10 is kept focused in a pencil shape by the uniform axial field of a solenoid magnet coil 22. The flux return path is provided by a surrounding iron shell 24. After transit of drift tube 20, beam 10 leaves the magnetic field, spreads out and is collected in a hollow collector 26. 
     Spaced along drift tube 20 are a number of resonant interaction cavities having gaps 30 which are crossed by beam 10. These cavities include an input cavity 32 having a coupling loop 34 for introducing an input microwave signal, an uncoupled cascade cavity 36 and an output cavity 38 having an output loop 40 to extract radio-frequency power. The cavities support the microwave signal in energy-exchanging relationship with the electron beam, with the beam undergoing linear velocity modulation in passing through the successive cavities as is well understood in the art. Of course, klystron cavities are not the only circuit means which can enable such linear velocity modulation; the slow-wave structures of traveling wave tubes are another typical example. 
     A portion of drift tube 20 between input cavity 32 and output cavity 38 is used for the inventive reflected-electron trap 42. Trap 42 comprises means for producing a periodic magnetic field transverse to the axis of beam 10, the periodicity being such that the direction of the transverse field rotates with distance along the beam. The pitch of rotation is equal to the axial distance an electron travels in one cyclotron period. In FIG. 1 this spiralling transverse magnetic field is produced by a bifilar pair of conductive helices 44, 46 wrapped around but insulated from an extended portion of drift tube 48. Helices 44, 45 are fed direct current in opposing rotational sense as shown by the arrows at the ends of the helices. The magnetic field of these currents traveling through the helices is mainly transverse to the axis of beam 10, and rotates with the pitch of helices 44, 46. 
     FIG. 2 illustrate the operation of the periodic magnetic field. They represent cross-sections taken at successive transverse planes labeled 0, 1/4, 1/2, 3/4 and 1, across drift tube 48 in FIG. 1, the fractions referring to the fractions of a cycle of rotation of helices 44, 46. The arrows 50, 52, into and out of the plane of the paper, indicate the angular position of helices 44, 46 and the direction of direct current in them. The vector B P  indicates the direction of the principal component of the spiralling transverse magnetic field. The vector F indicates the direction of the induced magnetic force on a forward electron 54 (represented by a small circle) as its axial motion into the paper cuts the transverse field B P . The dashed arc 56 indicates the cyclotron trajectory of forward electron 54 in the axial magnetic field B O , which is directed into the paper, and which is provided by solenoid 22. 
     FIG. 2A represents the forces on and motions of a forward electron 54 moving downstream from cathode to collector. At plane 0 the transverse field force is downward, tending to accelerate electron 54 in its clockwise cyclotron orbit. At plate 1/4, force F is to the right, opposing the cyclotron motion and decelerating it. At plane 1/2 the force is again accelerating the cyclotron motion, and at plane 3/4 again decelerating the cyclotron motion. At plane 1,the conditions are again the same as at plane 0. Thus for an electron of the primary beam, the transverse magnetic field has no net effect, since electron 54 has been accelerated half the time and decelerated the other half, averaging to zero then for a forward electron its normal cyclotron orbit under the influence of the axial magnetic field remains virtually unchanged. 
     FIG. 2B illustrates the forces and motions of a reflected electron 58, whose axial motion is out of the plane of the paper. Its cyclotron motion under axial field B O  will be in the opposite rotational sense to that of a forward electron 54, and is represented by lashed arc 56&#39;. At plane 0, force F is upward, accelerating reflected electron 58 in its cyclotron orbit. At plane 1/4, reflected electron 58 has completed 1/4 of a cyclotron orbit and the transverse field B P  has rotated the same amount, so force F is again accelerating the cyclotron motion. This condition continues through the entire orbit if the axial pitch of the transverse field rotation is approximately equal to the axial distance an electron travels during one cyclotron orbital period. As reflected electron 58 is continually accelerated, the diameter of its cyclotron orbit 56&#39; becomes even larger. Eventually it strikes the wall of drift tube 20 and is removed from the backstreaming beam. The principle is analogous to that seen at the first stage of the device of the Hirshfield patent referred to above, in which the transverse velocity of the electron beam is also increased at the expense of the axial velocity. But here, an electron filter or trap is provided, not amplification. 
     Since the electron trap 42 is essentially axially symmetrical as was seen above in the FIGS. 2 explanations, there is no net displacement of forward beam 10 from its axial symmetry. Thus, no forward electrons are collected, and the distribution of primary beam current reaching the collector is still axially symmetrical. This eliminates some of the problems of non-uniform dissipation encountered in prior-art traps which used lateral deflection of the whole beam. 
     FIG. 3 is an axial section of a slightly different embodiment wherein the spiralling transverse magnetic field is produced by a pair of permanent magnets 60, 62 spiralling longitudinally around drift tube 20&#39;. They are radially magnetized in opposite direction, so that at any given axial cross-section, their magnetizations are in the same direction, as shown. 
     FIGS. 4A and 4B are respectively a side view and a section perpendicular to the axis of another embodiment. Here, instead of the expensive long spiral magnets of FIG. 3, opposed pairs of discrete magnets 64, 66 are arrayed successively along drift tube 20&#34;. For each such approved pair, for example magnets 64 and 66, the magnetization is in the same direction (as in FIG. 3). The successive opposed pairs rotate in their orientation with distance along the axis, with a pitch as defined above. In the illustrated embodiments, the pairs are shown as spaced by 1/4 the pitch and rotated by 90° from the preceding pair. This is not a requirement. Any integral number of pairs greater than one could be used to make one axial pitch. 
     It will be obvious to those skilled in the art that the invention might be embodied in a variety of other forms. Other velocity-modulated linear-beam tubes other than those above discussed can benefit from the invention. Indeed, this invention is also applicable in other vacuum tube applications, including density-modulated electron-beam tubes, CRTs, and for ion-trap applications. The described embodiments are exemplary and not limiting. The invention is to be limited only by the following claims and their legal equivalents.