US3271618A

US3271618A - Depressed collectors for crossed field travelling wave tubes

Info

Publication number: US3271618A
Application number: US319606A
Authority: US
Inventors: Gerald P Kooyers
Original assignee: Litton Precision Products Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1963-10-28
Filing date: 1963-10-28
Publication date: 1966-09-06
Anticipated expiration: 1983-09-06

Description

Sept 6, 1966 G. P. KooYERs 3,271,618

DEPRESSED COLLECTORS FOR CROSSED FIELD TRAVELLING WAVE TUBES Sept 6, w66 G. P. KooYERs 3,271,618

DEPRESSED COLLECTORS FOR CROSSED FELD TRAVELLING WAVE TUBES Original Filed Nov. 50, 1962 3 Sheets-Sheet 2 arf/ef- Sept. 6, 966 G. P. KOOYERS 3,271,618

DEPRESSED COLLECTORS FOR CROSSED FIELD TRAVELLING WAVE TUBES Original Filed Nov. 30, 1962 l 1 I l l I 5 Sheets-Sheet 5 United States Patent O 3,271,618 DEPRESSED COLLECTURS FUR CROSSED FIELD TRAVELLENG WAVE TUBES Gerald l. Kooyers, Santa Clara, Calif., assigner to Litton Precision Products, Inc., a corporation of Delaware Continuation of application Ser. No. 241,396, Nov. 30,

1962. This application Oct. 28, 1963, Ser. No. 319,606

13 Claims. (Cl. S15-39.3)

This applica-tion is a continuation of my application Serial No. 241,396, tiled November 30, 1962, now abandcned.

This invention relates to electron discharge devices and more particularly to an improved electron collection arrangement in a crossed ield travelling wave electron discharge device.

There is a class of electron discharge devices in which a travelling electromagnetic wave is propagated along a slow wave structure such that a linear component of velocity of the wave is in the lrange of speeds which can readily be obtained by electrons from relatively simple electron guns. In such devices an electron beam is projected along or through the slow Wave structure and the individual electrons in the beam interact with the travelling wave to provide a net transfer of energy trom the electr-on beam to the travelling electromagnetic wave.

Such devices fall into two classes, these being the linear beam, or O-type devices, and the crossed held, or M-type devices.

In the linear beam, or O-type device, .a conventional electron gun accelerates electrons from the cathode and the resultant electron beam is projected along a slow wave structure. The classical slow wave structure for use in such devices is a helix, and the electron beam is conventionally projected along the axis of the helix. The accelerator electrode of the electric gun is usually maintained at ground potential, `as is the slow wave structure, and the cathode is maintained at a relatively high negative voltage with respect to ground. No magnetic iield is involved in such devices except as may .be used to maintain the electron beam in focus, and then the focusing magnetic iield plays no part in the interaction between the electron beam and any wave travelling on the slow wave structure.

At this point it is noted that the unfortunate arbitrary selection of electricity polarity at a time when little was known about the nature of electricity, results in much confusion of terminology in reference to voltages, potential, and charges. For present purposes, any electrical current is, in fact, to be understood as the unidirectional mo-tion of negatively charged electrons and that in electronic devices such motion of electrons occurs from negative t-oward more positive electrodes. As is well known to those skilled in the ar-t, an electron has greater potential energy when it is at a location in an electrostatic eld which is at a greater negative voltage and thus throughout the present specification the conventional designation of voltage polarity is utilized, but any references to electrons being at relatively higher or relatively lower potential refers to electrons being at iield locations which are at relatively Imore negative Ior relatively more positive voltages, respectively.

`Continuing now the description of an O-type travelling wave tube, the electrons are accelerated from a region of high potential, the cathode, past the accelerating anode and thus their entire energy is kinetic energy, since, as was previously stated, the anode and slow wave structure are being maintained at ground potential. The beam of electr-ons interacts with `an electromagnetic wave propagating on the slow wave structure and there is a net overall decrease in the -average velocity of the electrons ice and thus a net overall decrease in the kinetic energy of the electron beam, This energy is transferred to the electromagnetic wave, and, by proper choice of design parameters, the device lmay `function either as an amplilier to increase the strength of an applied electromagnetic wave or as an oscillator tube, i.e. operates as a source of electrom-agnetic wave energy.

When the electron beam has traversed the entire slow wave structure and has left the region of interaction with the electromagnetic wave it must then be collected and returned to the cathode of the electron gun. The conventional manner of collecting the electron beam in an O-type device is to position a c-ollector electrode, which also is maintained at ground potential, directly across the bath of the electron beam as it leaves the interaction region. The electrons impinging on the collector electrode are then returned through the power source to the cathode of the electron gun.

It was observed quite early in the development of such devices that the collector electrode became heated to high temperature by the impinging electron beam and it was frequently found necessary to provide some means for 'cooling the collector to prevent it from being melted by the impin-ging electron beam. Of course, `any such heating of the collector electrode, whether cooled or not, results in a decrease in the overall efliciency of the device, since the thermal energy represented by this heating, as supplied from the power source of the device, constitute an energy loss.

As the design of such devices became more sophisticated, it was recognized that this collector electrode heating could be reduced, and thus the overall eciency of the devices increased, by maintaining the collector electrode at some voltage intermediate the cathode voltage and the slow wave structure voltage. The potential of this collector electrode frequent-ly termed a depressed collector, is then higher than that of the slow wave structure and the accelerating electrode of the electron gun, but less than that of the -cathode of the electron gun. The electrons in the beam, upon leaving the interaction region, then approach the collector electrode by motion against a potential gradient and the electrons must give up kinetic energy by 4slowing down in order to approach or reach the collector, since the law of conservation of energy requires that the sum of the potential and kinetic energy of each electron remains constant. The electrons are thus collected at lower velocities and there is less heating of the collector electrode.

Ideally, the collector electrode would be maintained at such a potential that the electrons can just reach it and thus impinge upon it at zero velocity. However, since the electrons have delivered different amounts of energy to the electromagnetic wave on the slow wave structure and thus leave the interaction region with differing velocities, the collector electrode must be maintained at Ia potential no higher than that which sets up a field gradient such that the work to be done by the electron when moving against the gradient can be delivered by the kinetic energy of the slowest electron leaving the interaction region so that even this slowest electron can be collected, and it is not repelled. Any electrons faster than the slowest electron impinge upon the collector with a finite velocity and thus finite kinetic energy, and this kinetic energy is then lost as heat in the collector electrode.

A crossed eld, or M-type, travelling wave electron discharge device differs in several important respects from the linear beam, or O-type device, which was just described. In the crossed eld device, there is spaced from the slow wave structure, which may conventionally take the form of an interdigital delay line, a sole electrode, which is usually everywhere at an equal distance from .a the slow wave structure. Thus, the sole electrode may be parallel to the slow wave structure in a physically linear tube, or may be concentric with the slow wave structure in a physically circular tube. An electrostatic field is established between the sole electrode and the slow Wave structure. Conventionally, the slow Wave structure is maintained at ground potential and a relatively high negative voltage is applied to the sole electrode, thereby impressing on it a relatively high potential in the meaning of the term as defined above. A magnetic field is provided which is transverse to the electrostatic field throughout the interaction region between the slow wave structure and the sole electrode.

An electron gun is positioned at one end of the interaction lregion to inject electrons into the interaction Vregion in a direction substantially parallel to the slow wave structure and the sole electrode. Conventionally, the cathode of the electron gun is maintained at a potential intermediate that of the slow wave structure and that of the sole electrode.

As is well known to those skilled in the art, such a crossed electrostatic-magnetic field arrangement provides for velocity sorting of any electrons traversing the region, in which electrons having an initial velocity of E/B, where E represents the intensity of the electrostatic field and B represents the intensity of the magnetic field, continue down the interaction region parallel to the sole electrode and the slow wave structure, and in which the electrons having a velocity slower than E/B are drawn by the electrostatic field towards the slow wave structure and the electrons having a velocity faster than E/B are defiected by the magnetic field towards the sole electrode. In practice, the electron guns used in such devices are designed to supply electrons having as near this velocity as possible so that a maximum number of electrons traverses the length of the interaction region when there is no interaction between the electrons and any electromagnetic wave being propagated on the slow wave structure.

When the electron beam enters the interaction region, the individual electrons of the beam have a substantially uniform kinetic energy, which is a function of the acclerating field in the electron gun, and a potential energy which is a function of the electrostatic potential of the location in which the beam is injected into the interaction region. When an electromagnetic Wave having a phase velocity, i.e. a velocity component in the direction of the beam, which is substantially equal to the velocity of the electron beam, is being propagated on the slow wave structure, an injected electron may enter the interaction region at a time at which the electrical field component of the wave tends either to draw the electron towards the slow wave structure or to force the electron nearer the sole electrode.

Since the slow wave structure is being maintained at ground potential, as was previously described, those electrons which are attracted closer to the slow wave structure find themselves in a region of lower potential, and give up a portion of their energy to the electromagnetic wave. Some electrons actually strike the slow wave structure, thus being collected, and give all their potential energy to the wave. This energy transfer may be viewed in either of two ways. In the first of these, the crossed fields may be thought to maintain the electrons at a uniform velocity and as the electron is drawn by the Wave into a region of lower potential, that portion of the potential energy of the electron greater than that of the -region of lower potential into which it enters is delivered to the electromagnetic wave. In the second of these, the electron may be considered to interact with the travelling wave and deliver a portion of its kinetic energy to the wave. This results in a decrease of velocity of the electron and the slowed electron is drawn by the electrostatic field closer to the slow wave structure. In so doing the electron falls to a region of lower potential; thus it is immediately accelerated by the electrostatic field back to its initial velocity, but is now in a region of lower potential.

In either event it is seen that the net result is that the electron delivers potential energy to the electromagnetic wave and that in a crossed field device energy is delivered to the travelling wave in a fundamentally different manner than in the linear beam, or O-type device.

Conversely, those electrons which enter the interaction region in a region of an electric field which tends to force the electrons away from the slow wave structure and towards the sole electrode are driven into an area of higher potential. These electrons must extract energy from the travelling wave in order that they may take on a greater potential energy themselves. Those electrons which are attracted towards the slow wave structure, thereby delivering energy to the travelling wave, are termed favorably focused electrons; while those which are attracted towards the sole electrode, thereby extracting energy from the travelling wave, are termed unfavorably focused electrons. The `favorably focused electrons tend to be drawn more and more into the electromagnetic wave and thus continue to deliver their potential energy to the wave, while the unfavorably focused electrons are driven away from the electromagnetic wave and tend to take less and less energy from the wave as they approach the sole electrode. There is thus a net transfer of energy from the electron beam to the electromagnetic wave. By proper choice of design parameters, crossed field devices also can be designed either as amplifiers or oscillators.

When the electrons have traversed the entire interaction region they must be collected and returned to the cathode of the electron gun. Again, the conventional method of electron collection in the prior art is to provide a collector electrode at the end of the interaction region, which collector electrode is maintained at ground potential. Those skilled in the art are aware that a collector electrode at such a potential is heated by the impinging electron beam in a similar manner as was described in connection with the O-type devices previously, and that such heating, and the resultant thermal losses, decreases the overall efficiency of the device.

From a superficial examination it would appear that the collector electrode could again be operated at a depressed voltage, i.e. at a higher potential, than the slow wave structure and that a higher efiiciency in electron collection could then be obtained as is the case in O-type devices. However, because of the previously described fundamental difference in the nature of operation of the M-type and the O-type devices, such a collector electrode arrangement cannot be satisfactorily utilized. This is because electrons may leave the interaction region of an M-type device with any potential between that of the sole electrode and that of the slow wave structure, depending upon the position in the interaction region which the electron occupies at the time it leaves the interaction region. Thus, the collector electrode must be operated at a potential sufiiciently low to collect even those electrons which are at ground potential, because, if the collector is operated at any higher potential, some of the electrons will not have sutiicient energy to reach the collector electrode.

It is accordingly an object of the present invention to provide an improved crossed field travelling wave tube.

It is another object of the present invention to provide an improved electron collection arrangement for a crossed field travelling wave tube in which the electrons are more efficiently collected.

It is yet another object of the present invention to provide an improved crossed field travelling wave tube in which a collector electrode is efficiently operated at a higher potential than the slow wave structure and in which all electrons are collected in the collector region.

Briefly stated, the present invention relates to a crossed field travelling wave tube such as was described provided,

with two collector electrodes in the collector region. One of the collector electrodes is `a depressed collector, which means it is maintained at a potential substantially higher than that of the slow wave structure and efficiently collects all electrons which have sufficient energy to reach this electrode. The other collector electrode is maintained at the same potential as the slow wave structure and collects all other electrons which do not have such sufficient energy to reach the depressed collector of higher potential. Means are also provided to direct the largest possible number of electrons into the collector electrode of higher potential.

For a complete understanding of the invention, together with other objects and advantages thereof, reference may be had to the attached drawings, in which:

FIGURE 1 is a schematic representation of a crossed field amplifier and power supply circuit in accordance with the prior art;

FIGURE 2 is a schematic representation yof a similar crossed field amplifier and power supply circuit embodying the present invention;

FIGURE 3 is a cross sectional view of the collector region of a crossed field amplifier embodying the present invention;

FIGURES 4 and 5 are cross sectional views of the collector region of a crossed field amplifier u-tilizing additional embodiments of the present invention, wherein the horizontal scale has been increased;

FIGURE 6 is a cross sectional view of a collector region such as was shown in FIGURE 5, showing the trajectories of electrons in the collector region;

FIGURES 7, 8 and 9 are cross sectional views of the collector region of a crossed field amplifier and illustrate additional embodiments of the present invention; and

FIGURE illustrates a converging magnetic pole piece arrangement which may be used with certain embodiments of the present invention.

Referring now to FIGURE 1, therein is shown a schematic representation of a prior art crossed field amplifier such as was previously discussed. In the device of FIGURE 1 an electron gun consisting of a cathode 1 and an accelerating electrode 2 injects an electron beam into the interaction region bounded by slow wave structure 3 and sole electrode 4. An electrostatic field E is established between slow wave structure 3 and sole electrode 4 by power supplies 5 and 6. A transverse magnetic field B is established into the plane of the paper by any suitable means, such as an electromagnet or a permanent magnet. As was previously discussed, and is well known to those skilled in the art, the electrons traverse the interaction region with a synchronous velocity of E/B. In the shown device, which is a forward wave amplifier, a radio frequency input signal is applied to input 7, traverses the slow Iwave structure 3 with a phase velocity substantially equal to the velocity of the electron beam, and is removed from the slow wave structure at output 8. As was described previouslyand is well known to those skilled in the art, the electron beam interacts with the travelling electromagnetic wave on slow wave structure 3 and there is a net transfer of energy from the electron beam to the wave, resulting in an amplification of the wave. `After the electron beam has traversed the entire interaction region, those electrons which have not been collected on slow wave structure 3 are collected by collector electrode 9.

As shown in FIGURE l, slow wave structure 3 and collector electrode 9 are conventionally electrically connected together and connected to a common point of reference potential, such as ground. The electrons collected on slow wave structure 3 and collector electrode 9 are then returned through power supply 6 to cathode 1 for recirculation through the interaction region.

While the electronic characteristics of the device of FIGURE l are satisfactory, such a device suffers from the previously described defects of relative low efficiency and excessive collector electrode heating. FIGURE 2 shows a similar schematic representation of a forward wave crossed field amplifier which embodies the present invention and which obviates these previously discussed disadvantages of the prior art device. In FIGURE 2, the electron gun consists of cathode 10, accelerating electrode 11 and power supply 12, which establishes an accelerating field between cathode 111 and accelerating electrode 11. The interaction region again is bounded by slow wave structure 13 and sole electrode 14, with an electrostatic field E being established between slow wave structure 13 and sole electrode 14 by power supplies 15 and 1.6. A transverse magnetic field B is again established into the plane of the paper by any suitable means (again not shown). Input 17 and output 18 respectively allow a radio frequency input signal to be applied to slow wave structure 13 and an amplified output signal to be removed therefrom.

Again, electrons are accelerated from cathode 10 by a field established by accelerating electrode 11, and the electron beam is deflected by transverse magnetic field B into the interaction region, which it traverses with velocity E/B, delivering energy to the travelling wave on slow wave structure 13.

Two collector electrodes are provided at the end of the interaction region to collect all electrons which have not been collected by slow wave structure 13. These collector electrodes, conventional collector 19 and depressed collector 21), are positioned substantially as shown in FIGURE 2, with conventional collector 19 lying in the plane of slow wave structure 13 and depressed collector 20 having its end nearest the interaction region lying substantially in the plane of sole electrode 14 and having its surface diverge from the surface of conventional collector 19 as it leaves the interaction region.

Conventional collector 19 is maintained at the potential of the slow wave structure 13 and depressed collector 20 is maintained at some higher potential to collect as many electrons as have sufficient total kinetic and potential energy to reach this electrode. Devices in accordance with the present invention have been operated in which the potential of depressed collector 20 was varied from slow wave structure potential to sole electrode potential, but in practice it has been found most satisfactory to maintain depressed collector 20 at cathode potential, as is shown in FIGURE 2.

In accordance with the present invention, the surfaces of conventional collector 19 and depressed collector 20 diverge away from the interaction region. Since the two collector electrodes are maintained at constant potentials this divergence of surfaces results in a progressively decreasing electric field intensity away from the interaction region. Thus, electrons leaving the interaction region and entering the collector region see a eld of progressively diminishing value E/B as they progress along the collector region. The electrons then approach the surface of depressed collector 20 because, when the electric field intensity is reduced, -thus reducing the attraction of the electron towards conventional collector 19, the magnetic field B has a greater effect on the electron and defiects it downward towards depressed collector 20. As the electron is so deflected downward, it is forced by the magnetic field to move against the electrostatic field gradient. Since at this point the electron is neither receiving no1 giving up energy, its total energy must remain constant, and the electron velocity is reduced, thereby surrendering kinetic energy, in order to gain higher potential energy. All electrons which have sufficient total energy to reach depressed collector 20 are eventually collected on i-ts surface. The remaining electrons which do not have sufficient kinetic energy are eventually collected on the surface of conventional collector 19, because they are not, or less, deflected.

It will be recognized that the reduced velocity of the electrons along the collector region is still at all times equal to E/B, that is, the velocity of the electrons at any point is inversely proportional to the distance between the surfaces of the conventional collector 19 and the depressed collector 20. This velocity E/B may be termed the synchronous velocity of an electron in a crossed field. Thus, if the divergence of the surfaces of the collectors is such that the maximum spacing between them is four times the original spacing, thereby providing an electrical field intensity of 1A the original intensity, this synchronous velocity is reduced to 1A its original Value at this point and the kinetic energy of the electrons which have not yet been collected on the surface of depressed collector 20 is reduced to 1A@ of their original kinetic energy, since kinetic energy is proportional to the square of the velocity of the electron. The other /16 of the kinetic energy of these electrons has been converted into potential energy as the electrons moved against the field gradients while approaching the surface of depressed collector 20. Thus, this transformed kinetic energy may be represented by a potential through which those electrons have risen which are just collected at the point where the surfaces have diverged to a distance of four times their original distance, and those electrons leaving the interaction region at a point having a potential which is equal to the potential of depressed collector minus this potential are collected on the surface of depressed collector 20 at the point where the spacing is four times t-he original spacing. Those electrons which leave the interaction region at a potential higher than these first mentioned electrons have a total kinetic and potential energy greater than that represented by the potential of depressed collector 20 and strike the surface of depressed collector 20 with a finite velocity, thus causing some heating of depressed collector 20. Those electrons which leave the interaction region at a potential lower than the first mentioned electrons do not have a sufficient sum of energy to reach the depressed collector, having surrendered part of their energy to the wave while interacting with the wave. These electrons cannot be collected by depressed collector 20 but are instead eventually collected on the surface of conventional collector 19.

It will also be recognized by those skilled in the art that those electrons which leave the interaction region nearest the surface of depressed collector 20 are deflected almost immediately into this surface, while those electrons which leave the interaction region with progressively lower potential are deflected into the surface of depressed collector 20 progressively further away from the interaction region. Thus, the electron beam is spread out onto the surface of depressed collector 20 in what may be termed a potential spectrum, and this spreading of the beam prohibits undue concentration of the beam at any given point on depressed collector 20 and reduces any localized heating problems.

The utilization of the present invention results in a substantial increase of efficiency of the device of FIG- URE 2 over that of FIGURE 1. For example, if the magnitude of power supply 15 of FIGURE 2 and corresponding power supply 6 of FIGURE 1 is 10,000- volts and if the beam current in each application is 300 milliamperes, and if in each instance the tubes are delivering 900 watts of electromagnetic wave power, it has been found that the efficiency of the device increases from to 38%, with efficiency in this case being defined as the ratio of the electromagnetic wave power output to the D.C. power input to the device. Thus, in the prior art device the entire 300 milli-amperes must be returned through the 10,000 volts of power supply 6, requiring 3000 watts with an output of 900 watts, resulting in an efiiciency of 30%. In the device of FIGURE 2, obtained results have been the collection of 150 milliamperes on slow wave structure 13, 90 milliamperes on conventional collector 19, and 60 milliamperes on depressed collector 20. Thus only 240 milliamperes must be returned through the 10,000 volt power supply 15, requiring only 2400 watts to provide an output of 900 watts, thereby having an efficiency of 38%.

FIGURE 3 shows a cross sectional view of the collector region of a crossed field forward wave amplifier such as was schematically represented in FIGURE 2. As the electrons leave the interaction region between slow wave structure 13 and sole electrode 14, they enter the lcollector region defined by conventional collector 19 and depressed collector 20. Surface 21 of conventional collector 19 and surface 22 of depressed collector 20 diverge from each other in the direction away from the interaction space over a substantial portion of the collector region. At the remote end of the collector region, however, surface 23 of conventional collector 19 and surface 24 of depressed collector 20 converge in order to increase the electric field intensity in this region to attract the remaining uncollected electrons into surface 23 of conventional collector 19. Current from depressed collector 20 flows through conductor 25 which extends through and out of end housing 26 and bellows 27, which mechanical arrangement is provided to allow for positioning of depressed collector 20 and conductor 25 without breaking the vacuum of the tube.

Alternatively, depressed collector 20 could be internally connected to the cathode (not shown in this view) of the tube, in order that the device may be inserted directly into a socket in existing equipment to utilize the higher efficiency obtained by the present invention.

To adjust the potential gradients between conventional collector 19 and depressed collector 20, depressed collector 20 is supported for lateral movement relative to conventional collector 19. To this end, collector support 28 is secured by an internal thread arrangement to thrust bar 29 that extends through port 35 in tube housing 30. Lateral movement of thrust bar 29, and thus depressed collector 20, is achieved by -rotation of adjustor 31, which is connected to thrust bar 29 through member 32. The vacuum of the collector region is secured and maintained within the thrust bar housing 33 by diaphragm 34.

It will be appreciated that the space between conventional collector 19 and depressed collector 20 is relatively small so that adjustment of the space therebetween may be only a matter of a few thousandths of -an inch. Thus, depressed collector 20, collector support 28 and conductor 2S have sufficient resilience to accommodate such a small lateral movement and no pivotal arrangement at the remote end of the collector region is required.

FIGURE 4 is a cross sectional view of the collector of a crossed field amplifier in which the horizontal scale has been increased to better illustrate the principles of the present invention; thus, the horizontal dimensions have been reduced for purposes of clarity. As shown in FIG- URE 4, surface 22 of depressed collector 20 remains parallel to the surfaces of slow wave structure 13 and sole electrode 14, while surface 21 of conventional collector 19 diverges from surface 22 and again surface 23 of conventional collector 19 converges towards surface 22 to attract the Aremaining uncollected electrons at that point. Again, this arrangement results in a decreasing E/B ratio, thereby deflecting the maximum possible number of electrons at reduced velocities into depressed collector 20 for more efficient collection.

FIGURE 5 shows yet another embodiment of the invention similar to that of FIGURE 4 but in which conventional collector 19 includes a surface 40 which converges towards sole electrode 14 between slow wave structure 13 and depressed collector 20, which is to say between the interaction region and the collector region. This convergence of surface 40 results in `an increased electrostatic field intensity in this region which raises the electron beam away from sole electrode 14 to prevent its striking vertical surface 41 of depressed collector 20. At this point, surface 21 of conventional collector 19 and surface 22 of depressed collector 20 again diverge for the beneficial results previously described.

9 The dimensions d1 and d2 in FIGURE 5, as in the other figures, are chosen such that where VCC, VS, and VDC represent the voltages applied to conventional collector 19, sole electrode 14 and depressed col-lector 20, respectively, in order that the electric field intensity is not disturbed in this region and that the electrons see no abrupt discontinuity in the electric field.

FIGURE 6 shows an enlarged view of a portion of FIGURE 5 upon which has been superimposed plots of the trajectories of Several groups of electrons, or space charge groups, which represent portions of the electron beam. This figure illustrates the effects of the convergent and divergent surfaces in the collector region. The dashed lines in FIGURE 6 represent equal potential levels. The potential increases in a direction from the top of FIGURE 6 towards the bottom of FIGURE 6. As is illustrated at the left hand portion of FIGURE 6, the respective charge groups fall to lower potential levels as they are accelerated by the increasing electric field intensity resulting from convergent surface 40, and the upper portion of the beam, represented by trajectories a, is collected on surface 40 of conventional collector 19. As the remaining charge groups, those traversing trajectories b, enter the collector region of decreasing electric field intensity caused by

divergent surfaces

21 and 22, they rise to higher potential levels and approach depressed collector at reduced velocities, upon which those with sufficient total energy are collected. The remaining electrons which cannot reach depressed collector 2o are eventually collected on unshown portions of conventional collector 19.

FIGURES 7, 8 and 9 show specific collector geometries which utilize the principles of the invention which have previously been described.

The entire description to this point has assumed that the electrons are .affected only by the transverse electrostatic and magnetic fields. However, as is well known, the electrons themselves, being charged particles, establish electric fields of their own and if the density of electrons in a given region is sufficiently high, these fields so establish between electrons affect the movement of other electrons in the region. These are `known as space charge effects. It has also been assumed that the electron beam leaving the interaction region has the same uniform density with which it entered `the interaction region. In fact, the electron beam, in interacting with the travelling wave on the slow wave structure, has also been affected by the wave and has been hunched into rod-like groups having their axes parallel to the magnetic field, as is shown in FIGURE 7. The space charge effect within these rod-like groups of electrons, in conjunction with the magnetic field, causes the electrons in each group to rotate about the axis of the rod. For a magnetic field directed into the plane of the paper, as shown in FIGURE 7, the rotation is in the clockwise direction around the axis of the rods.

FIGURE 7 shows a collector geometry arrangement designed to take advantage of this rotation caused by space charge effects to increase the efficiency of electron collection. Depressed collector 20 is provided with a step consisting of surfaces 42 and 43, with surface 42 being perpendicular to surface 22 and surface 43 being parallel with surface 22. As the electron groups enter the collector region, their linear velocity is reduced and they move closer to surface 22 of depressed collector 2f), -as was previously described. When the groups move into the step region the clockwise rotation within the rod-like groups brings a higher percentage of electrons onto surface 42, thereby efficiently collecting these electrons at the higher potential of depressed collector 20.

Of course, the drawing of FIGURE 7 is idealized, and indicates greater bunching of the electrons than actually occurs and also that all of the electrons can be collected on the step in depressed collector 20. However, the collection on depressed collector 20 is still limited to those electrons having sufficient total energy to reach the potential thereof and all other lower energy electrons pass over the step for eventual collection on either surface 43 or

surfaces

21 or 23 of conventional collector 19, depending upon their total energy.

Calculations of electron beam profile at the location at which the beam leaves the interaction region indicate that the space charge groups are primarily deployed in two separate areas, with one of these being relatively close to the sole electrode while the other is slightly closer to the slow wave structure than to the sole electrode. FIGURE 8 shows a collector region geometrically designed to take advantage of this charge grouping. As shown therein, a jump field collector 44 is positioned between sole electrode 14 and depressed collector 20, with the surface 45 of jump field collector 44 lying in the same plane as the surface of sole electrode 14. Jump field collector 44 is maintained at cathode potential but since, as is shown in FIGURE 8, the space between jump field collector 44 and conventional collector 19 is the same as the space between sole electrode 14 and slow wave structure 13, there is an abrupt decrease or jump in the electric field intensity which is seen by the electron beam as it enters this region. Because of this abrupt decrease in electric field intensity, the magnetic field in this region exerts greater effect upon the electrons leaving the interaction region and those electrons in the region near the sole electrode are deflected into jump field collector 44 for efficient collection at cathode potential. The remaining electrons enter the collector region for collection by a similar collector geometry as was previously described in connection with FIGURE 4.

FIGURE 9 shows a cross section of the collector region of a crossed field amplifier in which a wedge-shaped depressed collector 47 is positioned between conventional collector 19 and a further electrode 50, which may be maintained at sole potential. Again, depressed collector 47 is maintained at some intermediate potential, preferably cathode potential. The geometry of conventional collector 19, depressed collector 47, and electrode 50 are chosen such that surfaces 21 and 51 diverge from each other, surfaces 21 and 48 converge and surfaces 49 and 51 converge, as is shown in FIGURE 9. In operation, the electron beam leaves the interaction region and, upon encountering the decreasing electric field intensity caused by divergent surfaces 21 and 51, is forced by the magnetic field closer to surface 51 into a region of higher potential, and the velocity of the electron beam is accordingly reduced as was previously described. A few electrons may even be collected on surface 51 if they have extracted sufficient energy from the electromagnetic wave to reach sole potential. After passing the leading edge of depressed collector 47, the electrons then enter regions of progressively increasing electric field intensity caused by converging surfaces 21 and 48 and converging surfaces 49 and 51. In these regions the electrons are drawn upward into regions of lower potential and those electrons which are above depressed collector 47 are drawn into surface 21 of conventional collector 19, while those electrons which are below depressed collector 47 are drawn upward into surface 49 of depressed collector 47 for efficient collection at cathode potential.

In all of the embodiments described thus far, the progressively decreasing E/ B ratio in the collector region has been provided by diverging surfaces maintained at constant potentials, thereby providing decreasing electric field intensities. Of course, a decreasing E/ B ratio could also be provided by a progressively increasing magnetic field intensity while maintaining the electric field intensity constant, or by a combination of decreasing electric field intensity and increasing magnetic field intensity. FIGURE l0 shows an external View of a cross field amplifier having means for increasing the magnetic field intensity in the collector region. As shown therein, magnetic pole pieces 52 and 53 are positioned on each side of the tube housing 30. Surfaces 54 and 55 of pole pieces S2 and S3, respectively, are opposite the interaction region and, being parallel, provide a constant magnetic field intensity in this region. However, opposite the collector region of the tube, surfaces 56 and 57 of pole pieces 52 and 53, respectively, converge to provide a progressively increasing magnetic field intensity in the collector region, thereby providing a progressively decreasing E/B ratio in the collector region of the tube and defiecting the maximum number possible of electrons into the depressed collector within tube housing 30.

While the invention is thus disclosed and several ernbodiments described, the invention is not limited to these shown embodiments. Instead, many modifications will occur to those skilled in the art which lie within the spirit and scope of the invention. For example, the invention is illustrated in connection with a forward wave amplifier. The invention is equally applicable to any crossed field travelling wave device such as a backward wave oscillator. Accordingly it is intended that the invention be limited in scope only by the appended claims.

I claim:

1. An electron discharge device comprising an interaction region, means positioned adjacent one end of said interaction region for forming a beam of electrons and for injecting said beam into said interaction region whereby said beam of electrons interacts with an electromagnetic wave in said interaction region, and means positioned adjacent the opposite end of said interaction region for collecting any electrons not collected in said interaction region, said collecting means including at least first and second electrodes positioned on opposite sides of the beam of electrons leaving said interaction region, means for establishing an electric field having an intensity E between said first and second electrodes, means for establishing a magnetic field having an intensity B transverse to both said electric field and the direction of travel of said beam of electrons, and means for providing a progressively diminishing ratio E/B in said collection region in the direction away from said interaction region.

2. The device of claim 1 in which said .means for providing a progressively diminishing ratio E/B comprises means for progressively reducing the electric field intensity E between said first and second electrodes in the direction away from said interaction region.

3. The device of claim 1 in which said means for providing a progressively diminishing ratio E/B comprises means for progressively increasing the magnetic field intensity B between said first and second electrodes in the direction away from said interaction region.

4. A electron discharge device comprising an interaction region bounded by a slow wave structure and a sole electrode, an electron gun positioned at one end of said interaction region for injecting an electron beam into said interaction region, and first and second collector electrodes having mutually facing surfaces positioned at the other end of said interaction region to define a collector region, the distance between said mutually facing surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collector region.

5. An electron discharge device comprising an interaction region bounded by a slow wave structure and a sole electrode, an electron gun positioned at one end of said interaction region for injecting an electron beam into said interaction region, first and second collector electrodes having mutually facing surfaces positioned at the other end of said interaction region to define a collector region, the distance between said mutually facing surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collector region, and means for establishing a potential difference between said first and second collector electrodes, whereby an electric field having a progressively diminishing intensity in the direction away from said interaction region is established between said first and second collector electrodes.

6. An electron discharge device comprising an interaction region bounded by a slow wave structure and a sole electrode, an electron gun positioned at one end of said interaction region for injecting an electron beam into said interaction region, first and second collector electrodes having mutually facing surfaces positioned at the other end of said interaction region to define a collector region, the distance between said mutually facing surfaces increasing in the direction away from said interaction region over at least portion of the length of said collector region, means for maintaining said second collector electrode at a potential higher than that of said first collector, thereby establishing an electric field having a progressively diminishing intensity in a direction away from said collector region between said first and second collector electrodes, and means for establishing a magnetic field having an intensity B transverse to both said electric field and the direction of travel of electrons in said beam, whereby the ratio E/ B progressively diminishes in said collector region in the -direction away from said interaction region and at least some of the electrons leaving said collector region are efiiciently collected at said higher potential by said second collector electrodes.

7. A crossed field travelling wave electron discharge device comprising a slow wave structure, a sole electrode spaced from said slow wave structure and defining therewith an interaction region, means for maintaining said slow wave structure at a first potential and said sole electrode at the second potential whereby an electric field having an intensity E is established between said slow wave structure and said sole electrode in said interaction region, an electron gun positioned adjacent one end of said interaction region for injecting an electron beam into said interaction region at a direction transverse to said electric field, said electron gun including a cathode which is maintained at a thirdpotential, an electron collection region positioned adjacent the other end of said interaction rcgion, said electron collection region including first and second collector electrodes positioned on opposite sides of an electron beam leaving said interaction region and having mutually opposed surfaces, the distance between said mutually opposed surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collection region, means for establishing a magnetic field in said interaction region and collection region transverse to both the direction of said electric field and the direction of travel of said electron beam and having a magnetic intensity B, and means for maintaining said second collector electrode at a higher potential than said first collector electrode, whereby the electric field intensity and thus the ratio E/B progressively decreases in said collection region in the direction away from said interaction region and at least some of the electrons entering said collection region are efficiently collected at said higher potential by said second collector electrode.

8. A crossed field travelling wave electron discharge device comprising a slow wave structure, a sole electrode spaced from said slow wave structure and defining therewith an interaction region, means for maintaining said slow wave structure at a first potential and said sole electrode at the second potential whereby an electric field having an intensity E is established between said slow wave structure and said sole electrode in said interaction region an electron gun positioned adjacent one end of said interaction region for injecting an electron beam into said interaction region at a direction transverse to said electric field, said electron gun including a cathode which `is maintained at a third potential, an electron collection region positioned adjacent the other end of said interaction region, said electron collection region including first and second collector electrodes positioned on opposite sides of an electron beam leaving said interaction region and having mutually opposed surfaces, the distance between said mutually opposed surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collection region, means for establishing a magnetic field in said interaction region and collection region transverse to both the direction of said electric field and the direction of travel of said electron beam and having a magnetic intensity B, means for maintaining said first collector electrode at said first potential, and means for maintaining said second collector electrode at a potential intermediate said first and second potentials, whereby the electric field intensity and thus the ratio E/B progressively decreases in said collection region in the direction away from said interaction region and at least some of the electrons entering said collection region are efliciently collected at said intermediate potential by said second collector electrode.

9. A crossed field travelling wave electron discharge device comprising a slow wave structure, a sole electrode spaced from said slow wave structure and defining therewith an interaction region, lmeans for maintaining said slow wave structure at a first potential and said sole electrode at a second potential whereby an electric field having an intensity E is established between said slow wave structure and said sole electrode in said interaction region, an electron gun positioned adjacent one end of said interaction region for injecting an electron beam into said interaction region at a direction transverse to said electric field, said electron gun including a cathode which is maintained at a third potential, an electron collection region positioned adjacent the other end of said interaction region, said electron collection region including first and second collector electrodes positioned on opposite sides of an electron beam leaving said interaction region and having mutually opposed surfaces, the distance between said mutually opposed surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collection region, means for establishing a magnetic field in said interaction region and collection region transverse to both the direction of said electric field and the direction of travel of said electron beam and having a magnetic intensity B, means for maintaining said first collector electrode at said first potential, and means for maintaining said second collector electrode at said third potential, whereby the electric field intensity, and thus the ratio E/B, progressively decreases in said collection region in the direction away from said interaction region and at least some of the electrons entering said collection region are efficiently collected at said third potential by said second collector electrode.

10. A crossed field travelling wave electron discharge device comprising a slow wave structure, a sole electrode spaced from said slow wave structure and defining therewith an interaction region, an electron gun positioned adjacent one end of said interaction region for injecting an electron beam into said interaction region at a direction transverse to said electric field, an electron collection region positioned adjacent the other end of said interaction region, said electron collection region including first and second opposed surfaces positioned on opposite sides of an electron beam lleaving said interaction region, the distance between said first and second surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collection region, and a wedge-shaped collector electrode positioned between said first and second surfaces and having its point facing said interaction region.

11. A crossed field travelling wave electron discharge device comprising a slow wave structure, a sole electrode spaced from said slow wave structure and defining therewith an interaction region, means for maintaining said slow wave structure at a first potential and said sole electrode at a second potential whereby an electric field having an intensity E is established between said slow wave structure and said sole electrode in said interaction region, an electron gun positioned adjacent one end of said interaction region for injecting an electron beam into said interaction region at a direction transverse to said electric field, an electron collection region positioned adjacent the other end of said interaction region, said electron collection region including first and second opposed surfaces positioned on opposite sides of an electron beam leaving said interaction region, the distance between said first and second surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collection region, a wedge-shaped collector electrode positioned between said first and second surfaces and having its point facing said interaction region, means for establishing a magnetic field in said interaction region and said collector region transverse to both the direction of said electric field and the direction of travel of said electron beam and having a magnetic intensity B, means for maintaining said first surface at said first potential, means for maintaining said second surface at said second potential and means for maintaining said wedgeshaped collector electrode at some potential intermediate said first and second potentials, whereby the electric field intensity, and thus the ratio E/B, progressively decreases in said collection region in the direction away from said interaction region between said interaction region and said wedge-shaped collector and at least some of the electrons entering said collection region are efficiently collected at said intermediate potentially by said wedge-shaped collector electrode.

12. A crossed field travelling wave electron discharge device comprising a slow wave structure, a sole electrode spaced from said slow wave structure and defining therewith an interaction region, means for maintaining said slow wave structure at a first potenti-al and said sole electrode at the second potential whereby an electric field having an intensity E is established between said slow wave structure and said sole electrode in said interaction region, an electron gun positioned adjacent one end of said interaction region for injecting an electron beam into said interaction region at a direction transverse to said electric field, said electron gun including a cathode which is maintained at a third potential, an electron collection region positioned adjacent the other end of said interaction region, said electron collection region including first and second opposed surfaces positioned on opposite sides of an electron beam leaving said interaction region, the distance between said first and second surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collection region, a wedge-shaped collector electrode positioned between said first and second surfaces and having its point facing said interaction region, means for establishing a magnetic field in said interaction region and said collector region transverse to both the direction of said electric field and the direction of travel of said electron beam and having a magnetic intensity B, means for maintaining said first surface at said first potential, and means for maintaining said second surface at said second potential, and means for maintaining said wedge-shaped collector electrode at said third potential, whereby the electric field intensity, and thus the ratio E/B, progressively decreases in said collection region in the direction away from said interaction region between said interaction region and said wedgeshaped collector and at least some of the electrons entering said collection region are efficiently collected at said third potential by said wedge-shaped collector electrode.

13. A crossed field travelling wave electron discharge device comprising a slow wave structure, a sole electrode spaced from said slow wave structure and defining therewith an interaction region, means for maintaining said slow wave structure at a first potential and said sole electrode at a second potential whereby an electric field having an intensity E is established between said slow wave structure and said sole electrode in said interaction region, an electron gun positioned adjacent one end of said interaction region for injecting an electron beam into said in- 15 teraction region at a direction transverse to said electric eld, said electron gun including a cathode which is maintained at a third potential, an electron collection region positioned adjacent the other end of said interaction region, said electron collection region including first and second opposed surfaces positioned on opposite sides of an electron beam leaving said interaction region, the distance between said first and second surfaces increasing in the direction away from said interaction region over at least a portion of the length of said collection region, a wedgeshaped collector electrode positioned between said first and second surfaces and having its point facing said interaction region, said wedgeshaped collector electrode having a third surface spaced from said first surface and a fourth surface spaced from said second surface, the distances between said first and third surfaces and said second and fourth surfaces decreasing in the direction away from said interaction region, means for establishing a magnetic field in said interaction region and said collector region transverse to both the direction of said electric field and the direction of travel of said electron beam and having a magnetic intensity B, means for maintaining said first surface at said first potential, and means for maintaining said second surface at said second potential, and means for maintaining said wedge-shaped collector electrode at said third potential, whereby the electric field intensity, and thus the ratio E/B, progressively decreases in said collection region in the direction away from said interaction region between said interaction region and the point of said wedge-shaped collector, and the electric field intensity, and thus the ratio E/B, progressively increases in said collector region in the direction away from said interaction region after the point of said wedge-shaped collector, and at least some of the electrons entering said collection region are efficiently collected at said third potential by said wedge-shaped collector electrode.

References Cited by the Examiner UNITED STATES PATENTS 2,600,509 6/1952 Lerbs 315-393 X JAMES W. LAWRENCE, Primary Examiner'.

R. SEGAL, Assistant Examiner.