CHARGE-PARTICLE COLLECTOR

Abstract: An electron collector (10) is shown having four axi-symmetric electrodes (12, 14, 16, 18), three of which have off-axis apertures (26, 28 and 30) therein which receive an electron beam (32). The application of a consecutively smaller voltage to each electrode as compared to the cathode (34) that generates the electron beam creates an electrostatic field that efficiently deflects the beam electrons onto the electrodes for their collection.

Description: The present invention relates to a charged-particle collector and, more particularly, to a multistage depressed electron collector BACKGROUND OF THE INVENTION Many electronic devices employ a traveling stream of charged-particles , such as electrons, formed into a beam as an essential function in the device's operation. For example, one type of vacuum device, a microwave traveling wave tube, incorporates a source of electrons that are formed into a beam, in which the electrons are accelerated to a predetermined velocity and directed along an axial path through an "interaction" region within the microwave tube body. In the interaction region, kinetic energy is transferred from the moving electrons to the high frequency electromagnetic fields, such as microwave signals, that are propagating along a slow wave structure through the interaction region at about the same velocity as the moving electrons. The electrons give up energy to the microwave field through the exchange process characterized as electronic interaction, evidenced by a lower velocity of the electrons exiting from the interaction region. The "spent" electrons pass out of the interaction region where they are incident upon and collected by a final tube element, termed the collector. The collector collects and returns the incident electrons to the voltage source. As is recognized, much of the energy in a moving particle is released in the form of heat when the particle strikes a stationary element, such as the collector. This produces undesired heating in the microwave tube and a lower overall electrical efficiency of microwave tube operation. The depressed collector and, more particularly, the multistage depressed collector is a collector that increases the electrical efficiency of traveling wave tube operation as well as reduces undesirable heat generation by a process of velocity sorting of the electrons controlled by a retarding electric field. The field slows the electrons so that the electrons are collected by electrodes at a reduced velocity and ideally at a zero velocity. As is known to those skilled in the art, the multistage depressed collector is characterized physically by a series of spaced metal electrodes, each containing a passage therethrough, a final electrode and a passage entry for receiving electrons. The electrodes are maintained at successively lower voltages with respect to the tube circuit taken as ground (or at successively higher negative voltages as otherwise viewed) so as to present a retarding electric field to the electrons which pass through the entrance into the collector region. Such types of devices are substantially well developed and hence are complex in nature as is known to the reader skilled in the art. One type of known multistage depressed collector employs a combination of a transverse electric field and a longitudinal magnetic field for sorting electrons as a function of electron velocity. See U.S. Letters Patent No. 3,526,805, by Okashi, et al.; No. 3,644,778, by Mihran, et al.; and No. 3,702,951, by Kosmahl. Another type of collector employs a retarding electric field established by a cuplike electrode and a pointed spike located in the center of the cuplike member. The effect of this structure with a voltage applied is to present an electron mirror with a negative focal length to electrons moving near the axis. Hence, the reflected beam is more divergent than the incident beam. See the paper entitled Multistage Depressed Collector Investigation For Travelling Wave Tubes , Tammaru, NASA CR-72950 EDDW-3207, Contract NAS-3-11536, Final Contract Report. The efficiency of the NASA collector is limited by the defocusing properties of the spikelike reflector. Further, the collectors shown in the patents mentioned above required the maintenance of an axial magnetic field of a critical magnitude for proper functioning. A multistage depressed collector utilizing electrodes that are asymmetrical was disclosed in U.S. Patent No. 4,096,409, by Hechtel. This electron collector has a high efficiency and utilizes the concept of focusing electrons to collection points on various electrodes depending upon the energy level of the electron. It comprised a plurality of electrodes spaced apart in sequence along a centre line of the collector, the first electrode, and any subsequent electrode other than the final electrode, having an aperture for the passage of charged particles, the aperture in the first electrode being offset from the centre line to define an entry to the collector for an electron beam travelling parallel to the centre line, the electrodes being able to define in the collector a particle retarding electrostatic field with concave equipotential surfaces. One disadvantage of the Hechtel patent is that it is difficult to fabricate and align the various asymmetric electrodes. According to the invention, there is provided a charged particle collector comprising a plurality of electrodes spaced apart in sequence along a centre line of the collector, each electrode having an axisymmetrical shape about the centre line to assist in stacking the electrodes in axial alignment and being shaped so as to be able to define in the collector an axisymmetrical particle retarding electrostatic field the equipotential surfaces of which field are concave as seen from a first, entry, electrode, the first electrode, and any subsequent electrode other than the final electrode, having apertures for the passage of charged particles, the aperture in the first electrode being offset from the centre line to define an entry to the collector for a charged particle beam. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example, to the accompanying drawings, wherein: Fig. 1 is a schematic diagram showing an electron collector as it might be used with a microwave tube and an electron gun; Fig. 2 is a curve showing the magnitude of the electrostatic field, V, and its first and second derivatives along the centerline (z axis) of an electron collector having focusing properties; Fig. 3 shows equipotentials at a radius, r, along the z axis of an axisymmetric electron collector; Fig. 4 illustrates a computer generated projection of the electron trajectories on the y, z-plane of an electron collector; Fig. 5 is a computer generated projection of the electron trajectories of Fig. 4 in the x, y-plane at a potential of 40% of circuit potential with respect to cathode; Fig. 6 is a computer generated projection similar to Fig. 5, showing electron trajectories in the x, y-plane at a potential of 25% of circuit potential with respect to cathode; Fig. 7 is a schematic, cross-sectional diagram of one embodiment of an electron collector along the r, z-plane thereof, showing the off-axis electron beam at an angle to the axis of the collector; Fig. 8 shows the preferred embodiment of the electron collector of the present invention in cross-section; and Fig. 9 is an exploded view of the electrodes used in Fig. 8. Referring now to the drawings, Fig. 1 shows a charged-particle collector 10 which may be used to collect electrons having a plurality of electrodes 12, 14, 16, and 18 formed from a metal such as copper, into a generally cuplike shape with each electrode nested into the other. While four electrodes are shown, as few as two and more than four electrodes may be used within the present invention. The left-most electrode 12 forms a particle entry wall of the collector 10, while the right-most electrode 18 forms the furthest electrode or back wall of the collector 10. The side walls of collector 10 are formed by ceramic cylinders 20 which mechanically separate and electrically isolate one electrode from the other. In some applications, electrodes 12 and 18 may be the only two electrodes required for the collector 10. Mounted at a slight distance from electrode 12 is a mounting plate 22 which may be fabricated from an insulating material. As seen in Fig. 1, the electrodes 12-18 are all symmetrical about a centerline 24 which forms the longitudinal axis of the collector 10. Electrodes 12, 14, and 16 are each provided with apertures; 26, 28 and 30, respectively, through which an electron beam 32 generated from a cathode 34 passes. It will be noted that apertures 26, 28 and, in some cases, 30 are offset from the axis 24 of the electron collector 10 for providing an off-axis injection of electron beam 32. As is well known in the art, the electron beam 32 is generated by an electron gun 36 which may comprise a cathode 34, control grids 38, and an anode 39. As the beam 32 exits the electron gun, it is directed into a vacuum device 40, such as a microwave device or, more particularly, a traveling wave tube. The spent electrons exit the microwave device 40 where they may be refocused by a magnetic field formed by permanent magnet 42 and/or an exit anode 44. The exit anode 44 may be mounted in close proximity to the left-most electrode 12 and is provided with an aperture therein which is in alignment with the offset aperture 26 of electrode 12. By reference to Fig. 1, the reader will now see the offset between an axis 46 of electron beam 32 and the centerline 24 of the electron connector 10. It will be understood that the precise configuration of the electrodes 12-18 within the electron collector 10 may vary as well as the number of such electrodes. The important feature of the electrodes 12-18 is that they focus the electron beam. Focus means a selective focus wherein different electrons which make up the beam 32 are selected by energy level for shunting within a generally circular area upon different and separate electrodes. In theory, an infinite number of electrodes provide a target for an infinite number of electron energy levels so that each electron strikes an appropriate electrode with a zero velocity. In practice, the infinite number of electrodes is reduced to meet the need for a simplified design. The key difference between the preferred embodiments described herein and the prior art is that many of the prior art devices rely on defocusing the electron beam which has a tendency to scatter the electrons into an annular area about the outer surfaces of the collector. Thus, electrons having the same energy level but traveling at different angles into the collector will land on different points around the peripheral inner surface of the collector. The concept of focusing the beam requires that two electrons with the same energy but a different entrance angle will land in the same circular area of an electrode, see Fig. 6. This concept permits higher efficiency within the electron collector. The prior art Patent No. 4,096,409 utilized this concept of focusing. However, the asymmetric, two-dimensional multistage collector disclosed therein is more difficult to build than conventional multistage collectors. The advantage of the novel axisymmetric collector of the present invention is that it is easy to fabricate and has the same efficiency as the prior art asymmetric collector. A basic equation which describes the electrostatic field, V, for the collector 10 shown and described in Fig. 1, is: V = 2/3z + 1/2z² - 1/12z⁴ - 1/4 r² + 1/4r²z² - 1/32r⁴ where the electrostatic potential, V, is described in an r, z-coordinate system. R is the radius from the longitudinal axis 24 of the collector 10, while z is the length along that axis. On the axis where r=o, the potential is: V r=o = -2/3z + 1/2z² - 1/12 z⁴ Its first derivative is: (∂V/∂z) r=0 = - 2/3 + z - 1/3 z³ while its second derivative is: (∂²V/∂z²) r=0 = 1 -z² Fig. 2 shows V,∂V/∂Z and ∂²V/∂Z² for the range of z = -1 to +1. Note, from Fig. 2 that ∂²V/∂Z² is positive over the entire range which is equivalent to the focusing action on the beam 32. Fig. 3 shows the equal potentials V, measured in arbitrary units, in an r vs. z system between z = -1 and z = 0.9. In designing a multistage collector, any equal potential surface can be substituted by a conducting electrode at the proper potential. Thus, the configuration of the focusing electrodes follows to some extent the contours shown in Fig. 3. Using a computer to project the various trajectories of an off-axis electron beam 32 as it enters the electron collector 10, it is possible to plot curves similar to that shown in Fig. 4 wherein a plot representing the projections of the electron trajectories on the y, z-plane is shown. Fig. 4 shows an electron beam 32 entering parallel to the longitudinal axis 24 of the collector 10 and assumes that all electrons within the beam have the same energy level which is 92% of the cathode voltage. A plot of the electron beam 32 in the x, y-plane at a particular z position is shown in Fig. 5. Fig. 5 shows the intensity of beam 32 as it passes into the electron collector 10 at a point where the potential of the electrostatic field is approximately 40% of the cathode. Similarly, Fig. 6 shows the pattern of the beam 32 at a point where the beam has a potential of 25% with respect to the cathode. Note, that the beam 32 includes two trajectory areas including a first area shown in the upper surface where the beam 32 is moving from left to right (Fig. 4) and a second portion wherein the beam 32' is moving from right to left. The return beam 32' is shown by squares which represent theoretical strike points of the spent electrons. It will now be understood from referring to Figs. 4-6 that electrons entering the electron collector 10 are focused in a generally circular area upon the rear or inner surfaces of the electrodes 12-16 depending upon the energy level of each electron. Referring to Fig. 7, a schematic design of a suitable electron collector 710 is shown having a plurality of electrodes 711, 712, 714, 718, and 719. Note, how the configuration of the electrode 712, 714, 718, and 719 comply with the equipotential lines shown in Fig. 3. In the design shown in Fig. 7, the potential applied to electrode 712 is 55% of the cathode voltage from ground or plus 45% when compared to the cathode voltage. Similarly, the voltage on electrode 714 is plus 35% the voltage on electrode 718 is plus 10% and the voltage on electrode 719 is 0 with respect to the cathode. That is, the grid 719 is 100% depressed. The electron beam 732 is offset from axis 724 and is shown entering electron collector 710 at an angle to the collector axis 724 of approximately 10°, although other angles between 6° and 14° may be used. Through experimentation, it was unexpectedly found that the zero voltage grid 719 is unnecessary within the present invention. That is, the electrode 719 which is 100% depressed has a tendency to turn the electrons around and send them back through the opening within the electron collector 710. Thus, it was unexpectedly found that the elimination of the 100% depressed electrode 719 not only retained the efficiency of the electron collector 710 but, in fact, improved it. Further, by experimentation, it was found that the efficiency of the electron collector remained the same whether the electron beam 732 entered the collector 710 at an angle, as shown in Fig. 7, or entered the collector parallel to its axis 724. This unexpected result was extremely useful as it simplifies the design of the collector. This simplified design makes it possible to fabricate all electrodes axisymmetrically about the centerline 724. The only feature of the electrodes that is not axisymmetrical is the offset apertures for the electron beam 732. Referring now to Figure 8, the preferred embodiment of the present invention will be described in greater detail. The electron collector 810 shown in Fig. 8 includes four electrodes 812, 814, 816, and 818. These cuplike metal electrodes are provided with outwardly extending flanges 848 which are mechanically and electrically separated from each other by insulators 820. The insulators 820 may be attached to flanges 848 by any suitable device such as by chemical bonding or electrical welding. The reader should note that electrodes 812, 814, 816, and 818 are symmetrical about a centerline 824 but for the apertures 826 and 828 in the left-most electrodes. Aperture 826 in electrode 812 is offset from the centerline 824 by a significant distance; while aperture 828 in electrode 814 is offset by a slightly smaller distance, although the aperture 828 is significantly larger. Experimentation has unexpectedly shown that it is not necessary to offset each of the apertures within the later stages of the electron collector. Thus, the aperture 830 in electrode 816 is shown as symmetrical even though it is utilized to capture an electron beam, such as beam 32 in Fig. 1 which is entering off-axis to the centerline 824 of the collector 810. The offset apertures 826 and 828 are circular in shape within the preferred embodiments. However, other shapes such as elliptical or oval may also be used. The left-most surface of electrode 812 is shown flat, while the inner surface thereof is made thicker toward the centerline 824 for purposes of focusing the electron beam. Similarly, the left-most surface of electrode 814 is dished; while the inner surface thereof is arranged in a parallel configuration thereto. This aids in focusing the beam 32 (Fig. 1). The aperture 828 passes through the flat portion of the dish in electrode 814 as well as part of the tappering surface thereof. Aperture 830 in electrode 816 is symmetrical, as stated above. The reader will note that the electrode 818 which forms the final electrode or rear wall of collector 810 is maintained at the same potential as electrode 816. As stated above, it was unexpectedly discovered that it is not desirable to depress the final electrode to a potential equal to the cathode. Rather, a potential slightly positive compared to the cathode is desirable for improved efficiency. In one of the preferred embodiments, the first electrode 812 was retained at 58% of the cathode voltage from ground, the second electrode 814 was retained at 80% of the cathode voltage from ground, and the third electrode 816 was maintained at 90% of the cathode voltage from ground along with electrode 818. The range of voltage on electrode 812 may vary from 30 to 65% of the cathode voltage from ground, the voltage on electrode 814 may vary from 55 to 85%, and the voltage on electrodes 816 and 818 may vary from 80 to 100%. However, as previously stated, it is preferred to retain the voltage on electrodes 816 and 818 at less than 100% of the cathode voltage from ground. Fig. 9 shows the electrodes of Fig. 8 in an exploded view to more clearly demonstrate the relationship of the off-axis beam injection through the offset apertures and the simplified fabrication of the axisymmetrical electrodes. The reader will understand that the equation set forth herein is but one of several equations which may be used to describe an electrostatic field for focusing electrons upon the plurality of electrodes. Further, the number and shape of electrodes may be varied within the teachings of the present invention. For example, electrodes 814 and 816 could be eliminated and only electrode 812 be provided with an offset aperture 826. One or more electrodes 814 and 816 may be used with or without offset apertures 826 and 830. Electrode 812 could be dished like electrode 814 in some application. It will be understood that the heat caused by the electron beam 32 as it strikes the electrodes may be dissipated by liquid cooling or by fins or other suitable arrangements. Finally, the electron gun 36 and the vacuum device 40 which are utilized with the electron collector 10 of the present invention should not be limited by the devices shown schematically herein. Accordingly, the present invention should be limited only by the appended claims.

Claims: A charged particle collector comprising a plurality of electrodes (12, 14, 16, 18) spaced apart in sequence along a centre line (24) of the collector, each electrode having an axisymmetrical shape about the centre line (24) to assist in stacking the electrodes in axial alignment and being shaped so as to be able to define in the collector an axisymmetrical particle retarding electrostatic field the equipotential surfaces of which field are concave as seen from a first, entry, electrode (12), the first electrode (12), and any subsequent electrode other than the final electrode (18), having apertures (26) for the passage of charged particles, the aperture (26) in the first electrode (12) being offset from the centre line to define an entry to the collector for a charged particle beam. A collector according to claim 1, and comprising a plurality of cup-like electrodes each with an integral side wall and base portion and which electrodes nest one into another, means being provided to insulate the cuplike electrodes from one another. A collector according to claim 1 or 2, wherein the shape of each aperture (26, 28, 30) is substantially circular. A collector according to claim 1, 2 or 3, wherein there is one of said electrodes between the first and final electrodes. A collector according to claim 4, wherein there are the first (12), a second (14), a third (16) and the final (18) one of said electrodes. A collector according to claim 5, wherein the second electrode has its aperture offset from the centre line. A collector according to claim 5 or 6, wherein the third electrode has an aperture that is substantially symmetrically arranged about the centre line. A collector according to any one of claims 5 to 7 wherein the electrodes are formed and disposed to define an electrostatic field V, where V = 2/3z + 1/2z² - 1/12z⁴ - 1/4r² + 1/4r² z ² - 1/32r⁴, in an r,z coordinate system, where z is the distance along the centre line and r is the radial distance therefrom. A collector according to any one of claims 3 to 8, and comprising means for applying different, consecutively smaller, voltages to the electrodes in the direction towards the final electrode. A collector according to any one of claims 3 to 8 and comprising means for applying different, consecutively smaller, voltages to the electrodes, apart from and in the direction towards the final electrode. A collector according to claim 10 wherein the final electrode and the adjacent electrode are coupled to operate at the same voltage. A collector according to any of the preceding claims wherein the electrodes are of metal. A device comprising a cathode (34) to generate said charged particles and a collector according to any one of the preceding claims. A device according to claim 13, and comprising means for maintaining the final electrode at a higher voltage than that on the cathode. A device according to claim 14, when appended to claim 5 and to claim 9 or 10, wherein the applying means are arranged so that, in use: the voltage applied to said first electrode is 30% to 65% of the cathode voltage with respect to ground; the voltage applied to said second electrode is 55% to 85% of the cathode voltage with respect to ground; and the voltage applied to said third and fourth electrodes is 80% to 100% of the cathode voltage with respect to ground. A device according to claim 14 or 15, and arranged for the charged particles to enter the aperture of the first electrode along a path parallel to the centre line. A device according to claim 14 or 15, and arranged for the charged particles to enter the aperture of the first electrode along a path at an angle to the centre line. A device according to any one of claims 14 to 17, wherein the cathode is such that the charged particles will comprise electrons. A device according to claim 18 and which is a microwave tube.

Inventors: HECHTEL, JOHANN RICHARD, HERRIOT, RONALD WILLIAM
Applicants: LITTON SYSTEMS, INC.
Application year: 1988
Main classification: H01J23
IPC codes: H01J23