Abstract:
A linear beam device comprises a cathode and an anode spaced therefrom, with the anode and cathode being operable to form and accelerate an electron beam. An RF interaction region having a drift tube is arranged relative to the anode to permit the electron beam to pass therethrough. A multi-stage depressed collector of the linear beam device has a plurality of collector electrodes successively arranged to collect spent electrons of the electron beam after passing through the RF interaction region. Each one of the plurality of collector electrodes has a distinct voltage level applied thereto defining a decelerating electric field within the collector. At least one of the plurality of collector electrodes further comprises a collecting surface having a shape that is normal to a coincident trajectory of the spent electrons, whereby a substantial portion of the collecting surface is covered with a plurality of narrow grooves. In an embodiment of the invention, the grooved collector electrode further comprises the final electrode of the collector. The final electrode has a surface that is substantially spherical, and the plurality of grooves may be arranged in a concentric pattern of circles on the electrode surface. The plurality of grooves may be formed to a depth that is approximately twice a corresponding width. A region adjacent to an opening of each of the plurality of grooves comprises electric fields defining a convergent lens, thereby focusing the spent electrons into the plurality of grooves.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to linear beam devices having multi-stage depressed collectors, and more particularly, the invention relates to a multi-stage depressed collector having grooved surfaces in order to suppress generation of secondary electrons. 
     2. Description of Related Art 
     Linear beam electron devices are used in sophisticated communication and radar systems to convert direct current (DC) power into radio frequency (RF) power. Conventional klystrons, traveling wave tubes and inductive output tubes are examples of such linear beam electron devices. In a linear beam device, an electron. beam originating from an electron gun having a cathode is accelerated by a DC voltage differential with an anode spaced from the cathode. The accelerated electron beam passes through a drift tube containing an RF interaction structure. The electron beam may become amplitude modulated by applying an RF input signal to a grid disposed between the anode and cathode. Alternatively, the RF interaction structure of the drift tube may further include an RF circuit used to induce a modulation on the electron beam. Either way, the modulation results in electron concentration or bunching due to electrons that have had their velocity increased gradually overtaking those that have been slowed. The accelerated electrons of the electron beam give up varying amounts of their energy to the RF electric fields of traveling or standing wave circuits of the RF interaction structure. The energy removed from the electron beam in this manner may be subsequently removed from the device in the form of an amplified RF signal. 
     It has long been desirable to increase the efficiency of linear beam electron devices. If it were possible to make the length of the electron bunches infinitesimal and the amplitude infinite so that the average electron current remained finite, then one could apply an RF decelerating field to the bunch that would stop all the electrons and yield a device that is 100% efficient. In actual practice, when a sinusoidally time varying RF electric field exists on or in an output circuit of a linear beam device and the time length of the electron bunch is finite, some of the electrons will necessarily pass through the output circuit at times when the decelerating force of the RF electric field is less than maximum. As a result, many of the electrons will give up less than all of their energy, and the efficiency of the tube will be reduced accordingly. 
     A known technique for recovering the energy of the electrons that emerge from the output circuit (referred to as the “spent beam” or “spent electrons”) and thereby increase the efficiency of a linear beam device is to use a multi-stage depressed collector. A multi-stage depressed collector includes plural collector electrodes having successively decreasing voltage potentials in order to define a steady (i.e., not time varying) decelerating electric field. The collector electrodes further include holes aligned with the electron beam axis providing a path for the spent electrons to penetrate into the collector. The decelerating electric field slows the spent electrons as they penetrate into the collector to thereby allow their collection on one of the collector electrodes. The movement of the spent electrons within the collector is analogous to the way balls of varying velocity might roll up a hill, then stop and reverse direction after converting all of their kinetic energy to potential energy. If an electron has a little momentum transverse to the electric field when they reverse direction, the electron is likely to be collected by one of the electrodes that has less than the maximum potential and some of the energy of the spent beam will therefore be recovered. Unlike balls, electrons exhibit mutual repulsion due to their similar charge (i.e., negative) to thereby provide the transverse momentum. 
     Multi-stage depressed collectors are generally constructed such that most of the spent electrons will strike the back side of each of the collector electrodes (i.e., the side facing away from the output circuit), with the exception of the final collector electrode. This is advantageous since it tends to minimize the adverse effects of secondary electron emissions from the electrodes. A secondary electron emission refers to electrons that are knocked out the metal material of the collector electrodes by the impact of an energetic electron. These secondary electrons can actually become accelerated by the electric fields in the collector in a direction opposite the flow of the electron beam back into the linear beam device. By configuring the collector such that electrons typically strike the back side of a collector electrode, the electric fields operative on any secondary electrons that are emitted generally cause the secondary electrons to simply return to the electrode. 
     The shape of the final collector electrode remains problematic in terms of its generation of secondary emissions. Because an electron can only give up kinetic energy to the component of the electric field that is parallel to its direction of motion, it is desirable to configure the surface of the final collector electrode to be normal to the incoming electron trajectories. This shape also tends to cause secondary electrons to be accelerated back to higher potential electrodes and thereby waste power that is dissipated when the secondary electrons strike the higher potential electrodes. It is also known to configure the final collector electrode as a deep “bucket,” sometimes having a spike extending along the beam axis to shape the electric fields at the back of the collector to disperse high-energy electrons. A drawback of this design approach is that equipotential electric field lines at the mouth of the bucket are rarely perpendicular to the electron trajectories. Electrons that strike the surface of the bucket or the spike will usually have a great deal of energy in momentum that is directed parallel to these surfaces that cannot be recovered. 
     Accordingly, it would be desirable to provide a multi-stage depressed collector for a linear beam device having an electrode shape that minimizes secondary emissions while otherwise promoting efficient electron collection. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a multi-stage depressed collector for use in a linear beam device having a plurality of grooves formed in the collecting surface of at least one of the collector electrodes. The grooves provide a substantially field-free region that tends to prevent any secondary electrons generated by electrons that impact the grooves from exiting the grooves. Moreover, the grooves distort the electric field lines closely adjacent to the electrode surfaces to direct electrons into the grooves. As a result, a substantial reduction of secondary emissions are expected with the multistage depressed collector of the present invention, thereby providing a corresponding improvement in collector efficiency. 
     More particularly, a linear beam device comprises a cathode and an anode spaced therefrom, with the anode and cathode being operable to form and accelerate an electron beam. An RF interaction region having a drift tube is arranged relative to the anode to permit the electron beam to pass therethrough. A multi-stage depressed collector of the linear beam device has a plurality of collector electrodes successively arranged to collect spent electrons of the electron beam after passing through the RF interaction region. Each one of the plurality of collector electrodes has a distinct voltage level applied thereto defining a decelerating electric field within the collector. At least one of the plurality of collector electrodes further comprises a collecting surface having a shape that is normal to a coincident trajectory of the spent electrons, whereby a substantial portion of the collecting surface is covered with a plurality of narrow grooves. 
     In an embodiment of the invention, the grooved collector electrode further comprises the final electrode of the collector. The final electrode has a surface that is substantially spherical, and the plurality of grooves may be arranged in a concentric pattern of circles on the electrode surface. The plurality of grooves may be formed to a depth that is approximately twice a corresponding width. A region adjacent to an opening of each of the plurality of grooves comprises electric fields defining a convergent lens, thereby focusing the spent electrons into the plurality of grooves. 
     In another embodiment of the invention, the grooved collector electrode further comprises an intermediate electrode other than the final electrode of the collector. The plurality of grooves are disposed on a front side of the intermediate electrode oriented toward the cathode. The plurality of grooves are arranged in a radial pattern by which the grooves are closely spaced at a region of the collector surface adjacent to the central beam hole. Since relatively few of the electrons strike the front side of the intermediate electrode, and the electrons that do strike the front side tend to impact close to the central beam hole, the radial arrangement of grooves will substantially reduce secondary emission even though a large percentage of the overall surface of the electrode is not covered by grooves. 
    
    
     A more complete understanding of the grooved multi-stage depressed collector for secondary electron suppression will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side cross-sectional view of a prior art linear beam device; 
     FIG. 2 is a side sectional view of a prior art multi-stage depressed collector showing secondary emission from a final electrode; 
     FIG. 3 is a side sectional view of a multi-stage depressed collector in accordance with an embodiment of the present invention; 
     FIG. 4 is a side sectional view of a grooved final electrode in accordance with another embodiment of the invention; 
     FIG. 5 is an end view of the grooved final electrode of FIG. 4; 
     FIG. 6 is a side sectional view of a grooved intermediate collector electrode in accordance with yet another embodiment of the invention; 
     FIG. 7 is an end view of the grooved intermediate collector electrode of FIG. 6; 
     FIG. 8 is an enlarged side sectional view of a grooved final electrode having a rectangular groove shape; and 
     FIG. 9 is an enlarged side sectional view of an alternative grooved final electrode having a triangular groove shape. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention satisfies the need for a multi-stage depressed collector for a linear beam device having an electrode shape that minimizes secondary emissions while promoting efficient electron collection. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more of the figures. 
     Referring first to FIG. 1, a prior art linear beam device  10  is illustrated. The linear beam device  10  includes a cathode  12  having a concave electron emitting surface and a heater filament  16  embedded within the cathode body. A focusing electrode  14  is disposed concentrically around the cathode  12 . An anode  18  is spaced from the cathode and forms a portion of an RF interaction region  22 . An axial beam tunnel  24  extends through the RF interaction region  22 . A filament voltage source E F  is coupled across the heater filament  16  causing the cathode temperature to rise to a level sufficient to permit thermionic emission of electrons from the cathode emitting surface. A cathode voltage source E K  is coupled between the cathode  12  and the anode  18  in order to define a highly negative voltage therebetween that is sufficient to draw and accelerate the emitted electrons into a beam. One or more grids may also be disposed between the cathode  12  and anode  18  in order to density modulate the electron beam. The electron beam extends coaxially along a central axis  20  of the device  10 . The device  10  may further include a magnetic field defined within the interaction region  22  that confines the electron beam within the beam tunnel  24 . The RF interaction region  22  may further include resonant characteristics, such as a slow wave structure, helix, resonant cavities, coupled cavities, and the like, in which the electron beam gives up energy to an RF current that is extracted from the device. Examples of known linear beam devices includes klystrons, traveling wave tubes, inductive output tubes, and other hybrid devices. 
     After passing through the RF interaction region  22 , the spent electron beam passes into a multi-stage collector including a first collector electrode  26  and a second collector electrode  28 . A first collector electrode voltage supply E C1  is coupled between the first collector electrode  26  and the cathode  12  in order to define a first voltage therebetween, and a second collector electrode voltage supply E C2  is coupled between the second collector electrode  28  and the cathode  12  in order to define a second voltage therebetween. It should be appreciated that a greater number of collector electrodes and corresponding voltage supplies could be-advantageously utilized. The voltages applied to the collector electrodes  26 ,  28  define a decelerating electric field within the collector that decelerates the spent electrons, causing them to be collected on one of the electrodes, thereby returning energy to the voltage supplies. 
     FIG. 2 illustrates an exemplary prior art multi-stage depressed collector  30  in greater detail. The multi-stage depressed collector  30  comprises five successive collector electrode stages, including a first electrode  32 , a second electrode  34 , a third electrode  36 , a fourth electrode  38  and a fifth electrode  42 . The electrode stages are separated by electrically insulating cylinders  33 ,  35 ,  37  and  39 , respectively. The first electrode  32  may actually be provided by an end portion of the RF interaction region (see FIG.  1 ). The first four electrodes  32 ,  34 ,  36 ,  38  have a generally annular shape with a hole aligned to the central axis  20  of the device, thereby permitting the spent electrons of the beam to pass therethrough. The fifth (i.e., final) electrode  42  comprises a generally spherical surface that encloses the back end of the collector  30 , though other shapes for the electrode surface such as parabolic, hyperbolic or planar may be selected depending upon the electric field characteristics of the collector so that the surface is normal to the trajectories of incoming spent electrons. The collector electrodes are generally comprised of an electrically and thermally conductive metal material, such as copper, and are each coupled to respective voltage supplies as described above with respect to FIG.  1 . 
     The exemplary trajectories of various ones of the spent electrons of the beam are further shown in FIG.  1 . Electron trajectory (k) depicts an electron that passes through the hole in the first electrode  32 , and then reverses direction and collides into the back side of the first electrode. Electron trajectory (a) depicts an electron that passes through the holes in the first and second electrodes  32 ,  34 , and then reverses direction and collides into the back side of the second electrode. Electron trajectories (b) and (j) depict electrons that pass through the holes in the first, second and third electrodes  32 ,  34 ,  36 , and then reverse direction and collide into the back side of the third electrode. Electron trajectory (c) depicts an electron that passes through the holes in the first, second, third and fourth electrodes  32 ,  34 ,  36 ,  38 , and then reverses direction and collides into the back side of the fourth electrode. The electron trajectories of each of the spent electrons tend to diverge as they penetrate into the collector  30  due to the repellent force of their like electrical charge. If any secondary emissions result from the aforementioned impacts between the electrons and the back sides of the electrodes, the secondary electrons would likely return quickly to the same electrode surface due to the decelerating electric field within the collector. 
     As further shown in FIG. 2, electron trajectories (d), (e), (f), (g), (h) and (i) depict electrons that penetrate all the way to the fifth electrode  42 , and secondary emissions are generated from the electrons having trajectories (g) and (h). A secondary electron is accelerated by electric fields within the collector to impact the second electrode  34  with trajectory (I), and another secondary electron is likewise accelerated to impact the first electrode  32  with trajectory (m). As described above, the secondary emissions represent a degradation of efficiency of the multi-stage depressed collector  30 . It should be appreciated that the electron trajectories depicted in FIG. 2 are merely illustrative of the paths of spent electrons based on mathematical modeling of the electric fields within the collector  30 , and that the actual trajectories of the electrons may be somewhat more complex and/or unpredictable. 
     Referring now to FIG. 3, an embodiment of a multi-stage depressed collector  130  in accordance with the present invention is shown. As with the prior art collector  30  of FIG. 2, the multi-stage depressed collector  130  comprises five successive collector electrode stages, including first electrode  132 , second electrode  134 , third electrode  136 , fourth electrode  138  and fifth electrode  142 . The electrode stages are separated by electrically insulating cylinders  133 ,  135 ,  137  and  139 , respectively. The first electrode  132  may actually be provided by an end portion of the RF interaction region (see FIG.  1 ). The first four electrodes  132 ,  134 ,  136 ,  138  have a generally annular shape with a hole aligned to the central axis  20  of the device, thereby permitting the spent electrons of the beam to pass therethrough. The diameters of the electrode holes increase successively with each of the first four electrodes  132 ,  134 ,  136 ,  138  in correspondence with the diverging paths of electrons within the collector  130  due to space charge effects in the absence of a confining magnetic field. The fifth (i.e., final) electrode  142  comprises a generally spherical surface that encloses the back end of the collector  130 , though other shapes for the electrode surface such as parabolic, hyperbolic or planar may be selected depending upon the electric field characteristics of the collector so that the surface is normal to the trajectories of incoming spent electrons. The collector electrodes may each be comprised of an electrically and thermally conductive metal material, such as copper, and are each coupled to respective voltage supplies as described above with respect to FIG.  1 . 
     Unlike the prior art collector  30  of FIG. 2, the multi-stage depressed collector  130  of FIG. 3 further comprises a plurality of grooves  150  formed in the surface of the final electrode  142 . The walls  152  separating individual grooves  150  have corresponding top surfaces  154 . The grooves  150  are generally narrow and deep such that the depth is greater than the corresponding width. In a preferred embodiment of the invention, the depth of the grooves  150  is at least twice the corresponding width of the grooves. The grooves  150  extend in a direction normal to the surface of the final electrode and are thereby aligned with the direction of trajectory of the incoming spent electrons. In view of the spherical shape of the final electrode  142 , an angle θ defined between the central axis  20  and the direction of each groove  150  measured in the depth dimension of the groove will increase as the distance from the central axis  20  increases. 
     FIG. 8 illustrates an enlarged portion of the final collector electrode of FIG.  3 . The grooves  150  are illustrated as having a generally rectangular cross-section. The thickness of the walls  152  between adjacent ones of the grooves  150  is kept to a minimum so that a large portion (i.e., greater than 75%) of the surface of the final electrode is covered by the grooves  150  as opposed to the tops  154  of the walls  152 . The equipotential electric field lines adjacent to the openings of the grooves  150  are also shown in FIG.  8 . The electric field lines for the collector  130  are substantially the same as in a conventional collector up to a distance very close to the surface of the final electrode. At this close distance to the final electrode, the equipotential-field lines  160   a ,  160   b ,  160   c ,  160   d  are distorted around the openings into the grooves  150  such that a convergent electrostatic lens is define that focuses into each one of the grooves. Corresponding force lines  165  are also shown which represent the direction of force applied by the negative of the electric field upon the electrons. The force lines  165  are normal to the equipotential lines  160 . By the time the spent electrons penetrate this far into the collector  130 , they have already given up most of their energy. The spent electrons then enter the grooves with very little energy, and any secondary electrons that are produced within the grooves tend to remain in the grooves  150  and are not accelerated back to the other collector electrodes. In this respect, the space defined at the bottom of the grooves  150  provides a substantially electric field free region in the same manner as a conventional deep “bucket” collector. 
     Since a large proportion of the surface of the final electrode is covered by the grooves  150 , it is expected that the secondary emissions will be reduced by at least the same proportion. Moreover, the convergent electron lens formed at the openings to the grooves  150  may actually guide electrons by bending their trajectories into the grooves and electron impacts onto the lands  154  separating the adjacent grooves would reduce accordingly. For this reason, the reduction in secondary emission will likely be greater than the actual proportion of the final electrode  142  covered by the grooves  150 , and may be in a range of 80% to 90% reduction of secondary emission. 
     FIG. 9 illustrates an enlarged portion of an alternative embodiment of the final collector electrode of FIG. 3, having grooves  170  illustrated as having a generally triangular cross-section. The walls  172  separating individual grooves converge to form an edge  174 , which may be sharp, squared or rounded. The converging shape of the walls  172  tends to further increase the portion of the surface of the final electrode covered by the grooves  170 . The equipotential field lines  180   a ,  180   b  are distorted around the openings into the grooves  170  in the same general manner as in the foregoing embodiment, and corresponding force lines  165  are also shown which represent the direction of force applied-by the negative of the electric field upon the electrons. It should be appreciated that other cross-sectional shapes of the electrode grooves would also be included within the scope of the present invention, such as semicircular. 
     Referring now to FIGS. 4 and 5, a grooved final electrode  240  in accordance with another embodiment of the invention is shown. The final electrode  240  corresponds generally to the final electrode  142  described above with respect to FIG.  3 . As in the embodiment of FIG. 3, the final electrode  240  includes a spherical collecting surface covered by a plurality of grooves  250  separated by lands  252 . The end view (see FIG. 5) of the surface of the final electrode  240  shows the plurality of grooves  250  as a pattern of concentric circles. It should be appreciated that other patterns, such as a radial or spiral pattern, could also be advantageously utilized. The electrode  240  further includes a plurality of cooling fins  260  that extend radially from the outer perimeter of the electrode collecting surface. The cooling fins  260  are enclosed within a ring  260 . It is anticipated that the final electrode  240  be machined from a workpiece of copper material using a turning tool and a lathe, such that the workpiece can be moved at an angle with respect to the lathe rotating the workpiece. This way, the spherical surface can be formed on the electrode  240  and the concentric grooves  250  can be cut into the. spherical surface in a direction normal to the spherical surface. The cooling fins  260  may be thereafter affixed to the machined electrode  240 , such as by brazing or soldering. 
     FIGS. 6 and 7 illustrate an intermediate collector electrode  300  in accordance with yet another embodiment of the invention. The intermediate collector electrode  300  corresponds generally to any one of the electrodes  134 ,  136 ,  138  described above with respect to FIG.  3 . The electrode  300  includes a generally funnel-shaped body  302  having a trailing surface  304  and a leading surface  305 . The shape of the electrode body  302  is determined by the desired electric field characteristics of the collector so that the electric field is normal to the trajectories of incoming spent electrons. The electrode  300  would be oriented with the leading surface  305  facing in a direction toward the cathode of the linear beam device. The electrode  300  further includes a hole  310  aligned to the central axis  20  of the linear beam device. The electrode  300  further includes a plurality of cooling fins  306  that extend radially from the outer perimeter of the electrode body  302 . The cooling fins  306  are enclosed-within a ring  308 . 
     As described above, most incoming spent electrons would pass through the hole  310  and impact onto the trailing surface  304  after reversing direction. To minimize secondary emissions from the relatively few electrons that strike the leading surface  305 , the leading surface is provided with a plurality of radially extending grooves  312  (see FIG.  7 ). The grooves  312  function in the same manner as the grooves  150 ,  250  in the final electrode described above. Particularly, any secondary electrons produced by electrons that enter into the grooves  312  will tend to remain within the grooves. The radial orientation of the grooves  312  provided on the leading surface  305  of the electrode  300  results in the grooves being relatively closely spaced together at the central edge of the electrode close to the hole  310 , and the spacing between grooves  312  becomes increasingly greater as, the distance from the hole  310  increases. As a result, the majority of the surface area of the leading surface  305  is not covered by the grooves  312 , unlike the final electrodes described above. Since it is anticipated that most electrons that strike the leading surface  305  of the electrode  300  will impact in the region close to the edge of the hole  310 , and will rarely strike farther outward on the electrode surface, it is, believed that the high concentration of grooves in the likely impact region will have a sufficiently beneficial effect in reducing most secondary emission from the front side of the electrode. It should also be appreciated that other groove configurations, such as concentric circles, could also be advantageously utilized. 
     Having thus described a preferred embodiment of a grooved multi-stage depressed collector for secondary electron suppression, it should be apparent to those skilled in the art that certain advantages of the aforementioned system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.