Abstract:
The invention relates to linear beam amplification devices having an electron emitting cathode and an RF modulated grid closely spaced therefrom, and more particularly, to a novel support structure for the grid that accommodates thermal expansion while maintaining an optimum grid-to-cathode spacing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to cathode-grid assemblies for linear beam microwave vacuum tube devices having an electron emitting cathode and a microwave modulated grid closely spaced therefrom, and more particularly, to such an assembly including a support structure for the grid, wherein the support structure accommodates differential thermal expansion of a cathode assembly and the grid while maintaining an optimum grid-to-cathode spacing. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art to utilize a linear beam microwave vacuum tube device, such as a klystron or traveling wave tube amplifier, to generate or amplify high frequency, microwave RE energy. Such devices generally include an electron emitting cathode, an anode spaced therefrom, and a grid positioned in an inter-electrode region between the cathode and the anode. Grid to cathode spacing is directly related to the performance and longevity of the linear beam device. A problem that has long existed in the art is that during initial heat up, the grid to cathode spacing changes as the cathode is heated, thereby causing performance and reliability problems. 
     Prior solutions to this problem suggested a grid support structure that is closely connected to the cathode button. These solutions however required complicated mechanical means to deal with the different radial thermal expansion of cathode and grid. In order to electrically insulate the cathode and the grid a plurality of ceramic members was needed to connect the grid to the cathode button. These ceramic members create a plurality of difficulties because the ceramic members are mechanically stressed from the expansion difference. Thus, it would be very desirable to provide a cathode support structure for a linear beam device that maintains a proper spacing between the cathode and grid across the operating temperature range of the device. It would be further desirable to provide such a grid support structure which is formed of a one-piece ceramic. Further, some cases are known where the cathode support cylinder has changed its shape over time due to thermal stress by many heat cycles. In a grided tube with a grid support independent from the cathode button this would cause the cathode to short out with the grid or at least change the initial cathode grid spacing. In both cases the tube will fail early. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect a grid support structure maintains a proper grid-to-cathode spacing across an operating temperature range of the linear beam device. 
     Another aspect of the present invention also provides a cathode grid connection that allows the grid to follow all cathode movements. 
     In one aspect of the present invention a linear beam device has an axially centered cathode and an anode spaced therefrom. The anode and cathode are operable to form and accelerate an electron beam. The linear beam device includes an axially centered grid positioned between the cathode and the anode. The grid is operable to accept a high frequency control signal to density modulate the electron beam. A grid support is in contact with the cathode and the grid and keeps the spacing between the cathode and the grid constant, while electrically insulating them. 
     It is another aspect of the present invention to provide a linear beam device having a cathode and an anode. A linear beam device includes a grid positioned at a predetermined distance from the cathode between the cathode and the anode. The grid is operable to accept a high frequency control signal to density modulate a beam. A grid support supporting the grid which is operable to maintain the predetermined distance between the cathode and the grid throughout the operating temperature range of the linear beam device. 
     Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: 
     FIG. 1 is a side cross-sectional view of a temperature compensated gun according to a preferred embodiment of the present invention; 
     FIG. 2 is an enlarged cross-sectional view of the cathode-grid assembly of the gun of FIG. 1; and 
     FIG. 3 is a side cross-sectional view of the grid support of FIGS.  1  and  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention satisfies the need for a grid support structure for a linear beam microwave vacuum tube device that maintains a proper spacing between the cathode and grid across the operating temperature range of the device. It should be understood that although terms such as “above” and “below” are used herein, these terms are to be interpreted in the relative sense as the linear beam device or temperature compensated gun is usable in any orientation. 
     Referring first to FIG. 1, the temperature compensated gun of a linear beam device, generally indicated at  10 , is illustrated according to the present invention. Because the gun operates conventionally, and the arrangement of the gun is known to one of ordinary skill, other than the inventive grid support structure of the present invention, the gun and the components illustrated in FIG. 1 will only be described briefly and generally. 
     As illustrated in FIG. 1, linear beam microwave vacuum tube device  10  includes a temperature compensated gun, i.e., cathode-grid, assembly, generally indicated at  12 , a heater assembly  14 , a cathode assembly  16 , a planar anode-pole flange  18  connected to an anode-drift tube  20 , an input ceramic  22 , a focus ring  24 , a grid connection  26  and a cathode support connection  28 . The heater assembly  14  extends into the cathode assembly  16  without touching it. The anode includes a central aperture, and by applying a high voltage potential between the cathode  40  and the anode-pole flange  18 , electrons can be drawn from the cathode surface and directed into a high power beam that passes through the anode aperture. Gun  12  is particularly useful in one class of linear beam microwave vacuum tube devices, referred to as an inductive output tube (IOT) which includes a grid  30  disposed in the inter-electrode region between the cathode  40  and the anode  20 . The electron beam can thus be density modulated by applying an RF signal to the grid  30  relative to a cathode  40 . As the density modulated beam is accelerated to the anode and propagates across a gap provided downstream within the TOT, RF fields are induced into a cavity coupled to the gap. The RF fields can then be extracted from the cavity in the form of a high power, modulated RF signal. An example of an TOT is disclosed by U.S. Pat. No. 5,650,751 to R. S. Symons, entitled “INDUCTIVE OUTPUT TUBE WITH MULTISTAGE DEPRESSED COLLECTOR ELECTRODES PROVIDING A NEAR-CONSTANT EFFICIENCY,” the subject matter of which is incorporated in the entirety by reference herein. 
     A grid support structure is illustrated in FIGS. 1-3, in which the linear beam microwave vacuum tube device  10  includes the axially centered grid  30  disposed in close proximity to the cathode  40 . To permit high RF voltage and high RF gain, it is desirable to space the grid  30  close to the cathode  40  surface. The grid support structure prevents, during start-up, the cathode  40  from moving toward the grid  30 . If the cathode  40  moves toward the grid  30 , then: 1) a change in perveance occurs during heat-up; 2) there is a possibility to short out the cathode and the grid; and 3) there is a variance in perveance. More particularly, the axially centered grid  30  is operable to accept a microwave high frequency control signal to density modulate an electron beam emitted by the cathode  40 . The grid  30  comprises a central active portion  34  and a peripheral portion or grid flange  36  with the peripheral portion comprising a plurality of evenly spaced mounting holes. The grid  30  is comprised of pyrolytic graphite material. The cathode  40  comprises a concave electron emitting surface  42  and the active portion  34  of the grid comprises a concave shape that corresponds with the emitting surface  42 . The concave electron emitting surface  42  and the grid  30  are concentric spheres, having the same center so that the grid  30  and emitting surface  42  are generally parallel to each other. The grid  30  is secured in place by a grid support structure (described below). The grid flange  36  is flat and lies in a plane that is substantially normal to the axis of the electron beam emitted by the cathode  40 . 
     The cathode assembly  16  is bolted to a cylindrical lower support  44  which in turn is connected to an upper support  46 . The lower support  44  has a plurality of threaded bolt holes  48  and is connected to a cathode flange  51  through corresponding bolt holes  55  in the cathode flange  51 . The cathode flange  51  has an annular recess  53  which receives one end  54  of a cylindrical molybdenum cylinder  56 . The end  54  of the molybdenum cylinder  56  is brazed to the recess  53  of the cathode flange  51 . An opposite end  57  of the molybdenum cylinder  56  is brazed to the cathode  40 . Since it is desirable to space the grid  30  closely to the cathode  40  surface, the grid  30  must be capable of withstanding very high operating temperatures. In view of these demanding operating conditions, it is known to use pyrolytic graphite material for the grid  30  due to its high dimensional stability and heat resistance. The pyrolytic graphite grid  30  may be made very thin, with a pattern of openings formed therein, such as by conventional laser trimming techniques, to permit passage of the electron beam therethrough. Because of the low coefficient of expansion of the pyrolytic graphite, heating of the grid  30  (by direct thermal radiation from the cathode  40  and by dissipation of RE drive power applied between the cathode  40  and grid  30 ) does not cause the grid  30  to expand into the cathode  40  and short circuiting of these two elements does not occur. As a result, the grid  30  can be positioned very close to the cathode  40  surface  42 , permitting high RE drive voltage and high gain. Nevertheless, a practical limitation on the efficiency of such linear beam devices has been the difficulty of supporting the cathode  40  in a proper position relative to the grid  30 . 
     Heater assembly comprises an insulated flange package  62  connected to two posts (one has heat shields). Posts are connected to a heating element  64 . The flange package is bolted to a heater connection  60  (upper flange) and a “ground” connection  66  (lower flange) which is at cathode potential. The heating element  64  is spaced from the cathode  40 . The grid  30  is mechanically connected through the grid support  114  to the cathode  40  and moves together with the cathode  40  as the cathode assembly expands. 
     As previously mentioned, in a linear beam device, such as a microwave electron beam vacuum tube with a gun, driven with RF applied to a grid, the spacing between the cathode  40  and the grid  30  must be precisely maintained because the spacing is in the range between 0.005 and 0.010 inches; this spacing is necessary to make the tube work at microwave frequencies, e.g., close to 1 GHZ, i.e., the grid to cathode spacing is a fraction of a wavelength of the tube operating frequency. 
     In operation, when the tube operation is started the cathode  40  is heated and expands towards the grid  30 . As depicted in FIG. 1, for example, the molybdenum cylinder  56  expands when the heating elements  64  are energized. Because the cathode  40  is rigidly connected to molybdenum cylinder  56  during a transient heat up condition, the grid cathode spacing would change if the cathode  40  were to move toward the grid  30 . If such movement is not prevented, the heating would cause a change in the cathode  40  to grid spacing if the grid support structure is not closely connected directly to the cathode  40 . The change in spacing would disadvantageously cause: 
     (1) A change in perveance during heat up. Applying constant beam and grid voltage the beam current would change during the first 15 to 20 minutes of operation after applying heater voltage. For many tube applications this long waiting time to get stable operation is unacceptable so that the only other solution is to constantly preheat the cathode (=stand by). This causes a constant evaporation of barium from the cathode  40  and limits the lifetime of the gun  10 . In many applications it would be desirable to reduce the total heat up time to less than five minutes. 
     (2) A possibility to short out the cathode  40  and the grid  30 . Especially in applications where the cathode  40  temperature is variable due to a variable heater voltage, cathode  40  might expand into the grid  30  to cause a short circuit between them. This will immediately damage both cathode  40  and the grid  30  and must be avoided. Tubes with tungsten dispenser type cathodes can usually be recovered from weak emission by overheating the cathode for the regeneration of barium on its surface. In the case of a tube with a grid, however, overheating might cause the cathode  40  to expand more than the gun was designed for and short out with the grid. This means that the useful tool of overheating the cathode cannot be used for a grided electron beam tube with small cathode to grid spacing. 
     (3) A variation in perveance depending on the cathode  40  temperature. As described with regard to the change in perveance during heat up, the expansion of the cathode  40  would decrease the spacing between cathode and grid. In many applications it is desirable to vary the cathode heating during the lifetime of the tube to optimize the barium production of the cathode and by this stabilize and secure the emission. Within the first couple hundred hours of operation the cathode should be heated slightly more to stabilize the barium production. Once the barium production is stable enough the cathode can be operated at lower temperature to evaporate less barium. This increases the lifetime of the cathode. When the tube reaches the end of its lifetime many operation hours can be added by increasing the cathode temperature to activate more barium. This procedure is well known for television klystrons and many other electron beam tubes. However, it is difficult or impossible to apply this procedure to a grided tube if the spacing between cathode and grid depends on the cathode temperature. So it is desirable to have a grided gun with constant cathode to grid spacing. 
     The electrical and mechanical connections of the grid  30  to cathode  40  via grid support structure  114  are illustrated in detail in FIG. 2. A copper foil  90  is disposed between a grid connection support  80  and the focus ring  24 . The thin copper foil  90  is used to provide electrical contact to the grid  30  through the grid connection support  26  (FIG. 1) and the grid connection support  80 . The copper foil  90  also has a plurality of evenly-spaced holes aligned with holes  84  of the grid connection support  80 . Tightening of the bolts  91  holding the focus ring  24  to the holes  84  in the grid connection support  80  compresses the copper foil  90  so that the foil conforms to support  80  and ring  24 . During high temperature “bake-out” of the linear beam device  10 , the copper foil  90  softens to reduce internal stress. The copper foil  90  has a portion  92  which extends inwardly and which has a plurality of substantially evenly spaced holes  94 . The foil is bolted together by bolts  96  with the grid flange  36  and the grid support  114  through corresponding bolt holes. The copper foil  90  provides for expansion and is flexible and has a fold or stepped portion  97  to provide for cathode  40  movement. For better heat transfer, the copper foil  90  can be constructed from a plurality of foils. An inner portion  98  of the copper foil  90  is positioned radially inwardly from bolts  96  and is clamped between a grid cover ring  110  and a flange  120  of grid support  114  together with the grid flange  36 . Disposed below and adjacent to a lower surface  106  of the stepped portion  97  is an upper surface  112  of the grid flange  36  of the grid  30 . The grid cover ring  110  is positioned below a lower surface  112  of the grid flange  36 . The grid cover ring  110  is made of a glassy carbon. The grid cover ring  110  could be left out if the grid flange  36  is thick enough to distribute the bolt  96  force evenly enough to get good contact between the grid flange  36  and the copper foil  90 . Also, instead of the glassy carbon, one could use small segments of stainless steel or any other metal or ceramic. Glassy carbon was chosen because it has the same expansion coefficient as the grid  30  and the grid support  114  while it is less expensive than PBN or pyrolytic graphite. The grid cover ring  110  is an annular member having a plurality of bolt holes matching the holes of the grid flange and grid support. The bolts  96  tighten the grid support  114 , the copper foil  90 , the grid flange  36  and the grid cover ring  110  together. 
     As depicted in FIGS. 2 and 3, the grid support  114  has an outwardly extending flange portion  120 , an intermediate vertically extending portion  122  and an inwardly extending lip  124  which together form a cup-like structure. Four (or more) circumferentially spaced and inwardly extending slots  126  are cut in the inwardly extending lip  124  and partially into the vertically extending portion  122  to provide flexibility in the grid support  114 . The cathode  40  has an outer button portion  86  which has an inwardly extending annular groove  88  which receives the lip  124  of the grid support  114 . 
     The grid support  114  is a one-piece ceramic structure to support the grid  30  and directly connect it to the cathode  40 . The grid support  114  is made from a pyrolytic boron nitride (PBN) ceramic. The grid support  114  has a cup shape with its bottom removed and has a thin slotted wall that is flexible enough to be clipped to the cathode  40  like a spring. The grid support  114  can also be brazed to the outside diameter of the cathode  40 . The slots  126  of the grid support  114  also cause the expanding cathode  40  to only bend the remaining tab formed sections of the cylindrical part of the grid support  114 , to prevent substantial stressing of the flange shaped portion. The material provides a minimal heat transfer characteristic so the grid  30  is not additionally heated by conduction. The flexibility and other mechanical properties of PBN are fairly stable up to 2000° C., so that grid support  114  does not substantially change size as a result of operation of device  10 . The machinable ceramic is machined to very small tolerances so no structure is necessary to align support  114  axially and radially to the cathode  40 . The ceramic of support  114  provides a non-moving, non-expanding mounting platform for the grid  30  that keeps the cathode  40  to grid  30  spacing stable at all temperatures. The surface of vertically extending portion  122  of the grid support  114  facing the grid  30  forms a mounting platform and is shaped as a flange. The flange  120  has a plurality of holes  128  through which the bolts  96  extend. The grid  30  is made of pyrolytic graphite which has nearly the same expansion coefficient as PBN which is used to form the ceramic support  114 . Therefore, the grid-ceramic connection remains unstressed at all operating temperatures of device  10 . A glassy carbon flange  110  on top of the grid flange  36  provides distribution of the clamping force. The glassy carbon flange could also be formed of thin stainless steel flange sections. 
     The grid  301  cathode  40  spacing can be adjusted by choosing the right number of shims between the grid rim  36  and ceramic flange  120 , i.e., the number of foils  90  between rim  36  and flange  120  determines the spacing between grid  30  and cathode  40 . The axial alignment is provided by the holes in the grid rim that are large enough to allow for adjustment before tightening the screws. 
     During operation of the linear beam device  10 , the pyrolytic graphite material of the grid  30  experiences slight thermal expansion. The cathode  40  on the other hand exhibits some thermal expansion in both the axial and radial directions. The material composition of the grid support  114  and the grid  30  and the grid cover ring  110  are selected to have similar coefficients of expansion and thus expand and contract at a uniform rate. As the cathode  40  expands in the radial direction, the grid support  114  flexes outwardly. Thermal expansion in the axial direction is basically caused by the molybdenum cylinder  56 . The axial expansion of cylinder  56  moves the cathode  40  together with the grid support  114  and the grid  30  and leaves the cathode  40  to grid  30  spacing basically constant. The only portion of cathode  40  that expands into the grid  30  is the part of the cathode  40  between the grid  30  and the inwardly extending annular groove  88  which is very small and causes only an acceptable variation in spacing. 
     It will be readily seen by one of ordinary skill in the art that the present invention fulfills all of the objects set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.