Abstract:
A coupled cavity circuit for a microwave electron tube comprises at least two resonant cavities adjacent to each other. An electron beam tunnel passes through the coupled cavity circuit to allow a beam of electrons to pass through and interact with the electromagnetic energy in the cavities. An iris connecting the adjacent cavities allows electromagnetic energy to flow from one cavity to the next. The iris is generally symmetrical about a perpendicular axis of the electron beam tunnel with the iris having flared ends and a central portion connecting the flared ends. The iris shape causes the iris mode passband to be lower in frequency than the cavity mode passband while still providing broadband frequency response.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to microwave amplification tubes, such as traveling wave tubes or klystrons, and more particularly, to a coupled cavity microwave electron tube that produces an inverted slot mode and broadband response. 
     2. Description of Related Art 
     Microwave amplification tubes, such as a traveling wave tube (TWT) or klystron, are well known in the art. These devices are designed so that a radio frequency (RF) signal and an electron beam are made to interact in such a way as to amplify the power of the RF signal. A coupled cavity TWT typically includes a series of tuned cavities that are linked or coupled by irises (also called notches or slots) formed between the cavities. A microwave RF signal induced into the tube propagates through the tube, passing through each of the respective coupled cavities. A typical coupled cavity TWT may have thirty or more individual cavities that are coupled in this manner. Thus, the TWT appears as a folded waveguide and the meandering path that the RF signal takes as it passes through the coupled cavities of the tube reduces the effective speed of the signal so that the electron beam can effectively operate upon the signal. Thus, the reduced velocity waveform produced by a coupled cavity tube of this type is known as a “slow wave.” 
     Each of the cavities is further linked by an electron beam tunnel which extends the length of the tube and through which an electron beam is projected. The electron beam is guided by magnetic fields which are induced in the beam tunnel region and the folded waveguide guides the RF field periodically back and forth across the drifting electron beam. Thus, the electron beam interacts with the RF signal as it travels through the tube to produce the desired amplification by transferring energy from the electron beam to the RF wave. 
     Klystrons are similar to coupled cavity TWTs in that they can comprise a number of cavities through which an electron beam is projected. The klystron amplifies the modulation on the electron beam to produce a highly bunched beam containing a RF current. A klystron differs from a coupled cavity TWT in that the klystron cavities are not generally coupled. A portion of the klystron cavities may be coupled, however, so that more than one cavity can interact with the electron beam. This particular type of klystron is known as an extended interaction output klystron. 
     For a coupled cavity circuit, the bandwidth over which the amplification of the resulting RF output signal occurs can be controlled by altering the dimensions of the cavities and irises, and the power of the RF output signal can be controlled by altering the voltage and current characteristics of the electron beam. More specifically for the bandwidth, as the cavity narrows it propagates higher frequencies and as the iris narrows it propagates fewer frequencies. 
     There are generally two frequency bands of interest in which propagation can occur. The lower frequency band is referred to as the “cavity passband” because its characteristics are controlled largely by the cavity resonance condition. The upper frequency band is referred to as the “iris passband” and its characteristics are controlled mainly by the iris resonance condition. Normally, the cavity passband is used for interaction with the electron beam. As the length of the iris increases, the cavity resonance condition, usually appearing at the 2π point on the lower passband of the dispersion curves, changes position with the iris resonance condition that appears at the 2π point on the upper passband. When this passband mode inversion occurs (cavity passband and iris passband trading relative positions—also known as inverted slot mode), it provides an advantage in preventing drive-induced oscillations and thus no special oscillation suppression techniques are required. Note that the mechanism of exciting the oscillations with a decelerating beam crossing a cavity resonance point is well known. 
     Unfortunately, to produce this passband mode inversion, the iris length is usually to such an extent that it wraps around the electron beam tunnel. This has the disadvantage of introducing transverse magnetic fields when the iris lies in an iron pole piece. Furthermore, a significant problem with RF amplification tubes is the efficient removal of heat. As the electron beam drifts through the tube cavities, heat energy resulting from stray electrons intercepting the tunnel walls must be removed from the tube to prevent reluctance changes in the magnetic material, thermal deformation of the cavity surfaces, or melting of the tunnel wall. The excessive iris length and corresponding reduction in the amount of metal results in a longer heat flow path around the iris. Thus the ability to remove heat is significantly reduced along with the overall coupled cavity circuit&#39;s thermal ruggedness. 
     Accordingly, it would be desirable to provide a coupled cavity circuit having an iris that produces the passband mode inversion without the excessive iris length. Also, it would be desirable for the coupled cavity circuit to have a broadband frequency response while preventing drive-induced oscillations so that no special oscillation suppression techniques are required. Furthermore, it would be desirable for such a coupled cavity circuit to offer a significant increase in the amount of metal that is provided around the electron beam tunnel such that a passband mode inversion occurs without an increase in transverse magnetic fields or degradation in thermal ruggedness. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a coupled cavity circuit is provided with an iris that produces passband mode inversion such that the iris mode passband is at a lower frequency than the cavity mode passband. In addition, the coupled cavity circuit also provides broadband frequency response while preventing drive-induced oscillations so that no lossy material is required within the coupled cavity circuit. Furthermore, the coupled cavity circuit provides these advantages without requiring an excessive iris length and thus avoids any severe increase in transverse magnetic fields or degradation in thermal ruggedness. 
     In an embodiment of the present invention, a microwave electron tube, such as a traveling wave tube or an extended interaction klystron, comprises an electron gun for emitting an electron beam through an electron beam tunnel to a collector that collects the electrons from the electron beam. A slow wave structure is disposed along the electron beam path and defines an electromagnetic path along which an electromagnetic signal interacts with the electron beam. The slow wave structure has at least one coupled cavity circuit comprising at least one iris disposed between a first cavity and a second cavity for coupling the electromagnetic signal between the first cavity and the second cavity. The iris is disposed between the electron beam tunnel and a sidewall of the tube with the iris being symmetrical about a perpendicular axis of the electron beam tunnel. The iris has a center portion with a first width and flared ends with a second width that is greater than the first width. The flared ends wrapping partially around the electron beam tunnel. 
     In a second embodiment of the present invention, the coupled cavity circuit of the slow wave structure has a rectangular shape. The iris has a rectangular central portion that extends substantially across one sidewall of the tube. The iris has flared ends that form a triangular region at each end of the central portion. The triangular regions have a hypotenuse that is adjacent to the electron beam tunnel and a side that extends part way along a sidewall of said tube that is adjacent to the one sidewall of the tube. 
     If there is more than one coupled cavity circuit, the irises can be in line, staggered, or on opposite sides of the tube. There can also be more than one iris per coupled cavity circuit with the irises in line or staggered from each other. The iris shape provides the inverted slot mode condition and broadband response without excessive iris length. 
     A more complete understanding of the coupled cavity circuit 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 that will first be described briefly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial perspective view of a typical coupled cavity portion of a cylindrical microwave electron tube; 
     FIG. 2 is a partial perspective view of a typical coupled cavity portion of a rectangular microwave electron tube; 
     FIGS. 3 a ,  3   b , and  3   c  are cross-sectional views of a polepiece taken along line  2 — 2  of FIG. 1; 
     FIGS. 4 a ,  4   b , and  4   c  are graphs illustrating the passband mode inversion that occurs as the iris length increases; 
     FIG. 5 is a cross-sectional view of a rectangular polepiece showing an iris according to an embodiment of the present invention; 
     FIG. 6 is a perspective view of an integral polepiece RF amplification tube utilizing an iris according to an embodiment of the present invention; 
     FIG. 6A is a perspective view of an alternative embodiment of an integral polepiece RF amplification tube; 
     FIG. 7 is an exploded view of the integral polepiece RF amplification tube of FIG. 5; 
     FIG. 8 is a cross-sectional view of the interior of the integral polepiece RF amplification tube, as taken through the Section  8 — 8  of FIG. 6; 
     FIG. 9 illustrates a side sectional view of a coupled cavity TWT amplifier with a standard PPM polepiece stack that utilizes an iris according to an embodiment of the present invention; 
     FIG. 10 illustrates a side sectional view of a coupled cavity microwave amplification tube assembled to an electron gun and a collector; 
     FIG. 11 is a graph illustrating the electric fields across the cavity gap at a cavity resonance frequency for a coupled cavity circuit that utilizes an iris according to an embodiment of the present invention; and 
     FIG. 12 is a graph plotting the frequency versus the normalized wave number for a coupled cavity circuit that utilizes an iris according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention satisfies the need for a coupled cavity circuit that provides passband mode inversion without requiring an excessive iris length. As a result, the coupled cavity circuit provides broadband response without introducing a severe increase in transverse magnetic fields or degradation in thermal ruggedness. Furthermore, the coupled cavity circuit prevents drive-induced oscillations and therefore no special oscillation suppression techniques such as lossy material is required in the circuit. 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 typical coupled cavity cylindrical traveling wave tube  10  is shown. Because the coupled cavity section may be of any desired length, the coupled cavity TWT  10  is shown broken away from an input or output section of the TWT. In addition, although the coupled cavity TWT  10  is shown as being cylindrical in shape, it should be understood that the coupled cavity TWT  10  may alternatively be rectangular or any other shape, as known in the art. The coupled cavity structure includes a plurality of adjacent cavities  26  separated by polepieces  34 . The polepieces  34  comprise disk shaped elements dividing the cylindrical shaped cavities  26 . The cavities  26  are coupled by coupling irises  35  that extend through a portion of each of the polepieces  34 , thus providing a meandering path  40  for the traveling RF wave. An electron beam tunnel  14  extends along an axis of the TWT through a central portion of each polepiece  34  permitting passage of an electron beam  13  through each cavity  26 . 
     FIG. 2 is a typical coupled cavity rectangular traveling wave tube  15  and, as with FIG. 1, is shown broken away from an input or output section of the TWT. The coupled cavity structure for the coupled cavity TWT  15  includes a plurality of adjacent cavities  24  separated by rectangular polepieces  32 . The rectangular polepiece  32  has an iris  33  and an electron beam tunnel  11 . As seen in FIG. 2, the iris  33  is typically rectangular in shape to correspond with the rectangular shape of the coupled cavity TWT  15 . 
     Referring now to FIGS. 3 a ,  3   b , and  3   c , each figure shows a cross sectional view, taken along line  2 — 2  of FIG. 1, of the polepiece  34 . Above each polepiece  34 , the respective length of the iris  35  is illustrated by L θ  where L θ  is the iris circumference length for a corresponding iris angle θ with origin centered at the electron beam tunnel. As discussed above, as the iris length L θ  varies, this changes the relative positions of the cavity mode passband and iris mode passband. This change in relative positions of the passbands is illustrated by the graphs of FIGS. 4 a ,  4   b , and  4   c . Specifically, FIGS. 4 a ,  4   b , and  4   c  graph the coupled cavity circuit response for frequency (ω c ) versus the normalized wave number (wave number β times the circuit period L divided by π) generated by the respective iris length L θ  of FIGS. 3 a ,  3   b , and  3   c.    
     FIG. 3 a  illustrates the typical iris length L θ  and FIG. 4 a  illustrates the corresponding coupled cavity circuit operation for the iris length L θ  shown in FIG. 3 a . As can be seen in the graph of FIG. 4 a , the cavity mode passband is lower in frequency than the slots mode passband. In this configuration, the cavity mode passband is typically the passband used to interact with the electron beam. As the iris length L θ  increases, the cavity mode passband and slots mode passband migrate closer to each other until the two unite, as shown in FIG. 4 b  for the corresponding iris length L θ  of FIG. 3 b . When the two modes merge, this is referred to as the coalesced mode. 
     As the iris length continues to increase, the cavity mode passband becomes the upper frequency band and the slots mode passband becomes the lower frequency band, as shown in FIG. 4 c  for the corresponding iris length L θ  of FIG. 3 c . This is referred to as inverted slot mode or passband mode inversion. Passband mode inversion allows the slots mode passband to function as the primary passband for interaction with the electron beam. Furthermore, passband mode inversion prevents drive-induced oscillations because, for the slots mode passband, the interaction impedance at the upper cutoff frequency is zero due to the vanishing axial electric field component on the axis. Thus, for the slots mode passband, no special oscillation suppression techniques are required such as lossy material placed within the coupled cavity circuit. 
     However, FIG. 3 c  shows that the iris length L θ  required to induce passband mode inversion is extensive. The iris within the polepiece wraps almost completely around the electron beam tunnel. This has the disadvantage of introducing transverse magnetic fields when the iris lies in an iron pole piece. In addition, due to current interception, heat is generated on the electron beam tunnel wall. Thus, the long iris length results in a longer heat flow path around the iris and therefore causes a decrease in the coupled cavity circuit&#39;s thermal ruggedness. 
     Referring now to FIG. 5, a rectangular polepiece  44  for a coupled cavity circuit shows the iris  55  according to an embodiment of the present invention. The large triangular opening  37  with a width W 2 , on each end of the iris  55 , increases both the bandwidth and the impedance of the circuit. This results, as noted above, because a broader iris allows the propagation of a greater number of frequencies. The iris  55  has an iris center width W 1 . The narrow separation of the iris center width W 1  increases the iris capacitance and thereby lowers the iris resonance frequency so that the coupled cavity circuit becomes stable in reference to drive-induced oscillations. Thus, the iris  55  induces passband mode inversion so that the iris mode passband is used to interact with the electron beam traveling through an electron beam tunnel  9 . Furthermore, the shape of the iris  55  induces the passband mode inversion without requiring the excessive iris length, such as illustrated in FIG. 3 c  for the prior art, and thus there is no severing of the magnetic flux from the periodic permanent magnet (PPM) focusing fields. 
     As can be seen in FIG. 5, the iris  55  according to an embodiment of the present invention has a much shorter iris length relative to the circumference of the electron beam tunnel  9  than in typical prior art irises such as illustrated in FIG. 3 c . The iris  55  thus produces passband mode inversion without the disadvantages discussed above. The shorter iris length results in a shorter heat flow path out from the electron beam tunnel wall and thus the coupled cavity circuit&#39;s thermal ruggedness is increased. Furthermore, the shorter iris length reduces any significant increase in transverse magnetic fields when the iris lies in an iron polepiece. 
     Referring now to FIG. 6, a perspective view of an integral polepiece RF amplification tube  20  is shown utilizing an iris in accordance with an embodiment of the present invention. The tube  20  comprises a plurality of non-magnetic plates  18  and magnetic plates  16  (also known as polepieces) which are alternatingly assembled and integrally formed together. The assembled tube  20  has end plates  12  disposed on either end and an electron beam tunnel  9  that extends through the end plates  12  and fully lengthwise through the tube  20 . The tube  20  has a top  23  and a bottom  25  opposite the top  23  that provide a planar surface for attachment of a heat sink. The tube  20  has a one side  27  and a second side  29  opposite the one side  27  which are flush with edges of the non-magnetic plates  18  and the polepieces  16  except for individual ones of the polepieces  16  that extend outward from the one side  27  and the second side  29  to provide ears  36 . The ears  36  provide a mounting position  38  for the installation of magnets (not shown). A more detailed description of integral polepiece RF amplification tubes is given in U.S. Pat. Nos. 5,332,947 and 5,534,750 and these are hereby incorporated by reference. 
     The polepieces  16  have an iris  55  (or notch), according to an embodiment of the present invention, disposed at an edge. As best shown in FIG. 7, the position of the notch  55  in polepiece  16   1  appears at the top  23 . The next polepiece  16   2  has a notch  55  disposed at the bottom  25 . The third polepiece  16   3  would again feature the notch  55  at the top side  23 , similar to that of polepiece  16   1 . Alternatively, the notch positions could all remain on a single side (the one side  27  or the second side  29 ), top  23 , or bottom  25  of the TWT  20 , or could be a combination of the two configurations having a portion of the notches  55  disposed at the top  23  and a portion disposed on the bottom  25 . Thus the notch  55  can be arranged in an in-line, staggered, alternating configuration, or any combination of the above or other geometric arrangement. In yet another embodiment, a single polepiece  16  could have more than one notch  55 , such as one at both ends of the polepiece  16 . FIG. 7 illustrates an exploded view of the integral polepiece RF amplification tube  20  of FIG. 6, and FIG. 8 illustrates a sectional view of the integral polepiece RF amplification tube  20  of FIG.  6 . 
     The notches  55  provide a coupling path for neighboring cavities  56  (see also FIG. 6) formed in the non-magnetic plates  18  that are adjacently positioned relative to the polepieces  16  and alternate with the polepieces  16 . The cavity  56  can be shaped, at each end, similar to notch  55  to aid in RF propagation and further the desired characteristics. Thus a continuous path  40 , visible in the sectional drawing of FIG. 8, through the tube  20  is provided that utilizes a notch shape according to an embodiment of the present invention as in FIG.  5 . 
     Alternatively, to vary the RF propagation characteristics, the cavity  56  could extend between the one side  27  and the second side  29  rather than the top  23  and the bottom  25  as shown in FIG.  6 A. The cavity direction could also alternate between a first direction extending between the top  23  and the bottom  25  and a second direction extending between sides  27  and  29  (not shown). Additionally, it should also be apparent that cavities  56  could be provided in polepieces  16  as well as the non-magnetic plates  18  (not shown). Likewise, the notches  55  could be provided in the non-magnetic plates  18  as well as the polepieces  16  as desired to produce desired tube characteristics (not shown). Therefore, as indicated above, there are a large number of arrangements and layouts for the cavities  56  in relation to the notches  55  that are in accordance with an embodiment of the present invention for the coupled cavity circuit. 
     It should also be understood that there are many variations of the iris  55  of FIG. 5 that are in accordance with embodiments of the present invention that would provide the required capacitive loading of the iris  55  in order to invert the cavity mode and slot mode passbands. Furthermore, the present invention can be utilized with one or more of the electron beam focusing schemes used in the art today, such as: 1) PPM focusing where the iron polepieces extend directly through to the electron beam tunnel, 2) PPM focusing where the iron polepieces are spaced from the electron beam tunnel, 3) permanent magnet focusing, and 4) solenoid focusing. FIG. 6 illustrated an example of the first type of focusing scheme, referred to as an integral polepiece structure, where the iron polepieces extended directly through to the electron beam tunnel. An example of the second type of focusing scheme, where the iron polepieces are spaced from the electron beam tunnel, is referred to hereinafter as a standard polepiece stack and is shown in FIG.  9 . 
     FIG. 9 illustrates a side sectional view of a coupled cavity TWT  30  with a standard polepiece stack that utilizes an iris according to an embodiment of the present invention. A RF input  78  and a RF output  79  are shown along with a PPM polepiece stack  70  that is spaced from an electron beam tunnel  77 . The meandering RF path  40  travels through the tuned cavities  76  that are linked by the irises  75 . The irises  75  are shaped according to an embodiment of the present invention as illustrated in FIG.  5 . The ends of the tuned cavities  76 , near the iris, may also be shaped according to an embodiment of the present invention to facilitate optimal RF propagation, as known in the art. For the TWT  30 , the irises  75  and the tuned cavities  76  may be formed as part of a pure copper circuit that is inserted into an assembly that includes the PPM polepiece stack  70 . 
     Using the standard polepiece stack as in FIG. 9 to generate the magnetic field, rather than the integral polepiece structure as in FIG. 6, allows the development of stronger magnetic field levels and the elimination of transverse fields in the electron beam tunnel  77 . Furthermore, the standard polepiece stack of FIG. 9 reduces the number of incipient stopbands that result from machining laminated blocks to fabricate the coupled cavity circuit as with the integral polepiece structure of FIG.  6 . In designing a lightweight, high-frequency amplifier, the integral polepiece structure may be preferred for low voltage applications while the standard polepiece stack may be preferred for high power applications. 
     An embodiment of the present invention can also be utilized in conjunction with a klystron. As known in the art, notches can couple a portion of the cavities in a klystron. The notches can be shaped according to an embodiment of the present invention, thus allowing the cavities to operate as an extended interaction output circuit for improved bandwidth. 
     To put the coupled cavity circuit into use, the coupled cavity circuit is placed within an amplification tube, usually along with a number of other similar coupled cavity circuits, to form a complete amplifier assembly. The amplification tube  60 , as shown in FIG. 10, can then be assembled to an electron gun  62  and an electron beam collector  64 . The electron gun  62  has a cathode  63  that emits electrons. The electrons are focused into an electron beam  66  by focusing electrodes  67  and an anode  68 . A magnetic field provided along the electron beam tunnel  65  maintains the focus of the electron beam  66  within the tube  60 . The collector  64  receives and dissipates the electrons after they exit the tube  60 . A RF input terminal  61  and a RF output terminal  69  are provided for amplification of a RF signal. 
     FIGS. 11 and 12 are graphs that provide performance data for a coupled cavity circuit in accordance with an embodiment of the present invention. FIG. 11 plots the axial component of the electric field in the coupled cavity circuit gap for a resonance frequency at 30 GHz. The equal amplitudes that correspond to a 2π phase shift between cavities identify this as a cavity resonance. This cavity resonance usually must be lossed out when it appears in the same passband as the operating frequencies. In this case, the circuit operates in the Ku frequency band using the iris mode passband. Thus, due to the iris producing passband mode inversion, the operating frequencies are far below the cavity passband that contains the cavity resonance and no lossy material is required inside the coupled cavity circuit. 
     FIG. 12 plots frequency as a function of the normalized wave number (wave number β times the circuit period P divided by π). The cavity mode passband and iris mode passband are plotted along with the slow wave dispersion for an electron beam. The plot shows how the slow wave circuit dispersion allows a broadband circuit to avoid drive-induced cavity resonances. As the electron beam loses energy during interaction, the phase velocity of the slow space charge waves decreases and the slope of the iris slow wave mode dispersion line drops. In prior art, the line would approach the cavity resonance. For this invention, the line moves away from the cavity resonance. Furthermore, the plot shows that an iris according to an embodiment of the present invention can be utilized not only for the forward wave, but also for the backward wave, as known in the art. 
     Having thus described a preferred embodiment of the coupled cavity circuit, it should be apparent to those skilled in the art that certain advantages of the within 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. For example, a rectangular waveguide shape has been illustrated to show an embodiment of the present invention, but it should be apparent that the inventive concepts described above would be equally applicable to circular waveguides or other shapes as known in the art. The invention is further defined by the following claims.