Abstract:
An overmoded distributed interaction network is provided that generates high peak and average RF power amplification at high frequencies. A series of overmoded cavities are bounded by parallel or concentric grids that may be separated by metallic spacers adapted to function as a photonic bandgap circuit to suppress competing electromagnetic modes. The selected electromagnetic modes have wavelengths much shorter than the lateral dimension of the grids, allowing the beam-wave interaction to be distributed transversely for improved interaction efficiency. The grids may optionally be slotted and arranged to provide a serpentine traveling wave tube configuration.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/243,010, filed Sep. 16, 2009. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to circuits for modulating an electron beam or for extracting power from a modulated electron beam. More particularly, it describes a system and method for creating an overmoded distributed interaction network comprising parallel or concentric grids. 
         [0004]    2. Description of Related Art 
         [0005]    The RF circuit of a microwave vacuum tube amplifier is used to modulate an electron beam and for extracting power from the modulated electron beam. For example, a typical klystron circuit includes a series of re-entrant cavities interacting with a beam propagating through an on-axis beam tunnel, or drift tube.  FIG. 1(   a ) depicts a cross section through a klystron output cavity  104 . Electron bunches  108  propagate through the drift tube along centerline  116  in the direction indicated at  110  and  112  from an electron source to a collector. The electron beam energy couples to the output cavity  104  at the location indicated by field lines  114 . Beam energy may be extracted through a waveguide  106  or other coupling circuit. In some implementations, such as that shown in  FIG. 1(   b ), grids  122  and  124  can be positioned across the drift tube noses of the klystron cavity  104 , confining the RF electric field  120  to the gap region and thereby enhancing interaction efficiency. However, the accompanying interception of current by the grids  122  and  124  restricts average power capability. A conventional, doubly re-entrant klystron cavity operating in the fundamental mode is typically about one free-space wavelength in diameter. The beam tunnel and electron beam passing through the center of the cavity  116 , however, are considerably smaller: the former is typically 0.1 to 0.2 wavelengths in diameter. This places a practical limit on the amount of beam current that can be focused through the beam tunnel, which in turn restricts the peak power of the device. Additionally, beam intercept by the RF circuit and, at higher frequencies, ohmic losses limit the average power capability. If the output circuit is configured so that the beam interacts with a higher order mode, an over-sized cavity can be used. While this may allow higher peak and average power operation, the interaction efficiency is substantially reduced. Accordingly, it would be useful to provide a system for extracting electron beam energy that overcomes many of these drawbacks of the prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    In a first aspect of the invention, an overmoded distributed interaction network (ODIN) is configured as at least one overmoded cavity bounded by a first grid and a second grid. The first and second bounding grids each include a plurality of apertures arranged to enable an electron beam to pass through them and into the overmoded cavity. The overmoded cavity is adapted to support an electromagnetic field mode within the cavity. The supported electromagnetic field mode has a wavelength that is smaller than the lateral dimension of the grids such that the interaction of the RF field and the electron beam is distributed transversely throughout the overmoded cavity. The overmoded cavity may optionally include an RF coupling circuit for coupling an RF signal to or from the overmoded cavity. 
         [0007]    In certain embodiments of an ODIN in accordance with the invention, the first and second grids are formed as concentric cylinders and configured to interact with a radial electron beam. In such a configuration, the supported electromagnetic field modes will generally have a transverse electromagnetic (TEM) character. In other embodiments, the ODIN comprises parallel planar grids oriented to be substantially perpendicular to the electron beam direction. In some embodiments, the distance between the grids may be maintained by spacers. The spacers may be made from dielectric material or metallic material of from a combination of both. The spacers may be arranged in such a way that a photonic bandgap circuit is formed that acts to attenuate certain electromagnetic modes. 
         [0008]    In another aspect of the invention, an ODIN may comprise multiple overmoded cavities formed between a stack of parallel grids, each one of the parallel grids having a plurality of apertures to allow passage of the electron beam and a plurality of spacers to maintain a selected distance between adjacent grids. The spacing between adjacent grids need not be uniform. Such a stack of adjacent overmoded cavities may be configured to operate as a coupled-cavity travelling wave tube. An input waveguide may be coupled to a cavity at one end of the stack, and an output waveguide may be coupled to a cavity at the other end of the stack. 
         [0009]    In another aspect of the invention, the parallel grids formed into a stack may further each include a coupling slot to facilitate coupling of the electromagnetic field between adjacent overmoded cavities. In one embodiment, the slots in adjacent parallel grids may be on opposite sides of the grid such that a serpentine path for the electromagnetic field through the stack is formed. Alternatively, the slots may be aligned with one another or placed in any other desired orientation with respect to one another. 
         [0010]    In some aspects of the invention, the incident electron beam may be divided into beamlets, wherein each beamlet is directed through a corresponding one of the plurality of apertures in the grid plates. This has the advantage of reducing beam loss due to impingement on the grid surfaces. In addition, the electron beamlets can be directed toward certain selected apertures in the grid plates that are near locations where a desired electromagnetic field mode would have peak field intensities. In this way, the selective direction of the electron beamlets can be used to excite specific desired electromagnetic modes. Further, the electron beam or beamlets may be bunched before entering the overmoded cavities, which may provide certain advantages for RF amplification. 
         [0011]    Certain other aspects and applications of the invention will be clear to those skilled in the art and would similarly fall within the scope and spirit of the present invention. The preferred embodiments will be described in detail below with reference to the attached sheets of drawings, which will first be described briefly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1(   a ) and  1 ( b ) depict cross sections of klystron output cavities typical of the prior art; 
           [0013]      FIG. 2  is a perspective drawing of an embodiment of an overmoded distributed interaction network (ODIN) in accordance with the present invention; 
           [0014]      FIG. 3  is an edge-on view of the embodiment of the ODIN depicted in  FIG. 2 ; 
           [0015]      FIG. 4  a cross section of an alternative embodiment of an ODIN in accordance with the present invention that has a coaxial grid structure; 
           [0016]      FIG. 5  is a cross section of an extended interaction output circuit known in the prior art; 
           [0017]      FIG. 6  is an alternative embodiment of an overmoded distributed interaction network (ODIN) in accordance with the present invention; and 
           [0018]      FIG. 7  is yet another alternative embodiment of an overmoded distributed interaction network (ODIN) in accordance with the present invention. 
           [0019]      FIG. 8  is a perspective drawing of a single element of an overmoded distributed interaction network (ODIN) in accordance with the present invention. 
           [0020]      FIG. 9  is a perspective drawing of an embodiment of an overmoded distributed interaction network (ODIN) configured as a coupled cavity traveling wave tube in accordance with the present invention. 
           [0021]      FIG. 10  is a perspective drawing of an embodiment of an overmoded distributed interaction network (ODIN) configured as a serpentine traveling wave tube in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    The overmoded distributed interaction network (ODIN) of the present invention addresses the need for high peak and average RF power amplification at high frequencies. An embodiment of the circuit comprises a series of overmoded cavities bounded by parallel or concentric grids that may be separated by an array of metallic or dielectric spacers. The wavelength of the mode supported between the grids is much smaller than the lateral dimensions of the gridded cavity, allowing the beam-wave interaction to be distributed transversely. The resulting improvement in power handling capability is of particular benefit to higher frequency devices. The spacers facilitate fabrication and may be configured as a photonic bandgap circuit for suppressing mode competition. In one embodiment, a cavity is formed between two parallel grids. In another embodiment, a coaxial cavity operates in a TEM-like mode for interaction with a radially directed beam. A series of grids can be arranged sequentially to form an extended interaction circuit, similar to those used in extended interaction klystrons. Alternatively, the overmoded cavities can be stacked and coupled together with the proper matched RF impedance at the first and last cavity, to form a network that will support a traveling wave mode. 
         [0023]      FIG. 2  illustrates a preferred embodiment of an ODIN in accordance with the present invention. An RF cavity is formed by creating a gap  208  between a first grid  202  and a second grid  204  placed parallel to the first. An electron beam is separated into a number of beamlets, each focused through an aperture  210  in the grids. One such beamlet is illustrated at  212  and comprises a series of electron bunches  206  that propagate through the two parallel grids  202  and  204 . The ODIN functions similarly to the cavity of a conventional klystron. As the electron beamlets  212  pass through the gap  208  between the grids  202  and  204 , they induce RF currents in the cavity, exciting one or more resonant modes. When the cavity is coupled to an external load, this interaction will extract microwave power from the beam. The dimensions of the cavity  208  transverse to the direction of beam propagation  212  determine the resonant frequency. As in other more conventional cavities, the interaction gap length  208  (i.e., the distance between the grids) is governed by the transit angle, which is preferably on the order of one radian for efficient interaction. For operation at 20 kV and 100 GHz, for example, this translates into a gap length  208  of approximately 0.005 inch. 
         [0024]      FIG. 3  is an edge-on view of the embodiment of the ODIN shown in  FIG. 2 .  FIG. 3  illustrates that metallic or dielectric posts  310  can be introduced between grids  202  and  204  to maintain the grid spacing. A convenient method of manufacture leaves spacers  310  machined on the first grid  202 , upon which the second grid  204  rests or indexes, as shown at  312 . 
         [0025]      FIG. 4  illustrates another embodiment of an ODIN in accordance with the present invention that utilizes a coaxial rather than a planar geometry. In this case, an inner grid  406  is separated from an outer grid  404  to create a gap  402 . As electron bunches propagate through apertures, e.g.,  408 , in the grid structure, they induce RF currents in the gap  408 , exciting one or more resonant modes from which energy can be extracted. Additional planar, coaxial, and other geometries are feasible. 
         [0026]    A method known in the prior art of increasing the efficiency and/or bandwidth of an output circuit is to couple a series of fundamental-mode cavities together to form an extended interaction output circuit (EIOC).  FIG. 5  illustrates a cross section one such structure, as described by Begum and Symons in U.S. Pat. No. 5,469,022. The EIOC includes an entrance tunnel  502  into which an electron beam is introduced. The EIOC includes multiple annular structures  520 ,  522 , and  524  that divide the interior into multiple resonant cavities  506 ,  508 ,  510  and  512 , with which the electron beam interacts before exiting through the output tunnel  504 . Energy is extracted from the cavities through waveguide port  514 . 
         [0027]    An alternative embodiment of an ODIN in accordance with the present invention uses multiple layers of grids to provide a sequence of cavities similar to an EIOC, thereby increasing the interaction efficiency.  FIG. 6  is a cross section of an exemplary embodiment of such an ODIN that uses four grids  602 ,  604 ,  606 , and  608  to create three interaction gaps  610 ,  612 , and  614 . The thickness of the grids sets the spacing of the multiple interaction gaps. When the grid thickness is chosen as an integer multiple of the distance traveled by the electron bunches  616  in one RF cycle, all gaps are excited in phase; other arrangements are feasible. Although the embodiment shown includes four grids, embodiments with other numbers of grids are possible and would also fall within the scope and spirit of the present invention. The multiple grids forming the overmoded distributed interaction circuit are typically at ground potential, allowing the RF output power to be transmitted without the need for a DC block. The spent beam exiting the ODIN is captured by a collector. The collector is a physically separate element, allowing it to be set at a potential below that of the output circuit for recovery of unused beam energy. 
         [0028]    In the preceding embodiments, the bandwidth of the ODIN can be controlled by the external Q, the degree of output coupling, or by changing the tuning of each cavity in the multilayer configuration. For high gain, each cavity is set to the same frequency (synchronous tuning), while for increased bandwidth, the cavity frequencies are offset. 
         [0029]    An alternative embodiment of an ODIN is shown schematically in  FIG. 7 . Here, a coaxial ODIN is presented having a multiple coaxial grid structure. In the embodiment shown, an inner grid  702 , a middle grid  704  and an outer grid  706  form two interaction gaps  708  and  710 . Of course, other numbers of grids are possible and such embodiments would also fall within the scope and spirit of the present invention. 
         [0030]    A coaxial structure such as the one depicted in  FIG. 7  can be operated in a TEM-like mode, allowing the diameter to be varied without changing the mode pattern and frequency, which are fixed by cavity height and the spacer distribution (not shown in  FIG. 7 ). This allows the output circuit diameter to satisfy other design constraints, such as voltage stand-off. However, a larger diameter increases the stored energy, reducing the shunt impedance and hence the energy extraction efficiency. As a result, a modest increase in current may be required to attain the same power levels as the circuit diameter grows. The power extracted from the cavities is coupled in parallel to a common coaxial transmission line. 
         [0031]    A single, stackable element for a planar embodiment of an ODIN is shown in  FIG. 8 . It consists of a grid  801 , multiple spacers  802  and multiple apertures  803 . 
         [0032]    An embodiment of an overmoded distributed interaction network (ODIN) configured as a traveling wave tube (TWT) is shown in  FIG. 9 . This structure is assembled by stacking a series of the elements introduced in  FIG. 8 . It combines the grid  901 , spacer  902  and aperture  903  components of the stackable element with an end plate  904 , an input waveguide  905  and an output waveguide  906 . This amplifier functions as a conventional coupled cavity TWT, though it is overmoded and has coupling through the grid apertures. A DC electron beam is modulated by interaction with the structure in response to the input signal; subsequent interaction between the modulated beam and the circuit causes the circuit wave to be amplified. Design details such as an electron gun, a magnetic focusing circuit, a sever and a collector for the spent beam are not shown. 
         [0033]    An embodiment of an overmoded distributed interaction network (ODIN) configured as a serpentine traveling wave tube is shown in  FIG. 10 . This structure is assembled by stacking a series of the elements introduced in  FIG. 8 , suitably modified with coupling slots. It combines the grid  1001 , spacer  1002  and aperture  1003  components of the stackable element with an end plate  1004 , an input waveguide  1005 , an output waveguide  1006  and a series of coupling slots  1007 . This amplifier functions as a serpentine coupled cavity TWT, though the cavities are overmoded. Coupling between cavities occurs through staggered slots located on opposite sides of each successive grid. This is equivalent to a 180° slot rotation angle, causing the electromagnetic wave to follow a serpentine path from the input to the output. The coupling slots need not be aligned with the grid apertures. Again, design details such as an electron gun, a magnetic focusing circuit, a sever and a collector for the spent beam are not shown. Slot rotation angles other than 180° are possible. For example in-line slots have a rotation angle of 0°, other angles may be chosen to achieve the desired dispersion characteristic. 
         [0034]    Other vacuum tube amplifiers that may be configured to utilize an ODIN include multi-beam klystrons and extended interaction klystrons. For the latter, the ODIN may support a standing wave or traveling wave. Furthermore, the coaxially configured ODIN allows implementation of radial amplifiers, in which the electron beamlets propagate radially inwards or outwards. Note that whereas amplifiers are mentioned above, oscillators using the ODIN likewise fall within the scope of the invention. 
         [0035]    The large physical size of the ODIN, in accordance with the multiple embodiments presented herein, allows distribution of the thermal loading, enabling higher average power operation. Additionally, focusing of the beamlets through the grid apertures provides a means of eliminating the limitation imposed on average power by grid interception. As with any overmoded circuit, preventing the excitation of unwanted modes close to the operating frequency may be necessary. To accomplish this, the array of metallic spacers can be designed to form a 2D photonic band gap (PBG) structure. By appropriately choosing the dimensions of the spacers, and the lateral distance between them, only electromagnetic fields within certain frequency ranges (the “bandgaps” of the array) are confined. Any mode or resonance outside of these bands will propagate outward. Materials such as lossy dielectrics or high resistivity electrical conductors can be located around the perimeter of the circuit to attenuate the unwanted modes. 
         [0036]    The size, shape and configuration of the spacers determine the bandgaps. A simple example is provided in  FIG. 5   a  of Smirnova, Chen, Shapiro, and Temkin (Journal of Applied Physics, 2002) which shows the confined frequency bands as a function of the ratio of diameter ( 2   a ) to center-to-center distance (b), for round spacers. In this case, it can be seen that a choice of a/b of slightly over 0.1 will provide two confined bands—one a low frequency band, and the other a narrow, higher frequency band. Operating in the high frequency band would prevent oscillations or other parasitic phenomena above the operating frequencies. It should be noted that this example is valid for round spacers, with a single missing spacer in an infinite array. The exact choice of ODIN dimensions required for mode control will depend upon the number, location and shape of the spacers. In a coaxial embodiment, this 2D photonic bandgap structure would be wrapped into a cylinder. 
         [0037]    There are additional opportunities for mode control. One technique is to preferentially excite the desired operating mode by propagating beamlets of electrons through the apertures corresponding to peaks in the field pattern. This approach becomes more effective if an emission-gated electron gun is used so that the electron beamlets are pre-bunched. Alternatively, for those cavities not coupled to an external load (i.e. not at the input, output or sever) cavity walls can be introduced to form fundamental mode cells around each aperture. 
         [0038]    In conclusion, the overmoded distributed interaction network provides a novel method for beam-wave interaction in high average power, high frequency vacuum tube amplifiers, with application in the terahertz regime.