FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to inductive output tubes. More particularly, the present invention relates to an inductive output tube adapted to operate in the L-band frequency range.  
       BACKGROUND OF THE INVENTION  
       [0002]     Since the late 1980s the Inductive Output Tube (also known as an “IOT” and a brand of which is marketed by Eimac under the trademark “Klystrode®”) has established itself as a useful device for broadcast, applied science and industrial applications in the UHF frequency range, typically operating in the 100 MHz-900 MHz range. Compared to a klystron, the IOT compensates for its lower gain with both superior efficiency and linearity, and it outperforms the tetrode, its next of kin in the electron device family, with regard to power capability and gain. However, it has long been thought that transit time effects limit the useful frequency range of IOTs to frequencies below 1000 MHz. It has been a commonly held belief in the industry that 1000 MHz is a hard threshold beyond which the performance of IOTs as fundamental frequency amplifiers would fall off rapidly.  
         [0003]      FIG. 1  is a simplified electronic schematic diagram of a typical IOT  10  in accordance with the prior art. A cathode  12  held at a high negative potential compared to ground (typically a dispenser-type barium cathode) emits a beam of electrons  14 . A control grid  16  fed by a radio frequency (RF) input source  32  density modulates the flow of the beam of electrons  14 . An anode  18  held at ground potential accelerates the modulated electron beam  14 . The modulated electron beam  14  passes through an output gap  20  where output power is extracted from the electron beam to an output resonator  19  by way of an induced electromagnetic field and directed to an output coupling  21  which is typically a coaxial feedline. A collector  22  receives the spent electrons. A grid bias supply  30  provides bias voltage to the grid, a beam power supply disposed between line  34  and line  38  provides the power to accelerate the electrons from the cathode to the anode, and a heater voltage supply  36  provides power to the heater of the cathode in a conventional manner. A solenoid magnet (not shown) typically surrounds the electron beam to focus it and reduce beam divergence. Input circuit  40  is shown schematically and acts to match the impedance of the input signal to the IOT  10 .  
         [0004]     The idea of employing higher-harmonic versions of IOTs at higher frequency bands was born early on. In a second-harmonic IOT, for example, the frequency-sensitive grid-cathode circuit (see, e.g., U.S. Pat. No. 5,767,625 entitled High Frequency Vacuum Tube with Closely Spaced Cathode and Non-Emissive Grid to Shrader et al.) could still be operated reliably in the well-experienced UHF regime, while the re-entrant output cavity could be tuned to a higher harmonic in an L-Band frequency. The main drawback to this approach is the relative length of the electron bunch that the low drive frequency forms. During its passage through the output gap the RF voltage in the output cavity changes its polarity twice: from the acceleration into the deceleration phase and back. Although the maximum of the current passes within the deceleration phase and thus ensures power conversion into the desired frequency, a considerable amount of electrons become accelerated, marginalizing efficiency and gain and causing problems with collector dissipation and X-ray radiation.  
         [0005]     An investigation was conducted to see how far up in frequency the fundamental-frequency IOT could be tuned in computer simulation without jeopardizing its performance characteristics, particularly the operation of its critical grid-cathode configuration. An existing one-dimensional IOT computer code of proven reliability was modified to include the effects of grid-cathode transit time into the simulation.  
         [0006]     As a first step an IOT electron gun with an established track record in UHF broadcast and science applications was analyzed to determine the change of electron bunch waveform and fundamental RF current versus frequency. The results of the simulation are shown in  FIG. 3  which is a graph of simulated fundamental frequency current of an existing IOT gun versus frequency at 22 kV beam voltage and 47.4 V peak RF grid voltage operating in class B. Also interestingly, the useful fundamental RF current carried by the bunches in the simulation does not drop significantly until about 2 GHz ( FIG. 3 ).  
         [0007]     Accordingly, it would be highly desirable to develop a fundamental mode L-band IOT with reasonable performance characteristics.  
       SUMMARY OF THE INVENTION  
       [0008]     An inductive output tube (IOT) adapted to operate at frequencies above 1000 MHz includes a cathode for emitting a linear electron beam; a grid comprised of non-electron emissive material for density modulating the beam, wherein an input RF signal is applied between the cathode and the grid; an anode for forming an electric field in combination with the cathode for accelerating the beam; a collector for collecting the spent beam (which may be of the single-stage or multi-stage depressed collector (MSDC) type); and an output cavity resonant to a frequency of the input RF signal, which is positioned between the anode and the collector. Electrons passing through the interaction gap within the cavity induce an RF field in the cavity. A coupler responsive to the RF signal couples the RF power from the cavity to the load.  
         [0009]     In an aspect of the invention an output window is provided to separate a vacuum portion of the IOT from an atmospheric pressure portion of the IOT, the output window being surrounded by a cooling air manifold, the manifold including an air input port and a plurality of apertures permitting cooling air to move from the port, through the manifold and across the window into the atmospheric pressure portion of the IOT.  
         [0010]     In another aspect of the invention the output cavity includes a liquid coolant input port; a lower coolant channel coupled to receive liquid coolant from the liquid coolant input port; a vertical coolant channel coupled to receive liquid coolant from the lower coolant channel; an upper coolant channel coupled to receive liquid coolant from the vertical coolant channel; and a liquid coolant exhaust port coupled to receive liquid coolant from the upper coolant channel.  
         [0011]     In yet another aspect of the invention the output cavity includes a vacuum tight diaphragm which can be moved into and out of the output cavity by manipulating a tuning control accessible on the exterior of the IOT. The tuning control may be bolt moving in threads or another mechanical component adapted to move the diaphragm in and out of the output cavity. Movement of the diaphragm causes a corresponding change in the resonant frequency of the output cavity.  
         [0012]     Other aspects of the inventions are described and claimed below, and a further understanding of the nature and advantages of the inventions may be realized by reference to the remaining portions of the specification and the attached drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention.  
         [0014]     In the drawings:  
         [0015]      FIG. 1  is a simplified electrical schematic diagram of a typical IOT in accordance with the prior art.  
         [0016]      FIG. 2  is a histogram plot of disc velocity and disc current versus reference phase for a simulated second-harmonic IOT operating at L-band frequencies.  
         [0017]      FIG. 3  is a graph of simulated fundamental frequency current of an existing IOT gun versus frequency at 22 kV beam voltage and 47.4 Volts peak RF grid voltage operating in Class B.  
         [0018]      FIGS. 4A and 4B  are diagrams offset with respect to each other by about 90 degrees showing the external configuration of an L-Band IOT in accordance with an embodiment of the present invention.  
         [0019]      FIG. 5  is a diagram showing an L-Band IOT in accordance with an embodiment of the present invention as it was configured for operation.  
         [0020]      FIG. 6  is a front elevational diagram of an L-Band IOT in accordance with an embodiment of the present invention as it would be configured as a product.  
         [0021]      FIG. 7  is a cross-sectional view of an L-Band IOT in accordance with an embodiment of the present invention.  
         [0022]      FIG. 8  is a cross-sectional view of the output cavity of the IOT illustrated in  FIG. 7 .  
         [0023]      FIG. 9  is a cutaway diagram of an output cavity of an L-Band IOT in accordance with an embodiment of the present invention.  
         [0024]      FIG. 10  is a cutaway diagram of an output cavity of an L-Band IOT in accordance with an embodiment of the present invention. The views of  FIGS. 9 and 10  are offset with respect to each other by about 90 degrees.  
         [0025]      FIG. 11  is a cutaway diagram of an output coupling of an L-Band IOT in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0026]     Embodiments of the present invention described in the following detailed description are directed at L-band IOTs. Those of ordinary skill in the art will realize that the detailed description is illustrative only and is not intended to restrict the scope of the claimed inventions in any way. Other embodiments of the present invention, beyond those embodiments described in the detailed description, will readily suggest themselves to those of ordinary skill in the art having the benefit of this disclosure. Reference will now be made in detail to implementations of the present invention as illustrated in the accompanying drawings. Where appropriate, the same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or similar parts.  
         [0027]     In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.  
         [0028]     Based on the findings discussed above, a complete 1300 MHz/15 kW continuous wave IOT was simulated, maintaining the above-described gun configuration. The simulated fundamental mode IOT in accordance with an embodiment of the present invention operating at 1300 MHz at a power output level of 16.4 kW results are in Table 1. Operational data for the simulated IOT is set forth in Table 1 set forth below.  
                                                                 TABLE 1                       Simulated Data for 15 kW CW L-Band IOT                                    Operational frequency   1300   MHz           Beam voltage   24   kV           Grid bias voltage   −50   V           Output power   16.4   kW           Collector dissipation   5.1   kW                Efficiency   68.3%                Drive Power   63   W           Gain   24   dB           Bandwidth   5   MHz           (double tuned, −1 dB)                      
 
         [0029]     Accordingly, a prototype unit was built in accordance with these principles by modifying an existing EIMAC K2 Series UHF IOT to operate at 1300 MHz. The external UHF output section was replaced with an internal 1300 MHz resonator. A 1⅝-inch diameter coaxial output feeder was used which contains an alumina window of the same type commonly used with L-Band klystron devices. The cavity is water-cooled as described in detail below in order to remove waste heat from the cavity as well as to provide stability against de-tuning which above 1000 MHz becomes much more critical than at lower frequencies.  
         [0030]     The input circuit is more complex. The input impedance of an IOT is of the order of 10 ohms, thus the input circuit has to transform the impedance downward from that of the input feeder (typically 50 ohms), instead of upward as in the case of a klystron. The input signal has to be transferred safely and reliably from the ground level to the high-voltage DC potential of the electron gun assembly. High-voltage-safe dimensions and low impedance are not easily married. The input circuit utilized on the 1300 MHz IOT is a modified version of a conventional UHF IOT input circuit. The tuning paddle has been removed and a stub tuner has been added for the purpose of matching the drive signal to the tube. This is shown in  FIG. 8  at reference no.  42 .  
         [0031]      FIGS. 4A and 4B  are diagrams offset with respect to each other by about 90 degrees showing the external configuration of the L-Band IOT  43 .  FIG. 5  is a diagram showing the L-Band IOT  43  as it is configured for operation.  FIG. 6  is a front elevational diagram of the L-Band IOT as it would be configured as a product. In  FIG. 5  the IOT is shown mounted within its magnetic focusing circuit  44 . The box  45  on top contains the conventional high-voltage connections (cathode, heater, grid bias, ion getter pump) and the input circuitry. The magnetic circuit is supported by a cart shown in detail in  FIG. 6  which also contains the cooling water connections. The output coupling  54  leads to a coax-waveguide transition  47  on top of which a directional coupler  48  and a water-cooled load  49  are visible ( FIG. 5 ).  
         [0032]      FIG. 7  is a cross-sectional view of the IOT  43 .  FIG. 8  is a cross-sectional view of integral output cavity  52  of IOT  43 .  FIGS. 9 and 10  are cutaway diagrams of output cavity  52  of IOT  43 . The views of  FIGS. 9 and 10  are offset with respect to each other by about 90 degrees.  FIG. 11  is a cutaway diagram of output coupling  54 . Coupling loop  53  couples RF energy from within output cavity  52  to output coupling  54 .  
         [0033]     Turning now to  FIGS. 4A, 4B ,  5 ,  6 ,  7 ,  8 ,  9 ,  10  and  11 , the IOT  43  includes an output coupling  54  disposed at 90 degrees to a longitudinal axis of IOT  43 . Output coupling  54  provides an interface to a 1⅝inch diameter circular waveguide at flange  55 . Output coupling  54  includes a manifold  56  fed with cooling air by a pair of input nipples  58   a ,  58   b . The manifold is formed about alumina output window  60 . The vacuum side  62  of output coupling  54  is held at vacuum. Alumina output window  60  separates the vacuum side  62  from the atmospheric pressure side  64  of output coupling  54 . Manifold  56  has a number of apertures  57  passing from manifold  56  into the atmospheric pressure side  64  of output coupling  54  in a region immediately adjacent to output window  60 . These apertures are provided to blow cooling air over output window  60  which air is, in turn, exhausted down the output coupling module and circular waveguide attached thereto (not shown). By providing this output window cooling mechanism, the thermal gradient across the ceramic window is minimized, thus reducing thermal stress that may cause window failure over time.  
         [0034]     Operating the IOT  43  at L-Band frequencies results in a relatively large amount of waste heat being deposited in the structure of the output cavity  52 . Absent an efficient mechanism for removing this waste heat, the waste heat would result in distortion of the structure of the output cavity  52  and consequent undesired distortions in the output signal. For example, any shift in the size or shape of the output cavity  52  would likely change the resonant frequency of the structure and thus its impedance at a given operating frequency. To reduce or eliminate these distortions, a cooling system is provided for the output cavity  52 . A liquid coolant such as pressurized deionized water (or another suitable liquid coolant such as a cooling oil, air, polyethylene glycol, polyethylene glycol mixed with water, mixtures of deionized water and other materials or other well-known non-corrosive coolants) is provided to the cooling system through input port  70 . From port  70  the liquid coolant passes into lower chamber  72  where it circulates about the lower chamber (which may be formed in a circular or other convenient shape) to remove heat from the structure, then passes through port  74  into vertical channel  76  (there is preferably a single vertical channel) and up through vertical channel  76 , through port  78  and into upper chamber  80  (which may be formed in a circular or other convenient shape) where it circulates to remove heat from the structure, through port  82  and out water exhaust port  84 . The structure of the output cavity  52  may be constructed, for example, of oxygen-free high-conductivity copper to provide good thermal conductivity and low corrosion so that the waste heat is efficiently removed by the output cavity cooling system.  
         [0035]     The output cavity  52  can be tuned slightly in frequency. In order to accomplish this, a diaphragm  88  is mounted on a flexible flange  90  ( FIGS. 9 and 10 ). The flange  90  makes a vacuum seal with the body  94  of the output cavity. A mechanical device  92  such as a bolt moving in threads or any other convenient mechanism for urging the flange  88  into the cavity  52  is used to push the flange  88  into cavity  52 . Flexible flange  90  acts as a biasing element to push diaphragm  88  back from cavity  52 . Adjustment of the position of diaphragm  88  slightly adjusts the resonant frequency of cavity  52  and provides a frequency adjustment for the IOT. Other biasing mechanisms, such as an exterior mounted spring coupled to the diaphragm could also be used as will now be apparent to those of ordinary skill in the art.  
         [0036]     As with all linear beam types, the L-Band IOT design can be fabricated with a multi-stage depressed collector (MSDC), fed with a plurality of power supplies if desired.  
         [0037]     The integral output cavity  52  used in the present invention includes its resonant structure as a part of the vacuum envelope, whereas the more common method for IOTs is to use an external tuning box to adjust the resonant frequency. This approach yields a tube of a relatively fixed frequency, but manufacturing variations may result in the tube having a resonant frequency that is slightly different than that desired. Accordingly, the diaphragm and flange tuning system described in detail above is used herein to adjust the volume of the integral output cavity  52  for the purpose of fine-tuning the resonant frequency of the IOT.  
         [0038]     Table 2 lists typical test results for output power levels in the 20-30 kW range.  
                                     TABLE 2                           Typical Prototype Test Results            Beam Voltage   Beam Current   Output Power   Gain   Efficiency               30 kV   1.23 A   20.1 kW   21.1 dB   54.4%       34 kV   1.58 A   29.5 kW   22.5 dB   59.0%                  
 
         [0039]     It is believed that these tests mark the first time that an IOT had been operated at a frequency beyond the UHF band (i.e., above 1000 MHz).  
         [0040]     While embodiments and applications of this invention have been shown and described, it will now be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein. Therefore, the appended claims are intended to encompass within their scope all such modifications as are within the true spirit and scope of this invention.
An inductive output tube (IOT) operates in a frequency range above 1000 MHz. An output window may be provided to separate a vacuum portion of the IOT from an atmospheric pressure portion of the IOT, the output window being surrounded by a cooling air manifold, the manifold including an air input port and a plurality of apertures permitting cooling air to move from the port, through the manifold and into the atmospheric pressure portion of the IOT. The output cavity may include a liquid coolant input port; a lower circular coolant channel coupled to receive liquid coolant from the liquid coolant input port; a vertical coolant channel coupled to receive liquid coolant from the lower circular coolant channel; an upper circular coolant channel coupled to receive liquid coolant from the vertical coolant channel; and a liquid coolant exhaust port coupled to receive liquid coolant from the upper circular coolant channel.