Abstract:
Generating and frequency tuning of modulated high current electron beams and a specific efficient, high current, frequency-tunable device for generating intense radio frequency (RF), microwave electromagnetic fields in a standard rectangular waveguide. The invention utilizes current multiplication of a seed electron beam, comprising an energetic electron beam to impact a thin foil surface with high electric field. The transmissive-electron-multiplier foils also mitigate both space charge expansion and improve beam propagation effects, by shorting of the radially directed electric field at the axial location of the foil(s). Foil thinness and intensity of the exit fields provide for a multiplication process occurring in a fraction of an RF period. Both self-excited oscillator and amplifier configurations are envisaged. Also included is both a self-excited microwave generator and an amplifier, using a temporally modulated laser to generate a seed electron beam that is amplified. Methods to tune the oscillator are described that allow tunability over a full waveguide band.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on U.S. Provisional Application Ser. No. 60/475,727, entitled High Power, Current Amplified, Tunable Post Accelerated Split Cavity Microwave Oscillator, filed on Jun. 4, 2003, the teachings of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention (Technical Field):  
           [0003]    The invention relates to microwave generation and more particularly to a resonant frequency (RF) generator that operates at low impedance, amplifies the current to increase the RF output power, allows tuning the frequency of the apparatus, and a method to allow operation as an amplifier  
           [0004]    2. Background Art  
           [0005]    The efficient generation of microwaves from modulated electron beams requires electron beam velocity spectrums with low ratios of perpendicular energy to axial energy. Devices which violate this criteria pay a large price in terms of efficiency. For example, the virtual cathode oscillator (D. J. Sullivan, “High Power Microwave Generation using a Relativistic Electron Beam in a Waveguide Tube,” U.S. Pat. No. 4,345,220,17 August 1982) has a very high ratio of E-perpendicular/E-parallel at the nominal axial location of the virtual cathode, potentially exceeding unity. Due to challenges in extracting usable RF power from such beams the practical efficiency of this device, a few percent typically, is poor, and no efficient means of harnessing the high modulated currents, often exceeding a few 10s kA at voltages of order 500 kV, has been developed.  
           [0006]    A highly efficient device for modulating electron beams is known as the Split Cavity Oscillator, as described in U.S. Pat. No. 5,235,248. While this device has a high ratio of E-perpendicular/E-parallel at its exit port, this ratio is substantially reduced with acceleration of the modulated electron beam to voltages of order MV. Post-acceleration of a spatially modulated electron beam, as a means to lock in a spatial modulation while substantially increasing axial kinetic energy and thus reducing E-perpendicular/E-parallel, has been used for many years. As far back as 1940 Haeff and Nergaard described post-acceleration in their Inductive Output Amplifier device, as shown in “A wide-band inductive-output amplifier,” A. V. Haeff and L. S. Nergaard,  Proc. of the IRE, vol.  28, pp. 126-130, March 1940. With post-acceleration, the SCO modulated beam kinetic energy can be converted to RF electromagnetic fields quite efficiently, exceeding 50%. However, virtual cathode formation limits the attainable current, due to space charge limitations in the modulating cavity of the device.  
           [0007]    The operation of the prior art transit time oscillator (TTO), split cavity oscillator (SCO), and post accelerated split cavity oscillator (PASCO) are next briefly described in order to enable a distinction between previous techniques and the new methods described in the present invention.  
           [0008]    The geometry of the TTO microwave oscillator is depicted in FIG. 1. Its operation relies on the interaction of a direct current (DC) electron beam  10  and the field of a cavity formed by a cylindrical pill box with perfectly conducting walls. The DC electron beam is often produced by a thermionic or field emission cathode  12 . The geometry of the cavity is such that the time of flight  18  across the cavity  20  and the interaction of the beam  10  with the oscillating axial electric field associated with the cavity&#39;s axially symmetric mode  22  produce a spatially modulated electron beam  14 . The spatially modulated electron beam  14  is converted to an electromagnetic wave  16 ; this conversion process is depicted symbolically in FIG. 1 since the details of the extraction/conversion process vary depending on the device and/or application. The operation of the TTO is described in detail in “The Split Cavity Oscillator: a high power e-beam modulator and microwave source,” B. Marder, et al., pg. 312,  IEEE Trans. Plasma Sci ., vol. 20, 1992. This device is an extremely inefficient oscillator, a problem that is addressed by the SCO.  
           [0009]    The geometry of the SCO microwave oscillator is depicted in FIG. 2. Its operation also relies on the interaction of a direct current (DC) electron beam  40  and the field of a cavity  41 . In this case the cavity  42  is formed by a cylindrical pill box cavity with an intermediate electrically conducting septum  44  (placed exactly midway across the pill box) that extends part ways across the interior of the cavity  42 , as shown in FIG. 2. Distinct from the operation of the TTO, the SCO operates according to a well known energy instability that exists between the electron beam  40  and the cavity  42 , and an externally applied field is not required, as described in “The Split Cavity Oscillator: a high power e-beam modulator and microwave source,” B. Marder, et al., pg. 312,  IEEE Trans. Plasma Sci ., vol. 20, 1992. Again, the DC electron beam  40  is often produced by a highly inefficient thermionic or field emission cathode  46 . As with the TTO, the geometry of the SCO cavity is such that the time of flight across the cavity  48  and the interaction of the beam with the Pi-mode of the cavity&#39;s oscillating electric field cavity produce a spatially modulated electron beam  50 . The SCO geometry and resulting electromagnetic field mode structure permits the axial length  48  of the cavity to be much shorter than the axial length of the TTO oscillator  18 . Finally, the spatially modulated electron beam  50  is converted to an electromagnetic wave  52  as depicted symbolically in FIG. 2. Note that cathode  46  must produce the entire charge of electron beam  40 , and consequently, operation at high powers requires that cathode  46  be capable of producing high current densities at high voltages. This often results in short cathode lifetimes, and pulse shortening due to gap closure caused by plasma drift across the anode-cathode gap, as shown in “Results of research on overcoming pulse shortening of GW class HPM sources,” K. Hendricks, et al., pg. 81, Digest of Technical Papers, International Workshop on High Power Microwave Generation and Pulse Shortening, Edinburgh, Scotland, 1997. The main disadvantage of the SCO is the large axial velocity spread, and the substantial unwanted perpendicular velocities of the electrons that exit the cavity, leading to poor beam-to-RF power conversion efficiencies. The SCO is described in U.S. Pat. No. 5,235,248. However, this prior art patent describes an apparatus with a very specific geometry (cylindrical pill box with an electrically conducting septum placed exactly midway along the axial length of the cavity) that operates at a single frequency. No capability to adjust the frequency of operation of the device is disclosed or implied in the prior art patent.  
           [0010]    The geometry of the PASCO microwave oscillator is depicted in FIG. 3. Its operation to produce a spatially modulated electron beam  60  is equivalent to the SCO described above. However, once the spatially modulated electron beam  60  is produced, the PASCO uses an accelerating screen or grid  62  at a high relative potential, typically 100&#39;s of kV or more, to accelerate the electron beam to relativistic velocities  64 , i.e., close to the speed of light. This greatly reduces the relative axial velocity spread intrinsic to the SCO and represents a major improvement for potential high power and high efficiency operation. The relativistic spatially modulated electron beam  64  is then converted to an electromagnetic wave  66  as depicted in FIG. 3. This technique results in a more tightly velocity-modulated beam, while maintaining excellent spatial bunching allowing more efficient beam-to-RF extraction. The main disadvantages of the PASCO are: (1) the device has an inherent limitation on total current due to space charge depression in the modulating cavity, which ultimately can lead to virtual cathode formation, but at more modest current levels, reduces modulation efficiency; and (2) the PASCO is a fixed frequency device, in that there is no ability to tune its frequency while maintaining axisymmetry.  
           [0011]    Post-acceleration of an electron beam for high power and high efficiency operation is described in U.S. Pat. No. 5,101,168. However, this patent describes methods that were well known prior to the patent&#39;s application date. As an example, post-acceleration of an electron beam was described by Haeff and Nergaard, “A wide-band inductive-output amplifier,” A. V. Haeff and L. S. Nergaard, Proc. of the IRE, vol. 28, pp. 126-130, March 1940. Furthermore, post-acceleration of an electron beam was described by Preist and Shrader, “The Klystrode—an unusual transmitting tube with potential for UHF,” D. H. Preist and M. B. Shrader, Proc. of the IEEE, vol. 70, no. 11, pp. 1318-1325, November 1982.  
           [0012]    The present invention, a Current Amplified, Tunable, Post Accelerated, Modulator (CATPAM) apparatus uses techniques of the well known transit time oscillator (TTO) as described in “Interchange of energy between an electron beam and an oscillating electric field,” J. Marcum, Journal of Applied Physics, vol. 17, January, 1946, a split cavity oscillator (SCO) shown in “The Split Cavity Oscillator: a high power e-beam modulator and microwave source,” B. Marder, et al., pg. 312, IEEE Trans. Plasma Sci., vol. 20, 1992, and the post accelerated split cavity oscillator (PASCO) (the PASCO is also known as the Reltron described in “Super RELTRON theory and experiments,” R. Miller, et al., pg. 332, IEEE Trans. Plasma Sci., vol. 20, 1992, in conjunction with unique techniques to operate at low impedance, amplify the current to increase the RF output power, tune the frequency of the device, and a method to allow operation as an amplifier, as opposed to just an oscillator. The disclosed apparatus spatially modulates a direct current (DC) electron beam using instabilities associated with device geometry and transit time effects; or, it directly generates a spatially modulated electron beam using laser-induced electron emission. It then amplifies the resulting electron beam (current), accelerates the spatially modulated beam to relativistic velocities, and converts the kinetic energy of the spatially modulated relativistic electron beam to electromagnetic fields at microwave frequencies. In addition, methods are disclosed that allow the device to be tuned to a desired operating frequency while maintaining nominal axisymmetry. None of the prior art teaches or implies these novel features.  
         SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)  
         [0013]    Disclosed is a CATPAM RF generator device that allows for substantial levels of frequency tunability, without the need to break vacuum, while maintaining axisymmetry, and retains all the advantages of the PASCO devices as discussed in the Background Art section of the specification. Additionally, the use of a transmissive electron multiplier allows substantially higher current operation compared with PASCO, reducing the impedance and output power by the multiplication factor. Finally, the use of a RF-modulated laser to generate a seed current permits the use of the device as an amplifier, and greatly increases the output RF pulse width of the device.  
           [0014]    A primary object of the present invention is to provide the ability to tune the frequency of the output microwave signal of the apparatus when it is operated as an oscillator.  
           [0015]    Another object of the present invention is to provide a technique to amplify, or multiply, the electron beam current of the CATPAM, or other, device which creates a modulated electron beam. This method increases the microwave output power of the device, enhances the low impedance properties and efficiency of the device.  
           [0016]    Another object of the present invention to provide a method for amplifying electron beams from an arbitrary device which has previously created a modulated electron beam current.  
           [0017]    Yet another object of the present invention is the provision of a RF-modulated, laser-induced emission of electrons from a cathode.  
           [0018]    An advantage of the present invention is that it increases the microwave output power of the apparatus, enhances the low impedance properties and efficiency of the apparatus.  
           [0019]    Yet another advantage of the present invention is the allowance of the CATPAM to operate without a field emission cathode and without a RF modulator, and helps the CATPAM achieve greater operational efficiency in less volume and with less weight than otherwise would be the case.  
           [0020]    Another advantage of the present invention is the ability of the CATPAM apparatus to operate as an amplifier.  
           [0021]    Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings:  
         [0023]    [0023]FIG. 1 depicts the geometry of the prior art Transit Time Oscillator.  
         [0024]    [0024]FIG. 2 depicts the geometry of the prior art Split Cavity Oscillator.  
         [0025]    [0025]FIG. 3 shows the geometry of the prior art Post Accelerated Split Cavity Oscillator.  
         [0026]    [0026]FIG. 4 illustrates the geometry of the preferred Current Amplified, Post Accelerated Modulator, with frequency tuning method no. 1.  
         [0027]    [0027]FIG. 5 depicts the geometry of the preferred Current Amplified, Post Accelerated Modulator, with frequency tuning method no. 2.  
         [0028]    [0028]FIG. 6 gives details of the geometry of the preferred Current Amplified, Post Accelerated Modulator, with frequency tuning method no. 3.  
         [0029]    [0029]FIG. 7 depicts the geometry of the preferred Current Amplified, Post Accelerated Modulator, with frequency tuning method no. 4.  
         [0030]    [0030]FIG. 8 illustrates the geometry of the preferred Current Amplified, Post Accelerated Modulator, with frequency tuning method no. 2.  
         [0031]    [0031]FIG. 9 shows the conceptual geometry of the current amplification process of the Current Amplified, Post Accelerated Modulator.  
         [0032]    [0032]FIG. 10 depicts the geometry of the preferred Current Amplified, Tunable, Post-Accelerated Modulator, with laser-induced electron emission from the cathode illustrated. The laser is pulsed at an RF frequency, so the electrons that are emitted from the surface of the cathode constitute a spatially-modulated electron beam, obviating the need for a modulating cavity.  
         [0033]    [0033]FIG. 11 graphically provides details of the laser-induced electron emission from the cathode of the Current Amplified, Tunable, Post-Accelerated Modulator. The laser is pulsed at an RF frequency, so the electrons that are emitted from the surface of the cathode constitute a spatially-modulated electron beam, obviating the need for a modulating cavity. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     BEST MODES FOR CARRYING OUT THE INVENTION  
       [0034]    Disclosed is a Current Amplified, Tunable, Post Accelerated, Modulator (CATPAM) that is frequency tunable, high power capable, highly efficient in operation, and exhibits low impedance operation. The CATPAM can operate either as an oscillator or an amplifier, depending on the particular configuration.  
         [0035]    The geometry of the CATPAM microwave oscillator, without current amplification, and with the first of four frequency tuning schemes, is depicted in FIG. 4 (please note that current amplification is not indicated in this figure). As shown, its operation relies on the interaction of a direct current (DC) electron beam  100  and the field of a closed rectangular pill box cavity  101  that contains an intermediate conducting septum  104  that extends partly across the interior centerline  106  of cavity  102 . Again, DC electron beam  100  is often produced by a highly inefficient thermionic or field emission cathode  108 . As with the TTO, the geometry of the modulating cavity  102  is such that the time of flight across the cavity  110  and the interaction of the beam  100  with the oscillating electric field  101  in cavity  102  produce a spatially modulated electron beam  112 . The modulator geometry and resulting electromagnetic field mode structure promotes an instability that generates an oscillating electromagnetic field  101  at the frequency of the modulator&#39;s cavity mode. The interaction of the electron beam  100  with the cavity&#39;s electromagnetic field and the time of flight across the cavity  110  generate a spatially bunched electron beam  112  on the output side of the cavity  116 . Spatially modulated electron beam  112  is subsequently accelerated by an accelerating grid  118  to a relativistic velocity producing a relativistic spatially modulated electron beam  120  and converted to an electromagnetic wave  122 ; this process is depicted symbolically in FIG. 4 since the details of the extraction/conversion process vary depending on the device and/or application. It is well known that the spatial modulation frequency of the modulator cavity is governed by the resonant frequency of the cavity. As shown in FIG. 4, included is a tuning annulus  124  for tuning (changing) the resonant frequency of cavity  102 , and consequently tuning the spatial modulation of the electron beam  112 . This, in turn, allows one to tune the output frequency of the electromagnetic signal that is ultimately extracted from the apparatus. The width of the tuning annulus  126 , adjustable from outside of the apparatus (not shown) governs the resonant frequency of cavity  102  and the resulting electromagnetic signal that is ultimately extracted from the apparatus. Tuning annulus  124  is introduced into cavity  102  from the interior wall  128  of rectangular pill box cavity  102 , as indicated in FIG. 4.  
         [0036]    The geometry of the CATPAM microwave oscillator, without current amplification and the second of four frequency tuning schemes is depicted in FIG. 5. Its operation relies on the interaction of a direct current (DC) electron beam  200  and cavity  202  in the same manner as described above, though frequency tuning of the device is accomplished in a different manner. As shown in FIG. 5, a second embodiment for a tuning annulus  204  is shown to tune (change) the resonant frequency of the modulator&#39;s cavity  202 , and consequently tune the spatial modulation of the electron beam  206 . Tuning annulus  204  is introduced into cavity  202  from the center septum  208  of rectangular pill box cavity  202 , as indicated in FIG. 5. Again, the width of the tuning annulus  210 , adjustable from outside of the device (not shown in the figure) governs the resonant frequency of the cavity and the resulting electromagnetic signal that is ultimately extracted from the device.  
         [0037]    The geometry of the CATPAM microwave oscillator, without current amplification and the third of four frequency tuning schemes is depicted in FIG. 6. Its operation relies on the interaction of a direct current (DC) electron beam  300  and cavity  302  in the same manner as described above, though frequency tuning of the device is accomplished in a different manner. As shown in FIG. 6, a dielectric material  304  with a relative dielectric constant greater than unity (ε r , &gt;1) is disposed in the internal volume of cavity  302 . In an alternative embodiment, plasma which has a dielectric constant less than unity (ε r &lt;1) can be disposed in portions of cavity  302  (not shown). The presence of dielectric material  304  depresses the resonant frequency of cavity  302 , and in turn, reduces (or with plasma, increases) the frequency of the electromagnetic signal that is ultimately extracted from the apparatus. Also included is a tuning annulus  306  used to tune (change) the resonant frequency of cavity  302 , and consequently tune the spatial modulation of electron beam  312 . Tuning annulus  306  is introduced into cavity  302  from the center septum  308  of the rectangular pill box cavity  302 , as indicated in FIG. 6. Again, the width of the tuning annulus  310 , adjustable from outside of the device (not shown in the figure) governs the resonant frequency of cavity  302  and permits the tuning of the resulting electromagnetic signal  314  that is ultimately extracted from the apparatus.  
         [0038]    The geometry of the CATPAM microwave oscillator, without current amplification and the fourth of four frequency tuning schemes is depicted in FIG. 7. Its operation relies on the interaction of a direct current (DC) electron beam  400  and cavity  402  in the same manner as described above, though frequency tuning of the apparatus is accomplished in a different manner, in yet another alternative embodiment. As shown in FIG. 7, a tuning annulus  404  is provided that extends from the wall  406  of pill box cavity  402  and a second annulus  408  that is introduced into cavity  402  from a center septum  410  of rectangular pill box cavity  402 . In this configuration, tuning (changing) the resonant frequency of the cavity, and consequently tuning the spatial modulation of electron beam  412  is accomplished. The widths  414 ,  416  of each tuning annulus  404 ,  408  work in concert to govern the resonant frequency of cavity  402  and permit the tuning of the resulting electromagnetic signal  430  that is ultimately extracted from the apparatus.  
         [0039]    The geometry of the CATPAM microwave oscillator, with current amplification and the second of four frequency tuning schemes is depicted in FIG. 8. Note that current amplification is possible with any of the tuning schemes, as described above. The invention utilizes current multiplication of a seed beam  601 , which is achieved by allowing an energetic electron beam to impact a thin foil surface  602  with high electric field on its downstream side. The foil is sufficiently thin and of such materials that the forward directed secondary electron cascade process, initiated by the seed beam, results in more electrons being ejected from the downstream surface than are incident on the front surface, per unit area, as described in “Reflection and transmission secondary emission from silicon,” R. Marti nelli, Applied Physics Letters, pp. 313-314, vol. 17, no. 8, 15 October 1970 and “The application of semiconductors with negative electron affinity surfaces to electron emission devices,” Proc. of IEEE, vol. 62, no. 10, pp. 1339-1360, Oct. 1974. The output secondary electron cascade from the exit surface of the foil can be accelerated subsequently by an accelerating grid  608  for further multiplication in a similar manner with a similar foil  606 , and so forth, yielding multiplication factors limited primarily by space charge and beam propagation effects, with the neglect of foil heating. The transmissive, electron multiplier foils as described are also beneficial in mitigating both space charge and beam propagation effects, due their shorting of the radially directed electric field as described in, “Image-field focusing of intense relativistic electron beams in vacuum,” R. J. Adler, Particle Accelerators, vol. 12, pp. 39-44,1982. Because the foil is sufficiently thin, and exit fields sufficiently intense, the multiplication process can occur on a small fraction of an RF period, even for frequencies as high as many GHz. Thus, any pre-existing modulation of the electron beam is well-preserved during the multiplication process. A large electric field on the final foil, provided by a large accelerating voltage  608 , even to fractional or multi-MV levels, can produce high flowing powers in the electron beam; these can be converted into extracted microwave power  620  using a variety of traditional methods, including tuned cavities driving one or more rectangular waveguides, or transmission lines, for example. Without foil current enhancement, the apparatus is limited to modulated currents of approximately 1 kA, for voltages up to approximately 200 kV. With foil enhanced current multiplication, multiplicatively higher currents will be obtainable with CATPAM. The particular nature of electron multiplication is indicated in FIG. 9 in which it is shown that a single electron  600  strikes the transmissive electron multiplier  602 , and three (for explanatory purposes) electrons are emitted on the other side  604 . This process is repeated onto further transmissive electron multipliers  606  until the desired current level is achieved. The resulting high current, spatially modulated electron beam  607  is subsequently accelerated by the high voltage of the accelerating screen  608  which yields a high current, relativistic, spatially modulated electron beam  610 . This configuration provides a method to operate a high current, high power microwave generator using an initial low value of seed current. This technique, in principle, allows for multiplicatively higher currents and current densities than would be available from PASCO or SCO devices. Both self-excited oscillator and amplifier configurations using the current multiplication method can be envisaged.  
         [0040]    To eliminate the modulating cavity (thereby saving weight and volume) the scheme whereby a spatially modulated electron beam is directly produced is illustrated in FIG. 10. Note the absence of a modulating cavity in FIG. 10. There, an intense, temporally-modulated laser light  700 , temporally modulated at RF frequencies, is depicted. A detail drawing of the laser-cathode interaction is depicted in FIG. 11. FIG. 11 shows a laser  701 , which illuminates the cathode; laser light  702  is oscillating at light frequencies, but is modulated (turned on and off) on an RF time scale  704 . The laser initiates the emission of a small number of electrons  706 , a seed current. The small seed current generated in this fashion is subsequently amplified in the manner described previously. Traditional field emission cathode oscillators are typically limited to high power operation in the microsecond regime due to gap closure caused by unwanted plasma generated in the electron generation process. Since the CATPAM oscillator can operate with a small seed current that is subsequently multiplied as described above, the generation of high power RF pulses can occur over much longer times. Production of modulated electron beams on a time scale of order fractional to several microseconds is foreseen.  
         [0041]    Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above, are hereby incorporated by reference.