Abstract:
A frequency-selective stabilization system for a gyrotron traveling wave tube amplifier using a dielectrically loaded lossy auxiliary waveguide coupled to the interaction waveguide of the gyrotron traveling wave tube. The auxiliary waveguide has its cutoff frequency in the range of that of the interaction waveguide where instability is likely to occur, but has a significantly different dielectric constant, which lets it absorb stray oscillations in the range of the interaction waveguide cutoff frequency without significant interaction loss in the range of the operating frequency.

Description:
BACKGROUND OF THE INVENTION 
     The invention is directed to gyrotron traveling wave tube amplifiers (gyro-TWTs), and particularly to arrangements for preventing oscillations near the cutoff frequency of the waveguide interaction structure in such gyro-TWTs. 
     The gyro-TWT is a relatively new type of amplifier in which an electron beam with a large component of velocity transverse to the axial direction of travel interacts with the transverse components of RF electric field in a waveguide to provide amplified power output. To obtain maximum gain and power, the value of magnetic field should be adjusted so that the relativistic cyclotron frequency is slightly below the waveguide cutoff frequency or to a subharmonic of this frequency. This is prevented by a tendency for parasitic oscillations to occur in the range of the waveguide cutoff frequency, where both forward and backward wave interaction can occur and where the low group velocity causes the interaction impedance to be very high. Present methods to reduce oscillations include (1) detuning the magnetic field to avoid synchronism with the waveguide cutoff frequency, (2) reducing the beam current, (3) reducing the transverse velocity, and (4) use of lossy coatings such as carbon or Kanthal (an Fe-Cr-Al-Co alloy) to increase circuit RF loss. All of these present methods have features which limit the gain, power, and efficiency. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of this invention to increase power, bandwidth and efficiency in gyro-TWTs using uniform or axially tapered interaction circuits. 
     Another object of this invention is to prevent oscillations in gyro-TWTs. 
     Still another object of this invention is to implement frequency-selective loss near the cutoff frequency of the gyro-TWT interaction citcuit waveguide while minimizing radio frequency (RF) loss above the cutoff frequency. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are attained by coupling a high-loss auxiliary waveguide to an interaction circuit waveguide which has a similar cutoff frequency but a much higher effective dielectric constant. Because of the difference in dielectric constants between these waveguides, the auxiliary waveguide only absorbs power from the interaction circuit waveguide in the range of the waveguide cutoff frequency. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the interaction circuit waveguide of a typical gyrotron tube. 
     FIG. 2 is a graphical representation of the gyrotron interaction circuit waveguide and cyclotron electron beam frequencies as a function of axial propagation constant. 
     FIG. 3 is a cross-sectional view of the lossy auxiliary waveguide coupled to the interaction circuit waveguide according to the invention. 
     FIG. 4 is a cross-sectional view of one example of an alternately configured interaction circuit waveguide, lossy circuit waveguide and coupling between them according to the invention. 
     FIG. 5 is a cross-sectional view of another example of an alternately configured interaction circuit waveguide, lossy circuit waveguide and coupling between them according to the invention. 
     FIG. 6 is a graphical representation of the gyrotron interaction circuit waveguide, auxiliary waveguide and cyclotron shifted electron beam frequencies as a function of axial propagation constant. 
     FIG. 7 is a cross-sectional view of the auxiliary waveguide coupled to the interaction circuit waveguide according to the invention, including provisions for adjusting the dielectric constant and degree of coupling of the auxiliary waveguide. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to the drawings, wherein like reference characters designate like or corresponding parts throughout the views, FIG. 1 shows an end view of a typical gyrotron tube interaction circuit. An electron beam 10 rotating at the cyclotron frequency interacts with electric fields 12 of a waveguide-type interaction circuit or interaction waveguide 14 to provide amplified RF power as the beam moves axially. 
     Dispersion curves of the electron beam 10 and interaction waveguide 14 as a function of axial propagation constant β are shown in FIG. 2. Curve 16 represents electron beam 10 frequency, shifted upward by the electron cyclotron frequency, as a function of axial propagation constant. Curve 18 represents the frequency of interaction waveguide 14 as a function of the axial propagation constant. Synchronism between the electron beam 10 and interaction waveguide 14 occurs at frequency ω t  where curves 16 and 18 intersect, represented by point 20. However, parasitic oscillations tend to occur at frequencies in the range of the interaction waveguide 14 cutoff frequenc ω o , represented by point 22. These parasitic osc can only be eliminated by reducing the beam transverse velocity or shifting the cyclotron frequency ω c , represented by point 23, further from ω o  by decreasing the magnetic field, in either case reducing amplifier efficiency. 
     A cross-sectional view of the preferred embodiment of the invention is shown in FIG. 3. An auxiliary waveguide 24 containing a lossy dielectric material 26 is coupled to the interaction waveguide 14 discussed above by a well known waveguide coupling 28, which may be, by way of example, a coupling slot or a series of coupling apertures between the walls of interaction waveguide 14 and auxiliary waveguide 24. In this case, coupling of power between the interaction waveguide 14 and auxiliary waveguide 24 occurs via a common axial magnetic field component through the coupling 28. The ends of the auxiliary waveguide 24 should extend slightly beyond the ends of the interaction region of the interaction waveguide 14. The ends of the auxiliary waveguide 24 should be terminated with well known RF attenuator loads. 
     Although FIG. 3 illustrates sidewall coupling of two rectangular waveguides both in the TE 10  (transverse electric) mode, either circular or rectangular waveguides can be employed with either sidewall or topwall coupling using either the TE or TM modes. Also, a plurality of lossy auxiliary waveguides can be employed to maintain symmetry or increase the RF loss. Some examples of suitable alternatives are shown in FIGS. 4 and 5. 
     FIG. 4 shows topwall coupling of a lossy dielectric filled auxiliary circular waveguide 24 in the TM 01  mode to a rectangular interaction waveguide 14 in the TE 10  mode. FIG. 5 shows sidewall coupling from a lossy dielectric filled circular auxiliary waveguide 24 in the TE 01  mode to a rectangular interaction waveguide 14 in the TE 10  mode. Similarly, other combinations and configurations may be used with the present invention, as will be apparent to those skilled in the art. 
     Dispersion curves for the above-described electron beam 10, interaction waveguide 14 and auxiliary waveguide 24 are shown in FIG. 6 for the case of no loss or coupling between the interaction waveguide 14 and auxiliary waveguide 24. Curve 30 represents the electron beam 10 cyclotron shifted frequency as a function of axial propagation constant. Curve 32 represents interaction waveguide 14 frequency as a function of axial propagation constant. Curve 34 represents auxiliary waveguide 24 frequency as a function of axial propagation constant. 
     The principle and methodology of the invention is evident by close inspection of FIG. 6. As is well known in the art, synchronism between the electron beam and waveguide circuit in a gyro-TWT occurs at a frequency ω t , represented by point 36, where the interaction waveguide curve 32 intersects the cyclotron shifted electron beam curve 30. Frequency ω t  is very close to the interaction waveguide cutoff frequency ω o , represented by point 38, which tends to induce parasitic oscillations, because at frequency ω o  both forward and backward wave interaction can occur and the low group velocity causes the interaction impedance to be very high. Consequently, the auxiliary waveguide 24 is coupled to the interaction waveguide 14 via the waveguide coupling 28, and has sufficient internal loss to absorb energy from the interaction waveguide 14, preventing parasitic oscillations from occurring in the range of frequencies close to ω o . The auxiliary waveguide 24 has its cutoff frequency tuned to nearly match that of the interaction waveguide 14, so as to maximize coupling between them at their mutual cutoff frequency ω o . The auxiliary waveguide 24 is selected to have a significantly different dielectric constant than the interaction waveguide 14 so that coupling only occurs about their mutual cutoff frequency ω o . This is apparent in FIG. 6, where it may be noted that the interaction waveguide curve 32 only intersects the auxiliary waveguide curve 34 at frequency ω o , represented by point 38. 
     At frequency ω o , the axial propagation constants of interaction waveguide 14 and auxiliary waveguide 24 are closely matched, and coupling between them is correspondingly high. However, at the gyro-TWT operating frequency ω t , represented by point 36, the interaction waveguide curve 32 and auxiliary waveguide curve 34 are significantly deviated from each other, indicating that their axial propagation constants are greatly different, and therefore coupling between interaction waveguide 14 and auxiliary waveguide 24 are minimal at the operating frequency ω t . In consequence, the auxiliary waveguide 24 only absorbs energy from the interaction waveguide 14 in the range of its cutoff frequency ω o  where parasitic oscillations are likely to occur, and not at the operating frequency ω t , where the attenuation of the auxiliary waveguide 24 would hamper operating efficiency and performance of the gyro-TWT. 
     Adjustability of both the effective dielectric constant and the degree of coupling between the interaction waveguide 14 and the auxiliary waveguide 24 is desirable to introduce as little insertion loss as possible to maintain amplifier stability. One example of means for adjusting the effective dielectric constant and coupling to the auxiliary waveguide 24 is shown in FIG. 7. Coupling between the interaction waveguide 14 and the auxiliary waveguide 24 via the waveguide coupling 28 as discussed above may be adjusted by changing the exposed area of the coupling slot 40 in the coupler 28. Alternatively, coupling slot 40 may be series of coupling apertures, in which case the number of coupling apertures may be varied to adjust the coupling. 
     The auxiliary waveguide 24 may use any of the well known means for establishing a higher dielectric constant than that of the interaction waveguide 14. As one example, shown in FIG. 7 the lossy dielectric 26 described above may comprise a rod 42 of lossy dielectric material, such as, by way of example, Berlox (a mixture of berylium oxide and silicon carbide), attached to adjusting rods 44 and placed within the auxiliary waveguide 24 in axial coincidence with it. The ajusting rods 44 pass through bellows 46 mounted in one wall of the auxiliary waveguide 24. Moving the adjusting rods 44 to displace the dielectric rod 42 from axial coincidence with the auxiliary waveguide 24 reduces the dielectric constant of the waveguide circuit. 
     Alternatively, dielectric rod 42 may be fabricated of other well known dielectric materials, such as, by way of example, a mixture of alumina and carbon, a mixture of alumina and silicon carbide or a mixture of berylium and silicon carbide. The dielectric material need not be lossy if an optional lossy coating 48 is applied to the inner surfaces of the auxiliary waveguide 24. This optional lossy coating 48 may be any of the well known coatings having radio frequency attenuation properties, such as, by way of example, carbon or Kanthal. 
     Although the invention may be applied to any gyro-TWT system, it is expected to be particularly effective in systems operating in the Ka (26.5 to 40 GHz) and W (80 to 100 GHz) bands. As an example of a suitable interaction waveguide and interaction waveguide for the Ka band at a frequency of 30 GHz, referring to FIG. 7, a rectangular interaction waveguide 14 having a topwall dimension of approximately 0.5 cm and sideall dimension in the range of 0.4 to 0.45 cm may be either sidwall or topwall coupled to a lossy rectangular auxiliary waveguide 24 having an overall effective dielectric constant of approximately 9.5 with a topwall dimension of approximately 0.16 cm and sidewall dimension in the range of 0.13 to 0.14 cm. The overall effective dielectric constant of the auxiliary waveguide 24 should be at least 3 for normal operation of the invention. 
     changes in the 
     It will be understood that various details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art, within the principle and scope of the invention as expressed in the appended claims.