Abstract:
A slow-wave, cyclotron-type, travelling-wave-tube amplifier in which the  m-wave interaction is the result of a Weibel-type instability. The travelling wave is slowed down in its propagation through the waveguide by a dielectric liner located on the inner wall of the waveguide. The bunching mechanism is the result of the V.sub.⊥ ×B.sub.⊥ Lorentz force.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to travelling wave amplifiers and especially to a travelling-wave-tube amplifier of the cyclotron type which has wideband characteristics. 
     The ability of travelling wave and cyclotron-type tubes to amplify electromagnetic wave signals of high frequency is well known. However, to obtain wideband amplification in such devices, it has been necessary to employ complex and expensive periodic structures. The frequency of such devices is limited by the dimensions of such structures. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to increase the bandwidth of travelling wave tubes without employing complex and expensive structures. 
     This and other objects of the present invention are accomplished by the use of a dielectric-lined waveguide in a travelling wave tube. The beam-wave interaction in the device is the result of the Weibel-type instability instead of the Cerenkov instability as in the case of the travelling wave tube (TWT) and the cyclotron maser instability as in the gyrotron travelling wave amplifier (gyro-TWA). More specifically, the bunching mechanism in the present invention is due to the V.sub.⊥ ×B.sub.⊥ Lorentz force (V.sub.⊥ and B.sub.⊥ are the transverse electron velocity and RF magnetic field, respectively) rather than the eE z  electric force as in the TWT or the eE.sub.⊥ electric force as in the gyro-TWA. Furthermore, in contrast to the TWT, the free energy in the present device resides in the transverse gyrational motion of the electron instead of the axial streaming motion. In contrast to the gyro-TWA, where a fast-wave structure has to be employed, the present device requires a slow-wave structure. The dielectric in the waveguide acts to slow down the wave propagated through the waveguide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1(a) is a schematic side view of an embodiment of a TWT employing the invention. 
     FIG. 1(b) is a graph showing the axial magnetic field intensity in the TWT; 
     FIG. 2 is a cross-sectional view taken along the line 2--2 in FIG. 1(a); 
     FIG. 3(a) is a graph of frequency vs. wavenumber for the invention; and 
     FIG. 3(b) is a graph of gain vs. wavenumber for the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1(a) shows a side cross-section of an embodiment of the invention. A cyclotron-type travelling wave tube is shown comprising a magnetron-type electron injection gun 10 consisting of a cathode 12, an anode 14, and an intermediate electrode 16. An annular electron beam from the cathode 12 is propagated through the electrodes and a drift region 18 into a waveguide 20 the inner wall of which has a dielectric liner 22. The electromagnetic wave to be amplified is fed into the front end of the waveguide through another waveguide 24 fed by a driver. 
     To prevent static charge buildup, a metal mesh 26, or coating, may be attached to the inner surface of the dielectric liner 22. The entire system is placed inside a magnet 28 which generates a magnetic field the profile of which, relative to axial distance z along the tube, is shown in FIG. 1(b). Upon leaving the dielectric-liner waveguide, the electron beam is guided radially outward by the divergent magnetic field lines into the electron collector 30. 
     The dielectric material does not have to be in the form of a cylindrical liner on the inner wall of the TWT waveguide. It could be of any form and any location within the waveguide where it will interact with the propagating EM wave and slow it down. For example it could be in the form of a centrally located, longitudinal cylinder of dielectric. 
     The dielectric material should be one which has a low-loss tangent, such as alumina. 
     In operation, a driver wave is guided by the input waveguide 24 to the input ot the TWT waveguide 20 where it launches an EM wave in the interaction region (dielectric-lined region) where it is amplified by the electron beam. The amplified wave leaves through an output window 32, while any reflected wave is absorbed by a microwave absorber 34 located at the other end in front of the input to the TWT waveguide. The absorber 34 prevents spurious oscillations. 
     A typical slow-wave wideband cyclotron amplifier designed in accordance with the present invention employs the TE 01  waveguide mode and the fundamental beam cyclotron harmonic. It has the following design parameters: 
     
         ______________________________________wave frequency    f =      35 GHzbeam voltage      V.sub.b =                      71.5 kVbeam current      I.sub.b =                      9.2 Ampsmagnetic field    B.sub.o =                      5.9 kGaverage beam radius             r.sub.o =                      1.6 mmdielectric inner radius             r.sub.d =                      3.2 mmdielectric material (alumina)             ε =                      10waveguide wall radius             r.sub.w =                      4.6 mmpower gain        g =      2 dB/cm bandwidth              ##STR1##                       89.6% (10 dB)                      67.1% (20 dB)                      53.7% (30 dB)device efficiency 10%______________________________________ 
    
     The dispersion curves (frequency f vs. wavenumber k z ) and the gain curve for the above design are shown in FIGS. 3(a) and 3(b), respectively. The dielectric-lined waveguide can propagate slow waves with a nearly constant group velocity over a wide frequency range, as is evident from FIG. 3(a), the waveguide mode curve. Thus, if the electron beam also propagates at the same velocity, it can interact with the wave over the constant-group-velocity frequency band. This is shown in FIG. 3(b) where the calculated gain, g, is plotted against the wave number k z . It can be seen that a gain of over 1 db/cm is achieved over a frequency rannge of approximately 25-65 GHz. 
     Compared with the usual TWT amplifier, the present invention has a simple and inexpensive structure. It is expecially advantageous at millimeter wavelengths because the frequency is governed by the applied magnetic field rather than the dimensions of the structure. Compared with the gyrotron travelling wave amplifier, it has a much wider bandwidth (60% vs. 10%) and can use a magnetic field of lower intensity for a given frequency of operation. 
     Other slow-wave structures, such as the periodic waveguide, and a variety of wavelaunchers and beam collector geometries can be employed in conjunction with the dielectric liner. The present invention can also operate at cyclotron harmonic frequencies and with other waveguide modes (such as a TM mode, a higher-order TE mode, or a hybrid TE and TM mode). 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.