Coupled cavity gyrotron-traveling-wave-tube amplifier

#The present invention relates to traveling wave tube amplifiers (TWT Amplifiers) in general and in specific to a coupled cavity gyrotron-traveling-wave-tube amplifier for producing highly efficient, broad band millimeter wave radiation.

#Many of today's high-tech military and commercial applications require a high power, broadband radiation source in the millimeter wave frequency range. Military applications for such TWT amplifiers include, but are not limited to, high resolution radar, communications and electronic jamming equipments. Commercial applications include, but are not limited to, equipment for high resolution airborne and ship-board navigation and communication systems, high efficiency satellite communications systems, millimeter-wave material processing, millimeter wave imaging systems and radiation source for laboratory test and measurement to name a few. Light weight, compact design and low cost are also critical factors for practical use and commercial production.

#The use of free electron beams, linear and rotating, in vacuum tubes has been successful in producing multi-kilowatt high power, broadband radiation. Tens of kilowatts power in the millimeter wave frequency range with a large bandwidth, operating at a low beam voltage (<60 kV ) is more attractive in today's TWT community.

#FIG. 1 is a diagram of the basic architecture of the coupled cavity gyrotron traveling wave tube amplifier.

#FIG. 2 is a diagram of one section of the coupled cavity circuit.

#FIG. 3 is a graph of the dispersion relations (frequency vs. phase shift per cavity) of the coupled cavity circuit.

#Referring now to the figures wherein like reference characters indicate like elements throughout the several views, FIG. 1 illustrates a block diagram of the basic architecture of the coupled cavity traveling wave tube amplifier 100. Electron gun 200 injects a gyrating electron stream into coupled cavity interaction circuit 300 through rf input coupling 250. An rf-source (not shown), provides rf-energy (rf-wave) which is injected into rf-input guide 210. The rf-energy propagates through guide 210 an into rf-input coupling 250 from where the rf-energy is also injected into interaction circuit 300. The rf-energy and the gyrating electronic stream then copropagate through interaction circuit 300, the gyrating electron stream and the rf-wave interacting as both travel the length of interaction circuit 300. Interaction circuit is oriented so that the electric field of the rf-wave is perpendicular to the propagation direction of the gyrating electron stream. A magnetic field across the gun 200, interaction circuit 300, and couplings 250, 255, is produced by magnetic coil module 400 which encircles the system 100. The magnetic field is used to position the gyrating electron stream to ensure coupling of the gyrating electric stream and the rf-wave over a broad band. When the gyrating electron stream and the rf wave couple, the electron stream's transverse momentum transfers energy to the rf-wave resulting in a net gain in the rf-energy through the phenomena known as electron cyclotron instability. The coupled cavity design of interaction circuit 300 which slows the phase velocity of the rf-wave and the magnetic field which positions the gyrating electron stream provide the necessary coupling between the electron beam velocity and rf-wave's transverse electric field, in order to allow the transverse electric field and the gyrating electric stream to interact over a wide band. The electron stream then passes through rf-output coupling 255 and is collected by depressed beam collector 355, while the amplified rf-wave passes through rf-output coupling 255 and propagates through rf-output waveguide 260.

#FIG. 2 shows a detailed illustration of interaction circuit 300. Interaction circuit 300, features a coupled cavity structure, comprising rectangular cavities 310 which feature ridges 330 which protrude into the cavity 310. The ridged cavities 340 are connected through coupling slots 350 which may be arranged in a staggered or other configuration but must couple the inner volume of ridged cavities 310 to allow propagation of the rf-wave 600 and gyrating electron stream 500 through interaction circuit 300. The rectangular cavities 310 which form interaction circuit 300 support a TE.sub.10 like mode of rf wave 600, and unlike conventional linear beam coupled cavity devices, is oriented so that the electric field of the TE.sub.10 like mode 600 is perpendicular to the propagation direction of electron beam 500. TE wave 600 propagates through alternating rectangular coupling slots 350, placed in the side wall of cavities 310. A finite element code HFSS, is used to increase the accuracy of predicting of the wave dispersion characteristics in periodic coupled cavity circuits and allows the structural design to be tailored to maximize bandwidth (Hewlett Packard High Frequency Structure Simulator Reference, Santa Rosa, Calif. (1992)). The cavity resonant frequencies are found in a closed cavity structure to predict the dispersion characteristics of each coupled cavity structure. Each peak represents a phase shift per cavity which may be expressed as: ##EQU1## where k.sub..parallel. is the axial propagation constant, p=1, 2, 3, . . . , N.sub.c, and N.sub.c is the number of cavity 310 forming interaction circuit 300 and l is the distance defined by the combined length of the structure defined by cavity 310 and coupling slot 350.

#When a beam cyclotron mode, is tuned to be synchronized with the rf-wave dispersion, the electron cyclotron instability will take place. This relationship may be expressed as follows: ##EQU2## for the gyrating electron stream where .OMEGA..sub.c is the magnetic field frequency, .gamma. is the relativistic factor, k.sub..parallel. is the axial propagation constant, and v.sub..parallel. is the beam axial velocity. The rf-wave dispersion may be expressed as: