Progress has been reported in a continuing effort to develop pulsed, high-power microwave signal sources based on (1) oscillators in the form of relativistic magnetrons containing transparent cathodes, and (2) hybrid antenna/ amplifiers powered via electronbeam generators. The underlying concept is that a compact, high-power microwave source could be constructed by integrating an electron-beam accelerator with a radiating antenna and an electrodynamic-interaction space.

If, in particular, the electron beam were generated by a linear induction accelerator and had an annular cross section and the radiating antenna were of a dielectric-rod surface-wave type (see figure), then amplification of a drive signal generated by the magnetron could be achieved by means of the Cherenkov mechanism of interaction between the electron beam and surface waves on the antenna. In effect, the accelerator / antenna / electrodynamic - interactionspace unit would function as a travelingwave- tube amplifier. Moreover, by virtue of the physical principles of a linear induction accelerator, it would be possible to keep the cathode holder of the accelerator at ground potential, making it possible to connect the accelerator to the magnetron or other external drivesignal source. There would be no need for either a microwave transmission line between the amplifier and antenna or a mode converter for coupling the output of the amplifier to an external antenna. In addition, characteristics of the output radiation could be controlled.

This Block Diagram schematically depicts a conceptual proof-of-principle version of a microwave source of the type described in the text. Microwave drive pulses from the magnetron would go through a transmission system within a high-voltage electrode (not shown) of the linear induction accelerator (LIA) and would excite the HE11 electromagnetic mode of the dielectric rod. This mode would become amplified by interaction with the hollow beam, which would be guided along the rod by the magnetic field.
Recent development efforts have included experimental and computational- simulation studies of a hybrid antenna/amplifier of the type described above. The results of the simulations were interpreted as indicating the expected beam-field interaction and microwave amplification. The experiments were focused on study of propagation of the electron beam, the formation of plasma at the surface of the dielectric rod, and effects of the plasma upon the rod material. The results of the experiments were interpreted as indicating the practicality of the antenna/ amplifier as a high-power microwave generator.

In a parallel development effort, work was performed in preparation for experiments on a relativistic magnetron containing a transparent cathode. The intention is to demonstrate the rapid start of microwave oscillation in such a magnetron, as proposed and studied previously by means of numerical simulations by researchers at the University of New Mexico. In this effort, the magnetron electrodes were designed and manufactured on the basis of the simulation studies. At the time of reporting the information for this article, assembly and testing of the experimental apparatus was underway.

This work was done by Kiyoshi Yatsui, Weihua Jiang, Anatoli Shlapakovski, and Tsuneo Suzuki of Nagaoka University of Technology for the Air Force Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Electronics/Computers category. AFRL-0012


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Microwave Sources Utilizing Linear Induction Accelerators

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This article first appeared in the February, 2007 issue of Defense Tech Briefs Magazine.

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