Most dangerous explosive materials, both toxic and radioactive, contain nitrogen salts with resonant absorption lines in the frequency range 0.3-10 THz. Therefore, there has been growing interest in remotely detecting such materials by observing the spectrum of reflected signals when the suspicious material is interrogated by THz radiation. Practical portable THz sources available today generate only 20–40 mW output power. This power level is too low to interrogate suspicious material from a safe distance, especially if the material is concealed. Hence, there is a need for sources that can provide greater power in the THz spectrum. Generating and extracting high output power from THz sources is complicated and inefficient. The efficiency of vacuum electronic microwave sources is very low when scaled to the THz range and THz sources based on scaling down semiconductor laser sources have low efficiency as well, resulting in the well known “THz gap.” The reason for such low efficiencies for both source types is material losses in the THz band. In this article an efficient power combiner is described that is based on scaling to higher frequencies a microwave combiner that increases the output power in the THz range of interest in simulation studies. The proposed power combiner not only combines the THz power output from several sources, but can also form a Gaussian wavebeam output. A minimum conversion efficiency of 89% with cophased inputs in a lossy copper power combiner and maximum efficiency of 100% in a Perfect Electric Conductor (PEC)-made power combiner were achieved in simulations. Also, it is shown that the TE01 output mode is a reasonable option for THz applications due to the fact that conductive loss decreases for this mode as frequency increases.
Microwave sources transform the kinetic energy of an electron beam into microwaves through the interaction of electrons with a periodic slow wave structure (SWS). A metamaterial (MTM) waveguide is proposed for use in a microwave oscillator instead of a conventional periodic SWS that has been used for a long time to generate high power microwave radiation. MTMs have interesting properties such as the negative refractive index, low group velocity, and below cutoff propagation, among others. In this work, we study the interaction of a multibeam cathode with a set of MTM structures inside a cylindrical waveguide. We developed a structure comprising a number of MTM metallic plates that have periodicity in the axial direction and are repeated in the azimuthal coordinate. Using eigenmode simulations, we obtained negative dispersion around the operating frequency where the group velocity is negative and extremely small. The fully electromagnetic, relativistic particle-in-cell codes MAGIC and CST Particle Studio and the fully electromagnetic software tool CST Microwave Studio were used to obtain the results in this study.
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