High power microwave source loaded by a two-spiral metamaterial structure for Cherenkov radiation

Summary form only given. Metamaterials have unusual properties such as negative refractive index that provides backward wave propagation, also called Cherenkov radiation in vacuum electronics. They also have other unique properties such as period much less than operational wavelength, low group velocity, below cutoff propagation, among others. We are interested in studying metamaterial slow wave structures (SWSs) as a new application for high power microwave sources similar to [3]. For this reason, we developed a structure comprising two parallel and transversely periodic spiral metamaterials having resonant frequency of about 4.4 GHz. Using HFSS eigenmode simulation, we obtained negative dispersion around this frequency which is a TE-like mode (Ez = 0) where the group velocity is negative and extremely low. Dimensions of one period of the two-spiral structure are 22x22x2 mm. Since it has 2 mm thickness and is all metallic, it will be less susceptible to electrical breakdown. These two parallel plates are placed inside a standard WR187 rectangular waveguide with cross section 22x48 mm and the corresponding cut-off frequency for the second mode is 6.25 GHz. This suggests below cut-off propagation for this structure. We used a constitutive parameter extraction method using Sparameters for the corresponding mode [5] in order to verify simultaneously negative ε and negative μ and we obtain double negative behavior in the frequency region 4.2-4.9 GHz, which includes our resonant frequency. Even if the structure is capable of having only negative μ, it can be considered as double negative structure when it is placed inside a cut-off waveguide according to [2].