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Microwave and radio frequency (RF) sources have been a subject of intense research for nearly 100 years, with the advent of High-Power Microwave (HPM) sources based upon intense relativistic electron beams emerging in the 1970s. These sources act to transform kinetic energy in an electron beam into radiation through the interaction of the electrons with some form of external circuit. One form of HPM device, the Cerenkov maser, consists of a hollow cylinder loaded with a dielectric material. The dielectric forms a slow-wave structure with which the relativistic beam can interact. This paper considers a new design approach and interaction mechanism for a Cerenkov maser. For the first time, a dielectric Cerenkov maser formed using microwave metamaterials in the form of a concentric ring structure was considered in the study. The authors proposed that metamaterials provide a practical new opportunity for vacuum electron devices, opening the means for new sources operating in the RF and microwave to THz regimes.