Powerful narrow-band relativistic masers with Bragg resonators operating from mm to sub-mm wavelength band: resent results and prospectsN.Yu. Peskov, N.S. Ginzburg, A.K. Kaminsky, S.N. Sedykh and V.Yu. ZaslavskyEPJ Web Conf., 149 (2017) 04012DOI: https://doi.org/10.1051/epjconf/201714904012