This paper studies three types of coaxial slow wave structures(SWSs):(1)with ripples on both the inner and outer conductors;(2)with ripples on the outer conductor and smooth on the inner one;and(3)with ripples on the inner conductor and smooth on the outer one.The frequencies,coupling impedances,time growth rates and beam-wave interaction efficiencies of the three types of coaxial SWSs are obtained by theoretical analysis.Moreover,the relativistic Cerenkov generators(RCGs)with the three types of coaxial SWSs are simulated with a fully electromagnetic particle-in-cell code,and the results verify the theoretical analysis.It is proved that the RCG with double-rippled coaxial SWS has the highest conversion efficiency and the shortest starting time.