A physical mechanism of electron beam excitation of surface plasmon polaritons (SPPs) on the circular cylindrical structure and transformation into coherent radiation is proposed. Here SPPs on the circular cylindrical structures are excited by a cyclotron electron beam (CEB) rather than by the linearly moving electron beam (LEB). This change leads to an essential consequence due to the natural periodicity of $2\pi$ in structure and CEB, and this dual natural periodicity makes the SPPs transformation possible and brings significant excellences. HEM hybrid modes and $\text{TM}_{0n}$ modes SPPs can be excited and propagate along a cyclotron trajectory together with the CEB to attract energy from CEB continuously to compensate the energy loss; the phase velocity of SPPs synchronizes the CEB; the process of the excitation and transformation is longer. Therefore, the transformed power density is enhanced and reaches up to $10^{10}\ \text{W/cm}^{2}$ . The cyclotron frequency of the electron beam is 1 THz, but the frequency regime of the SPPs and the radiation are much higher, up to hundreds of terahertz. The mechanism presented in this letter opens the way for developing the desired room temperature, powerful and coherent light radiation sources from the infrared to the ultraviolet frequency regime.
