Terahertz (THz) radiation can revolutionize modern science and technology. To this date, it remains
big challenges to develop intense, coherent and tunable THz radiation sources that can cover the
whole THz frequency region either by means of only electronics (both vacuum electronics and
semiconductor electronics) or of only photonics (lasers, for example, quantum cascade laser). Here
we present a mechanism which can overcome these difficulties in THz radiation generation. Due to
the natural periodicity of 2π of both the circular cylindrical graphene structure and cyclotron electron
beam (CEB), the surface plasmon polaritions (SPPs) dispersion can cross the light line of dielectric,
making transformation of SPPs into radiation immediately possible. The dual natural periodicity
also brings significant excellences to the excitation and the transformation. The fundamental and
hybrid SPPs modes can be excited and transformed into radiation. The excited SPPs propagate
along the cyclotron trajectory together with the beam and gain energy from the beam continuously.
The radiation density is enhanced over 300 times, up to 105 W/cm2. The radiation frequency can be
widely tuned by adjusting the beam energy or chemical potential. This mechanism opens a way for
developing desired THz radiation sources to cover the whole THz frequency regime.