Comments on “Cerenkov Instability due to Unmagnetized Electron Beam in Periodically Corrugated Waveguide” [Jpn. J. Appl. Phys. 42 (2003) 7095]

It is the purpose of the present comments to show that the boundary condition on the discontinuity of the wave electric field radial component at the interface of a relativistic electron beam with vacuum, used recently in Ogura et al. [Jpn. J. Appl. Phys. 42 (2003) 7095], as derived by Bobroff [IRE Trans. Electron Devices 6 (1959) 68] from the definition of an electric polarization, can be directly derived from Maxwell's equation. The equivalent boundary condition on the discontinuity of the wave magnetic field tangential component can also be directly derived from Maxwell's equations.