Energy distribution of the 14.7MeV protons, which has energy spread of about 2 MeV due to the thermal motion of fuel ions, is derived analytically. Curvature drift of charged particles in an open magnetic field with a spiral configuration is estimated for separation of the 15MeV protons from thermal components. Numerical orbital calculation shows that amplitude of a wave about 1MV is necessary for trapping and deceleration of the proton beam in a traveling-wave direct energy converter (TWDEC). About 80% of the kinetic energy of the proton beam can be converted into electricity when bunching of the proton beam is improved by applying series of velocity modulations. Results of a computer simulation show that the TWDEC has desirable performance characteristics. The traveling wave with a designed frequency is excited spontaneously without any external power supply. The wave rapidly reaches an equilibrium state after loading, and stably responds to load fluctuations.