The first systematic study of the modulation process of traveling wave direct energy converter (TWDEC), an efficient energy recovery device for fast protons created in an advanced fusion, has been presented. The necessary conditions required for a practical TWDEC modulator were examined. The experimental investigation of modulation characteristics for half-wavelength and one-wavelength modulators was performed. In the one-wavelength modulator, the same effect as the half-wavelength modulator can be obtained by half modulation voltages of electrodes. The modulation effect is sensitive with the phase difference between two RF voltages of the one-wavelength modulator, which could be used for control of modulation effect with a fixed electrode structure.