HIGH-EFFICIENCY TWT DESIGN USING TRAVELING-WAVE BUNCH COMPRESSION

A bunch compression scheme designed to obtain high efficiency in relat ivistic traveling-wave tube (TWT) amplifiers is reported. Bunch compre ssion is achieved by making the bunches stay in more positive slopes o f the axial electric field in the amplifier. The resulting momentum gr adient across the bunch tends to oppose space-charge spreading and hel ps to sustain short bunch lengths for long distances. A faster than li ght structure is employed to produce the required bunching. At the opt imal bunching point, a transition is made to a lower phase velocity st ructure where the narrow bunches are decelerated. An RF conversion eff iciency of >50% in an X-band amplifier is achieved in particle-in-cell (PIC) simulations. This contrasts strongly with the 20%-30% efficienc ies achievable from typical relativistic TWT's, where the bunching is not optimized.