High-efficiency TWT design using traveling-wave bunch compression

A bunch compression scheme designed to obtain high efficiency in relativistic traveling-wave tube (TWT) amplifiers is reported. Bunch compression is achieved by making the bunches stay in more positive slopes of the axial electric field in the amplifier. The resulting momentum gradient across the bunch tends to oppose space-charge spreading and helps to sustain short bunch lengths for long distances. A faster than light structure is employed to produce the required bunching. At the optimal bunching point, a transition is made to a lower phase velocity structure where the narrow bunches are decelerated. An RF conversion efficiency of >50% in an X-band amplifier is achieved in particle-in-cell (PIC) simulations. This contrasts strongly with the 20%-30% efficiencies achievable from typical relativistic TWT's, where the bunching is not optimized.