Absfrucf-The design and performance of the first traveling-wave tube (TWT) to be built with a phase-adjusted taper (PAT) is discussed. By adjusting the phase of the electron bunch with respect to the RF wave for strong electron bunch formation at the beginning of the taper and strong power conversion at the end, the PAT achieves a high efficiency of power conversion from the electron beam to the RF wave. A PAT incorporated into the output section of a baseline 29-30 GHz ferruleless coupled-cavity TWT experimentally increased the peak RF power from 420 to 1000 W and the peak RF interaction efficiency from 9.6 to 22.6%. * John P. Vaszari received the B.S. and M.S. degrees in physics from Purdue University, West Lafayette, IN, in 1968 and the University of California, Los Angeles, in 1970, respectively.He has been working with the Electron Dynamics Division of Hughes Aircraft Company since 1969. In his capacity as Project Engineer and Project Manager, he has been engaged in the development and production of a wide variety of Ku-band and millimeter-wave, coupled-cavity travelingwave tubes. These devices include high-peak and average-power gridded TWT's for radar applications and new product development of 30 to 94-GHz communication TWT's.