Abstract: An accelerator for a linear beam of charged particles has a first accelerating section upstream which modulates and accelerates the dc beam. This section is a traveling-wave circuit through which the entire rf power flows from the driving source. Output power from the other end of the traveling-wave section flows through a transmission line to a standing wave accelerating section downstream of the input section. An attenuator and a phase shifter between the two sections allow adjustment in the energy added to the particles in the downstream standing-wave section without disturbing the synchronism of the beam with the upstream accelerating section. As a result a high efficiency of acceleration and narrow energy spread of the final accelerated beam are achieved over a wide range of particle energies.