A new configuration is proposed wherein a low-current beam is accelerated to high energies (tens of amps, tens of MeV) by a driver beam of high current and low energy (a few kiloamps, <1 MeV). The annular driver beam excites the ${\mathrm{TM}}_{020}$ cavity mode of an accelerating structure which transfers its rf power to the on-axis secondary beam. Systematic variation of the driver beam radius provides the secondary beam with phase focusing and adjustable acceleration gradient. A proof-of-principle experiment is suggested.

• Received 1 November 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.3025