[en] The double-drift beam bunching system consists of two bunchers that are separated in space, independently driven but phase-locked together. The second buncher to be encountered by the beam is driven at twice the frequency of the first. This system offers an attractive alternative to conventional one- and two-frequency systems since its bunching efficiency is about twice that for a single frequency system and about 25% larger than that for a two-frequency system in which both harmonics are imposed on the same electrode. The independence of the two bunchers provides for greater ease in the adjustment and stabilization of the rf amplitudes and phase to the accuracy of +-1% and +-0.5⁰ that is required. A double-drift system, designed to operate as 4.5 to 14 MHz, has been installed on the ORNL EN-tandem and evaluated using ¹⁶O, ³²S, ⁵⁸Ni, and ⁶³Cu ion beams. Performance was in close agreement with predictions; about 60% of the dc beam was bunched into a phase angle of 6⁰ of the fundamental frequency. A brief discussion of the principles of operation, predicted performance and practical design considerations are given. Results of a theoretical study of the dynamic focusing effects and energy-modulation imperfections of ungridded klystron bunchers are presented as approximate formulas