  Radio-frequency quadrupole (RFQ) linear accelerators appeared on the
accelerator scene in the late 1970s and have since revolutionized the domain of
low-energy proton and ion acceleration. The RFQ makes the reliable production
of unprecedented ion beam intensities possible within a compact radio-frequency
(RF) resonator which concentrates the three main functions of the low-energy
linac section: focusing, bunching and accelerating. Its sophisticated electrode
structure and strict beam dynamics and RF requirements, however, impose severe
constraints on the mechanical and RF layout, making the construction of RFQs
particularly challenging. This lecture will introduce the main beam optics, RF
and mechanical features of a RFQ emphasizing how these three aspects are
interrelated and how they contribute to the final performance of the RFQ.
