Radio-frequency quadrupole

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An RFQ, or radio-frequency quadrupole, is a part of a linear accelerator used at low beam energies—about 2 keV to 3 MeV. It looks similar to a quadrupole mass analyzer, but its job is to speed up a beam of a single type of particle, not separate species. As particles travel, electric fields in two perpendicular directions (X and Y) are applied in a phased pattern so there is a constant forward push along the beam direction (Z) and alternating focusing in X and Y. Four electrode assemblies run along the length of the device and have gaps that vary periodically to match the RF frequency with the particles’ speed. This makes the beam form into bunches that pass through regions when the field is strongest for acceleration. Two common shapes are four vanes with a wavy tip or four cylindrical electrodes with conical sections. The electrodes sit in vacuum and are powered by high-power RF signals that are properly phased. Unlike a conventional linac with separate cavities, an RFQ provides continuous acceleration with built-in focusing and bunching, making it more compact. The RFQ was invented in 1970 by I. M. Kapchinsky and Vladimir Teplyakov and is used as an injector in many laboratories and industries.