The beam bunching and transport system of the Argonne positive ion injector

A new positive ion injector (PII) is under construction at Argonne. It will replace the existing 9 MV tandem electrostatic accelerator as an injector into ATLAS (Argonne Tandem-Linac Accelerator System). It consists of an electron-cyclotron-resonance (ECR) ion source on a 350 kV platform injecting into a superconducting linac optimized for very slow (β⩽0.007 c) ions. This combination can potentially produce even higher-quality heavy-ion beams than are available from the tandem since the emittance growth within the linac is largely determined by the quality of the bunching and beam transport. The system implemented uses a two-stage bunching system, composed of a fourth-harmonic gridded buncher on the ECR high-voltage platform and a room-temperature spiral-loaded buncher. A sinusoidal beam chopper is designed to provide mass resolution of m/Δm>400, and a doubly isochronous beamline is used to minimize time spread due to path length differences