A low-charge-state injector linac for ATLAS

The design of a low-charge-state linac which is capable of accelerating, for example, ¹³²Sn¹⁺ for injection into the existing heavy-ion linac ATLAS is discussed. The injector linac is intended for radioactive beam applications, and will accelerate a low-charge-state beam to energies of 500 keV/nucleon, at which point the ions can be stripped to charge states sufficiently high to be injected into ATLAS. A primary design goal has been to extend the very good longitudinal beam quality typical of ATLAS to low charge state beams. The proposed injector linac consists of several elements. First is a gridded-gap four-harmonic buncher and a short (normally-conducting) 12 MHz RFQ structure, both operating on a 350 kV open-air variable-voltage platform. Then comes an array of 24 MHz and 48 MHz superconducting interdigital accelerating structures interspersed with superconducting quadrupole transverse focusing elements. Numerical ray-tracing studies indicate that a transverse acceptance greater than 0.25 π mm·mrad can be obtained while simultaneously limiting longitudinal emittance growth to a very few keV-nsec