The front end of any modern ion accelerator includes a radio frequency quadrupole (RFQ). While many pulsed ion linacs successfully operate RFQs, several ion accelerators worldwide have significant difficulties operating continuous wave (CW) RFQs to design specifications. In this paper we describe the development and results of the beam commissioning of a CW RFQ designed and built for the National User Facility: Argonne Tandem Linac Accelerator System (ATLAS). Several innovative ideas were implemented in this CW RFQ. By selecting a multisegment split-coaxial structure, we reached moderate transverse dimensions for a 60.625-MHz resonator and provided a highly stabilized electromagnetic field distribution. The accelerating section of the RFQ occupies approximately 50% of the total length and is based on a trapezoidal vane tip modulation that increased the resonator shunt impedance by 60% in this section as compared to conventional sinusoidal modulation. To form an axially symmetric beam exiting the RFQ, a very short output radial matcher with a length of 0:75 was developed. The RFQ is designed as a 100% oxygen-free electronic (OFE) copper structure and fabricated with a two-step furnace brazing process. The radio frequency (rf) measurements show excellent rf properties for the resonator, with a measured intrinsic Q equal to 94% of the simulated value for OFE copper. An O 5þ ion beam extracted from an electron cyclotron resonance ion source was used for the RFQ commissioning. In off-line beam testing, we found excellent coincidence of the measured beam parameters with the results of beam dynamics simulations performed using the beam dynamics code TRACK, which was developed at Argonne. These results demonstrate the great success of the RFQ design and fabrication technology developed here, which can be applied to future CW RFQs.
A new large-acceptance RF beam sweeper was designed, constructed and put into operation with the goal to remove the energy-degraded primary beams tails from radioactive beams (RIB) produced by inflight transfer or charge-exchange reactions. The system makes use of the velocity difference between the RIB beam of interest and the remaining tails of the primary beam after momentum selection by a bending magnet. The time-delayed primary beam components are deflected vertically out of the beam path by the RF sweeper, significantly reducing the stable beam background. Beam purity for the inflight radioactive ion beams as high as 96% has been achieved with this system.
A new cryomodule containing seven low-beta superconducting radio frequency (SRF) cavities has been added to the ATLAS heavy ion linac, providing an additional 15 MV accelerating potential to the existing accelerator. We describe the final stages of cryomodule assembly, commissioning, and installation in the ATLAS accelerator. The clean techniques used to achieve low-particulate rf surfaces are presented, as are the module design features which enable clean assembly and reliable high-gradient operation. The thermal performance of the cryomodule is described, along with performance data for the SRF cavities. Details on subsystem performance including helium and nitrogen systems, vacuum systems, thermal and magnetic shields, slow and fast tuners, and survey/alignment systems are given.
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