components of the proposed facility included isotope separators for isotopic separation of in-flight fragmentation-produced exotic beams, a gas catcher/ ion guide for preparing these in-flight beams for subsequent injection into an accelerator, and a postaccelerator facility for varying the energy of these rare isotopes.

The 1999 report recommended conducting modest preconstruction research and development (R&D) on key elements of the facility to enhance the predicted performance and to reduce costs. The subsequent R&D has enhanced the concept, verified that the concept is robust, expedited the readiness to proceed to detailed engineering, and reduced the need for large financial contingencies. Key developments were made in the areas of ion source technology, superconducting cavity design, accelerator design, beam target and stripper technology, and gas catcher technology. The baseline concept design for the accelerator now includes about 1,200 major elements (300 radio-frequency [RF] resonators, 90 solenoids, 100 quadrupoles, and 16 magnetic dipoles) to achieve at least an energy of 400 MeV/A for all ions. The final energy for an ion depends on its charge-to-mass ratio (that is, hydrogen, with a charge-to-mass ratio of 1, will reach more than twice the energy/ mass unit of the heaviest ions). The lower-energy (200 MeV/A) driver, proposed for a FRIB, would merely be a shortened version of this existing design.

At the time of the NSAC task force, there was no ion source that had demonstrated the heavy-ion current to realize the 400 kW specification for the heaviest ions. To reach this specification required nearly an order-of-magnitude improvement in uranium ion current. Subsequently, with DOE-supported R&D, a group at the Lawrence Berkeley National Laboratory demonstrated that its Electron Cyclotron Resonance Ion Source meets the required specifications. The ion source is shown in Figure 4.1. Beam dynamics calculations have shown that the beam characteristics from the ion source are, in fact, so excellent that it is even possible to accelerate two charge states simultaneously. A unique radio-frequency quadrupole (RFQ) linac that accommodates the acceleration of multi-charge-states has been prototyped at the Argonne National Laboratory (ANL). The ability to simultaneously accelerate ions of different charge-states is important for reaching high beam powers.

The velocity of the accelerated ions varies considerably over the length of the accelerator, and the technology to accelerate these ions has been optimized to achieve cost-efficient acceleration. The concept design is unique in that it proposes to use superconducting RF cavities throughout the acceleration process. To reduce the size and cost of the accelerator, various cavity structures have been proposed and prototyped. The cavity structures are grouped into several accelerator sections according to the respective betas (β [beta] = v/c, ion velocity/speed of light) and resonating frequency. The structures include fork, quarter wave, half wave, triple



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