A 12-MHz CW RFQ for the AEBL Project

Abstract

The Advanced Exotic Beam Laboratory (AEBL) at the Argonne National Laboratory (ANL) will provide a research facility for studies of nuclear phenomena by using beams of short-lived isotopes for research on the nature of nucleonic matter and the origin of the elements, for tests of the Standard Model, for applications in medicine and industry, and for other applied physics research. The proposed design of the AEBL driver linac evolved from the Rare Isotope Accelerator (RIA) project. The AEBL will be a CW linac capable of accelerating uranium ions up to 200 MeV/u and protons to 580 MeV with 400 kW beam power. The AEBL facility also includes a post-accelerator which is designed for acceleration of radioactive ions with charge-to-mass ratios in the range from 1/238 to 1/6. Very low charge-state ions can be most efficiently bunched and accelerated by using normally-conducting radio-frequency quadrupole (RFQ) for the first few MV of the post accelerator. A two-meter long, 12-MHz CW RFQ was designed, built, and tested in the late 1990s as the first section of a three-section RFQ [18]. This RFQ achieved inter-electrode voltages of 110 kV CW (the peak surface field was ∼15 MV/m) and accelerated beams with A/q as large as 132 (132Xe). The AEBL requires a similar RFQ for the post-acceleration of singly-charged unstable nuclides. Our plan is to replace the vanes of this RFQ with a design that incorporates a stronger focusing and that will achieve a higher peak surface field (16 MV/m) at 82.2-kV inter-vane voltage. The objectives of this project are 1. to confirm the possibility of a low injection energy of 0.4 keV/u which significantly reduces the voltage required for a high-voltage deck; 2. to test the highest possible peak surface field on the RFQ electrodes designed for the lowest frequency of 12 MHz compared to existing RFQs worldwide; 3. to provide a technical base for the design of a post-accelerator for the future Advanced Exotic Beam Facility. At the present time, the design is complete, and the fabrication of the 12 MHz RFQ is scheduled to commence in October 2007 with testing planned in 2008. The physics and engineering design of the RFQ is discussed.