Four-harmonic buncher for effective combination of radioactive and stable beams

The Argonne Tandem Linac Accelerator System (ATLAS) is the only national user facility for low-energy stable heavy ion beams. There are plans to convert ATLAS into a multi-user facility to simultaneously accelerate both stable and radioactive beams. This update requires a multi- harmonic buncher (MHB) before the radio-frequency quadrupole (RFQ) that can provide enough voltage of the highly linear saw-tooth signal to bunch heavy ion beams with low charge to mass ratio. The existing MHB is realized with lumped elements and cannot withstand the required voltage of ~6 kV due to the thermal issues. In response to this problem, RadiaBeam Systems in proposes to design and build a four-harmonic buncher based on the three coaxial quarter wave resonators. We will implement the folded resonator design to significantly reduce the dimensions of the system, which otherwise would be too large due to the very low frequency of the fundamental harmonic (12.125 MHz). In Phase I, we will perform electromagnetic design of the buncher cavity and coaxial resonators, and develop the conceptual engineering design with 3D thermal analysis to ensure that the cavity will be capable to operate at the design levels of the fields. The conceptual design of the LLRF control system will be generated to ensure the signal stability. Commercial Applications and Other Benefits: The results of this work will be of immediate benefit to the ATLAS facility. In addition, a similar buncher could be used at the Facility for Rare Isotope Beams (FRIB) for transmission improvement. Low-frequency multi-harmonic cavities are also of the interest for cyclotrons as they can provide nearly flat-top voltage profile that significantly improves beam quality and increases the maximal achievable energy. The industrial applications of multi-harmonic cavities range from the use in accelerators for ion therapy to high power accelerator-driven systems such as waste transmutation or power generation facilities.