Design and optimization of the RF input-coupler for a low-frequency, split-coaxial RFQ

Abstract

The Isotope Decay-At-Rest experiment (IsoDAR) is a proposed underground experiment which is expected to be a definitive search for sterile neutrinos. In order to be decisive within 5 years, high rates of neutrinos must be produced, byimpinging a 10 mA continuous wave proton beam at 60 MeV on a high power target.Due to space restrictions, a compact cyclotron was chosen as an accelerator to producethis driver beam.To overcome space charge limitations during injection, H2 + ions are accelerated andlater stripped into protons by means of a carbon foil.IsoDAR uses an especially designed low-frequency (32.8 MHz) split-coaxial Radio-FrequencyQuadrupole (RFQ) to effectively bunch H2 + ions before injecting them into this cyclotron.The RFQ will be embedded vertically in the cyclotron yoke,facilitating a very compact design. This puts stringent limits on RFQ size, type, andaccessibility.Here, we present the design and optimization of the low-frequency (32.8 MHz) RF input-couplerfor the IsoDAR RFQ.The design is challenging due to the necessarily small diameter of the RFQ (28 cm)and the split-coaxial type, as well as limited access to the RFQ.We have determined the optimal position and shape for the coupler, leading to a low powerconsumption of < 6 kW for an inter-vane voltage of 22 kV. The highest calculated fieldsare safely below the Kilpatrick limit for this structure.