Prototyping of a 352 MHz, Beta=0.17 Superconducting Coaxial Half Wave Resonator

We have designed a 352 MHz superconducting coaxial half wave resonator, with optimum β=0.17. The cavity has a mechanical design similar to the β=0.31 one developed at LNL in 2004. It is very compact (232 mm real-estate length) and it is equipped with a side tuner not exposed to liquid helium, to make it insensitive to pressure fluctuations. Operation is foreseen at 4.2 K. This cavity fills the gap from 5 to ∼ 25 MeV between the LNL proton RFQ, under construction, and and the existing β=0.31 half wave resonator. This allows a 5÷100 MeV proton linac working at 352 MHz with 2 types of coaxial HWR cavities with large velocity acceptance, thus able to accelerate also other ion species (e.g. deuterons). A similar scheme was previously proposed for Spoke resonators; the aim of the HWR choice is compactness and cost reduction. The same design scheme is suitable for 176 MHz HWRs with β=0.09 and β=0.15. The β=0.17 cavity is presently under construction in the SPES R&D program at LNL; first test results are expected by the end of 2005.