Yao, Z.Y. Keir, J.J. Kishi, D. Lang, D. Laxdal, R.E. Liu, H. Ma, Y. Matheson, B. Waraich, B.S. Zheng, Q. Zvyagintsev, V. JACoW Publishing Geneva, Switzerland 2226-0366 978-3-95450-194-6 10.18429/JACoW-LINAC2018-TUPO039 English 417-419 TUPO039 Contribution to a conference proceedings 2019 2019-01 https://doi.org/10.18429/JACoW-LINAC2018-TUPO039 http://jacow.org/linac2018/papers/tupo039.pdf A balloon variant of the single spoke resonator (SSR) has been designed, fabricated and tested. The cavity is the SSR1 prototype for the Rare Isotope Science Project (RISP) in Korea. It is specifically designed to reduce the likelihood of multipacting barriers near the operating point. A systematic multipacting study leads to a novel geometry, a spherical cavity with re-entrant irises and a spoke. Other than eliminating multipacting around operational gradient, the balloon shape also provides competitive RF parameters and robust mechanical structure. Cryostat cold tests demonstrated cavity performance on each design aspect. The cold tests will be reported in this paper.