Development of a superconducting radio frequency double spoke cavity for CSNS

China Spallation Neutron Source (CSNS) has been in operation for about 2 years, and an upgrade is proposed to compensate the space charge effect. A superconducting radio frequency (SRF) linac with double spoke cavities and medium-beta elliptical cavities has been designed to increase the injection energy from 80MeV to 300MeV, and institute of high energy physics (IHEP) has developed a prototype double spoke cavity at ${\beta }_0$ 0.5. The cavity shape was optimized to minimize Ep/Eacc while keeping Bp/Eacc reasonably low. Meanwhile, mechanical design was applied to check stress, Lorentz force detuning and microphonic effects, and to minimize pressure sensitivity. A new input power coupling scheme was proposed to avoid electrons hitting directly on ceramic window. After fabrication and post processing of cavity, the cavity was vertical tested for several times, and mechanical polishing was applied to improve the performance. The cavity finally reached Bp of 140mT at Eacc = 16.1MV/m and $Q_0=1.42\times 10^{10}$ under vertical test at 2K.