Study On The Twin Input Coupler For A Multi-cell 1.5 Ghz Superconducting Cavity

High current, low emittance and continuous wave(CW) beam are the promising features of accelerator, which leads high-luminosity and stable synchrotron radiation to ensure high-quality imaging, materials analysis and materials processing, and provides a steady stream of high energy particles for nuclear energy and isotopes applications. All those features enable high gradient, high average beam current with low emittance, and CW accelerator a tendency in the near future.The 5-cell 1.5 GHz superconducting cavity designed by Shanghai Key Laboratory of Cryogenics & Superconducting RF Technology was optimized for an accelerating gradient of 15-20 MV·m-1, adopting at least two input coupler for development of a CW accelerator with its current close or up to 10 m A. Therefore, it is necessary to design the corresponding 1.5 GHz input coupler and to find out how to maintain twininput coupler.External quality factor of three kinds of schemes for input coupler placement were simulated by Computer Simulation Technology(CST). A slight difference between those schemes was showed in the results. Those differences resulted from structural change and associated electric field and magnetic field distortion have been discussed theoretically. Field distributions on axis and transverse coupler kick were computed, which indicated that coupler kick was dramatically diluted even when the difference of insert depths is 1mm by using twin input coupler. Moreover, by adopting a symmetric twin input coupler, dipole modes were suppressed.A derivation based on dual-ports device analysis theory was implemented to shed light on the effects of coupling parameters and incident power on power transmission. The results showed an enhancing on coupling when each coupler was of under coupling, but a decreasing to the weaker coupled one when two ports were over coupled. If the ratio of two coupling parameters was less than 0.16, the weaker coupled one should be measured as under coupled even when actually it was over coupled. Those conclusions were testified by measuring coupling parameters on a 1.5 GHz cooper cavity with one coupling variable coaxial input coupler assembled on each beam pipe.The optimized design procedure of a 1.5 GHz coaxial input coupler was presented. Geometric dimensions of the feasible coupler whose bandwidth of VSWR better than 1.05 to be 18 MHz was finalized. Tip of the inner conductor was enlarged as golf-tee- like to increase coupling strength without increasing insert depth.Strict requirements for performance of input coupler should be reduced by adopting twin input coupler as it can halve the power transmitted through per coupler. In addition, transverse kick will be diluted even if there was a difference of insert depths, which will help to keep the emittance from unexpected increasing. Therefore, it is the only option for accelerators operated in high current with low emittance and CW mode to use twin input coupler.