Study On The Pre-tuning Technology Of The Superconducting Cavity

Superconducting cavity has many advantages over normal cavity, such as highaccelerating gradient, high quality factor, etc. The superconducting cavities areapplied more and more widely in the world nowadays, especially the multi cellsuperconducting cavities, which have become a hot research topic in the field ofsuperconducting cavity. Pre tuning is a key technology in developing asuperconducting cavity. Due to the influence of such factors as machining tolerances,for a multi cell superconducting cavity, the volume of each cell are different afterelectron beam welding, which not only makes the cavity’s resonant frequencydeviate from the designed frequency, but also lets the electric field amplitude bedifferent in each cell. Therefore the surface electric field can’t be minimized and theaccelerating gradient can’t be maximized. These problems can be tackled bypre tuning. Pre tuning can effectively improve the performance of a multi cellsuperconducting cavity.The500MHz5cell cavity developed in Shanghai Key Laboratory ofCryogenics&Superconducting RF Technology is a big aperture, low loss typesuperconducting cavity. It can be used for the high current, high beam qualityaccelerators in the future. The researches in this paper are based on this500MHz5cell superconducting cavity.According to the universally used “bead pull” pre tuning theory, we set up acomplete pre tuning system. This system consists of two parts: hardware andsoftware. The hardware can be divided into three subsystems, including RF system,stepping motor system and auxiliary system. The RF system is used to stimulateelectromagnetic field in the superconducting cavity, the stepping motor system pullsthe perturbation passing through the cavity, and the auxiliary system contains someequipments, which are used for tuning. Software is a LabVIEW and MATLABhybrid program. The LabVIEW part is in charge of instrument control and dataacquisition, the MATLAB part is responsible for data processing. Taking advantage of the fabricated “bead pull” pre tuning system, we pre tunedcavities fabricated in our laboratory. On the argon arc welding500MHz5cellcopper prototype cavity, the established pre tuning system was debugged to ensureits reliability and stability; then the electron beam welding500MHz5cell copperprototype cavity was pre tuned, the field flatness was tuned as better as99.1%andits resonant frequency was adjusted to498.798MHz, which was only0.002MHzlower than the target frequency498.80MHz; finally the500MHz5cell niobiumcavity was pre tuned, the field flatness was adjusted to98.5%, which ensured thecavity’s final vertical testing performance.In this paper, the electron beam welding500MHz5cell copper prototypecavity was pre tuned many times, and some related problems were studied. A lot ofexperimental datas and much pre tuning experience were got. A complete pre tuningprocess was mastered. We explored a simple pre tuning step, and we found arelatively simplified pre tuning operation. All these laid a solid foundation for thefollowing multi cell superconducting cavity’s pre tuning in our laboratory.