Study Of Mechanical Stability And Frequency Tuning For Low Beta Superconducting Half-wave Resonator In High Intensity Proton Linac For China ADS

The superconducting (SC) half-wave resonator (HWR) was firstly proposed andfabricated at Argonne National Laboratory in1990s. It has a compact structure. Themain advantage of HWR cavity is that there is no vertical beam steering effect becauseof the geometry symmetry in vertical direction. Considering the advantage, it has beendeveloped as one of the primary choice of accelerating structures in low and medialbeta sections. The SC HWR is a coaxial type element based on the TEM mode. Thesuperconducting linac of China ADS (Accelerator Driven sub-critical System) is about10mA of current and50MeV of energy. Two different types of superconductingHWR are employed by injector-II in ADS project, which is in the charge of Institute ofModern Physics. However, because the load quality factor of SC cavity usually isextremely higher than that of room temperature cavity, its bandwidth is quite narrow(generally scores of Hz). Furthermore, the resonant frequency of the SC HWR ishighly sensitive to mechanical deformations. Therefore, the study on mechanicalstability of a superconducting cavity is an important and difficult issue around theworld.Mechanical stability of the HWR (optimum beta=0.10and the frequency162.5MHz) is studied in the thesis. Mainly, the multi-physics coupled analysis basedon the finite element method is used to calculate the detuning effects of the HWR. Generally, it includes the helium pressure coefficient, the Lorentz force detuningcoefficient, microphonics and the cooling down. A novel analysis method of the elasticboundary condition effect is adopted during the cavity detuning study, which makes asuperposition between the simulation and the measurement. According to thesimulation results, the structure optimization for a squeezed HWR cavity has beenfinished and a new design for higher mechanical stability is obtained. In addition, theinitial test results show that the improvement of the cavity is highly effective.Based on the design of a162.5MHz and beta=0.15taper HWR, the contributionof electric field and magnet field to the helium pressure coefficient are analyzedrespectively. Then a new conceptual design, which the helium pressure coefficientequals zero, is introduced in the thesis.The frequency tuning for the low beta HWR is also focused by the thesis. Firstly,the parameters of the tuning are all introduced such as frequency sensitivity, range,resolution and the stability of amplitude and phase and so on. A slow tuner driven bystep motor is designed based on the studies above. The results of the measurementduring room temperature and horizontal test prove the tuner can operate stably. Animprovement plan for the mechanical tuner is proposed after many tests. Finally, theHWR can operate at the correct frequency and suitable accelerating voltage range withthe slow tuner and fast tuner, which are controlled by low level RF. Also two kinds ofpiezo tuner design, operating at room temperature and low temperature, are achieved.The fast tuning systems, which meet the requirement of fast response, improve theperformance of the tuner and reduce the backlash effect.