Development Of A Dual Warm Windows Power Coupler

Nuclear fission energy is a strategic choice to solve China's future energy supply and ensure sustainable economic and social development.Accelerator-driven subcritical system is an effective means to safely dispose of nuclear waste.It puts forward strict requirements for the reliability and stability of the accelerator system.Power coupler(PC)is a key component of superconducting radio frequency(SRF)system of accelerator.Improving its reliability and stability is the inherent requirement of ADS transmutation system.According to the operation status of power coupler on superconducting cavity(SC)at home and abroad,there are three main reasons that affect the reliability and stability of power coupler.One is that power coupler on superconducting cavity often operate in total or most reflective state due to the detuning of superconducting cavity,the error of external quality factors,the disparity of operating voltage and the difference between beam current intensity and design value;the other is that the electronic activity near the ceramic window cause breakdown,including the surface flashover caused by the charge accumulation of field emission electrons from the superconducting cavity on the ceramic window.Third,low temperature gas condensation will lead to the increase of secondary electron emission coefficient on the material surface,which will affect the performance and stability of the coupler running online for a long time.Due to the influence of field emission electrons in the superconducting cavity,the vacuum leakage of the ceramic window occurs during the operation of the ADS special injector II coupler,which seriously affects the reliability and stability of the accelerator.In order to improve the reliability of SRF system,we have developed a dual warm windows coupler.In the third chapter,firstly,the dual-warm-windows coupler gone through electromagnetic design to meet the requirement of high-efficiency transmission power,and the antenna head is changed to shield the electrons emitted from the superconducting cavity.Secondly,the secondary electron multiplication characteristics of the dual-warm-windows coupler are analyzed to confirm the necessity of applying bias voltage,and then the mechanical design is carried out according to the electromagnetic model.Finally,the dual-warm-windows coupler is implemented on the test stand.The high power test of the couplers has been carried out,and the design target of traveling and standing wave power exceeding 12 kW has been achieved.DC bias voltage is an effective means to suppress secondary electron multiplication,but the bias voltage scheme for couplers with coaxial feeders is not mature enough.In the fourth chapter,a simple bias structure is proposed,which can not only achieve the purpose of applying bias voltage into the inner conductor,but also avoid the influence on power source output circuit.Then,the RF leakage field from bias insulated capacitor is studied by electromagnetic field equivalent principle and CST simulation.Finally,according to the transmission characteristics of coaxial transmission line with gap in the inner conductor,a new series resistance filter circuit is proposed,which passes through the on-line and off-line test.The stability of RF superconducting system are improved due to the successful bias structure.The secondary electron multiplication effect caused by low temperature gas condensation and the pollution of the coupler to the superconducting cavity were studied by on-line processing and operation of the double warm windows coupler.The field emission of the superconducting cavity was improved by high power short pulse processing.