Research Of Key Issues Of Radio Frequency System For ADS Superconducting Linac

China initiative Accelerator Driven System(CiADS)is one of the National Large Research Infrastructures Projects in the 12th Five-Year-Plan,and is also one of the Ac-celerator Driven System(ADS)theoretical verification facility.ADS accelerates a high-current proton beam with energy 0.5?1.5 GeV.Protons bombard a heavy metal spal-lation target,in order to produce fast neutrons for the transmutation reaction in the fast reactor.The purpose of ADS is to develop a method for safe and efficient disposal and recycling of nuclear waste.The CiADS includes a full-superconducting proton linac,with the designed energy of 500 MeV,and the beam current of 5 mA.ADS superconducting linac has extremely high requirements on operating stability and beam loss control.The radio frequency(RF)system,providing accelerating energy to proton beam,is one of the important parts of accelerators and has an important im-pact on the beam stability.Based on the key characteristics of the ADS accelerator,this dissertation puts forward the requirements for the stability of the RF devices of the super-conducting linac for CiADS and conducts research on the key issues affecting the stability of the RF system and its optimal design.The main contents include stability analysis of RF system;prototype development and testing of RF front-ends;RF power requirements of superconducting cavity and optimization of power level of standard module for solid-state power amplifiers to match each phases of CiADS.Different frequency acceleration structures in different energy sections are used to adapt the velocity of protons.In order to meet the requirements of the RF control sys-tem for amplitude and phase stability at different frequencies,the dissertation proposes a unified structure and wideband RF front-end circuit.A low-noise,wide-band mixer and amplifier were used to achieve multi-frequency down and up conversion.A LO dis-tribution network was carefully designed to improve the isolation between channels.A replaceable external band-pass filter design for up-conversion,which achieved well sup-port for wideband under a unified structure design finally.The prototype has been de-veloped and tested.The results are as follows:the down conversion linearity coefficient of determination R~2reaches 99.991%,and isolation reaches 65?89 dBc in the input signal range of-60?10 dBm.The phase noise and the closed-loop control with cav-ity emulator were tested on Low Level RF(LLRF)test platform in Lawrence Berkeley National Laboratory(LBNL).The phase noise is better than 80 dBc in range of 1 MHz off center frequency,and the accumulated noise is 0.006?.An online buncher cavity on CAFe(China ADS Front End Superconducting Demo Linac)was controlled by LLRF with wideband front-end.The results shows that the amplitude stability is 0.026%and the phase stability is 0.037?.The issues found during testing were summarized and ana-lyzed in the dissertation,and suggestions for improvement were also put forward.The RF front-end supporting multiple frequencies can meet the LLRF requirements of CiADS on availability,maintainability,modularity,standardization,low cost and mass production,and its performance tested can meet the overall requirements of CiADS RF system.The superconducting linac is coordinated working by 141 sets of RF devices.The reference phase distribution system plays an important role of the operation-stability of the entire RF system.It has the characteristics of low phase noise and low phase drift by ambient temperature variation and serveing as a”clock”.By analyzing error sources existing in the software and hardware of LLRF control systems,and also phase reference systems,it is clear that the accuracy and stability of the phase reference line are the main factor affecting the cooperative work stable of multiple RF system components.With refering to the international and domestic application status in accelerators,the coaxial phase average reference line with phase-locked loop(PLL)terminal short-circuit mode is proposed to be much more in line with the CiADS project special requirements in long-distance transmission,multi-node,scalable,etc.In order to improve the limitation of the number of nodes in the traditional scheme,based on the theoretical analysis of the phase average implementation principle,a non-PLL phase average reference line scheme is proposed and some experiments were excuted to verify its feasibility.The results of the new scheme show that RMS phase accuracy can reach 0.1?0.3?for days,0.02?for hours and 0.005?for minutes.The design beam power of the superconducting linac is 2.5 MW,and the solid-state power amplifier accounts for more than 30%of the equipment cost.Due to the large vari-ation of energy gain among superconducting cavities,how to maximize the RF power uti-lization when upgrading the beam current between different phases under the fixed power coupling condition brings great challenges towards the standardization and level design of solid-state power amplifier.Starting from a equivalent circuit of the radio frequency sys-tem,this dissertation established a mathematical model for the RF power requirements of the superconducting cavity and a grading strategy.Taking the minimization of reflected power corresponding to the matching current for all upgrade processes as the criterion,considering the bandwidth requirements of the stable operation of the superconducting cavity,and putting the discrete requirements of the beam power of each cavity required by the accelerator design into consideration,an optimization algorithm is developed to minimize the output power level steps of the standardized solid-state power amplifier modules and maximize the utilization of the accelerator RF power system.Evaluation was also performed for the RF power redundance allowed for detuning compensation of the superconducting cavities under the above optimized parameters.The optimization method described in this thesis can apply to the evaluations on different accelerator design operating parameters and conveniently generates the corresponding optimized RF power grades,based on this method,the upgrade strategy of the RF power source of CiADS superconducting linac beam current from 2.5 mA to 5 mA and even to 10 mA also be predicted.A versatile RF solid-state power amplifier configuration guidance for a variety kinds of superconducting linac operating under different beam power grades can thus be concluded.In summary,the research results presented in this dissertation provide a feasible scheme for the design and construction of the RF system for the CiADS superconducting linac.