Experimental Research On LLRF Control Algorithms For High Current Superconducting Accelerator

High beam current superconducting accelerator is the leading edge research field in the world, the proton linac injector II of accelerator driven subcritical system(ADS) is a high current superconducting accelerator which is under constructing, the beam current goal of ADS linac injector II is 10 m A, and the low beta superconducting cavity accelerating technology was being used, the working mode of ADS injector II is continuous wave(CW) mode.The intrinsic quality factor of superconducting cavity(Q0) is very high, usually more than 109, high Q0 value means the narrow bandwidth, so the superconducting cavity is very sensitive to the change of cavity resonant frequency, microphonics is a major cause of the superconducting cavity resonance frequency shift during CW operation. In addition, when the high current beam was accelerated by superconducting cavity, the beam will absorb RF power that are stored in the superconducting cavity, this process will result in the drop of cavity voltage, the amplitude and phase fluctuations of cavity electric field will affect the beam quality, the beam which arrived later can not be effectively accelerated. This thesis mainly studies and resolves these problems mentioned above which occurs during high beam current superconducting accelerator operating.Firstly, strarting from the superconducting cavity equivalent circuit model, the cascading coupling model of superconducting cavity – RF power source and the discrete difference model was built in this paper. Basing on the mathematical model of the superconducting cavity, the related numerical calculation and theoretical analysis of cavity was done, the calculation results shown that the small changes in the cavity resonance frequency and the long pulse high current beam will heavily affect the electric field of cavity.Then, a new digital microphonics measurement system was developed in this thesis for the microphonics measurement of low beta half wave resonator(HWR), the performance test results show that the measurement accuracy of this system is better than 0.2 Hz, and on this basis, we made the on-line microphonics measurement for the half wave resonators which were installed in the test cryostat module(TCM) and cryostat module 6(CM6), and the experimental results was analyzed.In addition, on the basis of the RF cavity self excitated loop theory, a new argument was put forward in this thesis for the problem of its theory application, and on the basis of this theory, a digital self excited low level RF system was developed in this thesis, and this system was successfully tested with the HWR superconducting cavity, this system can greatly shorten the frequency tracking time after turning on superconducting cavity and quickly built up the electric field in superconducting cavity, the self excited LLRF system is also necessary for the RF conditioning of superconducting cavity.Finally, the beam loading compensation method was discussed in the fifth chapter of this thesis. For the purpose of overcoming the beam loading caused effect on the electric field in the RF cavity, a simple and effective feedforward compensation scheme was developed based on the negative feedback control theory, and this scheme was successfully used for the 10 m A pulsed proton beam commissioning.