The Study On Unbalanced Distribution Of Reflected Power And Impedance Mismatch In Solid State Amplifier

Solid state amplifier(SSA)widely used in many accelerators because of many advantages,such as modular design which has more compact structure,more convenient for installation,maintenance,assembly and upgrade.The 25 MeV superconducting proton linac prototype accelerator known as CAFe adopted SSA driven normal conducting and super-conducting cavities and ensured the beam power and system stability.But there are still some problems during operation of SSA,for instance,some modules damaged that maybe caused by lacking of design structure and design parameter.In addition,the mismatching will affect the stability of amplitude and phase in cavity controlled by low level radio frequency.This thesis will focus on the problems mentioned above,and analyze corresponding phenomenon specifically and give a reasonable explanation,which will provide beneficial reference for design of SSA in CiADS(Chinese initiative Accelerator Driven Sub-critical System).Firstly,we derive the S parameter of multiple stage power combiner based on signal flow method from microwave network theory,and obtain a relationship between voltage transmission efficient and structure of power combiner.The calculation result shown that,phase shift in neighbor stages has important effect on voltage transmission efficient,that may cause unbalanced power distribution in some situations.Secondly,we analyzed the distribution of reflected power in the combiner under different output state of power amplifier modules.The result showed that the distribution of reflected power is balance while all power amplifier module in a normal output status,unbalanced situation occurred when single or several modules close which caused by protected of modules,for the SSA with one-stage,only failure module may undertake over loading reflection,for the SSA with multiple stage,any module in the combining branch where the faulty module was located may undertake over loading reflection.Reflected power will be four times as normal reflected power in some combiner while load is full reflection,which usually occurred in super-conducting cavities,and this power beyond the limitation that power amplifier can undertake.This result provided reasonable explanation for destroyed of current CAFe's amplifier module,and also pointed out the shortage of current SSA design.Thirdly,the over loading reflection will cause the damage of circulator of modules,which make corresponding input arm of combiner mismatch and change the impedance of SSA.A new method of resolve linear equations was developed to analyze the impedance mismatch problem directly in this thesis,and it is more conveniently analyze impedance mismatch problem in complex combiner compared to the method of degradation of S parameter.According to this method,the unbalanced distribution of reflected power also happened when some of input port mismatch,this is not only cause the critical damage of power amplifier module and output circuit belong to it,but also cause the change of SSA's output and module's working state,which will lead difficult control to super-conducting cavity.