Design And Experiment Of X-band High-gradient Choke-mode Accelerating Structure

X-band high-gradient accelerating technology is a hotspot in the research of room-temperature accelerating structures having wide ranging application prospects in electron-positron linear colliders,X-ray free electron lasers(XFEL),small light sources,and compact medical accelerators.The size and cost of such devices can be reduced by increasing the accelerating gradient.The Radio Frequency(RF)breakdown phenomenon,which will lower beam quality and damage the structure surface,is one of the main limitations to achieving high gradients.In addition the strong wakefield excited by multibunch beam in X-band accelerating structures will cause beam instability or even beam break up.Several kinds of structures with higher-order-mode(HOM)suppression have been researched in the effort to damp the wakefield effect.The Choke-mode accelerating structure is one of these HOM damping structures.It has the advantage of relatively simple fabrication and low surface magnetic field.C-band Choke-mode accelerating structures have been successfully applied in multibunch XFEL.However,the X-band Choke-mode study remains in the theoretical design stage.The breakdown characteristics and high-gradient performance of the choke are still unknown.In the thesis,a universal high-gradient testing and analyzing method was established by conducting high-gradient experiments on a multi-cell traveling-wave structure.This structure reached 110 MV/m which validated Tsinghua high-gradient technology.Based on the same X-band high-gradient technology,several different single-cell Choke-mode accelerating structures were designed,fabricated and high-gradient tested to study the related RF breakdown characteristics.The absence of field emission current flash was proposed to be the sign of breakdowns occurring inside the choke,this was verified by the post-mortem observation.Evaluation of the breakdown rate revealed that there is memory effect with pulse width and electric field.The breakdown rate in a single RF pulse did not have the 5th order pulse width and 30th order electric field dependency predicted by the empirical formula.An RF pulse evolution constant of the order of 10~7was observed from the conditioning rate of the structure.It was observed that high electric field and small choke dimension caused serious breakdowns in the choke which was the main limitation of the high-gradient performance.The Choke-mode accelerating structures reached 130 MV/m by decreasing the electric field and increasing the choke gap.A new quantity was proposed to give the high-gradient performance limit of Choke-mode accelerating structures due to RF breakdown.The new quantity was obtained from the summary of the high-gradient experiments and could be used to guide high-gradient Choke-mode accelerating structure design.