Investigations On RF Breakdown Phenomenon In High Gradient Accelerating Structures

Recently,there is a pressing need of high gradient accelerating structures in compact electron positron colliders,X-ray free electron lasers,compact medical and industrial accelerators.Radio-frequency(RF)breakdown phenomenon is one of the main limitations to achieve high gradient structures.RF breakdown needs to be suppressed as it often leads to problems including surface damage,cavity detuning,incorrect phase shift,low energy gain,and beam loss,etc.RF breakdown is a sophisticated physical phenomenon.It is usually believed to be triggered by field emission,followed by plasma formation,explosive emission,and surface melting,etc.The breakdown rate is determined by multi-factors including electric field,magnetic field,pulse length,and modified Poynting vector,etc.As the position and moment of RF breakdown occurrence is random,the usual method to study RF breakdown is to test an accelerating structure for long period so as to perform statistical study based on a large amount of data.Such researches face many problems,such as extremely long experiment period and difficulty of data analysis as the multi-factors can not be well separated.This thesis applies high power laser to trigger RF breakdown.The experiment period can be shortened and the conclusion can be clarified as the RF breakdown can be controllably triggered in terms of position and moment.With an S-band 1.6 cell photocathode RF gun,the time structure of the breakdown current has been obtained and multi breakdown events within one RF period has been observed.Based on equivalent circuit model with breakdown current,this thesis has analyzed the origin of multi breakdown events and proposed a new accelerating structure to suppress it.Besides,the thesis has also carried out innovative studies on RF breakdown and field emission with an L-band single cell high gradient photocathode RF gun.The evolution of emission parameters during RF conditioning has been observed.Using a pin cathode which can be moved longitudinally,this thesis has discovered that there is a dependence of field emission current on the stored energy when the electric field of the fundamental mode was kept the same.This result shows the local field emission can be influenced by the global parameters of the system.It reveals the limitation of the conventional application of the Fowler-Nordheim equation in accelerator studies.This thesis also carried out the study of high resolution in-situ field emission imaging.A novel method has been proposed and systematic dynamics study has been performed.In experiment,clear field emission pattern with 100 ?m resolution has been achieved.The image reveals that the field emission is dominated by separated emitters.The field enhancement factor,?,of each single emitters has been measured.By post surface analysis,it is discovered that most emitters overlap with RF breakdown spots.