Study Of The X-band High Gradient Accelerating Structure And Wakefield Effects

The development of high energy physics and photonic science has proposed higher requirements for the energy of experimental facilities,which greatly promote the development of particle accelerators.In order to discover and study new physical phenomena,the center of mass of particle colliders needs to be increased by one order of magnitude.Due to limited installation length,the accelerating gradient must be improved correspondly.It caters to the development for compact free electron laser(FEL)facility and is beneficial to miniaturization of the laboratory facilities and medical equipment.Thus,the high accelerating gradient is the key factor for the compact linear acceleration facility.In order to achieve high gradient,the Stanford Linear Accelerator Center(SLAC),the High Energy Accelerator Research Organization(KEK),the European Organization for Nuclear Research(CERN)and other research institutes have been devoting to studying the accelerating structure at X-band or even higher operation frequency.They carry out a large amount of research on the test facilities of NLCTA(Next Linear Collider Test Accelerator),XTF(New X-band Test Facility)and CTF(Compact Linear Collider Test Facility),and obtain 120 MV/m unloaded stable gradient of X-band structure,which is far greater than that of S-band and C-band.The gradient can reach 150 MV/m at 30 GHz operation frequency with extremely short pulse,but it is unable to operate stably in multi-bunch mode.With the development of precision machining technology and high power sources,the X-band FEL international cooperation group is founded by lots of institutes and the X-band high gradient accelerating structure is considered as the preferred scheme for the compact FEL facilities.However,the X-band accelerating structure still has problems on breakdown and transverse wakefield suppression.The breakdown usually occurs on cell wall,related to the surface electromagnetic field,and restricts the accelerating gradient.The transverse wakefield effect is closely related to the bunch spacing and bunch number,especially serious in the multi-bunch mode.For the facility in single bunch mode,liking FEL facility,the transverse wakefield suppression is decided by the beam aperture.In this paper,a disk-loaded structure with the gradient of 80 MV/m is designed,fabricated and made tuning.Due to the change of project item,the test accelerating structure is installed on the Shanghai soft X-ray FEL facility to realize linear compression of bunch,instead of specially designed for the compact FEL facility.During the period of study,a multi-objective structural optimization procedure based on polynomial fitting is developed.The suppression of the surface electromagnetic fields,wakefield effects and the multipole field effects in couplers are analyzed.The measurement and tuning of non-resonant perturbation are carried out.High power experiment and bunch experiment are completed after the manufacture of sample tube is finished.This paper also introduces the design of X-band waveguide damped structure at the first stage of CLIC project(380 GeV CLIC).Transverse wakefield effects and surface electromagnetic fields are suppressed by optimizing the shape of cavity contour,the distribution of waveguide size and the position of higher order mode load.Based on Gaussian detuned principle,the wakefield effects of the disk-loaded structure are analyzed under the two bunch operation mode and multi-bunch mode.