Study On Key Issues In Cavity Failure And High Energy Beam Transport Line In High Power Proton Accelerators

China initiative Accelerator Driven sub-critical System(CiADS)is the first Mega-watts(MW)level ADS device in the world used for the principle verification of nuclear waste disposal safely.The designed power for CiADS system is 10 MW,in which the energy of the superconducting(SC)proton linear accelerator(linac)is 500 MeV,with the beam current of 5 mA in continuous wave(CW)mode,and beam power of 2.5 MW.The preliminary design for CiADS has been finished and the construction will start soon.As for designed power,European Spallation Source(ESS)is currently the most powerful neutron source device in the world.The designed energy of the SC proton linac is 2 GeV,with the average current of 2.5 mA,and beam power 5 MW.ESS is under construction now,and the first beam has been extracted from the iron source(IS)in September 2018There are many challenges for High Power Proton Accelerators(HPPA)represented by the two accelerators,including space charge effect of high current beam,beam loss control for high power beam,the design and stable commissioning for high power RFQ and SC accelerating section,and beam distribution requirements from the high power spallation source.These challenges mainly focus on the SC section and High Energy Beam Transport line(HEBT).Considering the common problems in the accelerator of CiADS and ESS,key issues for the SC section and HEBT are studied in the thesis:cavity failure and rematch study in the SC section,power loss study and control in HEBT,and the homogenization method for the beam entering the targetWhen an SC cavity fails,the synchronous phases for downstream cavities will become disorganized.To avoid beam loss in SC cavities,it should be rematched for the beam.Taking the ESS SC linac as an example,a new rematch method is presented in the thesis with the optimization goal of minimizing the transfer matrix differences before cavity failure and after the rematch.Based on the method,a program is fulfilled with genetic algorithm,and the goal of matrix difference minimization is achieved The rematched results in zero current mode are close to that with the current of 62.5 mA.Based on the program,cavity failure and rematch is calculated and the beam properties are in line with expectations,including the envelope,emittance growth,Tune depression,etc.,verifying the validity and feasibility of the algorithm and program.The same rematch method is applied to single cavity failure for all the cavities in ESS SC section,and no power loss is observed in the multiparticle tracking simulation code for all the cases.It indicates that the program is useful for single cavity failure rematch without power loss.Comparing with lower energy section,the same percent beam loss leads to more power loss in HEBT.Power loss in HEBT is studied in the thesis,and the HEBT dynamic design for 571 MeV proton beam at ESS is accomplished.Regular problems are studied,including achromat in bending sections,the remnant field for dipoles,smooth transition of phase advance,especially power loss on the beam entrance window(BEW)with both error and scan in Accelerator to Target(A2T)section.The concept of beam loss scan factor(BLSF)is put forward for fast evaluation of power loss on BEW with raster scan.In the same way,the dynamic design for the CiADS HEBT including the vacuum differential system(VDS)is fulfilled,focusing on the collimation and beam homog-enization method study.In the collimation method study,based on the idea of the upstream emittance well inside the downstream acceptance,beam halo is collimated,ensuring that no power loss in vacuum holes and beam tube before the target.In the beam homogenization method study,different methods are simulated,compared,selected and optimized according to the CiADS target requirements of beam shape,distribution,peak current density(PCD),the utilization rate of the target area,etc.,and hollow spiral wobbler scan method is proposed.The radius function is extended up to the fourth harmonic through Fourier transform,in which the problem of the infinity of dI/dt is solved,and global smooth approximation is achieved for engineering feasibility.Hollow round distribution is formed after the scan and the PCD is controlled within 35?A/cm2,with high utilization rate of the target area.Based on CiADS and ESS project,key issues in cavity failure rematch and power loss control in HEBT are studied in the thesis.Because of the similarity of the two accelerators,the methods and conclusions of the study share the same reference value for both of them.