The Study Of Some Key Beam Dynamic Problems In The Recirculating Proton Linac

Project-X of the Fermi National Accelerator Laboratory(FNAL),which is an integral part of the U.S.Intensity Frontier Roadmap,is a world-leading program of mega-watt continuous wave(CW)high energy proton facility.The concept design is based on an 8 GeV superconducting linac using the superconducting radio frequency(SRF)technologies.It can support many experiments simultaneously,including Kaon,Muon,Nucleon,and Neutron Precision Experiments,Material Science,Neutrino Experiments,Nuclear Energy Applications and developing SRF accelerator technology.In order to satisfy the beam requirement of the precision experiments,the project proposed to use CW linac to accelerate the beam.Although linear accelerator is architecturally simple,but the beam only passes the cavities once,which is inefficient in the use of RF cavities.In order to increase utilization of the cavities,the concept of a recirculating proton linac was recently proposed by Ji Qiang of the Lawrence Berkeley National Laboratory(LBNL).The start-to-end beam dynamic design of the first section of the recirculating proton linac,the double-pass recirculating proton linac,was carried out recently.As the recirculating proton linac is a new type of the linac,there are some physics issues such as two bunches overtaking collision space charge effects,robustness of the linac design under imperfect realistic conditions and correction of two different energy bunch,multi-pass recirculating proton linac synchronization condition that need to be carefully studied.This dissertation focuses on studying some key physics problems in the recirculating proton linac based on the theory of linear accelerator and the designed Lattice of the double-pass recirculating proton linac.In order to study the change of the beam quality from repulsive nonlinear space-charge forces,the author derived a theory of space-charge effects during the overtaking interaction of the two bunches and calculated analytical estimation in the proposed double-pass proton linac.The author also simulated two proton bunches transporting through the linac with interbunch space-charge effects.Compared with analytical estimated results,the numerical simulation space-charge interaction effects might be somewhat small.As the reference design of the double-pass recirculating proton linac assumes perfect layout from the requirements,it is necessary to verify the robustness of this design under realistic imperfect conditions.Here we studied the static and dynamic errors from RF cavities and magnetic focusing elements in the double-pass linac.In the error study,we consider three important factors: beam power losses,root mean square(RMS)beam size,and emittance growth in the linac design.The results show that the average particle losses are 2.13E-6,the average beam power losses are 5.88 W.A new correction method named Optimation Correction Method(OCM)is proposed to corrector the error effect of low and high energy beams.With corrector simulation results show that the centroid shift,RMS beam size,RMS emittance,maximum beam size is smaller than before.For the multi-pass sections of the recirculating linac,the adjustment of separation between cavities will not be sufficient.A variable field phase-shifter between two cavities is needed to shift time delay between multiple passes to meet the synchronous condition.The author finished the conceptual design of the phase-shifter that varies the path length with the energy range in the four-pass linac section.The author compared the solutions with different maximum fields and found the needed parameters for the shifter.The results suggest that the higher maximum field reduces the total length of the phase-shifter,and establish the conceptual design of the four pass recirculating proton linac.