Physical Design And Key Issues Study Of HIAF RFQ

As the injector of " High Intensity Heavy-ion Accelerator Facility(HIAF)",a major national scientific and technological infrastructure during the 12 th Five Year Plan period,the high-power superconducting accelerator ‘i Linac’ has very strict requirements for beam loss control in the superconducting acceleration section.In order to meet the requirement of beam loss-free acceleration in the superconducting section,the beam must be controlled during its room-temperature front-end accelerator.The room-temperature front-end of i Linac contains a radio frequency quadrupole(RFQ)accelerator which relies on the almost uniform and continuously changing electro-focusing acceleration cells to focus,bunch and accelerate the lowenergy beam at the same time.The RFQ plays a significant role in constraining the transverse emittance and forming the longitudinal emittance of the beam.According to the RFQ requirements of the HIAF project,this thesis has carried out the optimal design of the transverse and longitudinal matching sections and the dynamics of the room-temperature front-end RFQ accelerator system,and conducted the RFQ error analysis and beam commissioning simulation study.The HIAF RFQ adopts an external bunching design,which can not only shorten the length of the RFQ,but also reduces the longitudinal emittance by losing part of the beam.The acceleration of the HIAF RFQ system employs a 3-harmonic buncher and a-45° RFQ accelerator,respectively.Transverse matching of the HIAF RFQ system is performed using a single solenoid.In the thesis,the layout and parameters of the transverse and longitudinal matching sections,as well as the RFQ transverse and longitudinal dynamics design including the external bunching,were optimized.The overall acceleration efficiency of the HIAF RFQ accelerator system is 86.1%,and the longitudinal emittance(99.9%)is 2.34 Pi.mm mrad,which meets the RFQ design requirements for HIAF superconducting linear accelerators.Misalignments and manufacturing errors of the RFQ acceleration cells,and changes in upstream beam parameters and the working environment,can cause an increase in beam emittance and beam loss.An automatic parameter scanning program was developed to conduct a redundancy analysis of beams with different input currents,emittances and twiss parameters.In addition,since the RFQ is a continuous structure,the error analysis method in the Trace Win program,which is only applicable to discrete elemetns,cannot be used in our RFQ design directly and the conventional international RFQ alignment error analysis cannot correctly simulate the displacement and tilt errors between the segments of the RFQ accelerator.Thus,an error generation program based on the segments alignment error model was developed using Python.First,RFQ cells within each segment are packaged together by Tracewin.Then,random errors are assigned to each segment by the error generation program.By this method,the maximum tolerance range of RFQ alignment and assembly error was redetermined,and the reason for the difference between the collimation assembly error simulation and the actual working condition was clarified.In practice,it is found that a range of transverse matching parameters can achieve RFQ high transmission efficiency,but output emittances corresponding to these parameters are very different,which means that it is almost impossible to rely on output emittance measurements to achieve beam matching.To solve this problem,a beam commissioning simulation study based on the HIAF RFQ design was conducted.The simulation results show that the center of the region with high transmission efficiency coincides with the position where beam RMS and 99.9%emittance are minimized.According to these results,the author proposes a method to realize the optimal matching of RFQ accelerator by using the transmission efficiency.The method was verified at the China Accelerator Driven System Front-end Demonstration Facility(CAFe).The simulations and experiments are in good agreement with each other.In beam commissioning,the scheme proposed by the author can achieve the maximum beam transmission efficiency and the minimum beam emittance at the same time.