Structure Optimization And Process Research Of Rf Electric Focusing CH-DTL

The radio frequency electric focusing CH-DTL（RFC）accelerating structure integrates the CH-DTL and the four vane RFQ which have the same electromagnetic field mode as TE210 in the same cavity.The CH-DTL with high shunt impedance is used for longitudinal acceleration and bunching,and the powerful electric focusing method in the low energy section is used for transverse focusing.It can maintain a high acceleration gradient and shunt impedance while ensuring stable transmission of the particle beam,extending the electric focusing method to higher energy section.This normal temperature accelerating structure is compact,efficient,stable and convenient in operation.It can be used as an intermediate transition structure connecting the RFQ and the main acceleration section in large linear accelerators,and is also suitable for medical and industrial accelerators,which has good prospects for application in general.The research on RFC is still at the theoretical stage and there is no precedent for engineering implementation.In order to conduct process route exploration and RF structure verification,the first international 325 MHz aluminum cold model cavity was developed.The cold model cavity is about 1 m long and contains 12 drift tube components and two RFQ segments.The processing of the cold model cavity is subjected to structural over-positioning,assembly and inspection difficulties.By choosing a reasonable machining process,both the drift tube assembly and the cavity wall were machined with high accuracy.A bolted connection between the drift tube assembly and the girder is provided to eliminate part of the over-positioning on the one hand,and on the other hand the positioning of the drift tube assembly is moved from the upper face of the girder to the outer cavity wall,allowing for fine adjustment during the assembly process.After assembly,the measured concentricity of the drift tubes in the cavity isΦ0.22 mm,which meets the design requirements.The low power test of the cold model cavity included frequency,unload quality factor Q₀ and field distribution measurements as well as tuning tests.The measured frequency is 326.72 MHz,0.53%above the design value,and the measured Q₀ is 6061,reaching 65%of the design value.The axial acceleration field and the quadrupole field in RFQ section were measured using the bead-pull method,the initial field distribution had an unevenness of 1.3%,which was in good agreement with the simulation results.A tuning test using the response matrix method improved the overall field flatness.Based on the problems encountered during the development and measurement of the cold model cavity and the experience gained,the RF structure of RF electric focusing CH-DTL was further optimized.The optimization of the CH-DTL cell,the RFQ cell,the transition cell and the whole cavity was carried out in the order from the cell to the whole cavity,using the equivalent circuit method.The objective of the CH-DTL cell is to improve the RF efficiency and to improve the process feasibility and operational stability of the cavity.After parametric scanning and analysis of the structural dimensions,the high girder with thin stem configuration used for the cold model cavity was changed to a configuration of low girder with thick stem.The RFQ cell was optimized for maximum surface fields.The presence of field defects in the transition cell was identified,and their causes were analyzed and solutions were proposed.The main content of the whole cavity simulation is to improve the field flatness,and the adjustment of the field flatness was achieved by combining the use of inclined stem in the endcell and the adjustment of the acceleration gap.Based on the RF structure optimization results,a 162.5 MHz RFC power cavity is designed with an effective acceleration gradient of 1.6 MV/m,an effective shunt impedance of 46 MΩ/m,a maximum surface electric field of 1.6E_k,and a maximum thermal power density of 20.2 W/cm².The cavity water cooling scheme is designed and multiphysics field simulation is performed.The results show that the maximum temperature rise is 14.431℃,the maximum deformation is 0.062 mm,and the maximum thermal stress is 47.985 MPa under 50%duty cycle condition,and the cavity can operate safely and stably.The design of the mechanical structure of the power cavity adopts the idea of the fitting method,which solves the problems encountered during the development of the cold model cavity,and the process scheme is practical and feasible.This thesis carries out research work on the RFC,a new accelerating structure.The first RFC cold model cavity was designed and developed,and systematic rf measurements were completed.Further optimization of the RF structure based on the cold model cavity measurement results has improved the RF efficiency,reduced the maximum surface field and improved the cooling performance and process feasibility of the cavity.The work of this thesis strongly promotes the development of the RF electric focusing CH-DTL accelerating structure toward the application direction.