The Study On Key Physical Issues Of High-current Compact Ion Linear Accelerator

The ion linear accelerators have a wide range of uses.For example,a high-brightness neutron source based on an ion linear accelerator is an important research tool in the fields of material irradiation,neutron imaging,and tumor treatment.The output energy of the accelerator is usually in the range of Me V to tens of Me V.With the continuous strengthening of application requirements,neutron sources are becoming more and more compact,integrated and mobile.There are challenges from two aspects.The first is to achieve high transmission efficiency of high beam current physically,the second is to achieve intensification and miniaturization technically.There are two main difficulties to achieve high transmission efficiency of high-current beams.One is to overcome the strong space charge effect.The other is to realize the matching between RFQ and DTL with insufficient adjustable parameters,which is due to the pursuit of compactness and integration.For the first question,high transmission efficiency of the high current beam in the RFQ,the control of longitudinal emittance and the separation of low-energy particles from the main beam bunch are studied.Taking the design of RFQ with current of 10 m A as an example,the different methods of the four-section,small energy acceptance and all-particle optimization are carried out.The results show that the all-particle optimization design method can not only reduce the 99.9%longitudinal emittance of the RFQ,but also realize the separation of low-energy particles from the main bunch.The nonlinear compensation of the space charge effect in RFQ is studied.Taking the transmission in RFQ with current of 30 m A as an example,the effective potential function of RFQ with time and space periodicity is solved.For the parabolic beam,study of numerical compensation is carried out,the RFQ horizontal and vertical electrode processing data is obtained with the compensation considered.A comparative study of RFQ transmission efficiency and exit beam quality is carried out,when beams with different transverse distributions pass through the RFQ.Using the Particle-Core model,the reasons for the high transmission efficiency of RFQ with hollow beam are analyzed.The difference in behavior between passing particles and lost particles is studied,and the results show that the periodic phase advance is significantly different,or the initial four-dimensional emittance is relatively larger when the particles are lost.Aiming at the second question,the compact matching of RFQ and DTL was studied.Taking the compact physical design of deuterium ion beam of 30 m A with energy of 11 Me V without MEBT as an example,the smooth transition of the phase advance per length,the ability of RFQ transition cell to rotate the horizontal phase space,the horizontal and longitudinal acceptance optimization of DTL have been studied.The results show that RFQ and DTL can be matched horizontally and vertically after removing MEBT.There is almost no online matching ability,the influence of various operating conditions on the transmission efficiency and exit beam quality are analyzed,and the boundary conditions of each parameter are proposed.The coupling of RFQ and DTL is a feasible way for intensification.The direct coupling of four wane RFQ and CH-DTL has been studied,the research is focused on the realization of the coupling,the tuning of the coupling cavity and the influence of the superimposed field on the beam transmission.The factors affecting the field amplitude ratio of the RFQ and CH-DTL are analyzed.Based on the results of RF research,the constrained dynamics of the coupling cavity is completed,and finally a complete set of dynamics and RF design methods of the coupled cavity are obtained.The research results show that the field amplitude ratio between RFQ and CH-DTL is from 16.5 to 36.2.Under the influence of the superimposed field,the difference in emittance is less than 1%.The machining and cold testing of the 750 MHz coupling cavity have been carried out.The measurement results show that the normalized electric field coefficients measured and simulated are basically in agreement.The improving of the working frequency of the cavity is one of the ways to realize the miniaturization of the accelerator.Taking the physical design of RFQ and DTL with energy of 10 Me V as an example,the dynamics,RF design and multi-physics analysis of the 750 MHz RFQ and DTL accelerator are carried out.The results of the multi-particle simulations from end to end show that the proton beam energy can be accelerated to greater than 10 Me V within a length of less than 3 m.Through the closed-loop design of DTL cell,the rationality of the effective voltage design of each gap is determined.Through the processing and testing of a small-aperture,high-gradient permanent magnet prototype,the reasonable range of the quadruple gradient is determined.This dissertation conducts a preliminary study on the key physical problems in the high-current compact ion linear accelerator,and carries out the machining and measurement of the model cavity and the prototype of permanent magnet.The test results are in good agreement with the simulation.A certain amount of experience on the miniaturization,integration and mobility of an ion linear accelerator is accumulated.