Study on the effects of chromaticity and magnetic field tracking errors at CSNS/RCS

The Rapid Cycling Synchrotron (RCS) is a key component of the China Spallation Neutron Source (CSNS). For this type of high intensity proton synchrotron, the chromaticity, space charge effects, and magnetic field tracking errors between the quadrupoles and the dipoles can induce beta function distortion and tune shift, and induce resonances. In this paper, the combined effects of chromaticity, magnetic field tracking errors and space charge on beam dynamics at CSNS/RCS are studied systemically. 3-D simulations with different magnetic field tracking errors are performed by using the code ORBIT, and the simulation results are compared with the case without tracking errors.     

中国散裂中子源快循环同步加速器中色品校正及六极磁铁非线性效应

中国散裂中子源（CSNS）快循环同步加速器（RCS）把能量为80 MeV的束流储存并加速到1.6 GeV然后引出到靶站。为了减少RCS中的束流损失，有必要对RCS做色品校正，并减小动量偏移对束流光学的影响。尝试了多种色品校正方案并对不同色品校正方案做了比较。用三维跟踪程序SIMPSONS研究了色品校正六极铁与空间电荷效应对束流的影响。色品校正六极铁可以有效减小色品引起的频散，但是由于六极铁为非线性元件，导致不同振幅的粒子间有一定频散。模拟发现同时存在空间电荷效应和色品校正六极铁时，会有少量的束流损失。     

Longitudinal RF capture and acceleration simulation in CSNS RCS

China Spallation Neutron Source (CSNS) is a high power proton accelerator-based facility. Uncontrolled beam loss is a major concern in designing the CSNS to control the radioactivation level. For the Rapid Cycling Synchrotron (RCS) of the CSNS, the repetition frequency is too high for the longitudinal motion to be fully adiabatic. Significant beam loss happens during the RF capture and initial acceleration of the injection period. To reduce the longitudinal beam loss, beam chopping and momentum offset painting methods are used in the RCS injection. This paper presents detailed studies on the longitudinal motion in the RCS by using the ORBIT simulations, which include different beam chopping factors, momentum offsets and RF voltage optimization. With a trade-off between the longitudinal beam loss and transverse incoherent tune shift that will also result in beam losses, optimized longitudinal painting schemes are obtained.     

Studies of dual-harmonic acceleration at CSNS-Ⅱ

The Rapid Cycling Synchrotron(RCS) of the China Spallation Neutron Source(CSNS) complex is designed to provide 1.56×1013 protons per pulse(ppp) during the initial stage,and it is upgradeable to 3.12×1013 ppp during the second stage and 6.24×1013 ppp during the ultimate stage.The high beam intensity in the RCS requires alleviation of space charge effects to reduce beam losses,which is key in such high beam power accelerators.With higher intensities in the upgrading phases,a dual-harmonic RF system is planned to produce flat-topped bunches that are useful to reduce the space charge effects.We have studied different schemes to apply the dual-harmonic acceleration in CSNS-,and have calculated the main parameters of the RF systems,which are presented in this paper.     

Theoretical study of a dual harmonic system and its application to the CSNS/RCS

Dual harmonic systems have been widely used in high intensity proton synchrotrons to suppress the space charge effect,as well as reduce the beam loss.To investigate the longitudinal beam dynamics in a dual rf system,the potential well,the sub-buckets in the bunch and the multi-solutions of the phase equation are studied theoretically in this paper.Based on these theoretical studies,optimization of bunching factor and rf voltage waveform are made for the dual harmonic rf system in the upgrade phase of the China Spallation Neutron Source Rapid Cycling Synchrotron(CSNS/RCS).In the optimization process,the simulation with space charge effect is done using a newly developed code,C-SCSIM.     

The compensation of quadrupole errors and space charge effects by using trim quadrupoles

The China Spallation Neutron Source (CSNS) accelerators consist of an H-linac and a proton Rapid Cycling Synchrotron (RCS). RCS is designed to accumulate and accelerate proton beam from 80 MeV to 1.6 GeV with a repetition rate of 25 Hz. The main dipole and quadruple magnet will operate in AC mode. Due to the adoption of the resonant power supplies, saturation errors of magnetic field cannot be compensated by power supplies. These saturation errors will disturb the linear optics parameters, such as tunes, beta function and dispersion function. The strong space charge effects will cause emittance growth. The compensation of these effects by using trim quadruples is studied, and the corresponding results are presented.     

Simulation on buildup of electron cloud in a proton circular accelerator

Electron cloud interaction with high energy positive beams are believed responsible for various undesirable effects such as vacuum degradation, collective beam instability and even beam loss in high power proton circular accelerators. An important uncertainty in predicting electron cloud instability lies in the detailed processes of the generation and accumulation of the electron cloud. The simulation on the build-up of electron cloud is necessary to further studies on beam instability caused by electron clouds. The China Spallation Neutron Source(CSNS) is an intense proton accelerator facility now being built, whose accelerator complex includes two main parts: an H-linac and a rapid cycling synchrotron(RCS). The RCS accumulates the 80 Me V proton beam and accelerates it to 1.6 Ge V with a repetition rate of 25 Hz. During beam injection with lower energy, the emerging electron cloud may cause serious instability and beam loss on the vacuum pipe. A simulation code has been developed to simulate the build-up,distribution and density of electron cloud in CSNS/RCS.     

快循环同步加速器射频加速电压幅度的数字化控制

 中国散裂中子源(CSNS)快循环同步加速器（RCS）中的射频低电平控制系统是基于FPGA的全数字控制系统,旨在完成对射频频率、加速电压和同步加速相位的控制。介绍了CSNS/RCS射频系统的低电平数字化控制设计方案,并着重对射频加速电压幅度控制回路进行了分析与讨论。电压幅度控制环路通过射频电压幅度信号与电压幅度设定值的比较,得到误差信号。误差信号经过控制器来控制输入到射频腔的功率,以达到稳定和改变腔压的目的。通过对控制对象的分析和建模,得到了满足系统要求的控制器。详细介绍了数字系统的实现,尤其是信号的解调和控制算法的实现。用ALTERA公司的DSP builder进行数字控制系统开发,系统仿真结果表明,环路误差信号大约于10 μs（400个系统时钟）后归于0,整个电压幅度控制环路能稳定运行。     

Introduction to the overall physics design of CSNS accelerators

The China Spallation Neutron Source (CSNS) is an accelerator-based facility. The     

A prototype RF power source for CSNS/RCS

A prototype RF power source has been built to supply high RF power to a ferrite-loaded cavity, which is a part of R&D of the Rapid Cycling Synchrotron of China Spallation Neutron Source (CSNS/RCS). A direct fast RF feedback amplifier, a 4:1 impedance transformer and auto tuning grid were locally located to compensate the heavy beam loading of CSNS/RCS. Design and commissioning of the RF power source is discussed here, also with some advice on system improvement.     

Longitudinal RF capture and acceleration simulation in CSNS RCS

China Spallation Neutron Source (CSNS) is a high power proton accelerator-based facility. Uncontrolled beam loss is a major concern in designing the CSNS to control the radioactivation level. For the Rapid Cycling Synchrotron (RCS) of the CSNS, the repetition frequency is too high for the longitudinal motion to be fully adiabatic. Significant beam loss happens during the RF capture and initial acceleration of the injection period. To reduce the longitudinal beam loss, beam chopping and momentum offset painting methods are used in the RCS injection. This paper presents detailed studies on the longitudinal motion in the RCS by using the ORBIT simulations, which include different beam chopping factors, momentum offsets and RF voltage optimization. With a trade-off between the longitudinal beam loss and transverse incoherent tune shift that will also result in beam losses, optimized longitudinal painting schemes are obtained.     

Introduction to the overall physics design of CSNS accelerators

The China Spallation Neutron Source (CSNS) is an accelerator-based facility. The accelerator of CSNS consists of a low energy linac, a Rapid Cycling Synchrotron (RCS) and two beam transport lines. The overall physics design of CSNS accelerator is described, including the design principle, the choice of the main parameters and design of each part of accelerators. The key problems of the physics design, such as beam loss and control, are also discussed. The interface between the different parts of accelerator, as well as between accelerator and target, are introduced.     

Study of the design of CSNS MEBT

The design of CSNS MEBT has two objectives：（1） to match the beam both in the transversal direction and the longitudinal direction from RFQ into DTL;（2） to further chop the beam into the required time structure asked by RCS. It is very diffcult and critical to control well the emittance growth and in the meantime to match and chop the beam. Firstly,the optical design is done and optimized,and the multi-particle simulations show that the maximum emittance growth is successfully controlled within 14%. Secondly,based on the different beam envelopes obtained by TRACE-3D and PARMELA,the least deflecting angle of the chopper is determined by TRACE-3D. At last,the field of steering magnet is determined through simulations.     

中国散裂中子源注入系统电切割磁铁物理设计

利用OPERA/TOSCA大型有限元分析软件对中国散裂中子源/快循环同步环（CSNS/RCS）注入系统的直流C型特种电切割磁铁进行了2维/3维磁场计算和设计,通过放置1 mm厚的DT4铁磁性屏蔽板,可将循环束流轨道上的边缘漏场比降低至2.610-4,大大减小了切割板外侧的边缘漏场对循环束流动力学的影响。另外,对该类型切割磁铁的线圈做了详细的水冷计算,以保证磁铁能在适当的温度下运行。结果表明,无论从物理上还是工程上,该类型磁铁完全可以满足CSNS/RCS注入系统物理设计以及长期稳定运行的要求。     

Vibration research of the AC dipole-girder system for CSNS/RCS

The China Spallation Neutron Source (CSNS) is a high intensity proton accelerator based facility. Its accelerator complex includes two main parts: an H^- linac and a rapid cycling synchrotron (RCS). The RCS accumulates the 80 MeV proton beam and accelerates it to 1.6 GeV, with a repetition rate of 25 Hz. The AC dipole of the CSNS/RCS is operated at a 25 Hz sinusoidal alternating current which causes severe vibration. The vibration will influence the long-term safety and reliable operation of the magnet. The dipole magnet of CSNS/RCS is an active vibration equipment, which is different from the ground vibration accelerator. It is very important to design and study the dynamic characteristics of the dipole-girder system. This paper takes the AC dipole and girder as a specific model system. A method for studying the dynamic characteristics of the system is put forward by combining theoretical calculation with experimental testing. The ANSYS simulation method plays a very important role in the girder structure design stage. With this method, the mechanical resonance phenomenon was avoided in the girder design time. At the same time the dipole vibratory force will influence the other equipment through the girder. Since it is necessary to isolate and decrease the dipole vibration, a new isolator was designed to isolate the vibratory force and decrease the vibration amplitude of the magnet.     

CSNS RCS注入涂抹凸轨磁铁脉冲电源设计

中国散裂中子源(CSNS)快循环同步加速器(RCS)是一台高束流功率质子加速器, RCS注入系统是将直线加速器预加速的负氢离子束流通过剥离的方式注入到RCS环中, 注入系统采用凸轨方案和相空间涂抹技术并要求涂抹凸轨磁铁脉冲电源输出脉冲电流的下降沿能够被程序控制, 电源采用IGBT功率放大的方式产生脉冲电流, 用程序控制电源的给定波形, 通过电源反馈控制系统, 使电源的输出波形跟随电源的给定波形, 达到控制电源输出脉冲电流下降沿的目的. 电源输出脉冲电流的跟踪误差是涂抹凸轨磁铁脉冲电源的重要指标, 为了满足跟踪误差小于2%的指标, 要求IGBT拓扑频率大于400kHz. IGBT拓扑采用IGBTH桥串并联错相工作的方式达到分压、分流和提高频率的作用. 高功率、高频率、快速的响应时间和最佳的反馈控制策略是涂抹凸轨磁铁脉冲电源具有良好性能的关键.     

An overview of design for CSNS/RCS and beam transport

The China Spallation Neutron Source (CSNS) is the first accelerator-based pulsed neutron source in China. Its accelerators are made up of an 80 MeV H⁻ linac, a Rapid Cycling Synchrotron (RCS) and two beam transport lines. RCS accumulates and accelerates protons to the design energy of 1.6 GeV, and extracts high energy beam to strike the target. The overview of RCS is presented, and the key problems of the physics design are discussed. The two beam transport lines, from linac to RCS and from RCS to the target, are also introduced.

     

Study of the design of CSNS MEBT

The design of CSNS MEBT has two objectives: (1) to match the beam both in the transversal direction and the longitudinal direction from RFQ into DTL; (2) to further chop the beam into the required time structure asked by RCS. It is very difficult and critical to control well the emittance growth and in the meantime to match and chop the beam. Firstly, the optical design is done and optimized, and the multi-particle simulations show that the maximum emittance growth is successfully controlled within 14%. Secondly, based on the different beam envelopes obtained by TRACE-3D and PARMELA, the least deflecting angle of the chopper is determined by TRACE-3D. At last, the field of steering magnet is determined through simulations.     

中国散裂中子源/快循环同步加速器束流负载效应的补偿

针对强流质子同步加速器中的束流负载效应,基于中国散裂中子源/快循环同步加速器射频系统样机,在不降低腔体Q值的前提下,以数字化低电平控制为主要技术手段,对束流负载效应进行补偿。提出完整的由多控制环路组成的束流负载效应补偿方案。该方案主要由引入自适应算法的束流前馈和高带宽低延时的射频直接反馈,以及在束流负载下对腔体谐振状态进行控制的腔体预失谐和动态调谐等组成。     

Beam collimation for CSNS/RCS

The current design of the CSNS/RCS beam collimation system consists of a two-stage betatron collimation and a single momentum collimator. This paper summarizes various aspects of collimator design, including collimation principle and layout, material choice and collimator mechanical structure, etc. At last, radiation and thermal analysis are carried out to illustrate the feasibility of the current beam collimation scheme.