A design study on the CSNS LEBT pre-chopper

The China Spallation Neutron Source (CSNS) front end incorporates a pre-chopper in the Low Energy Beam Transport line (LEBT) that will remove a 530 ns section of beam at approximately 1 MHz rate, which is the RF frequency of the ring at injection. It’s one of the most critical devices for properly controlling the injecting beam loss. Physical designing of the pre-chopper is carried out, and the RFQ itself is used as the beam dump of the chopper system. In order to examine the reliability of the pre-chopper design, the beam study of a similar chopper system is successfully performed. The results of physical design and experiments are presented.

     

Study on the pre-chopper in CSNS LEBT

Physical designing of the pre-chopper in CSNS LEBT is carried out,which includes the deflecting voltage,the length and the width of the deflecting plates,and the gap between the deflecting plates.The most outstanding feature of the design is that both the gap and the width vary with the beam envelope size.So both the requried deflecting voltage and the loaded capacitance are lowered.In order to avoid destruction of the space charge neutralization by the pre-chopper in the whole LEBT,an electron-trapping electrode is arranged to confine the electrostatic field of the pre-chopper to the local area.To examine the reliability of the pre-chopping design in CSNS LEBT,a similar pre-chopping design in ADS RFQ LEBT is set up and an experiment on the pre-chopper is prepared.3-dimensional simulations are carried out to determine the loaded capacitance and the applied voltage of the electron-trapping electrode.     

Design study of a radio-frequency quadrupole for high-intensity beams

The Rare isotope Accelerator Of Newness(RAON) heavy-ion accelerator has been designed for the Rare Isotope Science Project(RISP) in Korea. The RAON will produce heavy-ion beams from 660-MeV-proton to200-MeV/u-uranium with continuous wave(CW) power of 400 k W to support research in various scientific fields.Its system consists of an ECR ion source, LEBTs with 10 ke V/u, CW RFQ accelerator with 81.25 MHz and 500 ke V/u, a MEBT system, and a SC linac. In detail, the driver linac system consists of a Quarter Wave Resonator(QWR) section with 81.25 MHz and a Half Wave Resonator(HWR) section with 162.5 MHz, Linac-1, and a Spoke Cavity section with 325 MHz, Linac-2. These linacs have been designed to optimize the beam parameters to meet the required design goals. At the same time, a light-heavy ion accelerator with high-intensity beam, such as proton,deuteron, and helium beams, is required for experiments. In this paper, we present the design study of the high intensity RFQ for a deuteron beam with energies from 30 ke V/u to 1.5 MeV/u and currents in the m A range. This system is composed of an Penning Ionization Gauge ion source, short LEBT with a RF deflector, and shared SC Linac. In order to increase acceleration efficiency in a short length with low cost, the 2nd harmonic of 162.5 MHz is applied as the operation frequency in the D~+RFQ design. The D~+RFQ is designed with 4.97 m, 1.52 bravery factor. Since it operates with 2nd harmonic frequency, the beam should be 50% of the duty factor while the cavity should be operated in CW mode, to protect the downstream linac system. We focus on avoiding emittance growth by the space-charge effect and optimizing the RFQ to achieve a high transmission and low emittance growth. Both the RFQ beam dynamics study and RFQ cavity design study for two and three dimensions will be discussed.     

用于强流中子发生器的低能束流传输线的设计模拟

兰州大学设计研发的ZF-400强流中子发生器设计D-T中子产额6×1012 n/s,主要由ECR离子源、低能束流传输线（LEBT）、加速管、旋转靶等部分组成。LEBT负责将从离子源引出的束流进行分析聚焦并注入到加速管中。LEBT对束流的聚焦及分析的好坏程度决定了加速管中束流的损失程度、中子的产额以及靶的寿命。本工作就该强流中子发生器所需的低能传输线进行了设计。使用螺线管、分析磁铁和四极透镜组合的方案。利用TRACK软件对此方案进行模拟,得到符合要求的束流线及元件的参数。用TraceWin进行了验证模拟,验证结果符合要求。另外,通过软件模拟确定了杂质离子损失的位置,据此设计了束流管道冷却方案。通过模拟发现,这种透镜的组合方式可以让整个LEBT以较低的功率获得低损失、高纯度的打靶束流。The ZF-400 Intense Neutron Generator, which is designed by Lanzhou University with an expected neutron yield of 6×1012 n/s, is consist of ECR ion source, low energy beam transport (LEBT) line, accelerating tube and rotating target. The beam extracted from ECR source is analyzed and focused through LEBT, then, the beam is introduced into the accelerating tube. The focus and analysis ability of LEBT is very important for the beam loss, neutron yield and target's life. A LEBT line for intense neutron generator is designed in this paper, the project consisted of a solenoid, a bend magnet and three quadrupole lens. The qualified LEBT and its parameters have been got through the simulation with TRACK code. Then TraceWin program was employed to check this simulation and the result satisfied our requirement. Besides, the loss position of impurity ions was known through the simulation, and a project of beam line's cooling system was designed according to this result. Through the simulation, we find the optimal lens combination plan which can obtain lower beam loss and higher beam purity at low running power of LEBT.     

一台2.45GHz强流中子源

A 2.45GHz microwave-driven ion source is being used to provide 40mA of deuterium ion beam (peak current) for an RFQ accelerator as part of a neutron source system.We have also designed a 60kV electrostatic LEBT using computer simulations.In our experiment,we measured the hydrogen and deuterium ion beam currents as functions of discharge power,gas flow,and magnetic field strength.The required beam current was obtained using leas than 700W of net microwave power with a gas flow of less than 1.5sccm.From the rise time data,it was determined that in order to obtain a high percentage of atomic ions in the beam, the beam extraction should start after 1ms of switching on the microwave power.At steady state,the proton fraction was above 90%.     

Particle-in-cell mode beam dynamics simulation of the low energy beam transport for the SSC-linac injector

A new SSC-linac system (injector into separated sector cyclotron) is being designed in the HIRFL (heavy ion research facility of Lanzhou). As part of SSC-Linac, the LEBT (low energy beam transport) consists of seven solenoids, four quadrupoles, a bending magnet and an extra multi-harmonic buncher. The total length of this segment is about 7 meters. The beam dynamics in this LEBT has been studied using three-dimensional PIC (particle-in-cell) code BEAMPATH. The simulation results show that the continuous beam from the ion source is first well analyzed by a charge-to-mass selection system, and the beam of the selected charge-to-mass ratio is then efficiently pre-bunched by a multi-harmonic buncher and optimally matched into the RFQ (radio frequency quadrupole) for further acceleration. The principles and effects of the solenoid collimation channel are discussed, and it could limit the beam emittance by changing the aperture size.     

Accelerator development in India for ADS programme

At BARC, development of a Low Energy High Intensity Proton Accelerator (LEHIPA), as front-end injector of the 1 GeV accelerator for the ADS programme, has been initiated. The major components of LEHIPA (20 MeV, 30 mA) are a 50 keV ECR ion source, a 3 MeV Radio Frequency Quadrupole (RFQ) and a 20 meV drift tube linac (DTL). The Low Energy Beam Transport (LEBT) and Medium Energy Beam Transport (MEBT) lines match the beam from the ion source to RFQ and from RFQ to DTL respectively. Design of these systems has been completed and fabrication of their prototypes has started. Physics studies of the 20–1000 MeV part of the Linac are also in progress. In this paper, the present status of this project is presented.      

用于1MeV 分离作用RFQ加速器改造的ECR氧离子源及LEBT系统的研制

An ECR O~ ion source and LEBT system have been developed for the upgrade of 1MeV Integral Split Ring RFQ at Peking University.To satisfy the requirement of RFQ,a more than 10mA oxygen beam has been extracted at 22kV through a 5mm diameter aperture.Its normalized root-mean-square emmitance is less than 0.1π·mm·mrad,which is required by RFQ accelerator.The LEBT matching section is redesigned upon the bench test results.The preliminary results will be presented in this paper.     

Design of magnet and control of the beam emittance for Penning H- ion source

The design requirement and principle of the deflection magnet for Magnetron and Penning H- ion source are discussed.It is proved that there exists a maximum emittance for the beam that may be transformed by the magnet into a state with equal Twiss parameters of αr=αy and βr =βy,which is the requisite condition to get a minimum emittance at the entrance of RFQ after transporting by a LEBT with solenoids.For this maximum emittance,the corresponding magnetic field gradient index is 1.     

Beam dynamics study of RFQ for CADS with a 3D space-charge-effect

The ADS (accelerator driven subcritical system) project was proposed by the Chinese Academy of Sciences. The initial proton beams delivered from an electron cyclotron resonance ion source can be effectively accelerated by 162.5 MHz 4.2 m long room temperature radio-frequency-quadrupoles (RFQ) operating in CW mode. To test the feasibility of this physical design, a new Fortran code for RFQ beam dynamics study, which is space charge dominated, was developed. This program is based on Particle-In-Cell (PIC) technique in the time domain. Using the RFQ structure designed for the CADS project, the beam dynamics behavior is performed. The well-known simulation code TRACK is used for benchmarks. The results given by these two codes show good agreements. Numerical techniques as well as the results of beam dynamics studies are presented in this paper.     

LAPECR2离子源在320kV高压平台上的试运行

The high charge state all permanent Electron Cyclotron Resonance Ion Source(ECRIS)LAPECR2 (Lanzhou All Permanent magnet ECR ion source No.2)has been successfully put on the 320kV HV platform at IMP and also has been connected with the successive LEBT system.This source is the largest and heaviest all permanent magnet ECRIS in the world.The maximum mirror field is 1.28T(without iron plug)and the effective plasma chamber volume is as large as (?)67mm×255mm.It was designed to be operated at 14.5GHz and aimed to produce medium charge state and high charge state gaseous and also metallic ion beams.The source has already successfully delivered some intense gaseous ion beams to successive experimental terminals.This paper will give a brief overview of the basic features of this permanent magnet ECRIS.Then commissioning results of this source on the platform,the design of the extraction system together with the successive LEBT system will be presented.     

IH型漂移管直线加速器的束流动力学研究（英文）

为了提高兰州重离子加速器冷却储存环（HIRFL-CSR）的运行效率、改善加速器输出束流品质,并实现几个加速装置分时供束,提高整个重离子加速装置的利用率,特为（HIRFL-CSR）增建一台新的注入器--CSRLINAC。在108.48 MHz的RFQ之后的CSR-LINAC主加速段,主要由一台108.48 MHz和两台216.96 MHz的IH型漂移管直线加速器组成,用于加速荷质比为1/8.5~1/3之间的重离子,其最大的束流流强为3 mA,并将粒子从0.3 MeV/u加速到3.71 MeV/u。运用KONUS动力学原理,在满足设计指标的情况下,首先利用TraceWin程序进行中能束线MEBT设计,后针对高频腔体设计和束流匹配的基本参数的系列讨论,特别是对CSR-LINAC的中能束流匹配线、参数选择和IH型KONUS结构的漂移管直线加速器进行设计模拟优化。最终得出,在保证腔体设计指标和95.3%的传输效率的情况下,该紧凑型直线加速结构经过三个腔体的加速后,束流的纵向归一化均方根发射度增长仅有25%;同时发现,当流强达到3 mA时,存在空间电荷效应,导致其纵向相宽增长约25%,最大横向包络也存在16.5%的涨落。In order to improve the operation efficiency of the Cooling Storage Ring of Heavy Ion Research Facility in Lanzhou (HIRFL-CSR), a heavy ion linac (linear accelerator) was proposed and designed as a new injector for HIRFL-CSR. Following the 108.48 MHz Radio-Frequency Quadrupole (RFQ), three tanks in total with Interdigital H-mode drift tube linac (IH-DTL) structure are installed to boost the beam energy from 0.3 to 3.71 MeV/u, and the beam current of ions with charge-to-mass ratio from 1/8.5 to 1/3 can reach to 3 mA. The first tank operatesat the same frequency as the RFQ, and the rest two operate at 216.96 MHz. The “Combined Zero-Degree Synchronous Particle Structure” (KONUS) beam dynamics was used in the beam dynamics design. The overview of the physics design on the main accelerating components, including RF design and beam dynamics design are introduced in this paper. The optimized structure design, fabrication status and simulation results are presented in this contribution. It shows that under the condition of assurance of 95.3% transmission efficiency, the normalized rms emittance is about 25%. When the beam current is up to 3 mA, owing to the space charge effect, the increase of longitudinal phase spread and transverse envelope are about 25% and 16.3%, respectively.     

强流质子加速器束流剖面分布及束晕测量系统设计

 针对一台用于加速器驱动洁净核能源系统研究、高占空比的强流质子加速器,开展强流质子直线加速器束晕产生的研究工作,其中的束流剖面分布特别是束晕部分测量的束流诊断系统是研究工作的核心内容。束晕的产生在低能量段尤其重要,且对整个直线加速器的设计有重要影响。介绍了研究束晕增长的束流输运线和测量系统的布局设计,并根据所研究的加速器束流的情况进行束流剖面探测器和束晕测量的设计和预研,包括丝靶材料的模拟计算和选择、机械驱动的控制系统设计和研究、前端模拟电路的设计和仿真模拟、以及整个系统与EPICS和VxWorks的计算机控制接口等。     

带有质量分析系统的强流质子束低能传输线研究

强流质子源与低能传输线（LEBT）是作为CIADS注入器的超导强流质子直线加速器的关键前端系统。目前LEBT采用双螺线管匹配结构设计,并安装有限制锥,但仍然不能避免少量H₂+和H₃+进入后端加速装置,这对直线加速器长期运行稳定性与可靠性会产生一定影响。为此,在LEBT加入分析磁铁对混合束（H+,H+2,H₃+）进行分离再注入后端加速器腔体,将是一个有效的方案。本研究对经过带有30度分析磁铁的LEBT的强流质子束的束流品质进行了模拟与实验测量。结果表明,分析磁铁高阶磁场的影响使经过分析磁铁的强流质子束束流品质变差,并且该影响随着束流包络的增大而增大。这些结果为CIADS注入器的低能传输线设计提供了参考依据。High current proton source and the low energy beam transport(LEBT) are the key front-end systems for CIADS injector:high current proton linac accelerator. CIADS injector's LEBT adopts double solenoid matching structure, using a limit cone which can partially avoid H₂+ and H₃+ which injecting into the back-end linac accelerator may impact the long-term stability and reliability of the whole system. It will be an effective method to separate the hybrid ions (H+, H₂+, H₃+) by adding a dipole magnet at LEBT. In this article, we simulated and mesasured the high current proton beam quality behind the LEBT with a 30 degree dipole. The results show that the the proton beam quality is significantly effected by high-order magnetic fields of the dipole magnet, and the effect increases with the increase of the beam envelope. The achieved result is useful for the LEBT design of CIADS injector.     

加速器驱动次临界系统注入器离子源控制系统

强流质子源及低能传输线是加速器驱动次临界系统(ADS)项目注入器的重要组成部分,为了保证其工作效率设计了一种基于实验物理及工业控制系统(EPICS)架构的远程控制系统。根据被控设备硬件接口的特点及控制需求分别采用可编程控制器(PLC)和串口服务器等作为控制部件,在主控机中使用LabVIEW编程实现了对系统内所有设备的监控,并借助于DSC模块把设备状态和参数等以过程变量的形式进行网络发布。设计的控制系统具有结构简单、工作可靠的特点,已经在系统调试中发挥了重要作用。     

Power test of the separated function RFQ accelerator

The progress of the Separated Function RFQ (SFRFQ) accelerator, which can raise the field gradient of acceleration while maintaining the transverse focusing power sufficient for high current beam, is presented. In order to demonstrate the feasibilities of the novel accelerator, a prototype cavity was designed and constructed. Correspondingly, a code SFRFQCODEV1.0 was developed specially for cavity design and beam dynamics simulation. The prototype cavity will be verified as a post-accelerator for ISR RFQ-1000 (Integral Split Ring RFQ) and accelerate O+ from 1 MeV to 1.6 MeV. To inject a higher current oxygen beam for the prototype cavity, the beam current of ISR RFQ-1000 was upgraded to 2 mA. The status of high power and beam test preparation for the prototype cavity are presented in this paper.     

A particle-in-cell mode beam dynamics simulation of medium energy beam transport for the SSC-Linac

A new linear accelerator system, called the SSC-Linac injector, is being designed at HIRFL (the heavy ion research facility of Lanzhou). As part of the SSC-Linac, the medium energy beam transport (MEBT) consists of seven magnetic quadrupoles, a re-buncher and a diagnose box. The total length of this segment is about 1.75 m. The beam dynamics simulation in MEBT has been studied using the TRACK 3D particle-in-cell code, and the simulation result shows that the beam accelerated from the radio frequency quadrupole (RFQ) matches well with the acceptance of the following drift tube linac (DTL) in both the transverse and longitudinal phase spaces, and that most of the particles can be captured by the final sector focusing cyclotron for further acceleration. The longitudinal emittance of the RFQ and the longitudinal acceptance of the DTL was calculated in detail, and a multi-particle beam dynamics simulation from the ion source to the end of the DTL was done to verify the original design.     

SFC的新的轴向注入束流线

为了满足HIRFL-CSR对注入器SFC的束流强度和品种的越来越高的要求, 兰州重离子加速器国家实验室在研制超导ECR离子源的同时, 设计了一个新的SFC的轴向注入束流线. 这个系统可以分别使用现有的常规ECR离子源和新建造的超导ECR离子源, 期望把从C到U的各种离子的能量和束流强度提高到一个新的水平. 这个系统由二极磁铁, 四极透镜, GLASSER透镜, 螺线管, 螺旋形静电偏转器和两台丝网型线性聚束器组成. 在总结现有系统运行经验的基础上,无论在横向还是纵向, 其性能结构都做了必要的改进. 文章给出了新系统的设计思想, 系统的布局结构和束流光学计算结果, 并对进一步提高聚束效率和聚束器的改进设计作了简要的描述. 目前, 系统正在安装中.     

Beam dynamics and RF design of trapezoidal IH-RFQ with low energy spread

Beam dynamics and RF design have been performed of a new type trapezoidal IH-RFQ operating at 104 MHz for acceleration of ¹⁴C⁺ in the framework of RFQ based ¹⁴C AMS facility at Peking University. Low energy spread RFQ beam dynamics design was approached by the method of internal discrete bunching. ¹⁴C⁺ will be accelerated from 40 keV to 500 keV with the length of about 1.1 m. The designed transmission efficiency is better than 95% and the energy spread is as low as 0.6%. Combining the beam dynamics design, a trapezoidal IH-RFQ structure was proposed, which can be cooled more easily and has better mechanical performance than traditional RFQ. Electromagnetic field distribution was simulated by using CST Microwave Studio (MWS). The specific shunt impedance and the quality factor were optimized primarily.     

Power test of the separated function RFQ accelerator

The progress of the Separated Function RFQ (SFRFQ) accelerator, which can raise the field gradient of acceleration while maintaining the transverse focusing power sufficient for high current beam, is presented. In order to demonstrate the feasibilities of the novel accelerator, a prototype cavity was designed and constructed. Correspondingly, a code SFRFQCODEV1.0 was developed specially for cavity design and beam dynamics simulation. The prototype cavity will be verified as a post-accelerator for ISR RFQ-1000 (Integral Split Ring RFQ) and