Analytical and simulation studies for diode and triode ion beam extraction systems

This work is concerned with ion beam dynamics and compares the emittance to aberration ratios of two-and three-electrode extraction systems. The study is conducted with the aid of Version 7 of SIMION 3D ray-tracing software. The beam dependence on various parameters of the extraction systems is studied and the numerical results lead to qualitative conclusions.     

受空间电荷效应影响的离子束流从等离子体边界的最佳引出的一阶理论

The problem of extracting space-charge-limited ion beams from spherical emission boundaries is analyzed for simple,two electrode,parallel-plate and spherical sector electrode systems by application of Langmuir-Blodgett theory with account taken for the divergent lens effect caused by the aperture in the extraction electrode.Results derived from simulation studies for the three electrode system,designed for use with the Oak Ridge National Laboratory ECR ion source,complement predictions made from elementary analytical theory with or without magnetic field in the extraction region of the source.Under minimum half- angular divergence(minimum emittance)conditions,the plasma emission boundary has an optimum curvature and the perveance,P,(i.e,current density,j and extraction gap,d),has an optimum value for a given charge- state.From these studies,we find that the optimum perveance for any electrode system can be determined from the Child-Langmuir relation for the parallel-plate electrode system multiplied by a factor,F with value 0.49≤F≤1.     

HL-2M装置中性束离子源束光学红外成像实验研究

为了研究HL-2M装置中性束注入(NBI)加热用的80kV/45A/5s热阴极离子源束光学特性，采用红外电荷耦合元件(CCD)成像技术，测量离子源引出粒子束轰击量热靶板产生的温度分布，得到束功率密度空间分布区间特征参数1/e半宽度。在NBI热阴极离子源调试平台上，扫描离子源的放电和引出参数，利用CCD红外热像仪获得了对应参数下量热靶上的束功率密度分布。实验结果表明，HL-2M装置NBI加热系统80kV/45A离子源可用的导流系数范围为0.7~1.5μP。同样导流系数下，梯度电极与等离子体电极的分压比较高时，引出束流的半宽度较小。     

HL-2A装置NBI加热束线束光学特性

中性束离子源束光学特性直接影响中性束注入功率。通过理论计算和实验扫描的方式对引出束半宽度随导流系数的变化进行了研究。理论上, 采用IGUN程序对HL-2A装置的三电极引出加速器进行了束元轨迹模拟, 采用束几何光学计算程序对高斯束元进行叠加, 得到了在束线量热靶处引出束的1/e半宽度。实验上, 在放电气体为氘气, 弧放电电流分别为200, 300, 400 A时, 对引出高压进行扫描, 利用量热靶上的热偶测量了不同位置处的温升, 用高斯函数拟合得到引出束的分布和1/e半宽度。理论计算和实验结果都表明, 束半宽随着导流系数的增加先减少后逐渐增加。理论计算结果表明不同离子束流下的最佳导流系数值基本不变, 而实验上, 弧流为200, 300, 400 A时, 对应的最佳导流系数分别为1.6, 1.7, 1.85 P。     

离子推力器束流引出状态对栅极刻蚀的影响

为了研究30 cm离子推力器束流引出状态对栅极刻蚀的影响,建立了束流引出模型,并采用PIC-MCC方法对CEX离子造成的栅极腐蚀速率进行了计算,最后将计算结果与1500 h寿命试验结果进行比对分析。结果显示:束流正常聚焦时,在3 kW和5 kW两种工作模式下,加速栅和减速栅的质量刻蚀速率分别为(1.11～1.72)×10?15 kg/s及(1.22～1.26)×10?17 kg/s。在5 kW工况下,当屏栅上游等离子体密度达到4.03×1017 m?3时,束流出现欠聚焦现象,此时加速栅和减速栅的最大离子刻蚀速率分别为4.33×10?15 kg/s和4.02×10?15 kg/s;在3 kW工况下,当屏栅上游等离子体密度达到0.22×1017 m?3时,束流出现过聚焦现象,此时加速栅和减速栅的最大离子刻蚀速率分别为3.24×10?15 kg/s和5.01×10?15 kg/s。寿命试验结果表明,加速栅孔质量刻蚀速率的计算值与试验值比对误差较小,而由于束流离子对减速栅孔的直接轰击,导致减速栅孔刻蚀速率的计算值和试验值差异极大。经研究认为,对屏栅小孔采用变孔径设计,是降低当束流处于欠聚焦或过聚焦状态下,CEX离子造成加速栅孔和减速栅孔刻蚀速率,并提升推力器工作寿命的有效措施。     

强流ECR离子源引出系统研究

为了提高强流ECR 离子源的引出束流品质,分别设计了1# 和2# 引出系统,利用束流引出模拟软件PBGUNS 对1# 和2# 引出系统进行了质子束流引出与传输的模拟计算,结合实际测得的发射度数据分析引出系统,发现2# 引出系统比1# 引出系统引出束流品质高。对ECR 离子源引出系统的电势等位线分布等参数引起的球差进行了简单数学推导及MATLAB 绘图,并结合1# 和2# 引出系统束流相图模拟结果证明了球差会使引出束流品质有效发射度增长,通过适当加大电极孔径可改善束流聚焦情况,得到了束流光学聚焦较好的束流引出系统设计。To improve the quality of extracted ion beam from a high current ECR ion source, 1# and 2# extraction systems were designed and tested. The PBGUNS code was used to simulate the 1# and 2# extraction systems of proton ion beam. The emittance measurement results with the two different extraction systems were compared and analyzed with the simulation, the conclusion that more high quality beam extracted from 2# system than 1# system was got. The formula derivation of ECR ion source extraction system spherical aberration and MATLAB drawing was done by the analyzing on the distribution of extraction field equipotentials, effective emittance increasing caused by spherical berration was proved by 1# and 2# extraction systems beam phase space simulation result, beam focusing would be improved if electrode hole size increasing appropriately and a general concept on good optics focusing of ion beam extraction system was proposed finally.     

Optimization of Extraction System for Cold Cathode Ion Source

We have simulated ion trajectories with the SIMION 3D, version 7.0, package to optimize the extraction system of a cold-cathode ion source and compared the results with experimental data collected under the same operational conditions. The simulation determined the negative voltage applied to the extractor electrode and the extraction gap widths that maximize the ion-beam current for singly charged nitrogen ions with low emittance and small diameter. The experiment measured the input electrical discharge and output ion beam characteristics of the source at different nitrogen pressures. The extractor electrode voltage and the extraction gap width were determined at 8?×?10^?4?Torr nitrogen. The results of the simulation agreed well with the experimental data.     

栅极热变形对离子推力器工作过程影响分析

栅极热变形是影响离子推力器性能和寿命的重要因素. 采用三维粒子方法对栅极系统等离子体输运过程进行模拟, 对比、分析栅极热变形前后栅极系统的电子返流限制、导流系数限制、离子通过率和发散角损失. 结果表明: 栅极热变形增大了屏栅离子通过率和推力器推力值, 并由于加速栅截止电流阈值的提高拓展了推力器工作电流区间, 但电子返流阈值的明显降低对栅极系统可靠工作造成了不利影响.     

Performance of 300 s-beam extraction in the KSTAR neutral beam injector

The first neutral beam injector (NBI-1) has been developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. The first long pulse ion source (LPIS-1) has been installed in the NBI-1 for an auxiliary heating and current drive of KSTAR plasmas. The performance of 300 s ion beam extraction in the LPIS-1 was investigated on the KSTAR NBI-1 system, prior to the neutral beam injection for long pulse operation. The ion source consists of a magnetic bucket plasma generator with multi-pole cusp fields and a set of prototype tetrode accelerators with circular-type apertures. The inner volume of the plasma generator and accelerator column in the LPIS-1 is approximately 123 L. The nominal operation requirements for the ion source (IS) were a 100 kV/50 A deuterium beam and a 300 s pulse length. The extraction of ion beams was initiated by the formation of arc plasmas in the LPIS-1, called an arc-beam extraction method. A stable ion beam extraction of the LPIS-1 was achieved with 80 kV/27 A and a beam perveance of 1.19 microperv for a 300 s pulse length. Beam power deposition along the NBI-1 has been measured using water-flow calorimetry (WFC), and the sum of the deposited power on the ion source and beamline components was about 93% of the drained acceleration power (V_acc?I_acc). The beam power deposition was compared to the calculated results of the beam transport with re-ionization (BTR) code.     

Characteristics of a high frequency ion source used in a low-energy accelerator

This work presents the characteristics of a high frequency ion source operating on a low energy, 150 keV accelerator. The latter is to be used as a neutron generator and its design is based on a theoretical analysis which shows that if the axial potential in an electrostatic electrode system is made to increase with four thirds the power of axial distance, inward electric forces will compensate space charge forces tending to blow up the beam. This results in a simplified acceleration tube much shorter and of higher gradient than the conventional acceleration columns. The ion source itself is an ordinary type using axial extraction of the beam, and its main properties investigated are the beam current and beam quality (or emittance). Dependence of the two on different parameters is investigated in a series of tests.     

Simulation of ion beam extraction and focusing system

The characteristics of ion beam extraction and focused to a volume as small as possible were investigated with the aid of computer code SIMION 3D version 7.This has been used to evaluate the extraction characteristics(accel-decel system)to generate an ion beam with low beam emittance and high brightness.The simulation process can provide a good study for optimizing the extraction and focusing system of the ion beam without any losses and transported to the required target.Also,a study of a simulation model for the extraction system of the ion source was used to describe the possible plasma boundary curvatures during the ion extraction that may be affected by the change in an extraction potential with a constant plasma density meniscus.     

High Performance, Low Energy Ion Source

An ion accelerator system using one or two fine mesh tungsten grids has been used in a magnetic multipole containment ion source to produce a continuous positive ion beam variable over a wide range of beam currents and accelerator voltages. The ion source plasma is low noise and uniform to ± 1% density variation over 9 cm diameter. Beams up to 4 cm × 4 cm have been extracted which have low divergence (±1%), uniform current density (±2%), small ion energy speed, beam ion energy variable from 200V to several thousand volts, and current densities variable from ?A per cm2 to tens of milliamperes per cm2, limited only by acceleration grid power loading or sputtering.     

Numerical Modeling of Ion-Optical Systems for Extraction of a Beam of Negative Hydrogen Ions and Its Acceleration and Matching with a Radio-Frequency Quadrupole Accelerator

A source of negative hydrogen ions is constructed at the Institute for High Energy Physics in order to raise the intensity of the U-70 accelerator complex using multiturn charge-exchange injection. The 3D ion-optical IBSimu simulation package was used to model and optimize the system for extraction of an ion beam from plasma, its acceleration to 100 keV, and matching with a radio-frequency quadrupole (RFQ) accelerator. The system of extraction of an ion beam with a current of 50 mA and electron-flow suppression was modeled. The magnetic transport channels with two solenoids and six quadrupole lenses were compared. The end goals of analysis of the matching channel were to obtain the needed Twiss parameters and to minimize the emittance growth at the RFQ input.     

Numerical simulation and design of a thermionic electron gun

This paper reports the simulation of an electron gun. The effects on the beam quality of some parameters on the beam quality were studied and optimal choices were identified. It gives numerical beam qualities for a common electrostatic triode gun, and the dependencies on design parameters such as electrode geometries and bias voltages to these electrodes are shown. An electron beam of diameter 5 mm with energy of 5 ke V was assumed for the simulation process. Some design parameters were identified as variable parameters in the presence of space charge.These parameters are the inclination angle of emission electrode, the applied voltage to the focusing electrode, the gap width between the emission electrode and the focusing electrode and the diameter of the focusing electrode. The triode extraction system is designed and optimized by using CST software(for Particle Beam Simulations). The physical design of the extraction system is given in this paper. From the simulation results, it is concluded that the inclination angle of the emission electrode is optimized at 22.5?, the applied voltage to the focusing electrode was optimized and found to be Vfoc =-600 V, the optimal separation distance(gap between emission electrode and focusing electrode) is 4 mm, and the optimal diameter of the emission electrode is 14 mm. Initial results for these efforts aimed at emittance improvement are also given.     

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%.     

北京大学1.7 MV串列静电加速器的离子注入/辐照实验系统

北京大学1.7 MV串列静电加速器运行至今已有三十多年。该加速器配备有高频电荷交换负离子源和铯溅射负离子源,能够引出从H到 Au之间 的大部分元素的离子。离子能量可被加速至几百keV到若干MeV,主要开展离子注入/辐照实验和卢瑟福背散射(RBS)和沟道分析等离子束分析工作。基于辐照实验需求,建立了高温辐照系统,温度最高可达950 ℃。为了实现更加精确的离子注入,设计了直接式与间接式两种法拉第杯结构,使束流扫描面积精确控制,并且在测量束流强度时除了抑制次级电子,还考虑到了次级正离子的影响。利用不同能量的Au离子在单晶Si片上进行了注入实验,通过RBS分析显示测量剂量与期望剂量误差在4%以内,此外,注入均匀性的测试表明注入剂量的相对标准偏差为2%。     

反转永磁聚焦C波段高功率多注速调管

反转永磁聚焦C波段高功率多注速调管作为加速器系统的功率源，具有脉冲输出功率高、平均功率大、寿命长的优势，在工业辐照、放射治疗、无损检测等领域具有广阔的应用前景。设计并研发了一种可用于加速器系统的反转永磁聚焦C波段高功率多注速调管。其电子光学系统采用多电子注、周期反转永磁聚焦技术，高频系统工作在TM₂₂₀高次模式、多腔参差调谐，散热采用水冷方式。通过计算确定了结构参数，研制出速调管样管，通过测试，获得了大于3 MW的峰值输出功率，和计算结果相吻合。     

C波段50 MW行波输出速调管设计

设计了导流系数1.53 P的电子枪,枪内场强低于22.1 kV/mm,阴极平均负载小于6.3 A/cm2;采用部分屏蔽流均匀场电磁聚焦系统,实现了对电子注的良好聚焦;设计了C波段/2模盘荷波导行波输出结构,采用CST软件对其色散特性、耦合阻抗进行了计算分析。首先以单间隙输出腔代替行波输出结构对5腔注-波互作用系统进行计算,确定了前4腔的设计参数,然后采用PIC模拟软件对具有盘荷波导结构的输出系统进行了三维模拟,获得了大于50 MW的输出功率,效率大于45%,饱和增益大于50 dB,盘荷波导结构间隙电压比单间隙输出腔下降30%,效率提高4%。     

Experimental results of DPIS with a new RFQ

We have developed a new heavy ion production system which uses a combination of an RFQ and a laser ion source. Induced plasma by a laser shot is delivered to the RFQ without an extraction electrode. We named this new idea ‘direct plasma injection scheme (DPIS)’. In 2004, a new RFQ was built for demonstrating the capability of the DPIS. After a few months of commissioning period, we could obtain more than 60 mA of carbon beam from the RFQ. This new scheme could be applied to cancer therapy facilities and high energy nuclear physics accelerator complexes.     

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.