Beam-loss driven injection optimization for CSNS/RCS

This paper summarizes the painting injection optimization for the Chinese Spallation Neutron Source (CSNS) ring. This optimization focuses on two main design goals: the lower beam loss and a space-charge tune shift low enough to avoid strong resonances. Finally, the 3-D particle tracking is performed and we get some important results about the beam properties and beam loss.     

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.     

OPTIMIZATION OF ANISOTROPIC SANDWICH BEAMS FOR HIGHER SOUND TRANSMISSION LOSS

This paper presents a study on the optimization of sound transmission loss across anisotropic sandwich beams. It has been found in earlier studies that there is a significant increase in the sound transmission loss for sandwich beams with anisotropic materials compared to those with isotropic ones. The optimization studies presented in this work further validate this concept. The material and geometric properties of the structure are treated as the design variables with the objective to maximize the sound transmission loss across the beam. Appropriate constraints are imposed to maintain material and structural integrity.     

an Introduction to Mebt For JKJ and Its Upgrade Design

The medium-energy beam-transport line (MEBT) plays an important role in reducing the beam loss in the Japan Proton Accelerator Research Complex (J-PARC). A MEBT was designed and constructed,with good beam matching and lower beam loss. A brief introduction to the MEBT and its beam test results are given. To further reduce the beam loss during the transient time of the chopper,a medium-energy beamtransport line with an anti-chopper has been designed. It accomplishes three tasks: matching the beam from the RFQ to the acceptance of the DTL, chopping the beam to produce gaps for injection into the rapid cycling ring which follows the linac, and returning the partially deflected beam back to the acceptance of the DTL. An RF chopper and an anti-chopper have been adopted in the beam line, and the optimization of the design of the chopper cavity is discussed. Details of the beam dynamics analysis are given.     

JAERI和KEK联合项目中的中能束流传输线及其升级设计

中能束流传输线(MEBT)在日本质子加速器研究设施(J-PARC)中对控制束流损失起到非常关键的作用. 中能传输线已经成功设计和建造, 并进行了束流实验. 简要介绍了中能传输线及其束流实验的结果. 为进一步减少和消除传输线中的束流切割器的上升和下降过程中产生的不稳定粒子, 以满足更高流强下的束损要求, 给出了一个带有反束流切割器的中能传输线的升级设计, 该设计已被用作该项目的备用方案. 升级设计的中能传输线要完成三个任务：完成从RFQ到DTL的束流相空间匹配; 利用束流切割器把束流切割成注入快循环时所需要的束流结构, 以及利用反束流切割器把束流切割器上升和下降时间段产生的部分偏转的不稳定粒子作用回DTL的相空间接收度内, 以减少和避免粒子丢失. 在该中能传输线中, RF偏转器被用来作为束流切割器和反束流切割器. RF偏转器是中能传输线中最关键的部件, 进一步讨论了RF偏转器作为反束流切割器的优化设计. 也给出了该传输线的动力学分析的结果.     

双平面多圈注入模拟优化程序TPIS的初步开发与验证

为满足短时间内达到高累积流强要求,HIAF/BRing采用了一种新的注入方法--双平面相空间多圈注入。该注入方法与传统单平面多圈注入方法不同,而且在国际上是首次在实际项目中采用,尚无实际运行经验。因此,对注入过程进行程序模拟研究是验证双平面多圈注入方案可行性的必要手段。为详细模拟研究BRing双平面多圈注入过程,并克服已有程序跟踪速度较慢且注入参数修改不便的缺点,本文根据双平面多圈注入的特点,建立了双平面多圈注入模型,编写了双平面多圈注入模拟优化程序TPIS（Two-Planemultiturn Injection Simulation）。通过与ORBIT程序模拟结果对比,验证了TPIS程序模拟双平面多圈注入过程的正确性。在此基础上,在TPIS程序中加入了粒子群优化算法,并对BRing注入参数进行了优化。结果表明,TPIS程序可以对注入参数进行有效优化,经过优化后,束流损失减少了28%,最终剩余累积粒子数满足BRing的流强设计指标要求,进一步验证了双平面多圈注入设计的可行性。     

A new method for compensation and rematch of cavity failure in the C-ADS linac

For proton linear accelerators used in applications such as accelerator-driven systems,due to the nature of the operation,it is essential for the beam failure rate to be several orders of magnitude lower than usual performance of similar accelerators.A fault-tolerant mechanism should be mandatorily imposed in order to maintain short recovery time,high uptime and extremely low frequency of beam loss.This paper proposes an innovative and challenging way for compensation and rematch of cavity failure using fast electronic devices and Field Programmable Gate Arrays(FPGAs)instead of embedded computers to complete the computation of beam dynamics.A method of building an equivalent model for the FPGA,with optimization using a genetic algorithm,is shown.Results based on the model and algorithm are compared with TRACEWIN simulation to show the precision and correctness of the mechanism.     

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.     

被动调QNd:YAG/Cr:YAG微晶片激光器的优化设计与实验

用数值求解速率方程的方法对被动调Q微晶片激光器进行了优化设计,并在实验中验证了这一优化效果。首先对速率方程进行数值求解,从数值求解的结果中得出了几个关键性参数,例如可饱和吸收体的初始透过率、输出镜反射率、激光器腔长、激光器腔内损耗、抽运激光光斑大小等,给出了关键性的参数对被动调QNd∶YAG/Cr∶YAG激光器的影响;然后基于数值求解的结果对微晶片激光器进行了优化设计,例如缩短激光腔长度,减小腔内损耗,选择合适的输出镜反射率、初始透过率和抽运激光光斑大小等;最后完成了两种不同参数的微晶片被动调QNd∶YAG/Cr∶YAG微晶片激光器的制作。     

Broadband polarizing beam splitter with an embedded metal-wire nanograting

An embedded metal-wire nanograting is designed and fabricated for the first time to our knowledge. This artificial material could be used as a broadband polarizing beam splitter to reflect s-polarized light and transmit p-polarized light. An upper-cladding layer of the same material as the gratings is deposited on the ridge of the gratings, whereas the metal wire is deposited in the grating trenches. This embedded structure makes the grating more firm in its applications. High polarization efficiency and low insertion loss with a broad wavelength range (900-1700 nm) and a wide angular tolerance are obtained by optimization of the designed structure.     

X波段微带余割平方扩展波束天线阵赋形优化遗传算法研究

提出了一种改进型自适应遗传算法, 该算法用logistic函数拟合交叉概率和变异概率, 以赌轮盘选择和精英保留相结合的方式, 在全局寻找最优解. 与经典遗传算法相比, 改进型自适应遗传算法可以大大提高算法的求解质量. 本文基于改进的自适应遗传算法研究设计了-3 dB范围为0°-12°, -10 dB波束宽度为65°, 波束覆盖为65°, 天线频带范围为8.5-9.8 GHz, 中心频率为9.05 GHz的X波段微带余割平方扩展波束天线阵. 研究结果表明改进型自适应遗传算法对方向图的拟合程度具有较大提高, 适应度值可以从0.07以下提升到0.09以上.     

Experimental study of the possibility to increase the pitch factor of an intense relativistic helical electron beam

We present the results of experiments on the formation of a high-quality relativistic helical electron beam (HEB) in a magnetron-injection gun. It is shown that suppression of parasitic excitation of microwaves in the input part of the transportation channel allows eliminating high-voltage breakdowns in the gun and achieving greater beam compression. A modulation of the electron beam current at the frequency of longitudinal electron oscillations between the cathode and the magnetic mirror in the trap, which is related to the instability of the helical electron beam, has been observed for the first time. The modulation depth can reach tens percent. Pickup of reflected electrons by a special diaphragm makes it possible to increase the achievable pitch factor, eliminate the beam-current modulation and, as a result, form although with a current loss on the diaphragm, an HEB with record-breaking values of the pitch factor, which exceed 2. For a moderate HEB compression, when the portion of reflected electrons is relatively low, their pickup by the diaphragm allows one to form a beam in which the total energy of the transverse motion of electrons conserves despite the loss of part of the current. After the optimization, a beam with an electron energy of 300 keV, a current of 100 A, a pitch factor of 1.5, and a velocity spread of 20% is obtained for a 15% loss of the current on the diaphragm. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 855–863, October 2008.     

Numerical and Experimental Evaluation of Shine-Through Loss and Beam Heating Due to Neutral Beam Injection on EAST

This research applies experimental measurements and NUBEAM, ONETWO and TRANSP modules to investigate the shine-through(ST) loss ratio and beam heating percentage of neutral beam injection on EAST. Measurements and simulations confirm that the ST loss ratio increases linearly with beam energy, and decreases exponentially with plasma density. Moreover, using the multi-step fitting method, we present analytical quantitative expressions of ST loss ratio and beam heating percentage, which are valuable for the high parameter long-pulse experiments of EAST.     

Efficient method for evaluation of the diffraction efficiency upper bound of diffractive phase elements

The theoretical diffraction efficiency upper limit of diffractive phase elements (DPE's) with finite apertures is investigated. A successful numerical method of evaluating the efficiency upper bound of DPE's is proposed. The method includes a hybrid optimization procedure that combines a genetic algorithm with the conjugate gradient method. This efficient global optimization technique can also be used to design DPE's. Simulation computations are detailed for rotationally symmetric beam shaping in which a Gaussian profile laser beam is converted into a uniform beam. Numerical results demonstrate that the estimated diffraction efficiency upper bound is consistent with the design results.     

Diffractive beam splitter characterization via a power-recycled interferometer

We used the high-precision laser interferometer technique of power recycling to characterize the optical loss of an all-reflective grating beam splitter. This beam splitter was used to set up a Michelson interferometer with a power-recycling resonator with a finesse of 883. Analyzing the results obtained, we determined the beam splitter's total optical loss to be (0.193+/-0.019)%. Low loss all-reflective beam splitters might find application in future high-power laser interferometers for the detection of gravitational waves.     

BEPCⅡ电子环寄生模损失测量

BEPCII在设计阶段从束流不稳定性和寄生模损失角度对阻抗提出了限制,但在BEPCII运行中寄生模损失是影响高流强稳定运行的因素之一。针对BEPCII电子环的寄生模损失进行了测量,主要是基于同步相移随流强的微小变化、束流功率测量和高阶模吸收器的功率。测量结果表明:两种方法对比测量全环寄生模损失,结果重复可信,且全环寄生模损失是超导腔寄生模损失的4~5倍。     

The fate of the OH radical in molecular beam sampling experiments

The collisional history of ionized molecules in a molecular beam mass spectrometric flame experiment is target of our present investigation. Measurements in a double imaging photoelectron photoion coincidence spectroscopy (i²PEPICO) were performed at the Swiss Light Source (SLS) of the Paul Scherrer Institute to use the ion imaging device for separating the molecular beam ions from rethermalized ions. This enables the precise composition study of the individual types of ions. Results show clearly for the OH radical that the complete signal is obtained from the molecular beam, while the signal from other combustion compounds features additional rethermalized molecules. As for OH radicals, the mole fraction is reduced by sampling effects and contact with the ionization vessel walls significantly. Consequently, this leads to signal loss and lower mole fractions, when using ionization cross sections for the quantification. To improve on this, a beam fraction (BF) factor is presented. The factor describes the ratio of the separated beam signal without rethermalized ions with the total ion signal, consisting of the mass to charge ratio from the molecular beam and additional rethermalized ions. Since the detected OH radicals are solely from the molecular beam, a new method of comparing two molecular beam alignments using the OH to H₂O signal ratio is presented. This method has a decent potential for the optimization of the quality of molecular beams. Finally, the separated beam signal (without the rethermalized ions) was used to determine mole fraction profiles for the OH radical using ionization cross sections. These profiles are in good agreement with model predictions of the USC-II and the Aramco Mech 2.0 mechanisms, while the total signal leads to factor of 12 smaller OH mole fractions.     

直线感应加速器输运磁场遗传算法优化

以直线感应加速器（LIA）匹配磁场设计和束线调谐为背景,提出解决强流相对论电子束长距离、小波动、多元件磁约束的输运优化问题的数值优化办法,建立基于遗传算法的优化程序。结合束质心轨迹及束包络耦合模型,设计描述束传输半径波动大小的评价函数,采用励磁元件馈入电流为优化对象,解决LIA磁场配置组合爆炸优化问题。计算结果表明:优化程序可依据不同的初始束流,有针对性地快速给出一组符合束输运要求的励磁电流配置。研究成果为在建的LIA装置束线调谐提供一种重要的数值分析工具。     

Enhancing the accelerated beam current in the booster synchrotron by optimizing the transport line beam propagation

In this paper, we present the results of transverse beam emittance and twiss parameter measurement of an electron beam, delivered by a 20 MeV microtron which is used as a pre-injector system for a booster synchrotron in the Indus Accelerator Facility at RRCAT Indore. Based on these measured beam parameters, beam optics of a transport line was optimized and its results are also discussed in this paper. This beam transport line is used to transport the electron beam from the 20 MeV microtron to the booster synchrotron. The booster synchrotron works as a main injector for Indus-1 and Indus-2 synchrotron radiation facilities. To optimize the beam optics of a transport line for proper beam transmission through the line as well as to match the beam twiss parameters at the beam injection point of another accelerator, it is necessary to know the transverse beam emittance and twiss parameters of the beam coming from the first one. A MATLAB-based GUI program has been developed to calculate the beam emittance and twiss parameters, using quadrupole scan method. The measured parameters have been used for beam transport line optimization and twiss parameters matching at booster injection point. After this optimization, an enhancement of ～50% beam current has been observed in the booster synchrotron.     

Design of EPU oscillator cavity in femtosecond accelerator

We adopt the groove guide as the cavity of the undulator to reduce the diffraction effect. The groove guide has the advantages of lower surface energy loss, larger power capacity, less modes, and larger structure dimension over the traditional method waveguide. The attenuation calculation is given in this paper including the cavity optimization. And the dispersive character indicates that the oscillator can work in different modes with the change of the electron beam energy.