基于窗耦合结构的HIAF RFQ高频腔体研究

作为直线加速器的前级聚焦加速部分,RFQ(射频四极场) 在束流动力学和运行稳定性上都应表现良好,需要保持电场平整和电磁强耦合。为实现这两个目标,提出并研究了窗耦合结构。针对工作频率为81.25 MHz 的1 m长模型腔,利用三维电磁仿真软件CST MWS微波工作室对传统的四翼型、四杆型高频结构进行了仿真,并重点研究了窗耦合型结构。对开窗的对称方式、开窗的个数和大小等进行了分析,发现合适的窗耦合结构能保持较为紧凑的横向尺寸同时能耗适中,同四翼型结构相比二极模频率也远离了运行频率。为验证模拟结果,建造了一个铝模型腔,并对模型腔进行了冷模测试,实测频率为81.41 MHz,相邻模频率差为10.74 MHz,与模拟结果接近。仿真模拟和模型腔测试的结果表明,窗耦合四翼型结构可作为较低频率RFQ的一种设计。As a focusing and acceleration element in front part of the linear accelerator RFQ, should have ahigh performance in both beam dynamic and operation stability. It requires the electro-magnetic field of RFQ to keep uniform and strongly coupled. In this paper the window coupled structure is proposed and investigated to meet the requirements of RFQ design. Different structures have been compared and analyzed, including four-vane type and four-rod type, and the four-vane type with windows. It was concluded that window-coupled structure is more compact in the transverse dimension with modest power loss and the dipole frequency is far from the operation frequency compare to the normal four-vane structure. A one-meter long and frequency of 81.25 MHz model-cavity of alumimum was employed as a sample and simulated by using the microwave studio of CST. The low power RF test results show that the operating frequecy is 81.41 MHz and the nearest mode frequency separation is 10.74 MHz, which is in good agreement with the simulated values. It is concluded that the window-coupled structure is a candidate for low frequency RFQ.     

RF and field measurements of the SSC-LINAC RFQ

The 53.667 MHz continuous-wave heavy ion RFQ has been designed and manufactured for the SSC-LINAC project.This four-rod RFQ accelerates ions with maximum mass to charge ratio of 7 from 3.728 keV/u to 143 keV/u.Measurements have been carried out to check the RF performance of the cavity and the quality of the electric field.The S11 of the power coupler is adjusted to better than-44 dB,and the Q0 of the cavity is 6440.The quality of the electric field is evaluated by the perturbation method.The measurement procedure and data analysis will be discussed in detail.The error due to gravity of the perturbation bead has been corrected by averaging the fields in different quadrants.As a result,the unflatness of the electric field is±2.5%,and the dipole field component distributes from 0%to 20%in different longitudinal positions,which indicates the asymmetry of the quadrupole field.The unflatness of the quadrupole field distribution represents a good agreement with the simulation results.High power RF test and beam commissioning of the RFQ are on schedule in early 2014.     

Molecular properties and potential energy function model of BH under external electric field

Using the density functional B3P86/cc-PV5Z method, the geometric structure of BH molecule under different external electric fields is optimized, and the bond lengths, dipole moments, vibration frequencies, and other physical properties parameters are obtained. On the basis of setting appropriate parameters, scanning single point energies are obtained by the same method and the potential energy curves under different external fields are also obtained. These results show that the physical property parameters and potential energy curves may change with external electric field, especially in the case of reverse direction electric field. The potential energy function without external electric field is fitted by Morse potential, and the fitting parameters are obtained which are in good agreement with experimental values. In order to obtain the critical dissociation electric parameter, the dipole approximation is adopted to construct a potential model fitting the corresponding potential energy curve of the external electric field. It is found that the fitted critical dissociation electric parameter is consistent with numerical calculation, so that the constructed model is reliable and accurate. These results will provide important theoretical and experimental reference for further studying the molecular spectrum, dynamics, and molecular cooling with Stark effect.     

A new compact structure for a high intensity low-energy heavy-ion accelerator

A new compact accelerating structure named Hybrid RFQ is proposed to accelerate a high-intensity low-energy heavy ion beam in HISCL （High Intensive heavy ion SuperConducting Linear accelerator）, which is an injector of HIAF （Heavy Ion Advanced Research Facility）. It is combined by an alternative series of acceleration gaps and RFQ sections. The proposed structure has a high accelerating ability compared with a conventional RFQ and is more compact than traditional DTLs. A Hybrid RFQ is designed to accelerate 238U34＋ from 0.38 MeV/u to 1.33 MeV/u. The operation frequency is described to be 81.25 MHz at CW （continuous wave） mode. The design beam current is 1.0 mA. The results of beam dynamics and RF simulation of the Hybrid RFQ show that the structure has a good performance at the energy range for ion acceleration. The emittance growth is less than 5% in both directions and the RF power is less than 150 kW. In this paper, the results of beam dynamics and RF simulation of the Hybrid RFQ are presented.     

Polarization and structure dependent optical characteristics of the three-arm nanoantenna with C_(3v) symmetry and broken symmetry

The optical properties of a three-arm plasmonic nanoantenna with and without broken symmetry were analyzed in detail. For the symmetrical structure, the local electric field can be significantly enhanced and well confined within the feed gap, whilst the extinction spectrum illustrates polarization independence. With broken symmetry, multi-wavelength resonances are observed due to the single dipole resonance and dipole–dipole coupling effect, and wide tunability is also available through minor structural adjustment. Especially when illuminated by a circularly polarized light beam, the extinction and the electric field distribution can be effectively modulated by just varying the incident wavelength.     

Studies on RFQ accelerators and its applications

Development activities of Radio Frequency Quadrupole （RFQ） accelerators in China are presented. A 1 MeV O＋ RFQ and a 3.5 MeV ADS proton RFQ have been constructed. A novel separated function RFQ is under beam test, a 2 MeV D＋ RFQ is under construction and a CSNS RFQ is going to be constructed. The RFQ dynamics and the simultaneous dual beam acceleration with positive and negative ions were investigated and related codes were developed. The applications of RFQ will be further promoted in China.     

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.     

Preliminary design and simulation of a 162.5 MHz high-intensity proton RFQ for an accelerator driven system

A new procedure for the design and simulation of a Radio Frequency Quadrupole (RFQ) accelerator has been developed at the Argonne National Laboratory. This procedure is integrated with the beam dynamics design code DESRFQ and the simulation code TRACK, which are based on three-dimensional field calculations and the particle-in-cell mode beam dynamics simulations. This procedure has been applied to the development of a 162.5 MHz CW RFQ which is capable of delivering a 10 mA proton beam for the Accelerator Driven System (ADS) of the CAS. The simulation results show that this RFQ structure is characterized by the stable values of the beam acceleration efficiency for both the zero current beam and space charge dominated beam. For an average beam current of 10 mA, there is no transverse rms emittance growth, the longitudinal rms emittance at the exit of RFQ is low enough and there is no halo formation. The beam accelerated in the RFQ could be accepted easily and smoothly by the following super-conducting linear accelerator.     

New CMOS compatible super junction LDMOST with n-type buried layer

A new super-junction lateral double diffused MOSFET (LDMOST) structure is designed with n-type charge compensation layer embedded in the p$^{ - }$-substrate near the drain to suppress substrate-assisted depletion effect that results from the compensating charges imbalance between the pillars in the n-type buried layer. A high electric field peak is introduced in the surface by the pn junction between the p$^{ - }$-substrate and n-type buried layer, which given rise to a more uniform surface electric field distribution by modulation effect. The effect of reduced bulk field (REBULF) is introduced to improve the vertical breakdown voltage by reducing the high bulk electric field around the drain. The new structure features high breakdown voltage, low on-resistance and charges balance in the drift region due to n-type buried layer.     

Theoretical Design of a 104MHz Ladder Type IH-RFQ Accelerator

Beam dynamics and rf designs of a 104 MHz ladder type IH-RFQ （L-IH-RFQ） accelerator are finished at Peking University for the acceleration of 14C＋ from 40 keV to 500 keV. As a specific feature, the output beam energy spread is as low as 0.6% achieved with the internal discrete bunching method, which makes potential applications of RFQ feasible, such as accelerator mass spectrometry and ion implantation. Tolerances of the beam dynamics design are studied by means of changing the input beam parameters, and the results are quite satisfying. On the other hand, the L-IH-RFQ structure is employed, taking advantage of its mechanical stability and the absence of inter-electrode voltage asymmetry. Radio-frequency properties are studied and optimized for reducing power loss with Microwave Studio （MWS）. Tuning of the field flatness and frequency is investigated in principle.     

一米长四翼型RFQ调谐研究(英文)

为获取ADS工程中四翼型RFQ加速器在设计、调谐和运行等方面的经验,设计和建造了一台一米长四翼型RFQ加速器。该RFQ的设计频率为162.5 MHz,但是测量结果显示即使没有调谐器时,腔体的频率也为163.7 MHz。为降低腔体频率,使用四线模型理论进行了分析并重新设计了腔体的端板。使用了二极模稳定杆来加大四极频率与相邻二极频率的间隔。最终测量结果显示,在腔体电压分布没有大的波动的情况下腔体频率达到了设计频率。同时,腔体Q值降低了1%,这就要求需要更多的功率注入腔体。使用水对二极模稳定杆进行了冷却,以便腔体能够稳定工作。     

750 keV射频四极注入器射频结构的优化

 设计了750 keV,201.25 MHz的RFQ注入器的射频结构,对四杆型RFQ结构进行了简要的理论分析,在束流动力学设计的基础上,对射频结构进行了优化。研究了四杆型RFQ结构中支撑板高度、宽度、厚度、间距、形状、外腔体半径等因素对射频特性的影响,进行了优化设计并给出了主要的结构参数及射频特性的设计结果。优化设计得到的四杆型RFQ腔体长度126 cm,在极间电压80 kV时,峰值功率损耗为115.95 kW,二极场因子为1.004,电场沿轴向分布比较均匀,偏差小于3.5%,满足了物理需求。     

Frequency tuning with RFQ temperature in China ADS Injector Ⅱ

A 162.5 MHz four-vane radio frequency quadruple(RFQ) accelerator has been developed at the Institute of Modern Physics(IMP) for Injector II of the China ADS linac. The RFQ will operate in continuous wave mode at 100 k W. For the designed 10 mA beam, the additional RF power dissipation will induce a very large reflection of power. A water-temperature controlling system will be used to reduce the power reflection by tuning the frequency of the RFQ. The tuning capability of the water temperature is studied under different configurations of cooling water.Simulations and experiment are compared in this paper. The experimental results agree well with simulation using ANSYS. This can be used as a reference to tune the RFQ in beam commissioning.     

C-ADS注入器Ⅱ项目RFQ射频系统介绍（英）

C-ADS注入器Ⅱ的RFQ射频系统是该强流加速器的关键一环,它通过两台完全一样的耦合器为一台四边形四翼型RFQ提供和传送功率。该射频系统在设计之初就考虑为10 mA连续束流而进行特殊优化,从功率源、耦合器和功率传输系统几个方面对其进行详细介绍,尤其是针对CW模式下的系统稳定运行以及设备可靠性等方面进行的特殊考虑和模拟方法。该系统已经在2015年通过了10 mA连续束的测试,证实了该射频系统的设计和调试符合物理的实验需求。特别阐述了该系统中一种新的碗型陶瓷窗耦合器的设计思路和一种无环形器情况下的特殊的耦合系数调谐方法,同时推导了双端口耦合的计算方法的具体设置过程。     

Preliminary design and simulation of a 162.5 MHz high- intensity proton RFQ for an accelerator driven system

A new procedure for the design and simulation of a Radio Frequency Quadrupole (RFQ) accelerator has been developed at the Argonne National Laboratory. This procedure is integrated with the beam dynamics design code DESRFQ and the simulation code TRACK, which are based on three-dimensional field calculations and the particle-in-cell mode beam dynamics simulations. This procedure has been applied to the development of a 162.5 MHz CW RFQ which is capable of delivering a 10 mA proton beam for the Accelerator Driven System (ADS) of the CAS. The simulation results show that this RFQ structure is characterized by the stable values of the beam acceleration efficiency for both the zero current beam and space charge dominated beam. For an average beam current of 10 mA, there is no transverse rms emittance growth, the longitudinal rms emittance at the exit of RFQ is low enough and there is no halo formation. The beam accelerated in the RFQ could be accepted easily and smoothly by the following super-conducting linear accelerator.     

RFQ加速器中二极模稳定杆作用机理的研究

RFQ加速器中的二极模场给束流施加一个偏转力,导致束流损失在机器上.因此,必须采取降低二极模场的措施.人们提出采用二极模稳定杆使二极模频率远离四极工作模.在一台RFQ冷模上开展的实验研究中,发现了一些与二极模稳定杆作用机理相关的新现象,本文报告这些实验结果,并对其进行分析解释.根据这些结果,提出,利用非对称性地插入二极模稳定杆,可以降低非对称RFQ腔体中的二极模场分量.     

Tuner design and RF test of a four-rod RFQ

A mini-vane four-rod radio frequency quadruple (RFQ) accelerator has been built for neutron imaging. The RFQ will operate at 201.5 MHz, and its length is 2.7 m. The original electric field distribution along the electrodes is not flat. The resonant frequency needs to be tuned to the operating value. And the frequency needs to be compensated for temperature change during high power RF test and beam test. As tuning such a RFQ is difficult, plate tuners and stick tuners are designed. This paper will present the tuners design, the tuning procedure, and the RF properties of the RFQ.

     

Physics design of a 10 MeV, 6 kW travelling wave electron linac for industrial applications

We present the physics design of a 10 MeV, 6 kW S-band (2856 MHz) electron linear accelerator (linac), which has been recently built and successfully operated at Raja Ramanna Centre for Advanced Technology, Indore. The accelerating structure is a 2π/3 mode constant impedance travelling wave structure, which comprises travelling wave buncher cells, followed by regular accelerating cells. The structure is designed to accelerate 50 keV electron beam from the electron gun to 10 MeV. This paper describes the details of electromagnetic design simulations to fix the mechanical dimensions and tolerances, as well as heat loss calculations in the structure. Results of design simulations have been compared with those obtained using approximate analytical formulae. The beam dynamics simulation with space charge is performed and the required magnetic field profile for keeping the beam focussed in the linac has been evaluated and discussed. An important feature of a travelling wave linac (in contrast with standing wave linac) is that it accepts the RF power over a band of frequencies. Three-dimensional transient simulations of the accelerating structure along with the input and output couplers have been performed using the software CST-MWS to explicitly demonstrate this feature.