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.     

Experimental generation and characterization of uniformly filled ellipsoidal electron-beam distributions

For 40 years, uniformly filled ellipsoidal beam distributions have been studied theoretically, as they hold the promise of generating self-fields linear in the coordinate offset in all three directions. Recently, a scheme for producing such distributions, based on the strong longitudinal expansion of an initially very short beam under its own space-charge forces, has been proposed. In this Letter we present the experimental demonstration of this scheme, obtained by illuminating the cathode in a rf photogun with an ultrashort laser pulse (approximately 35 fs rms) with an appropriate transverse profile. The resulting 4 MeV beam spatiotemporal (x,t) distribution is imaged using a rf deflecting cavity with 50 fs resolution. A temporal asymmetry in the ellipsoidal profile, due to image charge effects at the photocathode, is observed at higher charge operation. This distortion is also found to degrade the transverse beam quality.     

Longitudinal momentum mining of beam particles in a storage ring

I describe a scheme for selectively isolating high density low longitudinal emittance beam particles in a storage ring from the rest of the beam without emittance dilution. I discuss the general principle of the method, called longitudinal momentum mining, beam dynamics simulations and results of beam experiments. Multi-particle beam dynamics simulations applied to the Fermilab 8 GeV Recycler (a storage ring) convincingly validate the concepts and feasibility of the method, which I have demonstrated with beam experiments in the Recycler. The method presented here is the first of its kind.     

Longitudinal beam emittance blow-up caused by injection errors in synchrotrons with a longitudinal feedback system

The problem of longitudinal beam emittance blow-up caused by injection errors in synchrotrons with a longitudinal feedback system is studied. The obtained results are used to estimate parameters of a longitudinal feedback system.     

Transverse damping systems in modern synchrotrons

Transverse feedback systems for suppression of transverse coherent beam oscillations are used in modern synchrotrons for preventing the development of transverse instabilities and damping residual beam oscillations after injection. Information on damper systems for the Large Hadron Collider (LHC; CERN, Geneva) and the accelerator complex FAIR (GSI, Darmstadt) is presented. The project for the LHC is being performed at the Laboratory of Particle Physics of the Joint Institute for Nuclear Research in collaboration with CERN. The information concerning the state of the project and the plans of its completion at the LHC is given. The results of the first design activity on transverse damping systems at the SIS100 and SIS300 synchrotrons, to be created in the framework of the new international project FAIR, are presented.     

Development of Software for Simulating and Analyzing the Dynamics of Charged-Particle Beams in Synchrotrons and Beam Lines

The paper considers software for the simulation and analysis of transverse dynamics of charged particle beams in synchrotrons and beam lines. It calculates the transverse beam dynamics in synchrotrons and beam lines based on the linear model, the structural functions of synchrotrons and beam lines, the beam acceptance and emittance, and the orbit correction. The software is capable of calculating the orbit-correction quality distributions and reconstructing the transport matrix parameters from the response matrices.     

合肥光源储存环束流截面测量系统改造和软件升级

 存储环束流截面及发射度是同步辐射加速器重要的基本参数。从束流截面大小的变化和束斑位置的移动规律，可以进行环上的束流不稳定性的研究。介绍了合肥光源，储存环束流截面测量系统局部改造和它的软件升级并且着重介绍西欧核子中心(CERN)的软件框架ROOT在数据处理和显示方面的应用。     

对受激布里渊散射激光进行组束的数值模拟及方案设计

 提出了时间上串行的多路激光脉冲通过受激布里渊散射(SBS)池进行组束的方法,并对其进行了数值模拟研究。利用6束每束能量为45J的KrF激光进行组束, 可获得能量为141.89J,脉宽为670ps的Stokes输出光。根据模拟结果设计出了时间上串行的SBS激光组束的两个方案, 对其进行了讨论。数值模拟还发现在介质的增益系数更大、声子寿命更短的情况下,输出激光脉冲的脉宽可以压缩得更窄。     

Beam dynamics studies in the driver LINAC pre-stripper section of the RIA facility

The RIA facility driver LINAC consists of about 400 superconducting (SC) independently phased rf cavities. The LINAC is designed to accelerate simultaneously several charge-state beams to generate as much as 400 kW of uranium beam power. The LINAC beam dynamics is most sensitive to the focusing and accelerating structure parameters of the pre-stripper section, where the uranium beam is accelerated from 0.17 keV/u to 9.4 MeV/u. This section is designed to accept and accelerate two charge states (28 and 29) of uranium beam from an ECR ion source. The pre-stripper section must be designed to minimize the beam emittance distortion of this two-charge-state beam. In particular, the inter-cryostat spaces must be minimized and beam parameters near transitions of the accelerating and focusing lattices must be matched carefully. Several sources of possible effective emittance growth are considered in the design of the pre-stripper section and a tolerance budget is established. Numerical beam dynamics studies include realistic electric and magnetic three-dimensional field distributions in the SC rf cavities and SC solenoids. Error effects in the longitudinal beam parameters are studied.     

Vibration analysis of a beam with PCLD Patch

An analytical approach for vibration response analysis of a beam with single passive constrained layer damping (PCLD) patch is presented. The governing equation of motion of the beam is firstly derived on the basis of an energy approach and the Lagrange equation. The noval contribution is that a third admissible function is introduced to represent the longitudinal displacements of the constraining layer in the PCLD patch when the assumed-modes method is applied for discretizing the governing equation. In conventional analytical approaches, only two admissible functions are used together with a longitudinal static equilibrium equation of a section of base beam or constraining layer. Comparison of the computational results from the proposed analytical approach and the conventional analytical approach as well as a commercial FEM code reveals that the proposed analytical approach can describe the vibration responses of the damped beam more accurately for commonly used viscoelastic material (VEM) layer in the PCLD patch while the conventional analytical approach, in general, overestimates the damping effects of the PCLD patch. The advantages and disadvantages of the proposed analytical approach and conventional analytical approach are discussed through some case studies.     

Stability of an ion beam in synchrotrons with digital filters in the feedback loop of a transverse damper

The stability of an ion beam in synchrotrons with digital filters in the feedback loop of a transverse damper is treated. A transverse feedback system (TFS) is required in synchrotrons to stabilize the high intensity ion beams against transverse instabilities and to damp the beam injection errors. The TFS damper kicker (DK) corrects the transverse momentum of a bunch in proportion to its displacement from the closed orbit at the location of the beam position monitor (BPM). The digital signal processing unit in the feedback loop between BPM and DK ensures a condition to achieve optimal damping. Damping rates of the feedback systems with digital filters are analysed in comparison with those in an ideal feedback system.     

An improved injector bunching geometry for ATLAS

The bunching system of the ATLAS positive ion injector (PII) has been improved by relocating the harmonic buncher to a point significantly closer to the second stage sine-wave buncher and the injector LINAC. The longitudinal optics design has also been modified and now employs a virtual waist from the harmonic buncher feeding the second stage sine-wave buncher. This geometry improves the handling of space charge for high-current beams, significantly increases the capture fraction into the primary rf bucket and reduces the capture fraction of the unwanted parasitic rf bucket. Total capture and transport through the PII has been demonstrated as high as 80% of the injected dc beam while the population of the parasitic, unwanted rf bucket is typically less than 3% of the total transported beam. To remove this small residual parasitic component a new traveling-wave transmission-line chopper has been developed reducing both transverse and longitudinal emittance growth from the chopping process. This work was supported by the U.S. Department of Energy under contract W-31-109-ENG-38.     

Balanced heterodyne signal extraction in a postmodulated Sagnac interferometer at low frequency

We describe a balanced-heterodyne postmodulated Sagnac interferometer signal extraction method that is suitable for gravitational wave detection. The method is simple to implement by placement of a polarization-selective modulator after the beam splitter in the dark port of the interferometer. The postmodulated Sagnac interferometer retains its common path advantage and exhibits insensitivity to laser frequency noise below, at, and above the heterodyne frequency. Balanced detection reduces sensitivity to laser amplitude noise. In this scheme mirror displacement signals were rf demodulated and observed from 0.2 to 10kHz.     

关于热阴极微波电子枪中电子反轰问题的研究

 提出了热阴极微波电子枪的设计中可以通过缩短腔长减小电子反轰功率，并通过模拟计算详细研究了腔长对电子反轰及枪出口束流品质的影响。     

Low-power MRI by Frank-sequence excitation

Recently, a new NMR method employing an rf excitation scheme with strongly reduced power has been introduced, which is based on modulating rf pulses according to Frank sequences. For many applications, a reduction of rf power is essential, e.g. to eliminate bulky rf pre-amplifiers or in medical high-field MRI to preserve patient safety. Another benefit of the new scheme are very short dead times allowing for measurements of samples with short relaxation times. In this work, Frank-sequence excitation is used for low-power imaging for the first time. Results of one-, two-, and three-dimensional imaging experiments are presented and compared to conventional images.     

Nonlinear longitudinal space charge oscillations in relativistic electron beams

In this Letter we study the evolution of an initial periodic modulation in the temporal profile of a relativistic electron beam under the effect of longitudinal space-charge forces. Linear theory predicts a periodic exchange of the modulation between the density and the energy profiles at the beam plasma frequency. For large enough initial modulations, wave breaking occurs after 1/2 period of plasma oscillation leading to the formation of short current spikes. We confirm this effect by direct measurements on a ps-modulated electron beam from an rf photoinjector. These results are useful for the generation of intense electron pulse trains for advanced accelerator applications.     

同步加速器中束流能量的修正

叙述了同步加速器中束流能量测量和修正的原理,推导出关于闭合轨道畸变和校正二极子强度的束流能量的修正公式,介绍了这些公式在北京正负电子对撞机（BEPC）上的应用,并对其结果进行了讨论．     

渡越辐射及其在北京自由电子激光置束测中的应用

 在北京自由电子激光器装置的性能改进中,采用了基于渡越辐射的电子束诊断方法。描述了渡越辐射应用于电子束参数测量的基本理论,介绍了BFEL中利用渡越辐射进行的实验方案,并报导了初步的结果。     

CFETR 中性束注入系统概念设计

为了满足中国核聚变工程实验堆(CFETR)对等离子体加热和电流驱动的要求,从总体布局、束传输 系统、束源系统三方面进行了中性束注入系统的概念设计。利用参数计算的方法,根据聚变等离子体的要求明确 了中性束注入系统的性能指标和基本布局;利用束传输空间分布程序评估了束传输性能,确定了各功能部件的空 间布局结构;在此基础上,确定了束源系统的性能指标和引出系统布局方式,结合当前研发进展,明确了束源的 基本技术方案。由此完成了中性束注入系统参数指标、束传输关键尺寸、束源性能指标等设计要求,为后续工程 设计奠定了基础。