Design of BEPC Ⅱ bunch current monitor system

BEPC Ⅱ is an electron-positron collider designed to run under multi-bunches and high beam current condition. The accelerator consists of an electron ring, a positron ring and a linear injector. In order to achieve the target luminosity and implement the equal bunch charge injection, the Bunch Current Monitor (BCM)system is built on BEPC Ⅱ. The BCM system consists of three parts: the front-end circuit, the bunch current acquisition system and the bucket selection system. The control software of BCM is based on VxWorks and EPICS. With the help of BCM system, the bunch current in each bucket can be monitored in the Central Control Room. The BEPC Ⅱ timing system can also use the bunch current database to decide which bucket needs to refill to implement "top-off" injection.     

BEPCⅡ储存环束团流强测量

针对北京正负电子对撞机二期工程(BEPCⅡ)原束团流强测量系统(BCM)不能稳定运行的问题,对系统进行了升级。利用高速ADC对束流位置探头(BPM)信号直接采样,在现场可编程门阵列(FPGA)内进行实时数字信号处理,进而得到加速器储存环内的束团流强值。基于反射内存网络,系统可以实现测量结果高速共享、实时显示。升级后系统实现了更高精度的测量,实时显示精度到0.1mA,最佳测量精度达到10μA。通过对系统的触发时钟和算法结构调整,系统实现了长时间的稳定工作,正负电子储存环均实现了对束团流强的均匀性控制,提高了加速器的对撞亮度。     

Progress of the Injector Linac Upgrade for the BEPCⅡ Project

BEPCⅡ—an upgrade project of Beijing Electron Positron Collider (BEPC) is a factory type of e+e- collider. It requires its injector linac to have a higher beam energy (1.89 GeV) for on-energy injection and a higher beam current (40 mA e+ beam) for a higher injection rate ( 50 mA/min). The low beam emittance (1.6π mm·mrad for e+ beam, and 0.2πmm·mrad for 300 mA e- beam) and the low beam energy spread (±0.5%) are also required to meet the storage ring acceptance. Thus the original BEPC injector linac must be upgraded to have a new electron gun with its complete tuning system, a new positron source with a flux concentrator, a new RF power system with its phasing loops and a new beam tuning system with orbit correction and optics tuning devices. These new components have been designed, fabricated, tested and will be installed in their final positions in this spring and summer, which are described in this paper.     

SPEAR3 Short-Pulse,Low-α Operation

The SPEAR3 storage ring at SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource, is delivering beam in a short-pulse, low-α operational mode. In order to deliver short photon pulses, the stored current in SPEAR3 must be reduced. In low-α mode, we presently deliver 100 mA total current with bunch lengths of 15 psec FWHM, compared to our standard operations with 500 mA and 46 psec FWHM bunch lengths. It is possible to deliver still shorter pulses at lower current. The low-α beam is stored in four bunch trains, interleaved with up to four “camshaft” timing bunches for pump-probe experiments. This simultaneously provides short pulses for timing experiments while ensuring 100 mA for experiments at other beamlines.     

BEPCⅡ Positron Source

BEPCⅡ— an upgrade project of the Beijing Electron Positron Collider (BEPC) is a factory type of e^＋e^－ collider. The fundamental requirements for its injector linac are the beam energy of 1.89GeV for on-energy injection and a 40mA positron beam current at the linac end with a low beam emittance of 1.6μm and a low energy spread of ±0.5% so as to guarantee a higher injection rate (≥50mA/min) to the storage ring. Since the positron flux is proportional to the primary electron beam power on the target, we will increase the electron gun current from 4A to 10A by using a new electron gun system and increase the primary electron energy from 120MeV to 240MeV. The positron source itself is an extremely important system for producing more positrons, including a positron converter target chamber, a 12kA flux modulator, the 7m focusing module with DC power supplies and the support. The new positron production linac from the electron gun to the positron source has been installed into the tunnel. In what follows, we will emphasize the positron source design, manufacture and tests.     

合肥光源单束团下束团长度和能散的测量

 根据同步光与储存环中的束流信号具有相同的时间结构的原理,测量同步光脉冲的半高全宽值可以计算出束团的长度。根据合肥光源的特点和实际需要,选择快速光电接收器搭配高速高带宽示波器作为在线测量束团长度和纵向分布等的主要手段。对单束团模式下束团长度随流强和高频腔腔压的变化趋势进行了测量。测量结果表明：束团长度与腔压的0.3次方成反比,比理论值0.5小;而束团长度随流强的增长率为2.0 ps/mA。通过测量纵向量子寿命进行了能散随流强变化的间接测量,结果表明,束团的拉伸是能散变化和势阱效应共同作用的结果。     

Progress of the intense positron beam project EPOS

EPOS (the ELBE POsitron Source) is a running project to build an intense, bunched positron beam for materials research. It makes use of the bunched electron beam of the ELBE radiation source (Electron Linac with high Brilliance and low Emittance) at the Research Centre Dresden-Rossendorf (40 MeV, 1 mA). ELBE has unique timing properties, the bunch length is <5 ps and the repetition time is 77 ns. In contrast to other Linacs made for Free Electron Lasers (e.g., TTF at DESY, Hamburg), ELBE can be operated in full cw-mode, i.e., with an uninterrupted sequence of bunches. The article continues an earlier publication. It concentrates on details of the timing system and describes issues of radiation protection.     

用于HLS单束团运行模式的高频剔除系统

 介绍了主要用于实现合肥光源单束团运行模式的高频剔除（RFKO）系统。该系统完全基于仪器设备(分频器、波形发生器、宽带放大器和具有I/Q调制功能的矢量信号发生器),无任何专门设计的电子线路。高频剔除的原理是激励粒子横向振荡而丢失。激励信号产生的过程是：将取自储存环高频系统的信号进行分频,得到束团同步信号;用束团同步信号触发波形发生器,生成窄脉冲;该窄脉冲调制信号发生器,输出高频剔除信号,放大后加在条带电极上,进行束团剔除。目前获得了18 mA的单束团最高流强。改变分频数和脉冲长度,可得到其它一些周期性填充模式,其中3串6束团以及非均匀填充模式得到了实验验证。     

An intense terahertz radiation source at the Compact ERL

The Compact ERL is an energy recovery LINAC (ERL) test facility that is planned for KEK. The circumference of the recirculation path will be 70 m. Initially, the beam energy will be about 65 MeV and the current about 10 mA. Although the primary purpose of the machine is to aid the development of the key technologies that are essential for building an ultra-brilliant new synchrotron light source based on an ERL, the Compact ERL itself has great potential as an intense source of terahertz radiation. To generate the intense terahertz radiation, an electron bunch of a very short bunch length is required and bunch compression is inevitable. We discuss the parameters of the Compact ERL, present the results of a simulation of bunch compression, and make an estimate of the generated coherent synchrotron radiation.     

Meeting Reports: Energy-Recovering Linacs for Next-Generation Light Sources

Energy-recovering linear accelerators (ERLs) have the potential of providing two orders of magnitude increase in brightness over third-generation sources while offering bunch lengths as low as tens of femtoseconds for timing experiments. ERLs are different from storage rings in that the electron bunches circulate once only. Thus they are not at equilibrium, and in particular can be packed much more tightly than in storage rings, resulting in brighter sources. However, dumping a 100 mA, 3 GeV beam that carries 300 MW of energy is not practical, so the energy is recovered, and the devices are in some sense energystorage rings.     

BEPC横向阻尼时间的测量

 利用逐束团测量系统在BEPC储存环中跟踪单个束团得到其多圈位置振荡信息,获得BEPC储存环的横向阻尼时间与束流流强、色品、八极子和束流能量的关系,并通过分析得到了BEPC储存环辐射阻尼时间为52 ms。实验结果表明：流强越高、色品越大,阻尼率也就越大;束流流强为4.7 mA时八极子强度的变化对阻尼率没有影响;束流流强为5 mA时,阻尼率随束流能量升高而减小。     

Broadband emission from a relativistic electron bunch in a semi-infinite waveguide

A study is made of the excitation of a transition radiation pulse during the injection of a charged particle bunch through the end metal wall into a semi-infinite cylindrical waveguide. Exact analytic expressions for the fields of a thin ring-shaped bunch are obtained in terms of the Lommel functions of two variables. The energy efficiency, power, and spectrum of radiation emitted from a finite-size charged bunch in a vacuum waveguide are calculated numerically with allowance for the multimode nature of the excited field. It is shown that, under certain conditions, the bunch can generate a short, high-intensity electromagnetic pulse with a broad frequency spectrum. The effect of various parameters of the charged bunch-waveguide system (such as the bunch current, bunch duration, and waveguide radius) on the generation efficiency of a transition radiation pulse is investigated.     

A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well vertical-cavity surface-emitting laser

A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well (QW) vertical-cavity surface-emitting laser (VCSEL) based on an intra-cavity contacted structure is fabricated. A threshold current of 1.5 mA for a 22 μm oxide aperture device is achieved, which corresponds to a threshold current density of 0.395 kA/cm². The peak output optical power reaches 17.5 mW at an injection current of 30 mA at room temperature under pulsed operation. While under continuous-wave (CW) operation, the maximum power attains 10.5 mW. Such a device demonstrates a high characteristic temperature of 327 K within a temperature range from -12℃ to 96℃ and good reliability under a lifetime test. There is almost no decrease of the optical power when the device operates at a current of 5 mA at room temperature under the CW injection current.     

合肥储存环注入状态下的高频系统研究

 对高频系统的束流负载效应和Robinson不稳定性条件进行了详细分析；通过对 失谐角、视在失谐角与储存环注入流强限的研究发现，对于合肥光源这样的中能储存环，由于注入状态的电子束能量较低，同步辐射相角接近90°，因此，只要给高频腔馈入一定的功率，再把腔调到一定的失谐角，就可以在环中注到较高的流强，达到二期工程的设计目标。     

Design studies on a 500 kV DC gun photo-injector for the BXERL test facility

BXERL is a proposal for a test facility(Beijing X-ray Energy Recovery Linac),which requires its injector to provide an electron beam of 5 MeV,77 pC/ bunch at a repetition rate of 130 MHz(average current of 10 mA).In this paper,we present the design of the injector,which consists of a 500 kV photocathodeDC gun equipped with a GaAs cathode preparation device,a 1.3 GHz normal conducting RF buncher,two solenoids,and one cryomodule containing two 1.3 GHz 2-cell superconducting RF cavities as the energy booster.The detailed beam dynamics show that the injector can generate electron bunches with a RMS normalized emittance of 1.49 7rmm-mrad,a bunch length of 0.67 mm,a beam energy of 5 MeV and an energy spread of 0.72%.     

Effect of the ponderomotive scattering and injection position on electron-bunch injection into a laser wakefield

For the purpose of laser wakefield acceleration, it turns out that the injection of electron bunches longer than the plasma wavelength can also generate accelerated femtosecond bunches with a relatively low energy spread. This is of great interest because such injecting bunches can be provided, e.g., by photo cathode rf linacs. Here we show that when an e-bunch is injected into the wakefield, it is important to take into account the interaction of the injected bunch with the laser pulse in the vacuum region located in front of the plasma. We show that at low energies of the injected bunch, this leads to ponderomotive scattering of the bunch and results in a significant drop of the collection efficiency. For certain injection energies the ponderomotive scattering may result in a smaller energy spread in the accelerated bunch. It is found that the injection position in the laser wakefield plays an important role. Higher collection efficiency can be obtained for certain injection energies, when the bunch is injected in plasma at some distance from the laser pulse; the energy spread, however, is typically larger in this case. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch. PACS 52.38.Kd; 41.75.Jv; 41.85.Ar     

High-performance InGaN/GaN MQW LEDs with Al-doped ZnO transparent conductive layers grown by MOCVD using H_2O as an oxidizer

In this study,the high performance of InGaN/GaN multiple quantum well light-emitting diodes(LEDs) with Aldoped ZnO(AZO) transparent conductive layers(TCLs) has been demonstrated.The AZO-TCLs were fabricated on the n~+-InGaN contact layer by metal organic chemical vapor deposition(MOCVD) using H_2O as an oxidizer at temperatures as low as 400 ℃ without any post-deposition annealing.It shows a high transparency(98%),low resistivity(510 ~4 Ω·cm),and an epitaxial-like excellent interface on p-GaN with an n+-InGaN contact layer.A forward voltage of 2.82 V @ 20 mA was obtained.Most importantly,the power efficiencies can be markedly improved by 53.8%@20 mA current injection and 39.6%@350 mA current injection compared with conventional LEDs with indium tin oxide TCL(LED-Ⅲ),and by28.8%@20 mA current injection and 4.92%@350 mA current injection compared with LEDs with AZO-TCL prepared by MOCVD using O_2 as an oxidizer(LED-Ⅱ),respectively.The results indicate that the AZO-TCL grown by MOCVD using H_2O as an oxidizer is a promising TCL for a low-cost and high-efficiency GaN-based LED application.     

Longitudinal dynamics of <Superscript>197</Superscript>Au<Superscript>32+</Superscript> and <Superscript>197</Superscript>Au<Superscript>79+</Superscript> ions in NICA injection chain

The main objective of the NICA project developed at the Joint Institute for Nuclear Research (JINR) is to conduct experimental studies with colliding heavy ion beams in an energy range of 1–4.5 GeV/nucleonucleon with luminosity on the level of 1 × 10²⁷ cm⁻² s⁻¹. In this paper the operation regime of the collider injection chain providing the bunch with experimentally desirable parameters at the output of the Nuclotron is considered for gold ions as an example.     

The performance of the series III 200kv high current industrial ion implanter

Two years of operation of the Series III Implanter have revealed the full potential of this machine.

The arsenic and phosphorus performance of this machine has greatly exceeded the initial 4 mA design criterion and 8 mA beams of these elements are readily obtainable. The boron beam currents are very dependent upon source history, but 1.2 mA is always available and 3 mA can be obtained from a well conditioned source.

The single gap post-acceleration system has proved very successful, a particular advantage over multi-gap tubes being the 100% beam transmission for any post-acceleration voltage and milliampere beams.

The processor system consists of a large capacity (54–3″. 27–4″, 18–5″ or 9–6″ silicon wafers) racetrack carousel which gives a mechanical scan free of geometric errors for wafers up to 61/2″ diameter. The wafers are mounted on plates with a clamping technique designed for use with an automatic loading system. The large implantation area (approximately 4,000 cm²) minimizes the temperature rise during implantation. The large beam area (approximately 25 cm² at the carousel) minimizes the pulse heating effect as the wafers sweep through the beam.     

合肥光源逐束团测量和横向束流反馈系统设计

 介绍了在合肥光源开展逐束团测量（横向和纵向）和横向束流反馈系统研究和研制的重要性,同时还介绍了设计思想。合肥光源高频频率为204 MHz,因此,系统至少需要100 MHz的带宽。还较详细地介绍了宽带部件和系统参数的选择原则。该系统不仅可用于研究由于高频腔中的高阶模和真空室的阻抗壁效应所引起的耦合束团不稳定性,而且还能抑制耦合束团不稳定性振荡、快速束流离子不稳定性和注入大幅度振荡等,从而将提高机器的运行性能。