Graphitization of inner surface of copper beam duct of KEKB positron ring

In order to evaluate the best technique to reduce electron cloud instability, we measured the electron cloud density in copper ducts coated with titanium nitride and non-evaporable getter used in the KEK B-factory (KEKB) positron ring. In this study, we developed a new system for the graphitization of copper beam ducts. The graphitization of a copper surface by intense electron bombardment resulted in the reduction in the secondary electron yield (SEY) to 1.0 in a laboratory experiment and to 1.1 in a real beam duct. The electron cloud density in the graphitized duct is 30% lower than that in an uncoated copper duct. Thus, it has been confirmed that the graphitization of a beam duct leads to a reduction in the electron cloud density in the duct.     

Development of alumina ceramics vacuum duct for the 3 GeV-RCS of the J-PARC project

An alumina ceramics vacuum duct has been developed for the 3 GeV-RCS of J-PARC project at JAERI. There are two types of alumina ceramics vacuum ducts needed, one being 1.5 m long with a circular cross-section for use in the quadrupole magnet, the other being 3.5 m long with a 15°, bend with a race-track cross-section for use in the dipole magnet. These ducts could be manufactured by joining several duct segments of 0.5-0.8 m in length by brazing. The alumina ceramic ducts have copper stripes on the outside surface to reduce the duct impedance. One of the ends of each stripe is connected to a titanium flange by way of a capacitor so to interrupt an eddy current circuit. The copper stripes are produced by an electroforming method in which a stripe pattern formed by Mo-Mn metallization is first sintered on the exterior surface and then overlaid by PR-electroformed copper (periodic current reversal electroforming method). In order to reduce emission of secondary electrons when protons or electrons strike the surface, TiN film is coated on the inside surface of the ducts.     

Improvement of beam lifetime and vacuum system of the PF-AR

Successive improvements have been performed on the vacuum system for the Photon Factory Advanced Ring (PF-AR) at the High Energy Accelerator Research Organization (KEK). The main purpose is to prolong electron beam lifetime for stable operation as an intense pulsed X-ray source. In the past three years, a total of 61 sputter ion pumps (SIPs) were additionally installed, and the increased effective pumping speed amounts to 13% of total. Comparison between calculated and observed beam lifetimes indicates that the lifetime is restricted mainly by the residual gas scatterings and that improvement of the vacuum will realize still longer lifetime. Sudden beam lifetime drops caused by dust trappings have been investigated for many years. The frequency of the lifetime drops has decreased as operation time elapsed after a large-scale reconstruction. Effect of distributed ion pumps (DIPs) on the lifetime drops has also been investigated experimentally.     

An in situ glow discharge cleaning method for the LEP vacuum system

The performance of the vacuum systems of present day electron storage rings is limited by synchrotron radiation induced gas desorption. A method to reduce the electron induced desorption yield (thought to be a measure of the radiation induced desorption) by glow discharge cleaning has been developed. Initial beam lifetimes for standard LEP operation (Large Electron Position Storage Ring) have been calculated based on measured electron stimulated desorption yields and two different proposed pumping systems. (One with distributed ion pumps, the other with distributed getter pumps). After bakeout, both systems would provide an initial beam lifetime of ～ 4 min. After a helium glow discharge for 3 h at 150°C (at 1.5 torr and with a discharge current of 0.7 A m^?1), a lifetime of ～6.7 h would be obtained in the ion pumped system as compared to ～1.7 h in the getter pumped version. Results obtained with Argon discharge proved to be inferior.     

Vacuum system for the 3-GeV RCS in J-PARC

The 3-GeV RCS (rapid cycling synchrotron) in the J-PARC (Japan Accelerator Research Complex) project accelerates a proton beam of 0.333 mA up to 3 GeV. At 25-Hz repetition rate, the RCS generates a high-power beam of 1 MW. In such an accelerated proton beam, neutrons as well as gamma rays are generated. The cumulative energy dose will be of the order of 100 MGy over 30 years of operation. So as to minimize the radiation exposure during maintenance, it is necessary to construct a vacuum system with reliable components which have a long life in such a high level of radiation. In addition, it is necessary to keep the operating pressure of the beam in ultra high vacuum to suppress pressure instability. Thus we should think not only of the outgassing mainly due to ion-induced desorption but also of the pumping efficiency. Based on the above, the vacuum system was designed as follows: The ring is divided by isolation valves into 6 sections (3 straight and 3 arc sections), which can be pumped down independently. To avoid any eddy current loss, ceramic ducts are used in the bending and focusing magnets. These ducts are connected to titanium ducts placing the Ti bellows between. Here, we adopt pure Ti as a material for the ducts and bellows because of its small residual radioactivity. The ring is evacuated with 20 ion pumps (0.7 m³/s) and 24 turbomolecular pumps (TMPs) (1.3 m³/s), which are attached to the Ti ducts. The TMPs are used not only for rough pumping but also for evacuation during the beam operation. Especially a collimator system for localizing beam losses in a restricted area is evacuated with the TMPs, because the outgassing from this region will probably be the greatest. In addition, the straight sections for beam injection and extraction are pumped down mainly by the TMPs. Each arc section is pumped by 4 ion pumps and 2 TMPs. To realize the above system, we developed components such as large aperture ceramic ducts and TMPs with high radioactive resistance, as well as several kinds of heat treatment to reduce the outgassing. Finally, we realized the UHV without baking in the RCS and the beam operation has been successful to date.     

Titanium flanged alumina ceramics vacuum duct with low impedance

We successfully developed alumina ceramics vacuum ducts for the 3GeV-RCS of J-PARC at JAERI. This duct has titanium flanges and exterior RF shield to reduce duct impedance. The temperature of titanium flange became 45 °C due to eddy current heating under dipole magnet operation. The longitudinal duct impedance was sufficiently small for stable beam acceleration as shown by impedance measurement by the coaxial method. It was found that the radiation damage of the capacitor used for the RF shield of this duct was small, because the capacitance decreased only by 7% after a gamma-ray irradiation of 30 MGy. This duct is usable for the 3GeV-RCS of J-PARC.     

定向凝固结合电子束制备高纯铜的研究

对定向凝固结合电子束制备5N高纯铜进行了研究。首先采用真空感应熔炼4N(99.99%)阴极铜并进行定向凝固实验,然后利用电子束对定向凝固后的铸锭进行重熔精炼,利用辉光放电质谱仪进行成分分析。实验结果表明:真空熔炼可以有效去除铜中饱和蒸气压高的杂质;通过定向凝固,杂质元素含量沿铸锭轴向逐渐升高;经过组合工艺提纯后,总杂质含量由13.6×10-6降低至4.0×10-6,铜的纯度提高至99.9996%。     

不锈钢的同步辐射光致脱附

电子储存环真空室壁及相连光束线的同步辐射光致脱附 (PSD)严重影响着束流质量和寿命。不锈钢是加速器真空室最常用的材料 ,合肥光源的真空系统即为不锈钢结构。合肥光源在B6a光出口建立了PSD光束线 ,选用长 12 1cm ,内径8 6cm的 30 4L不锈钢真空室作为首件样品 ,用于测试不锈钢不同表面状态的脱附情况 ,结果表明经辉光放电处理的表面脱附系数最低。     

Wake-field characteristics of a high-intensity, multibunched beam

A systematic experimental study on the wake-field characteristics of a high-intensity, multibunched electron beam was carried out in detail at the primary electron section of the KEK positron generator linac. The observed energy spectrum, which indicated the energy variation of one bunch from another, was explained fairly well by the multibunch effect of a longitudinal wake field; the importance of introducing a self-wake loss and a bunch-length effect into calculations of the energy loss due to a longitudinal wake field is noted. On the other hand, a transverse motion of each bunch, showing a peculiar behavior when the transverse instability occurred, was in good agreement with the results of a numerical calculation based on a multibunch version of Wilson's two-particle model. In both cases, the wake field for our cavity was estimated using a computer code called TBCI.     

Fast microwave detection system for coherent synchrotron radiation study at KEK: Accelerator test facility

A fast room temperature microwave detection system based on the Schottky Barrier-diode detector was created at the KEK ATF (Accelerator Test Facility). It was tested using Coherent Synchrotron Radiation (CSR) generated by the 1.28 GeV electron beam in the damping ring. The speed performance of the detection system was checked by observing the CSR from a multi-bunch (2.8 ns bunch separation time) beam. The theoretical estimations of CSR power yield from an edge of bending magnet as well as new injection tuning method are presented. A very high sensitivity of CSR power yield to the longitudinal electron distribution in a bunch is discussed.     

合肥光源真空控制系统

合肥光源（HLS）是第二代专用同步辐射光源，由200MeV直线加速器、输运线和800MeV电子储存环组成。真空控制系统是基于EPICS（Experimental Physicsand Industrial Control System）的分布式控制系统，3台10C分别控制储存环上的7台Varian真空规控制器、直线加速器上的15台溅射离子泵电源控制器和输运线上16台溅射离子泵的电源控制器。采用Channel Arciver实现了储存环真空度及直线输运线溅射离子泵离子流数据的采集、存档及检索的功能。利用开发的CGI程序，改善了数据检索的性能，并对合肥光源储存环真空管道的束流清洗过程进行了分析。     

铜与奥氏体不锈钢异种材料电子束焊接

目的研究不同参数下铜钢电子束异种焊接以获得符合要求的接头质量。方法以无氧高导热铜(OFHC)和奥氏体不锈钢(304)作为研究对象,控制扫描幅值、焊接速度等工艺参数,采用500 Hz真空电子束偏束"O"形扫描焊接的方式进行焊接。结果在电子束偏钢侧0.2 mm,电子束流为17 mA,聚焦电流为501 mA,焊接速度为600 mm/min的参数下,添加半径为1 mm,频率为500 Hz的圆形扫描波得到了抗拉性能为310.9MPa、硬度大于180 MPa的优质焊接接头。结论不同参数下的接头宏观均出现焊缝上表面下陷缺陷,接头铜侧热影响区存在大量颗粒状、块状、条状的铜钢固溶体析出相,接头钢侧热影响区存在宽度随扫描波幅值减小而减小的黑色过渡带。     

Vacuum stability and residual gas density estimation for the vacuum chamber upgrade of the ATLAS interaction region of the Large Hadron Collider

The CERN Large Hadron Collider (LHC) has 54 km of ultra-high vacuum (UHV) beam chambers out of which about 90% are at cryogenic temperature (1.9 K) and the rest at room temperature. During operation, the residual gas density in the beam pipes is dominated by beam induced effect such ion, electron and photon-stimulated gas desorption. Therefore, the computation of gas density profile is of great importance to confirm the vacuum stability, and to estimate the beam lifetime. Moreover, the gas density profiles are essential to determine the machine induced background in the experimental areas, and to define the pressure profile in the cryogenic sectors where there is no vacuum instrumentation available.In this paper, the vacuum stability is studied for a newly proposed upgrade of the vacuum chamber at the ATLAS interaction point, using the vacuum stability code called VASCO. The residual gas density profile along the ATLAS vacuum chambers and the effects of photon and electron flux hitting the vacuum chamber walls are presented and analysed.     

Scintillation ring hodoscope with WLS fiber readout

A simple tracking system for measuring the K⁺ decay vertex along the beam axis in the T-Violation Experiment (E246) at KEK is described. It consists of 32 grooved BC408 plastic scintillator rings with embedded Y11 wavelength-shifting (WLS) optical fibers. Both ends of each multi-clad WLS fiber are directly viewed by a green-extended photomultiplier. Light yield of 46 photoelectrons per minimum ionizing particle in the 5 mm thick ring has been achieved. This configuration gives 1.7 mm position resolution determined purely by the 6 mm width of each ring. Time resolution of 1.0 ns and detection efficiency of 98.4% were obtained.     

真空电子束焊接技术应用研究现状

简要概括了真空电子束焊接技术在不同材料连接方面的应用现状及研究发展动态,包括铝及其合金、钢铁材料、铜及铜合金、钛及钛合金、难熔金属钨/钽/铌/钼及其合金、金属间化合物及复合材料电子束焊接的发展现状。针对电子束焊接技术,简述了国内外学者已取得的部分研究成果,包括工艺试验、组织分析、数值模拟和力学性能等;分析了目前电子束焊接技术在材料连接方面还存在的问题,并展望了电子束焊接技术应向高温新型结构材料、异种材料、功能复合材料等方向发展,丰富了连接过程中的理论基础,揭示了工艺与组织及性能的对应关系,扩展了电子束焊接技术的应用领域。     

Scattering of electrons on thermal radiation photons in electron-positron storage rings

It is shown that Compton scattering on thermal radiation photons restricts the lifetime of high energy electron beams in storage rings to a level of 30 h. At a vacuum pressure of 10⁻¹⁰ Torr the probability that an electron is knocked out from the beam due to scattering on thermal photons exceeds the probability of bremsstrahlung on residual gas ( ) by one order of magnitude, i.e. this effect can cause a considerable background in detectors.     

Measurement of particle polarization in e+e− storage rings by means of synchrotron radiation scattering on the colliding beam

A new method for measuring the polarization of e⁺e^? in storage rings is described. It is based on the scattering of synchrotron radiation on the colliding beam. The method is simple in operation and allows a simultaneous measurement of the polarization of the electron and positron beam. This method was applied to the energy calibration of the storage ring VEPP-4 in the high precision measurements of the ?-mesons' masses with MD-1 detector.     

Molecular pumping properties of the LHC arc beam pipe and effective secondary electron emission from Cu surface with artificial roughness

A simplified 2D method of angular coefficients is applied to calculations of the molecular pumping properties of complex vacuum systems. An optimization of geometry for the cold LHC beam vacuum chamber with electron shields is performed.An additional interesting application of the angular coefficients method is the estimation of the effective secondary electron emission from surface with artificial roughness. This method allows to take into account re-reflection of electrons using experimental data of the secondary electron energy distributions and surface reflectivity. The suppression efficiency of the secondary electron emission from Cu as a function of roughness parameter is presented. This result is a good input for designing future accelerators and storage rings with potential electron-cloud problems.     

声学黑洞吸声体结构吸声性能的实验研究

通过在圆管内部布置圆环,使得管道的有效导纳沿轴向逐渐增大,从而实现管道的声学黑洞效应,即声波传播时声速逐渐减小至 0,不发生反射的现象,进而使声学黑洞吸声体成为宽带吸声结构。首先对声学黑洞吸声体的吸声性能进行理论分析,随后设计了不同尺寸的声学黑洞吸声体结构,通过实验研究声学黑洞吸声体物理参数对吸声性能的影响。实验结果表明,当圆环数量大于 20 时,声学黑洞吸声体结构能实现宽带吸声。当圆环数量相同时,有效吸声频带随着管道长度的增大向低频移动;当管道长度相同时,吸声性能随着圆环数量的增加而提高。     

Substrate-diffusion-controlled film growth: Silver and copper on lead (111)

Silver and copper were vapour deposited onto atomically clean lead (111) surfaces under ultrahigh vacuum conditions. Low energy electron diffraction, reflection high energy electron diffraction and Auger electron spectroscopy measurements in the same chamber showed that, for a 0.1 nm min^-1 impingement of silver or copper, the deposit completely re-evaporated. At 10 nm min^-1 both silver and copper condensed onto a lead (111) surface at room temperature. The silver deposit was a continuous-layer single crystal which had an atomically flat upper surface covered by an equilibrium monolayer of substrate atoms epitaxed to it. The results for the copper experiments were similar to those for silver, except for the presence of small amounts of misoriented copper (111) and polycrystalline copper.It is proposed that substrate diffusion during the condensation of silver or copper onto lead (111) is crucial in determining the mode of film growth. It is thought that two-dimensional nucleation results from a mixed mobile layer of adsorbate and substrate atoms.