Cold leak tests of LHC beam screens

In order to guide the high energy proton beams inside its two 27 km long vacuum rings, the Large Hadron Collider (LHC) at CERN, Geneva, makes use of superconducting technology to create the required magnetic fields. More than 4000 beam screens, cooled at 7-20 K, are inserted inside the 1.9 K beam vacuum tubes to intercept beam-induced heat loads and to provide dynamic vacuum stability. As extremely high helium leak tightness is required, all beam screens have been leak tested under cold conditions in a dedicated test stand prior to their installation. After describing the beam screen design and its functions, this report focuses on the cold leak test sequence and discusses the results.     

LHC: The world's largest vacuum systems being operated at CERN

With the successful circulation of beams in the Large Hadron Collider (LHC), its vacuum system becomes the world's largest vacuum system under operation. This system is composed of 54 km of ultra high vacuum (UHV) for the two circulating beams and about 50 km of insulation vacuum around the cryogenic magnets and the liquid helium transfer lines (QRL). The LHC complex is completed by 7 km of high vacuum transfer lines for the injection of beams from the SPS and their dumping.Over the 54 km of UHV beam vacuum, 48 km are at cryogenic temperature (1.9 K), the remaining 6 km are at ambient temperature and use extensively non-evaporable getter (NEG) coatings, a technology that was born and industrialised at CERN.The cryogenic insulation vacuum systems, less demanding technically, impress by their size and volume: 50 km and 15,000 m³. Once cooled at 1.9 K, the cryopumping allows pressure in the 10⁻⁴ Pa range to be attained.     

Electron beam testing

Electron-beam testing (EBT) is currently used at two stages in the development and manufacture of integrated circuits (ICs), namely for chip verification and failure analysis. These are explained with the aid of two examples. EBT is, however, only one of many diagnostic tools. In order to classify it more usefully, a scenario is described that takes account of trends in chip development and packaging, and the linking of EBT with CAD (computer-aided design) and CAT (computer-aided testing). The conclusion is reached that EBT will remain an indispensable diagnostic tool. In future, it will probably be used under increasingly more difficult conditions, however, due primarily to the greater complexity of specimen preparation and test stimuli generation.     

电子束焊接束流诊断用法拉第筒传感器研制

针对中高压电子束焊机5~100mA的电子束流诊断,完成了法拉第筒传感器的研制。该传感器由法拉第筒、微弱信号放大电路、增益控制电路构成。法拉第筒电极入射孔采用双孔结构,有效的阻止了二次电子和背散射电子的逸出,提高了电子束流的收集效率。通过控制微弱信号放大电路的增益将整个束流强度范围内的信号放大到伏级。在Ua=150 kV,Ib=10mA,If=360mA,H=315 mm条件下采集功率密度分布数据,得到束流密度的二维分布图形。由于受电子束焊机特性的影响,束流功率密度分布呈非对称性的近高斯分布。     

Electron beam weld parameters and thermal efficiency improvement

Thermal efficiency is considered in connection with welding regimes and seam parameters by applying a statistical approach. This approach allows one to establish empirically (by fitting a mathematical model) the type of relationship that is present between performance characteristics and its influencing factors. Optimal regimes are found through thermal efficiency optimisation. The study leads to new proposals for the position of the focus with respect to the surface of the welded material, under conditions of maximum thermal efficiency or maximum welding depth. The values of the ratio of power to weld depth (P/H) and weld width times velocity of weld front (vB) are confirmed to be the main characteristic parameters of electron beam welding, considered as a self-organising process.     

Electron beam deposition system for X-ray multilayer mirrors

We have developed a multi source electron beam evaporation system toprepare high quality X-ray multilayer mirrors. The system has three electron guns mounted in an ultra high vacuum chamber. The deposition system is evacuated by a turbo molecular pump and two sputter ion pumps. A movable masking system is mounted to deposit several kinds of multilayers. First results on niobium-carbon X-ray multilayers are presented.     

First results of the LHC longitudinal density monitor

The Large Hadron Collider (LHC) at CERN is the world's largest particle accelerator. It is designed to accelerate and collide protons or heavy ions up to the center-of-mass energies of 14 TeV.Knowledge of the longitudinal distribution of particles is important for various aspects of accelerator operation, in particular to check the injection quality and to measure the proportion of charge outside the nominally filled bunches during the physics periods. In order to study this so-called ghost charge at levels very much smaller than the main bunches, a longitudinal profile measurement with a very high dynamic range is needed.A new detector, the LHC Longitudinal Density Monitor (LDM) is a single-photon counting system measuring synchrotron light by means of an avalanche photodiode detector. The unprecedented energies reached in the LHC allow synchrotron light diagnostics to be used with both protons and heavy ions.A prototype was installed during the 2010 LHC run and was able to longitudinally profile the whole ring with a resolution close to the target of 50 ps. On-line correction for the effects of the detector deadtime, pile-up and afterpulsing allow a dynamic range of 10⁵ to be achieved.First measurements with the LDM are presented here along with an analysis of its performance and an outlook for future upgrades.     

Optimized annular triode ion pump for experimental areas in the LHC

The large hadron collider will be the world next generation accelerator to be operational in 2007 at CERN. The UHV requirements force the installation of ion pumps in the experimental areas of ATLAS. Due to the unacceptable particle background that standards ion pumps may generate, a reduction in the amount of material constitutive of the pump body is required. Hence, a stainless steel 0.8 mm thick body annular triode ion pump has been designed. A pumping speed of ∼20 l/s at 10⁻⁹ mbar is provided by 15 pumping elements. Finite elements analysis and destructive tests have been performed in its design. Final vacuum tests results are shown.     

Effect of filament heating current on electron beam of directly heated strip-type electron gun

The method of Electron Beam-Physical Vapour Deposition (EB-PVD) for handling large substrates is well established in the metallurgical industry, where sweeping axisymmetric electron guns or multiple pencil guns are routinely used to cover a large target area. The non-uniformity in current density in those methods can be overcome, to a large extent, by using the strip-type electron gun. In this paper, we propose to use an AC-heated strip-type electron gun to cover large target areas. The magnetic field generated by the alternating filament current oscillates the beam in a direction parallel to the filament length, thereby eliminating the need for applying an external electric or magnetic field for sweeping the beam. The non-uniformity in the current density within the strip electron beam, arising due to finite length of the filament, is reduced by the use of dummy filaments on both ends of the active filament. These results are supported by electron trajectory simulations.     

A new generation of power supplies for electron beam welding machines

Widely used electron beam welding machines are equipped with heavy power supplies, located in a special oil tank and connected with an electron gun to a high-voltage cable. A special system detects electric discharges in the electron gun space, which may arise during welding and then it tries to switch off the high voltage to interrupt an electric arc. Such disadvantages have been eliminated with the novel power supply described here. The Q of resonance circuit of this supply is stabilized and as a result circulating power appears. During an electric discharge in the gun, power is not sent to the electron gun but circulates between the electronic parts of the resonance circuit without losses and “waits” for the break in the short circuit. The power supply is much smaller and lighter than supplies of similar rating used nowadays. It is connected directly to the electron gun chamber without a high-voltage cable. The first construction of the supply was designed for electron beam welding machine of 5 kW power and 60 kV accelerating voltage. Tests of the new power supply in laboratory and industrial conditions have shown its usefulness for electron beam welding.     

高能电子束电磁透镜的研究进展

在电子束加工设备中,聚焦系统和偏转系统是决定电子枪性能的关键因素。介绍了电子束聚焦、偏转技术的原理和特点,归纳整理了电磁透镜结构的优化结果、电子束聚焦规律以及减小束斑像差控制等方面所取得的研究进展。分析了目前电磁透镜控制电子束加工中存在的不足,并对未来电子枪发展趋势做出了展望。     

Control of the electron beam active zone position in electron beam welding processes

It is known that charged particles emitted from the region of electron beam (EB) interaction with the material being processed, are an important source of information for the understanding of EB welding processes. Measurements for the three largest groups of charged particles, namely, backscattered electrons, true secondary electrons and ions are presented here. It was estimated that only the signals of the direct component amplitude of these particles’ currents, processed by neural networks, could be used to effectively control the EB welding process. Computer simulations of various models of neural networks are described. The best result was obtained for a network that determines an optimal value of focusing current for the weld being made, based on the amplitude of signals measured with a moderately defocused EB.     

Advanced beam model for fiber-bridging in unidirectional composite double-cantilever beam specimens

This work investigates the interlaminar fracture and fiber-bridging in double-cantilever beam specimens from the theoretical and experimental points of view. Crack initiation and propagation tests were performed on unidirectional E-glass/polyester double-cantilever beam specimens. The well-known classical beam theory-based solution agrees excellently with the experimental results in the case of crack initiation tests. In contrast, the classical model seems to be inadequate for the evaluation of the propagation test data. The apparent contradiction was attributed to the fiber-bridging phenomenon. Thus, a novel beam model was developed, which accounts for the effect of fiber-bridging. Based on the solution of beam theory, the number of the bridging fibers and the bridging force can be approximated. The former reaches a peak value and decreases notably, while the force tends to a plateau value as the crack grows.     

Electron beam surface treatment. Part II: microstructure evolution of stainless steel and aluminum alloy during electron beam rapid solidification

Surface rapid solidification microstructures of AISI 321 austenitic stainless steel and 2024 aluminum alloy have been investigated by electron beam remelting process and optical microscopy observation. It is indicated that the morphologies of the melted layer of both stainless steel and aluminum alloy change dramatically compared to the original materials. Also, the microstructures were greatly refined after the electron beam irradiation.     

Electron beam lithography developed resist profile improved by quality analysis

A numerical experiment of electron beam lithography (EBL) using the computer simulation tool SELID is presented and analysed. A layout of parallel lines with 0.3 μm width, situated 0.4 μm from each other, is obtained through exposure and development of the positive resist PMMA. Through a model-based approach an analysis of the influence of the exposure dose, the acceleration voltage and the resist thickness on: (i) the width of the developed resist profile, measured at the interface between the resist and the substrate, (ii) the measured width at a height of 5% of the initial resist thickness from the substrate, (iii) the average sidewall angle of the developed resist trench edge and (iv) the thickness loss of the developed resist in the non-irradiated areas is made. Concrete conclusions concerning the role of these factors are drawn. Optimisation is done using the quality criteria of obtaining developed resist profiles with parallel sidewalls. In order to find the parameter areas where the deviation from the target values of the performance characteristics above will be minimal in mass production two models, for the mean value and the variance, are proposed and estimated for tolerance limits of the factors of about 5%. The proposed approaches can be applied also with other simulation models, parameters and resists, as well as with real experimental EBL data.     

Direct evidence for electron beam irradiation-induced phenomena in nanowired ZnO thin films

In-situ electron beam induced microstructural transformation experiments, leading to porosity in nanowires of ZnO, have been performed under a TEM operated at an electron accelerating voltage of 200 kV. For this purpose, nanowired (diameter: 20 to 80 nm) films of ZnO with thickness ~ 100 to 120 nm, were grown via metal-catalyst free-vapor phase mechanism. The evolved porosity (pore size about 2 to 20 nm) in nanowires, under electron beam irradiation, has been attributed to different bond-breaking phenomena at molecular Zn-O. Such nanoporous objects of ZnO are beneficial for various optical and sensing devices.     

大尺寸电子束引出窗

详细介绍了一种新型平面压封结构的大尺寸电子束引出窗口的研究和设计,并给出了金属膜窗的基本参数的设计方法和计算公式,为大尺寸电子束引出窗的设计提供了依据,可作为同类膜窗设计的参考。     

Electron beam welding of titanium aluminides

This paper gives an account of research which has been carried out on electron‐beam welded specimens made of high‐Nb γ/α₂‐titanium aluminides (in short: TiAl). The high niobium content of this alloy (5–8 %) has a positive effect on the strength and the creep resistance and also on the oxidation resistance. The standard welding parameters weld speed and beam current and also additional beam oscillation have been used for the determination of factors which will effect the solidification morphology and structural constitution since those, in return, exert influence on the high cracking susceptibility of the alloy. In order to reduce the critical cooling rate, preheating which has a positive effect on crack formation has, in addition, been applied. In a direct comparison of both alloys, micrographs and electron beam micro‐analyses of the developing weld structure and of the HAZ the effects of the beam parameters on both alloys have been established.     

Electron beam surface treatment. Part I: surface hardening of AISI D3 tool steel

The effects of electron beam surface hardening treatment on the microstructure and hardness of AISI D3 tool steel have been investigated in this paper. The results showed that the microstructure of the hardened layer consisted of martensite, a dispersion of fine carbides and retained austensite while the transition area mainly consisted of tempered sorbite. Also, the microhardness of the hardened layer on the surface increased dramatically compared to that of base material. Finally, the hardening response of AISI D3 tool steel to electron beam surface treatment is closely related to the scanning speed of the electron beam.     

基于DIABEAM法的电子束能量密度测试分析

为了充分发挥电子束焊接技术在航空航天领域的作用,对反映束流品质的关键因素—电子束能量密度开展了研究;基于法拉第传感原理,通过DIABEAM测试法对真空电子束焊机电子束能量密度分布进行测试分析;结果表明:电子束能量密度分布呈非对称的近高斯分布,随着聚焦电流的增加,电子束能量密度趋于发散分布,达到焦点状态时dP90区域内电子束能量密度均值最高;电子束能量密度峰值随着灯丝加热电流增加而增加,但受灯丝尺寸等因素的影响,灯丝加热电流达到稳定值后能量密度分布不变。