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.     

Ti-Zr-V non-evaporable getter films: From development to large scale production for the Large Hadron Collider

Non-evaporable getter (NEG) alloys after dissolution of their native oxide layer into the bulk are able to pump most of the gases present in ultra-high vacuum systems. The dissolution process, commonly called activation, is obtained by heating in vacuum. NEG materials can be sputter-deposited as a thin film on the inner wall of a vacuum chamber, transforming it from a source of gas into an effective pump. The most significant advance in the development of NEG films was the discovery of a very low activation temperature (180 °C for 24 h heating) in a large range of compositions of the Ti-Zr-V system. This favourable property was correlated with nanometric grain size of the film (about 3 to 5 nm).In addition to pumping, NEG films lead to reduced induced gas desorption and secondary electron yields. As a consequence, Ti-Zr-V films provide the optimum solution to most of the problems encountered in vacuum systems of modern particle accelerators for high energy physics and for synchrotron radiation facilities. In the near future the most significant benchmark for Ti-Zr-V films will be the Large Hadron Collider (LHC) presently under construction at CERN, where about 6 km of beam pipe are being coated. A dedicated magnetron sputtering facility has been built to cope with the high number of vacuum chambers (about 1200) and the tight production schedule.     

Electron stimulated desorption from stainless steel at temperatures between −15 and +70 °C

The electron stimulated desorption (ESD) yield from 316LN stainless steel was studied as a function of surface temperature in the range from −15 to +70 °C. The ESD yields as a function of electron dose for three samples kept at −5, +20 and +70 °C were found to be very similar. After long-term electron bombardment at fixed temperature, the ESD temperature dependence was also measured for each sample at temperatures between −15 and +70 °C. The results show that desorption yields increase with temperature, this dependence is small for H₂ and increases with atomic mass number reaching the maximum difference of factor 3 for CO₂. The main conclusion is that the vacuum chamber temperature in the range between −15 and +70 °C is not critical in most cases for accelerator vacuum system design. The change of ESD yield with temperature is relatively small compared to desorption yield uncertainties and the significant reduction observed with an accumulated electron dose.     

NSRL电子储存环超高真空系统残余气体分析

在同步辐射电子储存环中,由于光电解吸的作用,将解吸大量的气体,而储存的束流电子与气体分子的碰撞将引起电子的用射,导致束流寿命的减小,因此残余气体分成为衡量一个电子储存环真空系统性能的重要的指标,本文将介绍有关ＮＳＲＬ储存环真空系统残气分析的部分工作。     

Studies of photon induced gas desorption using synchrotron radiation

In view of finalizing the design of the vacuum system of the Large Electron and Positron Storage Ring (LEP) we have studied synchrotron radiation induced neutral gas desorption. A 3 m section of an aluminum vacuum chamber has been exposed to the photon beam emerging from the electron storage ring DCI in Orsay, under conditions closely simulating the environment in a particle acceletor. In order of importance the gases desorbed were H₂, CO₂, CO and CH₄ with H₂O practically absent. Under the experimental conditions of an unbaked chamber and 11 mrad glancing incidence of the photons, the initial molecular desorption yields for these gases were typically 0.5, 8 × 10^?2, 2 × 10^?2 and 8 × 10^?3 molecules per photon respectively. These values could be reduced by about 1 to 2 orders of magnitude during continued photon exposure and most cases without evidence that this ‘beam cleaning action’ would be limited. After exposure to air and pumpdown of the previously cleaned chamber, we observe a significant memory effect. The dependence of the photon desorption on the angle of incidence has been studied down to a glancing angle of 11 mrad showing a definite deviation from the previously assumed 1/sin ø scaling. The implications of the results in terms of the expected beam-gas lifetime in LEP are discussed.     

重离子激发解吸测试装置设计与调试

束流与真空管壁碰撞后从腔体表面解吸出一定数量的分子和离子, 解吸出的粒子使加速器系统的真空度下降, 进而会影响束流的寿命。在重离子加速器中, 为了降低动态真空对束流的影响, 需找出一种低解吸率材料作为准直器镀膜材料。因此, 不同材料的解吸率测量已成为重离子加速器真空系统设计的关键所在。本文首先介绍了基于中科院近代物理研究所320 k V高电荷态离子综合研究实验平台设计的解吸率测量装置, 并利用Molflow+真空分析软件计算了该测试装置的静态真空分布。其次以无氧铜为测试靶材进行了解吸率实验研究, 得出了束流轰击靶材时动态真空变化的趋势, 同时计算了无氧铜在不同能量和不同流强Xe¹⁰⁺和O⁺束流作用下的解吸率和相对应的电子能损。结果表明粒子解吸引起的动态真空变化较明显, 无氧铜解吸率随着入射束流能量的增加而增加, 而且在长时间束流作用时会出现束流清洗效应, 即解吸出的粒子在束流长时间轰击下逐渐减少。电子能损的平方和解吸率成正比例关系, 与相关文献的结论一致, 说明实验装置运行稳定, 获得的数据具有一定的参考意义。     

Residual gas analysis in the TOF vacuum system

The residual gas composition in the time-of-flight (TOF) spectrometer vacuum system has been measured with a quadrupole mass spectrometer. All residual gases except hydrogen and helium are condensed and freezed on the windows of the liquid hydrogen target. As a result it increases the background during the reaction between the cooler synchrotron (COSY) beam and the target. These condensates have to be cleaned from the target windows by fast heating the target cell from 16 K up to room temperature. The partial pressure spectrums of the condensed gases on the liquid hydrogen target are also measured. The residual gas analysis shows that the majority of the condensates on the target windows are nitrogen, oxygen and water vapor. The target area has to be in a high vacuum <1×10⁻⁶ mbar in order to minimize the condensate. The target windows have to be cleaned with the fast heating cycle every 48 h.     

Ar beam induced desorption from different materials at TSL

This paper describes new experiments on the heavy ion desorption yield measurements with 5 MeV/u Ar⁸⁺ and summarizes all results of experiments with 5 MeV/u Ar⁸⁺ performed at The Svedberg Laboratory in Uppsala (Sweden). These results are important for the update and design of the FAIR facility at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt (Germany) where the required increase in beam intensity is limited by ion induced pressure instability.It was shown that lowest desorption yields can be achieved with gold coatings, whereas grazing incident loss increases the desorption yield by roughly an order of magnitude compared to perpendicular loss. The desorption yield of saturated NEG samples was measured to be higher compared to any non pumping samples. The desorption yield of copper can be lower and higher compared to stainless steel depending on cleaning procedure and sample history.Additionally the secondary electron and ion yield was measured to be a few tens of electrons and ions emitted per projectile impact in backward direction. Their influence on the desorption yield due to secondary effects was less than 5% compared to the primary desorption by the high energetic projectile.     

Electron stimulated desorption from aluminium alloy and aluminium coated stainless steel

The electron stimulated desorption (ESD) yield was measured at the internal surface of tubular samples made of aluminium alloy and aluminium coated stainless steel. The ESD yields were measured as a function of electron accumulated dose to a maximum of 3 × 10²² e⁻/m² at electron energy of 500 eV, then as a function of electron energy between 40 eV and 5 keV. The ESD yields as a function of electron energy were linear for all gasses except CH₄ and CO₂.     

Nanoscale effects on heterojunction electron gases in GaN/AlGaN core/shell nanowires

The electronic properties of heterojunction electron gases formed in GaN/AlGaN core/shell nanowires with hexagonal and triangular cross sections are studied theoretically. We show that at nanoscale dimensions, the nonpolar hexagonal system exhibits degenerate quasi-one-dimensional electron gases at the hexagon corners, which transition to a core-centered electron gas at lower doping. In contrast, polar triangular core/shell nanowires show either a nondegenerate electron gas on the polar face or a single quasi-one-dimensional electron gas at the corner opposite the polar face, depending on the termination of the polar face. More generally, our results indicate that electron gases in closed nanoscale systems are qualitatively different from their bulk counterparts.     

Application of low temperature desorption in systems for adsorptive purification of cryogenic gases

The application of low temperature desorption and reactivation of the adsorbent in adsorptive systems for the purification of cryogenic gases from gas impurities is considered. The degree of purification obtained using low temperature desorption is evaluated. Experiments for the determination of the rate and the degree of desorption of nitrogen and hydrogen on charcoal at 77.4 K were carried out. Practical schemes for purification of gases using low temperature desorption of the main part of the adsorbed substances are proposed.     

Experimental and computational studies on propane-propylene separation by adsorption and variable-temperature stepwise desorption

An experimental adsorption and desorption study was carried out using propane-propylene gas mixtures adsorbed onto 13X molecular sieves in order to obtain a better understanding of a variable-temperature stepwise desorption (VTSD) method that has been found to have potential for industrial separations. The experimental conditions used were based on estimates of possible industrial importance. Both desorption rates and desorption equilibrium data were obtained at one atmosphere with no dilution gases, in small U-tube columns (0.012 m diameter). The desorption rates appeared to be relatively rapid and not an important factor in pilot-scale desorption in comparison with heat transfer rates. A computational method has been developed in order to calculate the composition and amounts of gases that are desorbed during variable-temperature stepwise desorption. The computed predictions ofa pilot-scale system, based on equilibrium data from small scale column experiments, were compared with pilot-scale (0.04 m diameter × 1.8 m length) experimental results and were found to provide reasonable approximations.     

金属氧化物半导体气敏传感器稳定性研究进展

金属氧化物半导体(MOS)气敏传感器的稳定性是气敏器件实用化进程中最具有挑战性的因素。影响器件稳定性的主要因素包括颗粒尺寸、颈部宽度、微裂纹、电极、湿度和温度的变化。颗粒的长大和颗粒间颈部尺寸的变化降低了耗尽层对总电阻变化的贡献;微裂纹的加剧使器件的电阻发生漂移,并且为水蒸气、氧气和待测气体扩散到敏感膜内部提供便捷的通道;电极的退化影响电极与气敏材料之间的接触电阻Rc;温度和湿度的改变使气体的吸附、脱附、反应活性和电子迁移率等都发生变化,因而器件的稳定性得不到保证。在机理分析的基础上,分别介绍了提高器件稳定性的方法。     

TiZrV吸气剂薄膜的性能研究

采用独立设计组装的磁控溅射装置完成了对不锈钢管道的镀TiZrV薄膜处理。对TiZrV薄膜的相关性能包括二次电子产额(SEY)、光致解吸(PSD)产额和吸气性能进行了研究。在200℃下加热2h后TiZrV的SEY有所下降,峰值由2.03降到1.55。不锈钢真空室在镀TiZrV薄膜处理后其PSD效应显著降低,在200℃下加热24h后,各气体的PSD产额与初始值相比可降低两个量级左右。TiZrV薄膜对CO和H2有较好的吸气效果,相同激活条件下对CO的抽速比H2高一个量级,吸气容量则较之低两个量级;另外,随着加热温度的提高和时间的延长,其吸气能力会有所提高。     

Preparation and magnetic properties of spindle porous iron nanoparticles

Spindle porous iron nanoparticles were firstly synthesized by reducing the pre-synthesized hematite (α-Fe₂O₃) spindle particles with hydrogen gas. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption/desorption isotherms and vibrating sample magnetometry (VSM). A lattice shrinkage mechanism was employed to explain the formation process of the porous structure, and the adsorbed phosphate was proposed as a protective shell in the reduction process. N₂ adsorption/desorption result showed a Brunauer-Emmett-Teller (BET) surface area of 29.7 m²/g and a continuous pore size distribution from 2 nm to 100 nm. The magnetic hysteresis loop of the synthesized iron particles showed a saturation magnetization of 84.65 emu/g and a coercivity of 442.36 Oe at room temperature.     

Study of the co-pyrolysis of biomass and plastic wastes

This work aimed to study the recovery of two types of waste by the process of pyrolysis. The obtained results show that the adding of a plastic mix improves the overall efficiency of the slow pyrolysis of pine. Therefore, it was possible to achieve higher liquid yields and less solid product than in the classic slow pyrolysis carbonization of biomass. The obtained liquids showed heating values similar to that of heating fuel oil. The gas products had energetic contents superior to that of producer gas, and the obtained solid fractions showed heating values higher than some coals. There were also identified some typical products of fast biomass pyrolysis used as raw material in several industries. The effects of experimental conditions in product yield and composition were also studied. The parameters that showed higher influence were (with its increase): reaction time on gas product composition (increase of the alkane content) and on liquid composition (increase in aromatics content); reaction temperature on product yield (decrease of liquid yield with increase of solids and gases) and on gas product composition (increase in alkane content); initial pressure on liquid composition (increase in the aromatics content) and mainly the pine content of the initial mixture on products yield (increase of gas and solid yield with a decrease in liquids) and on the gas product composition (favouring CO and CO₂ formation). An erratum to this article can be found at     

Calculation of the pressure in the unit-cell vacuum system of Taiwan Photon Source

Taiwan Photon Source is a third-generation 3-GeV synchrotron light source under construction. The electron storage ring of circumference 518.4 m contains 24 unit cells and 24 long straight sections. Each cell of length about 14 m contains two bending chambers and two short straight chambers. To estimate the pressure distribution in the cell vacuum system, a program combining an iterative method and a Monte-Carlo method was used to calculate the pressure. In the modeling, the rate of thermal outgassing of the chamber surfaces and the yield of photon-stimulated desorption of the absorbers are obtained from the experimental results. To provide the effective pumping speed of various pumps that depends on the gas, the composition of residual gases from photon-stimulated desorption was assumed to be 80% H₂ and 20% CO. The pressure calculation for the vacuum cell compares the beam cleaning efficiency during the early commissioning stage when the accumulated beam dose attained 1 A h and 100 A h. The effects of the confined pumping design are evaluated through the modeling to assess whether the commissioning can be accelerated. The result of the pressure calculation obtained with the Monte-Carlo method shows that a mean pressure rise per beam current at 1.9 × 10⁻¹⁰ Pa mA⁻¹ after beam cleaning to a beam dose 100 A h is reasonable and within typical specifications of the synchrotron light source.     

Experimental study of the influences of degraded vacuum on multilayer insulation blankets

The paper presented experimental investigation on the heat transfer of MLI with different rarefied gases at different pressures. The investigations were carried out using an innovative static liquid nitrogen boil-off rate measurement system in the case of the small temperature perturbations of cold and warm boundaries. The heat fluxes for a number of inert and some polyatomic gases have been analyzed at different heat transfer conditions ranging from molecular to continuum regime, apparent thermal conductivities of the multilayer insulation were measured over a wide range of temperature (77 K–300 K) and pressure (10⁻³–10⁵ Pa) using the apparatus. The experimental results indicated that under degraded vacuum condition, the influences of rarefied gas on the MLI thermal performance very depend on the gas rarefaction degree which impacted by the MLI vacuum degree. Under the condition of molecular regime heat transfer, the MLI thermal performance was greatly influenced by gas energy accommodation coefficients (EAC), when under the continuum regime, the performances depend on the thermal conductivity of rarefied gas itself. Compared to the results of N₂, Ar, CO₂, Air and He as interstitial gases in the MLI, Ar was the better selection as space gas because of its low EAC and thermal conductivity characteristics on the different vacuum condition ranging from high pressure to vacuum. So different residual gases can be utilized according to the vacuum level and gas energy accommodation coefficient, in order to improve the insulation performance of low vacuum MLI.     

Electrical and gas sensing properties of self-aligned copper-doped zinc oxide nanoparticles

Electrical and gas sensing properties of nanocrystalline ZnO:Cu, having Cu X wt% (X = 0.0, 0.5, 1.0, and 1.5) in ZnO, in the form of pellet were investigated. Copper chloride and zinc acetate were used as precursors along with oxalic acid as a precipitating reagent in methanol. Material characterization was done by X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and inductive coupled plasma with optical emission spectrometry (ICP-OES). FE-SEM showed the self-aligned Cu-doped ZnO nano-clusters with particles in the range of 40-45 nm. The doping of 0.5% of copper changes the electrical conductivity by an order of magnitude whereas the temperature coefficient of resistance (TCR) reduces with increase in copper wt% in ZnO. The material has shown an excellent sensitivity for the H₂, LPG and CO gases with limited temperature selectivity through the optimized operating temperature of 130, 190 and 220 °C for H₂, LPG and CO gases, respectively at 625 ppm gas concentration. The %SF was observed to be 1460 for H₂ at 1% Cu doping whereas the 0.5% Cu doping offered %SF of 950 and 520 for CO and LPG, respectively. The response and recovery time was found to be 6 to 8 s and 16 s, respectively.     

Effect of coating morphology on the electron stimulated desorption from Ti-Zr-Hf-V nonevaporable-getter-coated stainless steel

The electron stimulated desorption (ESD) was studied for quaternary Ti-Zr-Hf-V alloy coated stainless steel samples with different surface structures: dense on one sample and columnar on another. The ESD yields were measured as a function of electron accumulated dose up to ∼10²³ e⁻/m² or greater and three different NEG coating activation temperatures: 150, 180 and 250 °C. After each ESD experiment the samples were saturated with a mixture of H₂, CO and CO₂. Both samples depicted lower ESD yields for all desorbed species compared to a ternary Ti-Zr-V alloy. It was also shown that although the columnar NEG coating demonstrated better pumping properties and, for NEG activated at 150 °C, lower initial ESD yields, the higher activation temperature may result in a significant H₂ yield increase with dose for the columnar NEG coating. This effect was demonstrated for the first time and should be considered for application in particle accelerators.