Measuring volume ratios of vacuum vessels using non-evaporable getters

Several methods have been used in vacuum metrology for volume determination. Volume ratios can be determined by different gas expansion methods, which were primarily developed for the precise determination of expansion ratios in static expansion systems. Measurements of the volume ratios using the gas expansion methods below 10⁻² Pa are influenced by outgassing from chamber walls. To reduce outgassing in the expansion chambers during measurements at low pressures, we have installed non-evaporable getter (NEG) pumps, which pump hydrogen and other active gases but have a negligible pumping speed for inert gases. The volume ratio of two chambers of an experimental static expansion system has thus been measured using the inert gases argon, helium and krypton. The obtained results were compared with the measurements without the use of an NEG pump. Measurements of the pressure ratios were performed with a spinning rotor gauge (SRG).     

Primary vacuum standard for UHV range — Standing experience and present problems

Accurate calibrations of gauges to ever lower pressures (higher vacuum) are increasingly becoming necessary. The calibrations over the entire ultra high vacuum (UHV) range i.e. up to 10⁻¹⁰ Pa are already sometimes demanded by the users of vacuum measuring devices. Standards covering part of this range are available in some national metrological laboratories while primary standards covering the entire UHV range are under development. However, the most suitable principle of the primary standard for generating such a low pressure is still not clear. In spite of some attempts during the last century no sufficiently accurate absolute (“calibration-free”) gauge for high vacuum (HV) and UHV ranges has been developed to date. Apparatuses for generating gas pressure in a dynamic flow control method will continue to be used as primary standards for the UHV range, but, by virtue of technical difficulties, the equilibrium (Maxwellian velocity distribution) will be severely disturbed during the process of pressure generation that the pressure in the chamber will not be defined as a simple scalar quantity. This principal problem must be solved first by the development of a primary UHV standard. Another serious problem in the design of an apparatusfor generating controlled pressures on the order of 10⁻¹⁰ Pa requires the uncontrolled changes of gas pressure to 10⁻¹² Pa or lower. Hence an extremely high vacuum (XHV) apparatus must be used in which the requirements concerning the pumps and the outgassing rate from the materials are kept very strict.     

Applications of non evaporable getter pump in vacuum metrology

Applications of non evaporable getter pump in vacuum metrology are reviewed and discussed with a special focus on static expansion primary vacuum standard, and flow division method ultra-high vacuum (UHV) and extremely high vacuum (XHV) standard. The results obtained show that the non evaporable getter pump is suited for extending the calibration lower limit, and it is a valuable supplement to the basic methods. The feasibility of use of non evaporable getter pump in constant conductance method vacuum leak standard is also discussed.     

非蒸散型吸气剂泵在真空计量中的应用研究

本文简要阐述了非蒸散型吸气剂(NEG)泵的吸气机理,着重介绍了NEG泵在超高/极高真空标准、静态膨胀法真空标准等真空计量标准中的最新应用结果。结果表明:利用NEG泵在室温下获得了10-10Pa的XHV;利用NEG泵将超高/极高真空标准校准下限延伸到了10-10Pa量级;利用NEG泵将静态膨胀法真空标准的校准下限延伸到了10-7Pa量级。最后还对用NEG泵延伸固定流导法微流量标准测量下限的可行性进行了分析。     

Measurement of conductance of a sapphire sealed variable leak valve

A variable leak valve (VLV) is used to a great extend to regulate throughput of process gases into the vacuum system. A gas flow through the VLV can be in different flow regimes: viscous, transition or molecular, depending on the upstream gas pressure. In this work the pressure dependence of conductance of a sapphire sealed VLV has been investigated.The conductance of the VLV was determined by measuring gas flow in known volume using the pressure rise method. The pressure rise was measured using a spinning rotor gauge while the upstream pressure was measured by capacitance diaphragm gauges. The VLV conductance was determined with a relative standard uncertainty of 1.2%.The measurements of conductance were done for different VLV settings. To reduce the influence of outgassing on the gas flow measurement by the pressure rise method we have used a non-evaporable getter in the measurement volume. Argon was used as the test gas. As a result, the throughput of VLV for Ar could be measured down to 5 × 10⁻¹¹ Pa m³ s⁻¹. The time stability of the VLV for conductance set at 5 × 10⁻¹⁰ l s⁻¹ and fill pressures of Ar ranging from 0.2 kPa to 5 kPa was also determined.     

Stabile Vakuumprozesse mit Dreh‐/Schiebedurchführungen

Stable vacuum processes despite rotary/sliding feedthroughs – Effects of motion speed and direction using the MeTraVac system as an example Components for ultra‐high (p < 1 · 10^?7 mbar) and extreme high vacuum (p < 1 · 10^?11mbar) conditions must have very clean surfaces with low outgassing rates. The determination of outgassing characteristics for static components is common practice. However, it is unusual to investigate the influence of the outgassing rates of dynamic components when they move in a vacuum. Various experiments with a self‐designed and built rotary/sliding feed‐through were carried out. The aim of these investigations was to monitor the pressure increases during movements and thereby draw conclusions on the quality of the vacuum conditions. One rotary and two translational movement types were selected exemplarily for practical vacuum applications. Each experiment was performed in an unconditioned and conditioned (in‐situ bake‐out) state. For the experiments a heatable test setup was assembled. The rotary motion does not affect the process vacuum perceivable. Two translational movements were carried out, which differed in the speed. The pressure increases during slower and faster translatory movements show that the speed of movement has no significant influence on the vacuum. Nevertheless, a significant change in pressure was observed with translational movements. By bake‐out, the amount of water released per movement will significantly be reduced. Most kind of vacuum‐sided movements do not affect the process pressure at all or only to a small extent.     

静态膨胀法真空标准容积比测量及不确定度评定

静态膨胀法真空标准是真空计量的基础标准,其校准的精度在很大程度上取决于容积比测量的精度。选用一台高压力气体活塞压力计测量膨胀前气体的压力;利用电容薄膜真空计作为比较器,用一台低压力数字式气体活塞压力计测量膨胀后气体的压力;对静态膨胀法真空标准的容积比进行了测定,并对测量结果进行了不确定度的评定。     

Outgassing measurement of an LHC collimator and estimation for the NEG performances

The outgassing rate of the collimators in the Large Hadron Collider (LHC) at CERN has an important role for the life-time of the Non-Evaporable Getter (NEG), and an accurate analysis allows the definition of future activities, like NEG vacuum activation. For these reasons, both, total outgassing rate and gas composition of a secondary collimator have been measured in the laboratory. The outgassing rate decreases by about two orders of magnitude by after bake-out and moreover, repeated bake-out further reduced the outgassing rate. The gas transmission through the NEG coated beam pipes and the resulting pressure distributions near the collimator were also measured in a dedicated setup. It is found that the main gas component after just 2 m of NEG coated beam pipe is CH₄ due to the extreme pumping speed of NEG for the other gases. Large amount of outgassing for H₂ and carbon related molecules are released when moving the collimator jaws. It is found that the NEG is very effective even in such case with large gas load.     

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.     

Outgassing characteristics of a polycarbonate core material for vacuum insulation panels

Performance deterioration with time is one of the most important issues in a vacuum insulation panel (VIP), which is mostly due to the inner gas pressure rise. Outgassing from the interior of core materials is the major gas source when the core material is a polymer. Outgassing characteristics of a polycarbonate as the VIP’s core material are examined theoretically and experimentally. To measure the outgassing rate, specific outgassing tests are carried out using a pressure rise method. Diffusive outgassing mechanism is discussed based on the Fick’s law. As the result, the total amount of dissolved gas and the diffusion coefficients of various gases in the polycarbonate are obtained by using the measured outgassing rate. Temperature dependence of the diffusion coefficient of nitrogen is also examined. It is shown that the outgassing rate of polymer core materials can be significantly reduced to a negligible level by a baking pre-treatment in vacuum and/or by a metal coating on the polymer surface.     

Wireless communication using detectors located inside vacuum chambers

Progress in modern vacuum technology requires an increasing number of measurements to be performed in the vacuum environment itself and in many applications data transfer outside the vacuum vessels is both difficult and expensive. This paper explores the potential of wireless technology to transmit data from and into ultra high vacuum (UHV) compatible vacuum chambers using glass viewport windows attached to normal standard flange fittings. The current tests focus in particular on Bluetooth technology, and its characteristics and limitations investigated. Signal propagation has been verified up to about 9 m and the outgassing of components shown to be insignificant in a clean metal-sealed vacuum system down to a pressure of 10⁻⁷ mbar. Some preliminary tests of noise and electromagnetic field immunity have been carried out, confirming the robustness of Bluetooth spread spectrum modulation technique. Signal data from Integrated Circuit Temperature Sensors and Photodiodes have been successfully transmitted from a vacuum chamber, proving the capability of this technology standard to handle both low and high frequency data. The future prospects and the further developments of the approach are also discussed.     

常见放气率测试方法的量化比较

对材料放气率的准确测量至关重要,本文对自主设计出的转换气路(SPP)法测试装置进行了试用,装置重复性较好。将此方法与工程上常用的静态升压法、小孔流导法进行了数量上的直观比较,SPP法比小孔流导法、静态升压法测得的放气率更低。结合molflow模拟,实测值与模拟值相互验证,分析了误差来源,明确了提升方向。     

An UHV system to study gassing and outgassing of metals under friction

An UHV dynamic expansion system to study gassing and outgassing of metals under friction in vacuum has been developed. The vacuum chamber of the system has two main parts separated by a wall with a central orifice of nominal conductance of 5.27 l s⁻¹ for N₂ at 300 K. The main part of the system is equipped with a quadrupole mass-spectrometer, ion gauge and the gas line supply. The chamber closer to the pumping systems is also equipped with an ion gauge. The pumping of the system is performed by: (I) a turbomolecular and (II) a cryogenic pump, which can operate either independently or in parallel. The ultimate pressure of chambers 1 and 2 are respectively 5.2.10⁻¹¹ mbar and 1.10⁻⁹ mbar. The friction cell allocated in chamber 2 consists of a sample holder and a linear motion feedthrough with a steel ball indent allows the friction experiments. The sample holder lets to apply a load to the sample surface with a normal force in the range of 0,02 N to 0,85 N. Heating of the sample up to 500°C is provided by electron bombardment. The temperature of the sample as well as the electrical resistance between the indent and the sample surface can be measured. The results show that pressure increase caused by the deformation of the sealing bellow of the friction cell, i.e. without friction, is 1.7×10⁻¹¹ mbar (0.6% of the base pressure). The outgassing of the sample caused by friction depends on the normal force as well as on the sweep frequency. The behavior of the total pressure is characterised by an oscillating current supported on a continuous current, which is not possible to explain at present time.     

Casting particulate and fibrous metal-matrix composites by vacuum infiltration of a liquid metal under an inert gas pressure

A new method of making metal-matrix composites is reported. This method combines the essentials of three liquid-phase fabrication methods: (i) vacuum infiltration, (ii) infiltration under an inert gas pressure, and (iii) squeeze casting. In this method, the particulate or fibrous preform is placed in a mould and the matrix alloy is placed above the preform. The matrix alloy is heated to the liquidus temperature together with the mould and the preform under vacuum. Then an inert gas like argon is compressed on to the top surface of the matrix-alloy melt, forcing the melt to infiltrate the preform. The pressure is 1000 to 2500 psi. As the melt is just at liquidus temperature, it is much lower than that used in squeeze casting. Moreover, the pressure is an order of magnitude lower than that used in squeeze casting. The low temperature lessens the interfacial reaction between the matrix and the filler, while the low pressure essentially eliminates preform compression. This method has been successfully used to fabricate aluminium-matrix composites reinforced by short ceramic fibres, continuous ceramic fibres, SiC particles, Al₂O₃ particles, graphite flakes and SiC whiskers.     

低温绝热气瓶真空寿命模拟试验研究

提出了对绝热气瓶真空夹层逐次充入模拟气体进行绝热气瓶漏气和材料放气的真空寿命模拟试验评价方法。试验实例表明:低温绝热气瓶静态蒸发率在低温下夹层压力>5×10^-2Pa后迅速上升,即5×10^-2Pa可视为夹层真空寿命终结的拐点(或阈值)。5A分子筛在液氮温度下对氮具有巨大的吸附潜力,对氢表现出弱的吸附能力。真空绝热夹层的材料放气对真空寿命的影响远远大于漏气的影响,提高绝热气瓶真空寿命的技术途径是减小夹层材料的放气率和改善内置吸附剂对氢的吸附能力。模拟试验能直观、实际、准确地研究漏气和放气对真空寿命诸因素的影响,为确定切合实际的设计参数和工艺提供参考数据,进而推广用于各类真空绝热型低温容器的真空寿命评价和应用。     

Gas dynamics modelling for particle accelerators

Design of an accelerator vacuum chamber requires an input from different scientific disciplines such as surface science, material science, gas dynamics, particle beam dynamics, and many others. Although vacuum scientists work on the boundary field between these disciplines, gas dynamics is the one that allows joining them to the vacuum science for particle accelerators. The vacuum requirements (usually UHV or XHV) in particle accelerators are defined by beam-gas interactions that should be negligible compared to other phenomena that limit the quality of the beam. At such low pressures the main source of gas in the vacuum chamber is a molecular desorption from materials used for the vacuum chamber and its components. The outgassing rates vary over a very wide range and depend on material, cleaning procedure, treatments, temperature, bombardment by particles and accumulated irradiation dose. The gas dynamics is used to design the research facilities to accurately measure and to study outgassing rates at different conditions. By applying these data to the accelerator vacuum design, one would have to consider that outgassing is often non-uniform and changes with time with different functions. The most time-efficient way of beam vacuum optimization is using a 1D diffusion model where all parameters are defined as a function of longitudinal coordinate (along the beam path). A full 3D modelling with TPMC codes provides much more accurate results, however, being time consuming work is not ideal for pumping and design optimization and is used for complex components and for finalized design.     

Carbon Aerogel-Based High-Temperature Thermal Insulation

Carbon aerogels, monolithic porous carbons derived via pyrolysis of porous organic precursors synthesized via the sol–gel route, are excellent materials for high-temperature thermal insulation applications both in vacuum and inert gas atmospheres. Measurements at 1773K reveal for the aerogels investigated thermal conductivities of 0.09W · m⁻¹ · K⁻¹ in vacuum and 0.12W · m⁻¹ · K⁻¹ in 0.1MPa argon atmosphere. Analysis of the different contributions to the overall thermal transport in the carbon aerogels shows that the heat transfer via the solid phase dominates the thermal conductivity even at high temperatures. This is due to the fact that the radiative heat transfer is strongly suppressed as a consequence of a high infrared extinction coefficient and the gaseous contribution is reduced since the average pore diameter of about 600nm is limiting the mean free path of the gas molecules in the pores at high temperatures. Based on the thermal conductivity data detected up to 1773K as well as specific extinction coefficients determined via infrared-optical measurements, the thermal conductivity can be extrapolated to 2773K yielding a value of only 0.14W· m⁻¹ · K⁻¹ in vacuum.     

基于标准漏孔比对法的漏率测量装置

针对真空密封件的密封性能评估,采用了基于标准漏孔比对的漏率测量方法,具体包括动态比对法和静态累积比对法,通过四极质谱仪比较标准漏孔和密封件流出气体产生的离子流,利用标准漏孔的已知漏率和四极质谱离子响应的线性外推,计算密封件的漏率。研制了一套密封件的漏率测量装置,装置极限真空可达10^-7Pa量级,漏率测量范围为:10^-7-10^-14Pa·m³/s,可用于具有检漏接口的各类密封件的漏率测试。采用该装置的两种方法分别进行不锈钢刀口密封件的漏率测试。结果表明:动态比对法对该不锈钢刀口密封件的漏率测试误差较大,低于10^-11Pa·m³/s量级的极低漏率密封件更适合采用静态累积比对法进行测试。     

Mass spectrometric determination of oxygen at partial pressures of less than 1.5 μPa (10−8 torr)

During an investigation of chemisorption and photodesorption reactions between oxygen and thin films of zinc oxide, the need arose for the determination of oxygen partial pressures in the range 1 μPa to 100 pPa (10^?8 to 10^?12 torr), in a static system, using a residual gas analyzer. Within this range the pumping actions for various components of the vacuum system were significant and this paper describes these actions and the techniques developed to lower the limit of detection of oxygen by mass spectrometric analysis to about 700 pPa .l (5 × 10^?12 torr .l)     

不同处理方法下铁氧体出气性能的研究

为减少加速器束流快引出元件铁氧体材料真空出气量,对铁氧体材料进行真空除气、镀膜以及烘烤工艺处理。采用旁路切换法(SPP)测量不同处理条件下的出气率。结果表明,铁氧体材料经真空除气后镀TiN膜,在测试罩中经烘烤,冷却后充氮气测得的出气率最低。