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.     

He leaks in the CERN LHC beam vacuum chambers operating at cryogenic temperatures

The 27 km long large hadron collider (LHC), currently under construction at CERN, will collide protons beam at 14 TeV in the centre of mass. In the 8 arcs, the superconducting dipoles and quadrupoles of the FODO cells operate with superfluid He at 1.9 K. In the 8 long straight sections, the cold bores of the superconducting magnets are held at 1.9 or 4.5 K. Thus, in the LHC, ∼75% of the beam tube vacuum chamber is cooled with He.In many areas of the machine, He leaks could appear in the beam tube. At cryogenic temperature, the gas condenses onto the cold bores or beam screens, and interacts with the circulating beam. He leaks creates a He front propagating along the vacuum chambers, which might cause magnet quench.We discuss the consequences of He leaks, the possible means of detections, the strategies to localise them and the methods to measure their size.     

Vacuum-system design for the large-scale cryogenic gravitational wave telescope

The large-scale cryogenic gravitational wave telescope (LCGT) requires ultra-high vacuum tubes which the laser beams pass through. Two 3-km vacuum tubes are kept in ∼10⁻⁷ Pa of vacuum pressure so as to reduce scattering-effects due to residual gas molecules. The stainless-steel material with electro-chemical buffing is the most available for use as long tubes of 1 m in diameter. To reduce noises due to scattered light from the surface of tubes, a lot of baffles are inserted into the tubes.     

Outgassing from stainless steel and the effects of the gauges

The true outgassing from surfaces is often masked by the production of gases and pumping within the measuring gauge. Quadrupole mass analysers have been used to measure the outgassing from carefully prepared samples of stainless steel. By using an isolation pressure rise technique coupled to non-evaporable getter pumping, the problem of production of methane by the gauge is overcome and it has been possible to measure the outgassing of methane to very low levels. The specific outgassing rate of methane from 316 L stainless steel is found to be ?5×10⁻²² mbar l s⁻¹ cm⁻². The outgassing of the other gases has been measured using several gauges. The specific outgassing of hydrogen is found to be 4.5×10⁻¹⁵ mbar l s⁻¹ cm⁻² and the total of all other gasses amounted to no more than 10% of this figure. Although the measurements of the other gases are more ambiguous than those for methane, it is concluded that, in all probability, only hydrogen is outgassed from stainless steel.     

Running experience with the vacuum system of the superconducting linac of the TESLA Test Facility

The superconducting linear accelerator of the TESLA Test Facility (TTF) at DESY/Hamburg is running successfully since several years. The main focus was so far on machine developments as well as on a proof of principle experiment for a self amplifying spontaneous emission free electron laser (SASE FEL) for the proposed 500 GeV e⁺e⁻ linear collider TESLA with integrated X-ray free electron laser (XFEL) laboratory. Presently, the machine is substantially modified to become a VUV-FEL user facility with tunable wavelengths in the nm range within 2004.The beam pipe of the accelerator contains sections operated at room temperature as well as at 2 K in the areas of the superconducting structures used for acceleration of the beam. Three cryogenic modules, each containing 8 solid niobium cavities have been tested. In addition to standard UHV requirements, the vacuum system for this machine needs to preserve the cleanliness of the superconducting cavity surfaces. Thus the preparation of all vacuum components includes cleaning steps to remove particle contaminations, installation of components into the machine under local clean rooms and special procedures for pump down and venting.     

Development of an ultra-high vacuum system for space application

The aim of this paper is to present the development of a very specific ultra-high vacuum system for the space application PHARAO. In order to reach the specified pressure (2.6×10⁻⁸ Pa) during 3 years in a self-contained system, specific solutions have been developed. A calculation of partial pressures of different chemical species (typically hydrogen, rare gases and cesium) in the different areas of the vacuum tube has been computed. Experiments have been also performed in order to verify the possibility of using different kind of materials (outgassing rates of bulk and porous titanium, silicon carbide reinforced aluminum, graphite, etc.) and pumping systems such as getters and a specially developed ion pump.     

Design of the vacuum system for Diamond, the UK third generation light source

The Diamond synchrotron, which is due to come into operation with beam for users in 2007, is being constructed at the Rutherford Appleton Laboratory, Oxford. The design status of the vacuum system of the storage ring as at 31st December 2002 will be reported.Diamond is based on a 24 cell 3 GeV electron storage ring of 561.6 m circumference. As is the case for most such machines, the operational pressure has been specified as 10⁻⁹ mbar to give a beam lifetime >10 h at the design current of 300 mA. The storage ring vacuum system will use conventional technology and most of the vacuum vessels will be constructed of stainless steel. With the exception of the insertion device (ID) vacuum chambers, the ring has not been designed to be baked in situ, but all components will be vacuum baked as sub assemblies before installation. The vacuum system is designed to achieve the required pressure after 100 A h of beam conditioning using the pumping scheme, which will be described.Twenty-one ID straights are available for ID, of which seven will be installed at the start of operations. Three types of vacuum vessel will be used in these straights, a stainless-steel make-up pipe, a NEG coated narrow-gap vacuum chamber for conventional IDs and a wider vacuum chamber to house in vacua IDs. The ID straights have an isolation valve installed at each end, and in situ baking can be used in these restricted locations, for example to activate the NEG coatings.The vacuum system for the beam line front ends has been designed to provide good vacuum isolation between the storage ring and the experimental beam lines, whether or not an interposing window is fitted.Pumping schemes, pressure measurement and calculated pressure profiles will be described.     

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.     

Vacuum performance of a carbon fibre cryosorber for the LHC LSS beam screen

A new carbon fibre material was developed at the Institute of Solid State Chemistry and Mechanochemistry at the Siberian Branch of the Russian Academy of Science (SB RAS) to meet the requirements of a cryosorber for the large hadron collider (LHC) vacuum chamber. The material must have a large sorbing capacity, a certain pumping speed, a working temperature range between 5 and 20 K, a low activation temperature (below room temperature), a certain size in order to fit into the limited space available and it should be easy to mount. The vacuum parameters of the LHC vacuum chamber prototype with a carbon fibre cryosorber mounted onto the beam screen were studied in the beam screen temperature range from 14 to 25 K at the Budker Institute of Nuclear Physics SB RAS. This carbon fibre material has shown sufficient sorption capacity for hydrogen at operational temperatures of the beam screen in the LHC long straight sections. It is also very important that this material does not crumble and makes a convenient fixation onto the beam screen in comparison to the widely used granulated charcoal. The problem of fluff and ways of reducing the quantity of fluff in the beam channel were studied. The results of these studies show that the carbon fibre material is a possible cryosorber-candidate for use in the LHC and other long vacuum system at cryogenic temperatures. The experimental set-up and results of measurements of the H₂ cryosorption capacity of this carbon fibre material are presented in this paper.     

Processing map for hot working characteristics of a wrought 2205 duplex stainless steel

Processing map on a wrought 2205 duplex stainless steel under hot compression conditions has been developed based on the dynamic material model theories in the range 1223–1473 K and 0.01–10 s⁻¹. The various domains in the map corresponding to different deformation characteristics have been discussed in combination of microstructural observations. The results show that the power dissipation efficiency (η) depends strongly on the dynamic recrystallization (DRX) of austenite which plays a dominant role in microstructural evolution, while the ferrite phase mainly continues to exhibit relatively well-developed dynamic recovery (DRV) at large strain. The optimum hot working domain of wrought 2205 duplex stainless steel is obtained to be in the temperature range 1373–1473 K and at strain rate of 0.01 s⁻¹, with peak efficiency 50% occurring at about 1423 K, in which more uniform microstructure is developed due to the occurrence of complete DRX of austenite. The unstable hot working regimes are predicted by Prasad instability criterion, in good agreement with the macro-and microstructural observations. As predicted, flow instability, which are manifested as twinning, bands of flow localization and the absence of DRX in austenite are observed at lower temperatures and higher strain rates (1223–1273 K and 1–10 s⁻¹); in other cases, wedge cracking is responsible for instability phenomena observed at the temperature range 1373–1423 K and strain rate of 10 s⁻¹.     

Thermal conduction measurement of miniature coaxial cables between 0.3 and 4.5 K for the wiring of superconducting detectors

Thermal conduction of thin coaxial cables made of 70-30 CuNi and stainless steel 304 with a PTFE insulator was measured in a temperature range between 0.3 and 4.5 K. Thermal conductivity of the 70-30 CuNi and PTFE was measured independently and their contribution to the thermal conduction of the coaxial cables was investigated. The thermal conductivity of the 70-30 CuNi differed from the literature by 25% at 0.36 K and 40% at 4.2 K, and the alloy exhibited a weak temperature dependence, which indicated the effects of mechanical treatment. It has been confirmed that the thermal conduction of the coaxial cables are low enough to keep a cold stage of ³He cryostats at a temperature below 0.3 K, even when one hundred cables are installed between 0.3 and 3 K for the read-out of superconducting tunnel junction arrays. The cables were installed in a cryogen-free ³He cryostat, and the operation below 0.3 K was successful.     

INRIM continuous expansion system as high vacuum primary standard for gas pressure measurements below 9 × 10 Pa

The continuous expansion system can be considered the state of the art for pressure measurement in the ultra high vacuum range. In the last years, at INRIM, a new continuous expansion system was designed, assembled and characterized. The system is the high vacuum primary standard in the pressure range from 1 × 10⁻⁶ Pa to 9 × 10⁻² Pa with relative standard uncertainty ranging from 2.1% at 1 × 10⁻⁶ Pa down to 0.4% at 9 × 10⁻² Pa. The system is based on the passing of a measured gas flow through a fixed and known conductance. The gas flow is generated and measured by a primary gas flowmeter based on the constant-pressure-variable-volume method.In the first part of the paper both a correction for the effect of transitional flow through the orifice and a new analytical evaluation of orifice conductance are presented. In the second part the accuracy of the system and the pressure uncertainty evaluation are described.     

Characteristics of a magnetron cold cathode gauge

The discharge characteristics of a cold-cathode gauge of the non-inverted magnetron type were studied in ultra-high vacuum. The experimental magnetron cell of length 56 mm and diameter 32 mm was made of stainless steel. The cathode with a diameter of 6 mm was placed along the anode axis. The diameter of the anode was 25 mm and the length was 50 mm. Discharge current versus voltage and magnetic field was measured in the pressure range between 1×10⁻⁸ and 1×10⁻⁶ mbar. It was found that the current at first slowly increased with increasing voltage, reached a maximum at a certain voltage, and decreased rapidly with further increase of the voltage. The voltage, at which the current reached the maximum, depended on the magnetic field density and slightly on the pressure. A novel type of a cold cathode gauge with a self-adjusting power supply is suggested.     

Tribological behavior of radio-frequency sputtering WS2 thin films with vacuum annealing

Thin films of tungsten disulfide (WS₂) were deposited on 3Cr13 martensitic stain less steel substrate by radio-frequency (RF) sputtering. The as-deposited films were annealed at 473, 673 and 873 K respectively for 2 h in 5 × 10^− 4 Pa vacuum. Composition of the films was inspected by energy dispersive spectroscopy. Surface morphology and structure properties were studied by scanning electron microscopy and X-ray diffraction techniques. Tribological behavior was also examined using tribometer. At 473 K, the films exhibited low crystallization structure and no significant improvement in the tribological performance. At 673 K, the tribological performance was improved and a transition from non-crystalline to hexagonal structure took in place. When the annealing temperature rose up to 873 K, the films cracked and fell off from the substrate. The results suggested that with suitable technical parameters vacuum annealing could promote crystallization and improve tribological performance of RF sputtering WS₂ films.     

Effect of the substrate on the electrodeposition of Bi2Te3−ySey thin films

The potentiostatic electrodeposition of n-type Bi₂Te_3−ySe_y thermoelectric films onto stainless steel and gold substrates from nitric acid aqueous solutions has been carried out at room temperature. The cathodic process during the electrodeposition of Bi₂Te_3−ySe_y films was investigated by cyclic voltammetric experiments. The structure and surface morphology of Bi₂Te_3−ySe_y films deposited on both substrates were characterized by X-ray diffraction (XRD) and environment scanning electron microscopy (ESEM) coupled with energy dispersive spectroscopy (EDS). Electrical and thermoelectric properties of as-deposited films were also measured at room temperature. The results show that the reduction process under the same depositing conditions on gold and stainless steel substrates is very different. On gold substrates, H₂SeO₃ in the electrolyte is firstly reduced to elemental Se, and then the deposited Se reacts with HTeO₂⁺ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. On stainless steel substrates, HTeO₂⁺ in the electrolyte is firstly replaced by elemental Fe to produce elemental Te, and subsequently the generated Te reacts with H₂SeO₃ and Bi³⁺ to form Bi₂Te_3−ySe_y alloy. Analysis of ESEM show that the surface morphology of the films electrodeposited on gold substrates is more compact than that on stainless steel substrates. The XRD patterns indicate that the films electrodeposited on both substrates exhibit preferential orientation along (1 1 0) plane, but the relative peak intensity of (0 1 5) and (2 0 5) planes on stainless steel substrates is stronger than that on gold substrates. The Seebeck coefficient and electrical resistivity of the films deposited on stainless steel substrates are higher than that on gold substrates.     

Ultra-thin zinc oxide film on Mo(100)

Zinc oxide films on a single crystal Mo(100) substrate were fabricated by annealing the pre-deposited metal Zn films in 10^− 5-10^− 4 Pa O₂ ambience at 300-525 K, and were characterized by in situ Auger electron spectroscopy, electron energy loss spectroscopy, low energy electron diffraction and high-resolution electron energy loss spectroscopy. The results show that the atomic ratio of oxygen to zinc in zinc oxide film is significantly dependent on sample annealing temperature and O₂ pressure. A stoichiometric zinc oxide film has been obtained under ∼10^− 4 Pa O₂ at about 400 K. A redshift of Fuchs-Kliewer phonon energy correlated with surface oxygen deficiency is observed.     

Thermal stability of the Mobil Five type metallosilicate molecular sieves—An in situ high temperature X-ray diffraction study

We have carried out in situ high temperature X-ray diffraction (HTXRD) studies of silicalite-1 (S-1) and metallosilicate molecular sieves containing iron, titanium and zirconium having Mobil Five (MFI) structure (iron silicalite-1 (FeS-1), titanium silicalite-1 (TS-1) and zirconium silicalite-1 (ZrS-1), respectively) in order to study the thermal stability of these materials. Isomorphous substitution of Si⁴⁺ by metal atoms is confirmed by the expansion of unit cell volume by X-ray diffraction (XRD) and the presence of Si-O-M stretching band at ∼960 cm⁻¹ by Fourier transform infrared (FTIR) spectroscopy. Appearance of cristobalite phase is seen at 1023 and 1173 K in S-1 and FeS-1 samples. While the samples S-1 and FeS-1 decompose completely to cristobalite at 1173 and 1323 K, respectively, the other two samples are thermally stable upto 1623 K. This transformation is irreversible. Although all materials show a negative lattice thermal expansion, their lattice thermal expansion coefficients vary. The thermal expansion behavior in all samples is anisotropic with relative strength of contraction along ‘a’ axes is more than along ‘b’ and ‘c’ axes in S-1, TS-1, ZrS-1 and vice versa in FeS-1. Lattice thermal expansion coefficients (α_v) in the temperature range 298-1023 K were −6.75 × 10⁻⁶ K⁻¹ for S-1, −12.91 × 10⁻⁶ K⁻¹ for FeS-1, −16.02 × 10⁻⁶ K⁻¹ for TS-1 and −17.92 × 10⁻⁶ K⁻¹ for ZrS-1. The highest lattice thermal expansion coefficients (α_v) obtained were −11.53 × 10⁻⁶ K⁻¹ for FeS-1 in temperature range 298-1173 K, −20.86 × 10⁻⁶ K⁻¹ for TS-1 and −25.54 × 10⁻⁶ K⁻¹ for ZrS-1, respectively, in the temperature range 298-1623 K. Tetravalent cation substitution for Si⁴⁺ in the lattice leads to a high thermal stability as compared to substitution by trivalent cations.     

Microanalyses on the hydrogen permeated 70% SiCC films

SiCC films with content of 70% SiC were deposited by rf magnetron sputtering on stainless steel or NaCl substrate followed by argon ion bombardment. Samples were then submitted to hydrogen permeation at 3.23×10⁷ Pa and 500 K for 3 h. Secondary ion mass spectroscopy (SIMS) was used to analyze hydrogen concentration with depth and to check the formation of hydrogen related bonds in the SiCC films with IR measurement. Auger electron spectra (AES) and X-ray photoelectron spectra (XPS) were carried out to check the effects of hydrogen participation on shifts of chemical bonding states of C, Si and O contamination.     

Diffusion bonding of Fe-28Al(Cr) alloy with low-carbon steel in vacuum

Fe-28Al(Cr) alloy and low-carbon steel were diffusion bonded in a vacuum of 10⁻⁴-10⁻⁵ Pa. The relationship of the bond parameters and shear strength at the interface was discussed. Microstructure characteristics and the reaction products at the interface were investigated by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The thickness of the diffusion reaction layer was measured with electron probe microanalysis (EPMA). The results indicated that controlling bonding temperature 1333 K for 3.6 ks, shear strength at the interface can be up to 112 MPa. Three kinds of reaction products were observed to have formed during the vacuum diffusion bonding, namely FeAl, Fe₃Al and α-Fe (Al) solid solution. The thickness (X) of the diffusion reaction layer increases with bonding time (t) according to a parabolic law X²=6.4×10³ exp(−104.1/RT)(t-t₀) (μm²).     

New leak element using sintered stainless steel filter for in-situ calibration of ionization gauges and quadrupole mass spectrometers

A new leak element using a sintered stainless steel filter with a pore size of less than 1 μm has been developed for in-situ calibrations of ionization gauges (IGs) and quadruple mass spectrometers (QMSs). The gas flow through this leak element realizes molecular flow at an upstream pressure of less than 10⁴ Pa. This new leak element, which is a kind of open-type standard leak, has four advantages. (1) Calibrations for various gas species are available only with this single leak element because the conductance is easily compensated for gas species by molecular mass. (2) Calibrations with multiple pressure points are easily available because the conductance is constant against changing upstream pressure. (3) Calibrations for a gas mixture are available because the interference effect between gas molecules in a gas mixture is negligible. (4) The dependence of flow rate on temperature is small and is compensated theoretically. These advantages were experimentally demonstrated. The stability and uncertainty of the leak element were also evaluated. The changes in the conductance of this leak element were less than 3% over one year. Since the conductance is typically 10⁻¹⁰ · m³/s, the reference gas flow in the range from 10⁻⁸ Pa · m³/s to 10⁻⁶ Pa m³ is obtained by changing the upstream pressure from 10² Pa to 10⁴ Pa with an uncertainty of approximately 6%.