From freeze-dried foods to flat-panel displays and space simulation, vacuum technology is essential in many fields of research and industry. Globally, vacuum technologies represent a multi-billion-dollar, and growing, market. However, it is only when vacuum is applied to particle accelerators for high-energy physics that the technology displays its full complexity and multidisciplinary nature – which bears little resemblance to the common perception of vacuum as being just about pumps and valves.
Particle beams require extremely low pressure in the pipes in which they travel to ensure that their lifetime is not limited by interactions with residual gas molecules and to minimise backgrounds in the physics detectors. The peculiarity of particle accelerators is that the particle beam itself is the cause of the main source of gas: ions, protons and electrons interact with the wall of the vacuum vessels and extract gas molecules, either due to direct beam losses or mediated by photons (synchrotron radiation) and electrons (for example by “multipacting”).
Nowadays, vacuum technology for particle accelerators is focused on this key challenge: understand, simulate, control and mitigate the direct and indirect effects of particle beams on material surfaces. It is thanks to major advances made at CERN and elsewhere in this area that machines such as the LHC are able to achieve the high beam stability that they do.
Since it is in the few-nanometre-thick top slice of materials that vacuum technology concentrates most effort, CERN has merged in the same group: surface-physics specialists, thin-film coating experts and galvanic-treatment professionals, together with teams of designers and colleagues dedicated to the operation of large vacuum equipment. Bringing this expertise together “under one roof” makes CERN one of the world’s leading R&D centres for extreme vacuum technology, contributing to major existing and future accelerator projects at CERN and beyond.
Vacuum technology for particle accelerators has been pioneered by CERN since its early days, with the Intersecting Storage Rings (ISR) bringing the most important breakthroughs. At the turn of the 1960s and 1970s, this technological marvel – the world’s first hadron collider – required proton beams of unprecedented intensity (of the order of 10 A) and extremely low vacuum pressures in the interaction areas (below 10–11 mbar). The former challenge stimulated studies about ion instabilities and led to innovative surface treatments – for instance glow-discharge cleaning – to mitigate the effects. The low-vacuum requirement, on the other hand, drove the development of materials and their treatments – both chemical and thermal – in addition to novel high-performance cryogenic pumps and vacuum gauges that are still in use today. The technological successes of the ISR also allowed a direct measurement in the laboratory of the lowest ever achieved pressure at room temperature, 2 × 10–14 mbar, a record that still stands today.
The Large Electron Positron collider (LEP) inspired the next chapter in CERN’s vacuum story. Even though LEP’s residual gas density and current intensities were less demanding than those of the ISR, the exceptional length and the intense synchrotron-light power distributed along its 27 km ring triggered the need for unconventional solutions at reasonable cost. Responding to this challenge, the LEP vacuum team developed extruded aluminium vacuum chambers and introduced, for the first time, linear pumping by non-evaporable getter (NEG) strips.
In parallel, LEP project leader Emilio Picasso launched another fruitful development that led to the production of the first superconducting radio-frequency (RF) cavities based on niobium thin-film coating on copper substrates. The ability to attain very low vacuum gained with the ISR, the acquired knowledge in film deposition, and the impressive results obtained in surface treatments of copper were the ingredients for success. The present accelerating RF cavities of the LHC and HIE-ISOLDE (figure 1) are essentially based on the expertise assimilated for LEP (CERN Courier May 2018 p26).
The coexistence in the same team of both NEG and thin-film expertise was the seed for another breakthrough in vacuum technology: NEG thin-film coatings, driven by the LHC project requirements and the vision of LHC project leader Lyn Evans. The NEG material, a micron-thick coating made of a mixture of titanium, zirconium and vanadium, is deposited onto the inner wall of vacuum chambers and, after activation by heating in the accelerator, provides pumping for most of the gas species present in accelerators. The Low Energy Ion Ring (LEIR) was the first CERN accelerator to implement extensive NEG coating in around 2006. For the LHC, one of the technology’s key benefits is its low secondary-electron emission, which suppresses the growth of electron clouds in the room-temperature part of the machine (figure 2).
Electron clouds had to be studied in depth for the LHC. CERN’s vacuum experts provided direct measurements of the effect in the Super Proton Synchrotron (SPS) with LHC beams, contributing to a deeper understanding of electron emission from technical surfaces over a large range of temperatures. New concepts for vacuum systems at cryogenic temperatures were invented, in particular the beam screen. Conceived at BINP (Russia) and further developed at CERN, this key technology is essential in keeping the gas density stable and to reduce the heat load to the 1.9 K cold-mass temperature of the magnets. This non-exhaustive series of advancements is another example of how CERN’s vacuum success is driven by the often daunting requirements of new projects to pursue fundamental research.
Preparing for the HL-LHC
As the LHC restarts this year for the final stage of Run 2 at a collision energy of 13 TeV, preparations for the high-luminosity LHC (HL-LHC) upgrade are getting under way. The more intense beams of HL-LHC will amplify the effect of electron clouds on both the beam stability and the thermal load to the cryogenic systems. While NEG coatings are very effective in eradicating electron multipacting, their application is limited for room-temperature beam pipes that needed to be heated (“bakeable” in vacuum jargon) to around 200 °C to activate them. Therefore, an alternative strategy has to be found for the parts of the accelerators that cannot be heated, for example those in the superconducting magnets of the LHC and the vacuum chambers in the SPS.
Thin-film coatings made from carbon offer a solution. The idea originated at CERN in 2006 following the observation that beam-scrubbed surfaces – those that have been cleared of trapped gas molecules which increase electron-cloud effects – are enriched in graphite-like carbon. During the past 10 years, this material has been the subject of intense study at CERN. Carbon’s characteristics at cryogenic temperatures are extremely interesting in terms of gas adsorption and electron emission, and the material has already been deposited on tens of SPS vacuum chambers within the LHC Injectors Upgrade project (CERN Courier October 2017 p32). By far, the HL-LHC project presents the most challenging activity in the coming years, namely the coating of the beam screens inserted in the triplet magnets to be situated on both sides of the four LHC experiments to squeeze the protons into tighter bunches. A dedicated sputtering source has been developed that allows alternate deposition of titanium, to improve adherence, and carbon. At the end of the process, the latter layer will be just 50 nm thick.
Another idea to fight electron clouds for the HL-LHC, originally proposed by researchers at the STFC Accelerator Science and Technology Centre (ASTeC) and the University of Dundee in the UK, involves laser-treating surfaces to make them more rough: secondary electrons are intercepted by the surrounding surfaces and cannot be accelerated by the beam. In collaboration with UK researchers and GE Inspection Robotics, CERN’s vacuum team has recently developed a miniature robot that can direct the laser onto the LHC beam screen (“Miniature robot” image). The possibility of in situ surface treatments by lasers opens new perspectives for vacuum technology in the next decades, including studies for future circular colliders.
An additional drawback of the HL-LHC’s intense beams is the higher rate of induced radioactivity in certain locations: the extremities of the detectors, owing to the higher flux of interaction debris, and the collimation areas due to the increased proton losses. To minimise the integrated radioactive dose received by personnel during interventions, it is necessary to properly design all components and define a layout that facilitates and accelerates all manual operations. Since a large fraction of the intervention time is taken up by connecting pieces of equipment, remote assembling and disassembling of flanges is a key area for potential improvements.
One interesting idea that is being developed by CERN’s vacuum team, in collaboration with the University of Calabria (Italy), concerns shape-memory alloys. Given appropriate thermomechanical pre-treatment, a ring of such materials delivers radial forces that tighten the connection between two metallic pipes: heating provokes the clamping, while cooling generates the unclamping. Both actions can be easily implemented remotely, reducing human intervention significantly. Although the invention was motivated by the HL-LHC, it has other applications that are not yet fully exploited, such as flanges for radioactive-beam accelerators and, more generally, the coupling of pipes made of different materials.
Technology advancement sometimes verges off from its initial goals, and this phenomenon is clearly illustrated by one of our most recent innovations. In the main linac of the Compact Linear Collider (CLIC), which envisages a high-energy linear electron-positron collider, the quadrupole magnets need a beam pipe with a very small diameter (about 8 mm) and pressures in the ultra-high vacuum range. The vacuum requirement can be obtained by NEG-coating the vacuum vessel, but the coating process in such a high aspect-ratio geometry is not easy due to the very small space available for the material source and the plasma needed for its sputtering.
This troublesome issue has been solved by a complete change of the production process: the NEG material is no longer directly coated on the wall of the tiny pipe, but instead is coated on the external wall of a sacrificial mandrel made of high-purity aluminium (figure 3). On the top of the coated mandrel, the beam pipe is made by copper electroforming, a well-known electrolytic technique, and on the last production step the mandrel is dissolved chemically by a caustic soda solution. This production process has no limitations in the diameter of the coated beam pipe, and even non-cylindrical geometries can be conceived. The flanges can be assembled during electroforming so that welding or brazing is no longer necessary.
It turns out that the CLIC requirement is common with that of next-generation synchrotron-light sources. For these accelerators, future constraints for vacuum technology are quite clear: very compact magnets with magnetic poles as close as possible to the beam – to reduce costs and improve beam performance – call for very-small-diameter vacuum pipes (less than 5 mm in diameter and more than 2 m long). CERN has already produced prototypes that should fit with these requirements. Indeed, the collaboration between the CERN vacuum group and vacuum experts of light sources has a long history. It started with the need for photon beams for the study of vacuum chambers for LEP and beam screens for the LHC, and continued with NEG coating as an efficient choice for reducing residual gas density – a typical example is MAX IV, for which CERN was closely involved (CERN Courier September 2017 p38). The new way to produce small-diameter beam pipes represents another step in this fruitful collaboration.
Further technology transfer has come from the sophisticated simulations necessary for the HL-LHC and the Future Circular Collider study. A typical example is the integration of electromagnetic and thermomechanical phenomena during a magnet quench to assess the integrity of the vacuum vessel. Another example is the simulation of gas-density and photon-impingement profiles by Monte Carlo methods. These simulation codes have found a large variety of applications well beyond the accelerator field, from the coating of electronic devices to space simulation. For the latter, codes have been used to model the random motion and migration of any chemical species present on the surfaces of satellites at the time of their launch, which is a critical step for future missions to Mars looking for traces of organic compounds.
Of course, the main objective of the CERN vacuum group is the operation of CERN’s accelerators, in particular those in the LHC chain. Here, the relationship with industry is key because the vacuum industry across CERN’s Member and Associate Member states provides us with state-of-art components, valves, pumps, gauges and control equipment that have contributed to the high reliability of our vacuum systems. On the other hand, the LHC gives high visibility to industrial products that, in turn, can be beneficial for the image of our industrial partners. Collaborating with industry is a win–win situation.
The variety of projects and activities performed at CERN provide us with a continuous stimulation to improve and extend our competences in vacuum technology. The fervour of new collider concepts and experimental approaches in the physics community drives us towards innovation. Other typical examples are antimatter physics, which requires very low gas density (figure 4), and radioactive-beam physics that imposes severe controls on contamination and gas exhausting. New challenges are already visible at the horizon, for example physics with gas targets, higher-energy beams in the LHC, and coating beam pipes with high-temperature superconductors to reduce beam impedance.
An orthogonal driver of innovation is reducing the costs and operational downtime of CERN’s accelerators. In the long term, our dream is to avoid bakeout of vacuum systems so that very low pressure can be attained without the heavy operation of heating the vacuum vessels in situ, principally to remove water vapour. Such advances are possible only if the puzzling interaction between water molecules and technical materials is understood, where again only a very thin layer on top of material surfaces makes the difference. Achieving ultra-high vacuum in a matter of a few hours at a reduced cost would also have an impact well beyond the high-energy physics community. This and other challenges at CERN will guarantee that we continue to push the limits of vacuum technology well into the 21st century.
Les prouesses du CERN dans le vide
À l’échelle mondiale, les technologies du vide représentent un marché de plusieurs milliards de dollars, mais ce n’est que lorsque le vide est utilisé pour des accélérateurs de particules destinés à la physique des hautes énergies que cette technologie révèle toute sa complexité et sa nature multidisciplinaire. Le CERN a réuni des spécialistes de la physique des surfaces, des experts des revêtements en couche mince et des professionnels des traitements galvaniques, ainsi que des équipes de concepteurs et d’autres spécialistes qui se consacrent au fonctionnement de grands équipements pour le vide. Avec toutes ces compétences spécialisées réunies sous le même toit, et le flux constant de projets complexes exigés par la recherche fondamentale auquel il doit faire face, le CERN est devenu l’un des centres de R&D les plus éminents au monde pour les technologies du vide poussé.