CERN Accelerating science

Updates to the CLIC performance studies

The CLIC study submitted a number of reports to the update of European Strategy for Particle Physics at the end of 2018, among them a detailed description of the CLIC accelerator complex and its performance. The CLIC project proposes electron-positron collisions in three energy stages: 380, 1500, and 3000 GeV in the center of mass. The baseline scenario for the initial CLIC stage consists of eight years of data taking at 380 GeV accumulating 1.0 ab-1 of data.

In subsequent studies, the collaboration has studied ways of increasing the luminosity performance at 380 GeV – by a factor of two to three – at modest additional cost and power consumption. Studies also confirm that high-luminosity running at the Z-pole is possible with a staged installation, or as a dedicated operating period with redistributed modules, and that gamma-gamma collisions at up to ∼315 GeV are possible with an interesting luminosity spectra for physics. These performance updates are summarized in a recent CLIC note.

Increasing the luminosity at 380 GeV

The CLIC baseline luminosity at 380 GeV is 1.5 x 1034 cm-2 s-1.  The key free parameter for the luminosity is the normalised vertical emittance εy at the interaction point (IP). The value of εy at the damping ring extraction is 5 nm, and a growth to 30 nm during the transport to the IP, due to various imperfections, has been taken into account for in the luminosity predictions. Considering only static imperfections, the luminosity would be 1.9 x 1034 cm-2 s-1; simulations of the expected performance show more than 2.3 x 1034 cm-2 s-1 with a 90% likelihood and an average value of 3.0 × 1034 cm-2 s-1. A machine without imperfections would reach a luminosity of 4.3 x 1034 cm-2 s-1. Future improvements of the technologies to mitigate imperfections, e.g. better pre-alignment and active stabilisation systems, will allow to come closer to this value. A machine without imperfections would reach a luminosity of 4.3 x 1034 cm-2 s-1. Future improvements of the accelerator technologies to mitigate imperfections – e.g. better pre-alignment and active stabilisation systems – will allow to come closer to such a value.

An important outcome of the technical studies made for the European Strategy update concerning the 380GeV initial energy stage was the realization that the repetition rate of the facility, and consequently its luminosity, could be doubled, from 50Hz to 100Hz, without major changes and with relatively little increase in the overall power consumption. This is because a large fraction of the power is used by systems where the consumption is independent of the repetition rate. Specifically, even though the power required by the RF systems increases by about a factor two, the total power consumption only increases from 170 to 220 MW, that is, around 30%. The associated cost increase must be evaluated in detail, but it is expected to be approximately 5%. Some components of the collider would require minor design modifications, and these are well understood. Specifically, a special study verified that we can control the impact of the stray fields will be larger at 100 Hz.

Running CLIC at the Z-pole

Operating the 380 GeV CLIC accelerator complex at the Z-pole results in an expected luminosity of about L = 2.3 × 1032 cm-2 s-1. In this scenario, the main linac gradient is reduced by about a factor of four, leaving the rest of the configuration. On the other hand, an initial installation of the linac modules needed for a Z-pole factory, and an appropriately adapted beam delivery system, would result in a luminosity of L = 0.36 × 1034 cm-2 s-1 for a 50 Hz operation. This option is worthwhile if one would operate for a couple of years at the Z-pole, at the beginning or end of the first energy stage.

Gamma-gamma collisions at 380 GeV

Recent studies have also revealed the potential of operating the CLIC linear collider in gamma-gamma mode, providing additional physics possibilities. In this mode, two electron beams are focused at the interaction point, but just before it an intense laser pulse collides with each beam. The electrons will Compton-scatter photons in the direction of the collision point, typically carrying 80% of the beam energy. The expected 80% electron polarization is important for this process. An example of a gamma-gamma luminosity spectrum for the 380 GeV stage is shown in figure 1.

figure 1.png

The luminosities discussed above can be delivered to one detector as in the baseline CLIC scenario, or shared on two detectors using either push-pull technology, or by constructing a second beam delivery system and interaction point. However, the latter would add substantially to the costs (∼ 15%) of the accelerator project.

Further technical details about these studies can be found in the recent CLIC study update:

  1. CLIC-Note-1143: CLIC study update August 2019:
Panos Charitos (CERN)
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UK’s Free Electron Laser Test Facility reached another significant milestone.

Improving access to FEL facilities through the CompactLight project

Group photo from CompactLight's Midterm Review Meeting. (Image: CompactLight)


The H2020 design study CompactLight gathered in Helsinki for its Midterm Review Meeting, a full review of the project held halfway through its duration, at the end of the project’s Month 18. CompactLight is an international collaboration that brings together world-leading experts in the fields of accelerator technologies and FEL radiation, with the goal of designing a new generation of Free-Electron Laser (FEL) facilities. These aim to be more compact, more power-efficient, and cheaper in construction and operation, compared to present-day facilities, based on conventional technologies. FELs are among the most powerful tools for investigating matter, but have been chronically overbooked by the users, due to the small number of existing facilities owing to their huge costs. By making FEL facilities cheaper, and thus affordable for institutions with less financial resources, the project aims to contribute to the larger diffusion of these light sources.

The key concept of the project is to combine cutting-edge technology for each different component of a FEL light source into a single machine. One of technologies used by CompactLight is the high-gradient normal conductive Radio-Frequency (RF) acceleration in the X band, developed at CERN within the context of the Compact Linear Collider (CLIC) study, as well as innovative undulator technologies developed by leading institutes like KIT and STFC.

A group of 24 academic and industrial partners bring the necessary expertise to fulfil the project’s goals. The collaboration, coordinated by Elettra Sincrotrone Trieste, involves eleven public European research institutes (Elettra, CERN, STFC, IASA, INFN, ENEA, ALBA-CELLS, CNRS, KIT, PSI, CSIC), eight European universities (Uppsala, Ankara-IAT, Lanchester, Eindhoven, Roma-Sapienza, Helsinki-HIP, Free University of Amsterdam, Strathclyde), two European industrial partners (Kyma and VDL), and three extra-European partners (SINAP, University of Melbourne and ANSTO-AS).

The successful meeting in Helsinki provided clear evidence of the large progress in defining the machine parameters and of the advancements made by the partners in the design of each single subsystem, since the Annual Meeting in Barcelona in December 2018. Options for a very compact beam injector in the S-, C- and X- band of frequencies have been presented and discussed. The definition of a standardised unit for the X-band linac is progressing, a component that could even be used as a stand-alone element for smaller projects, for instance, a university-scale Compton source, or smaller FELs for special applications that can be constructed and operated with smaller budgets. New concept undulators like superconductive undulators and cryogenic permanent-magnet undulators, as well as exotic schemes, like microwave undulators, are also under investigation.

A session dedicated to industrial partners, rarely occurring at such an early stage of a design study, demonstrated the great interest the project is gathering. World-leading industries attended the conference: CPI, Canon (ex-Toshiba), Thales, Scandinova, and Jema displayed their latest developments and carefully studied the new directions suggested by the CompactLight collaboration. Accelerator physicists and industrial partners gathered to exchange experiences, technology requirements, and the new machine parameters, to identify where industry must look at in the future.

The large demand for high-quality X-rays sources and the technological developments pushed by CompactLight make the project particularly appealing, not just for the accelerator community and for the numerous X-rays users, but also for European and worldwide industries. The industry session demonstrated that, despite its young age, CompactLight is already showing an impact in the field today. The success of CompactLight, determined by the promising results obtained so far, will affirm X-band technology as a new standard for accelerator-based facilities and advance undulators to the next generation of compact photon sources. A new generation of compact X-band-based accelerators and light sources is approaching, across and beyond Europe.

The next meeting will be held in Istanbul in January 2020. At the time, the collaboration is expected to demonstrate further progress and a concrete idea of the final CompactLight design.

Panagiotis Charitos
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Cryogenics and its applications took central stage during the EASISchool 2 that was held from the 30th of September to the 4th of October in two CEA sites, Paris-Saclay and Grenoble, France.

Highlights from CLIC Week 2019

Figure 1: the CLIC collaboration in January 2019. (Image: M Volpi)

The physics opportunities brought by of a high-luminosity linear electron-positron collider

The annual Compact Linear Collider (CLIC) workshop brings together the full CLIC community, this year attracting more than 200 participants to CERN on 21-25 January 2019. The workshop covered accelerator and detector R&D, as well as detailed studies of the physics opportunities at a high-luminosity linear electron-positron collider. The programme also included civil engineering aspects, infrastructure studies, industrial involvement, and technology spin-off activities.

CLIC occupies a unique position in both the precision and energy frontiers, combining the benefits of electron-positron collisions with the possibility of multi-TeV collision energies. The CLIC project covers the design and parameters for a collider built and operated in three stages, from 380 GeV to 3 TeV, with a diverse physics programme spanning 30 years. CLIC uses an innovative two-beam acceleration scheme, in which high-gradient X-band accelerating structures are powered via a high-current drive beam, thereby reducing the size and cost of the accelerator complex significantly.

Figure 2: The main electron beam is produced in a conventional radio frequency (RF) injector, which allows polarisation and is accelerated to 2.86 GeV. The beam emittance is then reduced in a damping ring. To produce the positron beam, an electron beam is accelerated to 5 GeV and sent into a crystal target to produce energetic photons, which hit a second target and produce electron-positron pairs. The positrons are captured and accelerated to 2.86 GeV. Their beam emittance is reduced in a series of damping rings. In the final injection step, a booster linac system accelerates both beams to 9 GeV, the bunch length is compressed, and the beams are delivered to the main linacs where they are accelerated to 190 GeV, using a unique and innovative two-beam accelerating scheme featuring 12 GHz X-band accelerating structures that reach accelerating gradients of 100 MV/m. The beam delivery system removes transverse tails and off-energy particles with collimators, and compresses the beam to the small size required at the interaction point (IP). After collision, the beams are transported by the post collision lines to their respective beam dumps. (Image: CLIC/CERN)

In Wednesday’s open plenary session, Steinar Stapnes, the CLIC project leader at CERN, reported that the key CLIC concepts, such as drive beam production, operation of high-efficiency radio-frequency cavities, and other enabling technologies, had all been successfully demonstrated. The afternoon session also included detailed presentations on the accelerator and detector status, and of the CLIC physics potential, including key motivations for an electron-positron collider that can be extended to multi-TeV energies.

“The CLIC project offers a cost-effective and innovative technology and is ready to proceed towards construction with a Technical Design Report. Following the technology-driven timeline, CLIC would realise electron-positron collisions at 380 GeV as soon as 2035,” says Stapnes.

A major focus in 2018 was the completion of a Project Implementation Plan (PiP), as well as several comprehensive Yellow Reports describing the CLIC accelerator, detector, and detailed physics studies. These reports were further distilled into the two formal input documents, submitted to the European Strategy for Particle Physics Update 2018 - 2020 (ESPP), which can be found at:

A central point of the workshop was an updated cost and power estimate, which for the first stage amount to around 5.9 billion CHF and 168 MW, respectively. The energy upgrade to 1.5 TeV adds a cost of approximately 5.1 billion CHF, including an upgrade of the drive-beam radiofrequency power. Further, the energy upgrade to 3 TeV would add approximately 7.3 billion CHF, including the construction of a second drive-beam complex. With the proposed staged approach these costs can be distributed over two or three decades.

Looking to the future, the workshop also discussed the next important step for the CLIC project: an initial five-year preparation phase focusing on further design, technical and industrial developments, with a focus on cost, power and risk reduction. Civil engineering aspects and infrastructure preparation, including site authorisation, will become increasingly detailed during the preparation phase. At the same time, system verifications will be made in dedicated CLIC prototype setups, as well as in other facilities, which have similar high-power radiofrequency linac systems and low emittance beams. These include normal-conducting Free Electron Laser (FEL) linacs and compact photon sources such as those based on Inverse Compton Scattering.

These facilities will provide powerful demonstrations and new benchmarks for reliability, technical parameters, simulation and modelling tools. The goal is to produce a Technical Design Report (TDR), enabling the start of construction for the first CLIC stage by 2026.

The technology-driven schedule is shown in Figure 2, showing the construction and commissioning period and the three stages for data taking, including the corresponding goal for integrated luminosity. The schedule for construction and installation shows that the CLIC project can be implemented well in time for first collisions in 2035, which would allow the exploration of Higgs physics at CLIC immediately after the end of the high-luminosity LHC programme.

2018 saw several significant achievements, including extensive X-band structure development and testing at CERN and in collaborating institutes; further developments of high-efficiency radiofrequency systems; overall system verification studies in CERN Linear Electron Accelerator for Research (CLEAR), Accelerator Test Facility (ATF2) at KEK, free-electron lasers and low emittance rings; comprehensive civil engineering and infrastructure studies; and numerous technical developments, optimising the most critical and cost/power-driving components of the CLIC accelerator.

In parallel, a systematic overview of potential industrial involvement in the CLIC core technologies is being compiled. Several agreements with collaboration partners support technical developments for smaller X-band based accelerators and elements with similar parameters. These include the CompactLight European Commission Design Study for an X-band based free-electron laser design, and the recently proposed eSPS project that aims to study dark sector physics with an Super Proton Synchrotron (SPS)-based primary electron beam facility at CERN, comprising a compact X-band electron injector linac using CLIC technology. These efforts, and many others, were highlighted throughout the week and the subject of a dedicated session on applications of high-gradient X-band technology and of advanced uses of electron beams.

The workshop also discussed interesting developments in plasma- and dielectric-based acceleration. The laser straight linear tunnel of CLIC can provide a natural infrastructure for long-term future projects that might use such novel acceleration techniques.

In the CLIC detector R&D session focusing on silicon pixel technologies, several new and refined test-beam analysis and simulation results were presented. These results enabled the design of two new monolithic detector technology demonstrators targeting the challenging vertex and tracker requirements at CLIC. Final design features for both chips, as well as plans for future tests, were presented and discussed. Many of these tests will take place at the test-beam facilities of DESY in Hamburg, where the CLICdp vertex and tracker group will be welcomed for several weeks during the second planned long shutdown of CERN’s accelerators in 2019/20.

The workshop heard reports on recent developments for Standard Model precision tests in the Higgs boson and top-quark sector, as well as on the broad sensitivity to effects from physics beyond the Standard Model. Updates were presented on benchmark scenarios with challenging signatures such as boosted top quarks and Higgs bosons, numerously produced at the  higher-energy stages of CLIC. Further, the Higgs self-coupling, which determines the shape of the Higgs potential, can be directly accessed at the multi-TeV collisions at CLIC through the measurement of double Higgs production. This measurement benefits from the excellent jet resolution and flavour tagging capabilities of the CLIC detector as well as the clean collision environment in electron-positron collisions. The workshop reported projections of the Higgs self-coupling, in a full simulation study, reaching a precision of 10%, an accuracy that is preserved in a global fit of the full Higgs programme of CLIC.

The CLIC physics programme continues to attract interest from the theory community. A series of talks in a dedicated mini-workshop, joint between theorists and experimentalists, reported on the CLIC potential to extend our knowledge of physics beyond the Standard Model. New results were presented showing the CLIC potential to probe the possible composite nature of the Higgs boson at the scale of tens of TeVs, to discover dark matter candidates such as the thermal Higgsino, and to study axion-like particles in a unique mass range. It was also shown how searching for neutral scalar particles coupled through the Higgs will allow CLIC to explore models relating to the nature of the Electroweak Phase Transition and with non-minimal supersymmetric models addressing the Naturalness Problem. The workshop also saw discussions of first studies of stub-tracks and long-lived particles searches, a domain in which CLIC has promising potential for future exploration.

There is widespread support for a lepton collider for high-precision Higgs boson and top-quark physics. CLIC is a mature contender that, in addition, can be extended to multi-TeV collisions, providing unique sensitivity to physics beyond the Standard Model.

Further reading:

  • CLIC 2018 Summary Report (CERN-2018-005-M)
  • CLIC Project Implementation Plan (CERN-2018-010-M)
  • The CLIC Potential for New Physics (CERN-2018-009-M)
Mauro Taborelli (CERN)
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Graeme Burt (Lancaster University), Donna Pittaway (STFC), Trevor Hartnett (STFC) and Peter Corlett (STFC)
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D. Gamba, A. Curcio, R. Corsini (CERN)
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CLIC technology lights the way to compact accelerators

What if accelerators could be more compact and more cost-effective? It would make their use in research, industry and medicine more affordable and more accessible. This is where the CompactLight project steps in. This new European project, which kicked off on 25 January at CERN, aims to use advanced linear-accelerator (linac) technology, developed at CERN and elsewhere, to develop a new generation of compact X-ray free-electron lasers (XFELs).

XFELs work by accelerating electrons at almost the speed of light before sending them through “undulators”, which are an array of magnets producing alternating magnetic fields. These fields deflect the electrons back and forth to produce high-intensity X-ray beams of unprecedented brilliance and quality. These X-ray beams provide novel ways to probe matter and allows researchers to make “movies” of ultrafast biological processes. The demand for such high-quality X-rays is large, as the field still has great and largely unexplored potential for science and innovation – potential that can be unlocked if the linacs that drive the X-ray generation can be made smaller and cheaper.
By using a technology known as “X-band”, linacs can accelerate electrons with higher accelerating-gradients, resulting in shorter accelerating cavities and hence a more compact machine. X-band technology is the result of years of intense research and development at SLAC in the US, KEK in Japan and at CERN in the context of the Compact Linear Collider (CLIC) project.

The latest developments in high-quality beam sources, as well as innovative undulators are also part of the recipe for achieving a significant reduction in facility cost. Compared with existing XFELs, the proposed facility can have a lower electron-beam energy (due to the enhanced undulator performance), so can be more compact (with both lower energy and a higher accelerating-gradient) and have lower electrical power demand.

Success for CompactLight will have a much wider impact: not just establishing X-band technology as a new option for accelerator-based facilities, but integrating advanced undulators to the next generation of compact photon sources. This can help the wider spread of a new generation of compact X-band-based accelerators and light sources, with a large range of applications including medical use, and enable the development of compact cost-effective X-ray facilities at national or even university level across and beyond Europe.

The three-year CompactLight project, funded by the European Commission’s Horizon 2020 programme, brings together a consortium of 21 leading European institutions, including Elettra, CERN, PSI, KIT and INFN, in addition to seven universities and two industry partners (Kyma and VDL). The CompactLight kick off took place during the 2018 CLIC workshop, held at CERN.

This article originally appeared on the CERN homepage.

Header image: A CLIC X-band prototype structure built by PSI using Swiss FEL technology. (Image credit: M Volpi)

Daniela Antonio (CERN)
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