This is the 8th conference in the IBIC series, the follow-on to the very successful regional BIW and DIPAC workshop series. IBIC brings together the world community of experts in instrumentation for particle accelerators, to explore the physics and engineering challenges of beam diagnostics and measurement techniques for charged particle beams. The conference program includes tutorials on selected topics, invited and selected talks, as well as poster sessions.
The North American Particle Accelerator Conference (NAPAC) brings together several hundred experts in all fields of accelerator science and technology.
It is the largest domestic particle accelerator conference and covers the entire spectrum of accelerator science and technology topics. As such, NAPAC is particularly useful for students, postdocs, technicians, and engineers as they can be exposed to the entire field in one conference.
Delegates present invited and contributed papers and posters, receive immediate feedback on their research, and get problem-solving suggestions. Mini-courses on highly-relevant topics are also offered. Everyone leaves with new ideas and possible solutions to their own technical problems. Attendees develop new contacts and strengthen existing collaborations with colleagues throughout the DOE complex and internationally. Many of the most prominent accelerator vendors also present at and help support NAPAC. It is an excellent venue for all conference attendees to bring themselves up to date with the newest developments in accelerator technology.
Some 120 physicists gathered in Orsay on 13–14 December 2018 for a workshop on additive manufacturing – popularly called 3D printing – with metals. The goal was to review the work being done in Europe (particularly at CERN, CEA and CNRS) on the application of the technique to high-energy physics and astrophysics.
3D printing makes possible novel and optimised designs that would be difficult to create with conventional methods. Embedded radio-frequency (RF) cavities such as those featured in spiral-shaped cooling channels are one example. Another comes from detector design: mesh structures, as required for many gas-filled ionisation tracking detectors, are often difficult to manufacture with traditional methods as the removal of material in one part of the mesh may destroy another part of it; but they are easy to build with additive manufacturing.
Despite the remaining challenges, which relate to ultra-high-vacuum properties, mechanical strength, electrical conductivity, new alloys and post-processing, the technique is beginning to be used for working accelerator components. Participants of the Orsay event heard about the beam test at an accelerator (LAL’s photoinjector PHIL), of a beam position monitor and about the performances of RF antennas designed at Université de Rennes for future space missions. Plans for employing additive manufacturing at future accelerators and HEP experiments were also discussed.
Although metal additive manufacturing is currently limited to a few applications, the workshop, which was the first of its kind, showed that there is strong potential for it to play a larger role in the coming years.
On 1 April more than 90 delegates gathered at CERN to discuss perspectives on superconducting magnet technology. The workshop marked the completion of phase 1 of the Future Superconducting Magnet Technology (FuSuMaTech) Initiative, launched in October 2017.
FuSuMaTech is a Horizon 2020 Future Emerging Technologies project co-funded by the European Commission, with the support of industrial partners ASG, Oxford Instruments, TESLA, SIGMAPHI, ELLYT Energy and BILFINGER, and academia partners CERN, CEA, STFC, KIT, PSI and CNRS. It aims to strengthen the field of superconductivity for projects such as the High-Luminosity LHC and Future Circular Collider, while demonstrating the benefits of this investment to society at large.
“The need to develop higher performing magnets for future accelerators is certain, and cooperation will be essential,” said Han Dols of CERN’s knowledge transfer group. “The workshop helps reiterate common areas of interest between academia and industry, and how they might benefit from each other’s know-how. And just as importantly,” continued Dols, “FuSuMaTech is seeking to demonstrate the benefits of this investment by setting up demonstrator projects.”
The successful preparation of 10 project proposals for both R&D actions and industrial applications is one of the main achievements of FuSuMaTech Phase-1, noted project coordinator Antoine Dael. These projects include new designs for MRI gradient coils, the design of 14 and 16 T MRI magnets, and a conceptual design for new mammography magnets. New developments are also included in the proposals, with the design for a hybrid low–high temperature superconductor magnet, an e-infrastructure to collect material properties and a pulsed-heat-pipe cooling system.
In phase 2 of FuSuMaTech, launched with the signing of a declaration of intention between the FuSuMaTech partners on April 1, the 10 project proposals prepared during phase 1 will evolve into independent projects and make use of other European Union programmes. “We were really impressed with the interest we got from organisations outside of the project,” said Dael. “We currently have six industrial partners, two more have already contacted us today, and we expect others.”
More than 1100 accelerator professionals gathered in Melbourne, Australia, from 19 to 24 May 2019 for the 10th International Particle Accelerator Conference, IPAC’19. The superb Melbourne Convention and Exhibition Centre could easily cater for the 85 scientific talks, 72 industrial exhibitors and sponsors, 1444 poster presentations and several social functions throughout the week. Record levels of diversity at IPAC’19 saw 42 countries represented from six continents, and a relatively high gender balance for the field, with a quarter of speakers identifying as women.
In the wake of the update of the European Strategy for Particle Physics in Granada in May, accelerator designs that advance the energy and intensity range of a next-generation discovery machine were discussed, but there is no clear statement as to which is best. It will be up to the particle-physics community to decide which capability is needed to reach the most interesting physics. Reports on mature hadron facilities such as Japan’s J-PARC and the LHC were balanced by the photon sources and electron accelerators that are becoming an increasingly robust presence at IPAC, and which comprised a fifth of contributions in 2019. Presentations on the most recently commissioned accelerators were a particular highlight, with Japan’s SuperKEKB collider, Korea’s PAL-XFEL free-electron laser and Sweden’s MAX IV light source taking centre stage.
Exciting progress in the field of plasma- wakefield accelerators was also reported. In particular, Europe’s EuPRAXIA collaboration is aiming to create a laser wakefield accelerator to drive a free-electron laser facility for users in the next few years. The scientific programme was bookended by local Australian-grown talent. Suzie Sheehy from the University of Melbourne described the successes of particle accelerators and some of the future challenges, while Henry Chapman, a director of the Center for Free-Electron Laser Science at DESY and the University of Hamburg, gave the closing plenary on how particle accelerators have enabled groundbreaking work in coherent X-ray science.
“In Unity” was chosen as the theme for IPAC’19 and art was commissioned from Torres Strait islander Kelly Saylor to symbolise this coming together of the particle-accelerator community. The success of IPAC’19 demonstrates the ongoing need for face-to-face meetings to share and communicate ideas and collaborate on pressing scientific problems. In a pioneering effort for the IPAC series, the opening and closing sessions were live-streamed to the world. The aim is to broaden the impact of the conference and highlight the importance of particle accelerators to many fields of science, industry and medical applications.
Student poster prizes were won by Nazanin Samadi, an Iranian PhD student at the University of Saskatchewan, Canada, and Daniel Bafia of Fermilab and IIT. Among other awards, the Xie Jialin Prize went to Vittorio Vaccaro of the University of Naples, the Nishikawa Tetsuji Prize was won by Vladimir Shiltsev of Fermilab, the Hogil Kim Prize went to Xueqing Yan of Peking University, and the Mark Oliphant Prize was taken by Stanford PhD student James MacArthur.
IPAC takes place annually and alternates between Asia, Europe and the Americas. Next year it will move to Caen in France, and then to Brazil in 2021.
A team at Cornell University in the US has demonstrated that high-frequency superconducting radio-frequency (SRF) cavities made from niobium–tin alloy can be operated more efficiently than conventional niobium designs, representing a step towards smaller and more economical particle accelerators.
SRF cavities are the gold standard for the acceleration of charged-particle beams and are used, for example, in the LHC at CERN and the upcoming LCLS-II free-electron-laser X-ray source at SLAC. Currently, the material of choice for the best accelerating cavities is niobium, which frequently has to be operated at a temperature of around 2 K and requires costly cryogenic equipment to cool the cavity in a bath of superfluid liquid helium. The technology is only heavily used at large-scale accelerators, and not at smaller institutions or in industry due to its complexity and costs.
Researchers around the world are striving to remove some of the barriers prohibiting broader uptake of SRF technology. Two major obstacles still need to be overcome to make this possible: the temperature of operation, and the size of the cavity.
Earlier this year, a team at Cornell led by Matthias Liepe demonstrated that small, high-frequency triniobium-tin (Nb3Sn) cavities can be operated very efficiently at a temperature of 4.2 K. While seemingly only slightly warmer than the 2 K required by niobium cavities, this small rise in temperature omits the need for superfluid-helium refrigeration.
The size of the cavity is inversely related to the frequency of the oscillating radio-frequency electromagnetic field within it: as the frequency doubles, the necessary transverse size of the cavity is halved. A smaller cavity with a higher frequency also demands a smaller cryomodule; what was once 1 m in diameter, the typical size of an accelerating SRF cryomodule, can now be roughly half that size.
The vast majority of SRF cavities currently in use operate at frequencies of 1.5 GHz and below – a region favoured because RF power losses in a superconductor rapidly decrease at lower frequency. But this results in large SRF accelerating structures. Cornell graduate student Ryan Porter successfully made and tested a considerably smaller proof-of-principle Nb3Sn cavity at 2.6 GHz with promising results. “Niobium cannot operate efficiently at 2.6 GHz and 4.2 K,” Porter explains. “But the performance of this 2.6 GHz Nb3Sn cavity was just as good as the 1.3 GHz performance. Compared to a niobium cavity at the same temperature and frequency, it was 50 times more efficient.”
“This is really the first step that shows that you can get good 4.2 K performance at high frequency, and it is quite promising,” adds Liepe. “The dream is to have an SRF accelerator that can fit on top of the table.”
The large amount of Run-2 data (collected in 2015–2018) allows the LHC experiments to probe previously unexplored rare processes, search for new physics and improve Standard Model measurements. The amount of data collected in Run 2 can be quantified by the integrated luminosity – a number which, when multiplied by the cross section for a process, yields the expected number of interactions of that type. It is a crucial figure. The uncertainty of several ATLAS Run-1 cross-section measurements, particularly of W and Z production, was dominated by systematic uncertainty on the integrated luminosity. To minimise this, ATLAS performs precise absolute and relative calibrations of several luminosity-sensitive detector systems in a three-step procedure.
The first step is an absolute calibration of the luminosity using a van-der-Meer beam-separation scan under specialised beam conditions. By displacing the beams horizontally and vertically and scanning them through each other, it is possible to measure the combined size of the colliding proton bunches. Determining in addition the total number of protons in each colliding bunch from the measurement of the beam currents, the absolute luminosity of each colliding bunch pair can be derived. Relating this to the mean number of interactions observed in the LUCID-2 detector – a set of photomultiplier tubes located 17 m in either direction along the beam pipe that detect the Cherenkov light of particles which come from the interaction – the scale for the absolute luminosity measurement of LUCID-2 is set.
The second step is to extrapolate this calibration to LHC physics conditions, where the number of interactions increases from fewer than one to around 20–50 interactions per crossing, and the pattern of proton bunches changes from isolated bunches to trains of consecutive bunches with 25 ns spacing. The LUCID-2 response is sensitive to these differences. It is corrected with the help of a track counting algorithm, which relates the number of interactions to the number of tracks reconstructed in ATLAS’s inner detector.
The final step is to monitor the stability of the LUCID-2 calibration over time. This is evaluated by comparing the luminosity estimate of LUCID-2 to those from track counting in the inner detector and various ATLAS calorimeters over the course of the data-taking year (figure 1). The agreement between detectors quantifies the stability of the LUCID-2 response.
Using this three-step method and taking into account correlations between years, ATLAS has obtained a preliminary uncertainty on the luminosity estimate for the combined Run-2 data of 1.7%, improving slightly on the Run-1 precisions of 1.8% at 7 TeV and 1.9% at 8 TeV. The full 13 TeV Run-2 data sample corresponds to an integrated luminosity of 139 fb–1 – about 1.1 × 1016 proton collisions.
The AVS International Symposium and Exhibition, held each fall, is attended by scientists, engineers, professors, senior managers, technicians and students from the US and abroad. More than 1300 papers and posters are presented in over 150 technical sessions.
An extensive exhibition of equipment, tools, materials, supplies, chemicals, services and consulting, technical literature and new technologies are showcased.
AVS is a society unique for its multidisciplinary coverage of science and technology – no place is this more evident than at the International Symposium and Exhibition which attracts international attendees from a variety of industries.
Vacuum technology for particle accelerators has been pioneered by CERN since its early days. The Intersecting Storage Rings (ISR) brought the most important breakthroughs. Half a century ago, this technological marvel – the world’s first hadron collider – required proton beams of unprecedented intensity and extremely low vacuum pressures in the interaction areas (below 10–11 mbar). Addressing the former challenge led to innovative surface treatments such as glow-discharge cleaning, while the low-vacuum requirement drove the development of materials and their treatments. It also led to novel high-performance cryogenic pumps and vacuum gauges that are still in use today, and CERN’s record for the lowest ever achieved pressure at room temperature (2 × 10–14 mbar) still stands.
The Large Electron Positron (LEP) collider opened a new chapter in CERN’s vacuum story. Even though LEP’s residual gas density and current intensities were less demanding than those of the ISR, its exceptional length and intense synchrotron-light power 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 cavities based on niobium thin-film coating on copper substrates. It was a great success, and the present accelerating RF cavities of the LHC and HIE-ISOLDE are essentially based on the expertise assimilated for LEP.
The coexistence at CERN of both NEG and thin-film expertise was the seed for another breakthrough in vacuum technology: NEG thin-film coatings, driven by the requirements of the LHC and its 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 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.
Synchrotron radiation-induced desorption and electron clouds at temperatures of around 4.3 K had to be studied in depth for the LHC, leading CERN’s vacuum experts to develop new concepts for vacuum systems at cryogenic temperatures, in particular the beam screen. The more intense beams of the high-luminosity LHC (HL-LHC) upgrade, presently under way, will amplify the effect of electron clouds on both the beam stability and the thermal load to the cryogenic systems. Since NEG coatings are limited for room-temperature beam pipes, an alternative strategy had to be found for the parts of the accelerators that cannot be heated, such as those in the HL-LHC’s inner triplet magnets.
Following an idea that originated at CERN in 2006, thin-film coatings made from carbon offer a solution and the material has already been deposited on tens of SPS vacuum chambers within the LHC Injectors Upgrade project. Another idea to fight electron clouds for the HL-LHC involves laser-treating surfaces to make them more rough, so that secondary electrons are intercepted by the surrounding surfaces. 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 (see image above). 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. Another benefit of this study is the development of small robots for the in situ inspection of long ultra-high vacuum beam pipes, such as those of the LHC’s arcs.
The Compact Linear Collider (CLIC) project, which envisages a high-energy linear electron–positron collider at CERN, demands quadrupole magnets with a very small-diameter beam pipe (about 8 mm) supporting pressures in the ultra-high vacuum range. This 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 in which 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.
Next-generation synchrotron-light sources share CLIC’s need for very compact magnets with magnetic poles as close as possible to the beam, so as to reduce costs and improve beam performance. CERN’s vacuum group collaborates closely with vacuum experts of light sources, MAX IV in Sweden and PSI in Switzerland being prominent examples, to develop the required very-small-diameter vacuum pipes. Further technology transfer has come from the sophisticated simulations necessary for the HL-LHC and the Future Circular Collider study, which have also found applications beyond the accelerator field, from the coating of electronic devices to space simulation.
Relations with industry are key to the operation of CERN’s accelerators, especially those in the LHC chain. The vacuum industry across CERN’s member countries provides us with state-of-art components, valves, pumps, gauges and control equipment that have contributed to the high reliability of the lab’s vacuum systems. In return, the LHC gives high visibility to industrial products. Indeed, the variety of projects and activities performed at CERN provide us with a continuous stimulation to improve and extend our competences in vacuum technology. In addition to future colliders are: antimatter physics, which requires very low gas density; radioactive-beam physics, which imposes severe controls on contamination and gas exhausting; and gravity-wave physics for which the tradeoff between cost and performance of vacuum systems is essential for the approval of next-generation observatories.
An orthogonal driver of innovation in vacuum technology is the reduction of costs and operational downtime of CERN’s accelerators. 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 involved in fundamental exploration are guaranteed to drive further advances in vacuum technology.
Muon colliders have been the topic of much discussion this week during the update of the European Strategy for Particle Physics (ESPP) in Granada. Proposals for such a machine are much less advanced than those for other projects under consideration for a post-LHC collider. A muon collider is therefore not being considered on the same footing as circular and linear electron—positron projects such as FCC-ee, CEPC, ILC and CLIC. Nevertheless, its potential to produce very high-energy collisions that can transfer all energy into the production of new particles in a relatively small facility is proving difficult for physicists to resist.
In one of 160 written inputs to the ESPP update, the Muon Collider Working Group points out that a 14 TeV muon collider provides an effective energy reach similar to that of a 100 TeV FCC proton—proton machine. In addition to its energy reach, the report states, a dedicated muon collider is able to precisely measure the Higgs boson’s mass and width, along with other observables: “A muon collider is therefore ideal to search for and/or study new physics and for resolving narrow resonances both as a precision facility and/or as an exploratory machine”. A 14 TeV muon machine would fit neatly into the tunnel that is currently occupied by the LHC, and in principle the technology could go to much higher energies.
Being some 200 times more massive than electrons, muons suffer significantly less from synchrotron-radiation losses and therefore can be accelerated efficiently in a circular machine. But reaching high luminosities is extremely tough owing to short muon lifetime at rest (2.2 μs) and the difficulty of producing large numbers of muons in suitably shaped bunches.
One way to conquer this problem is to “cool” an initial low-energy muon beam, which has large transverse and longitudinal emittances, by several orders of magnitude in the 6D phase-space and then rapidly accelerate it. Last year, the UK-based Muon Ionisation Cooling Experiment (MICE) demonstrated the principle of ionisation cooling, by observing 4D cooling in a low-flux muon beam. “The results point the way to an exciting programme in which muon beams of high-brightness are exploited to seek new insights into the properties of neutrinos and to explore the energy frontier with multi-TeV lepton-antilepton collisions,” says MICE spokesperson Ken Long of Imperial College, London, who was present at the ESPP symposium. Fermilab in the US also pursued such technologies in its dedicated Muon Accelerator Program (MAP), while an experiment at J-PARC in Japan last year accelerated muons by a radio-frequency accelerator for the first time.
Recently, physicists in Italy and France proposed an alternative concept for a muon collider called the Low Emittance Muon Accelerator (LEMMA), which offers a naturally cooled muon beam with a long lifetime in the laboratory frame by capturing muon–antimuon pairs created in electron–positron annihilation.
The Muon Collider Working Group, which was established by CERN in 2017, has performed a first, high-level review of these two muon collider schemes. “The focus has been on the positron-based scheme, which it was really promising but it has been found to require consolidation,” said Daniel Schulte of CERN, reporting the group’s findings in Granada this week. The group recommends that an international collaboration “promote muon colliders and organise the effort on the development of both accelerators and detectors and to define the road-map towards a CDR by the next ESPP update”. The production of muon neutrinos with a well-known flux and energy spectrum would also serve as the source for a neutrino factory.
Given the broad spectrum of views expressed in Granada this week, both during the sessions and in the sidelines, the path to a muon collider could be bumpy. “At the Higgs pole, it’s not competitive with any of the other machines, although the perspective for Higgs physics at a very high-energy muon collider is actually very good,” pointed out one participant during a discussion session about Higgs and electroweak physics. “It’s total fantasy!” said another. But the view from the expert working group is more positive. Can muon colliders at this moment be considered for the next project? “Not yet,” said Schulte, but enormous progress has been made and it is clearly worthwhile to continue R&D. “A muon collider may be the best option for very high lepton collider energies beyond 3 TeV, and has strong synergies with other projects such as magnet and RF development,” concluded Schulte. “We should not miss this opportunity.”
