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First light beckons at SLAC’s LCLS-II

**Intense** The LCLS undulator hall, where high-energy electrons run a gauntlet of 32 undulators each containing 224 magnets whose alternating poles force the electrons to zigzag violently and radiate X-rays. Credit: SLAC

An ambitious upgrade of the US’s flagship X-ray free-electron-laser facility – the Linac Coherent Light Source (LCLS) at SLAC in California – is nearing completion. Set for “first light” early next year, LCLS-II will deliver X-ray laser beams that are 10,000 times brighter than LCLS at repetition rates of up to a million pulses per second – generating more X-ray pulses in just a few hours than the current laser has delivered through the course of its 12-year operational lifetime. The cutting-edge physics of the new facility – underpinned by a cryogenically cooled superconducting radio-frequency (SRF) linac – will enable the two beams from LCLS and LCLS-II to work in tandem. This, in turn, will help researchers observe rare events that happen during chemical reactions and study delicate biological molecules at the atomic scale in their natural environments, as well as potentially shed light on exotic quantum phenomena with applications in next-generation quantum computing and communications systems.

Successful delivery of the LCLS-II linac was possible thanks to a multi-centre collaborative effort involving US national and university laboratories, following the decision to pursue an SRF-based machine in 2014 through the design, assembly, test, transportation and installation of a string of 37 SRF cryomodules (most of them more than 12 m long) into the SLAC tunnel. All told, this major undertaking necessitated the construction of forty 1.3 GHz SRF cryomodules (five of them spares) and three 3.9 GHz cryomodules (one spare) – with delivery of approximately one cryomodule per month from February 2019 until December 2020 to allow completion of the LCLS-II linac installation on schedule by November 2021.

This industrial-scale programme of works was shaped by a strategic commitment, early on in the LCLS-II design phase, to transfer, and ultimately iterate, the established SRF capabilities of the European XFEL in Hamburg into the core technology platform used for the LCLS-II SRF cryomodules. Put simply: it would not have been possible to complete the LCLS-II project, within cost and on schedule, without the sustained cooperation of the European XFEL consortium – in particular, colleagues at DESY, CEA Saclay and several other European laboratories as well as KEK – that generously shared their experiences and know-how.

Better together

These days, large-scale accelerator or detector projects are very much a collective endeavour. Not only is the sprawling scope of such projects beyond a single organisation, but the risks of overspend and slippage can greatly increase with a “do-it-on-your-own” strategy. When the LCLS-II project opted for an SRF technology pathway in 2014 to maximise laser performance, the logical next step was to build a broad-based coalition with other US Department of Energy (DOE) national laboratories and universities. In this case, SLAC, Fermilab, Jefferson Lab (JLab) and Cornell University contributed expertise for cryomodule production, while Argonne National Laboratory and Lawrence Berkeley National Laboratory managed delivery of the undulators and photoinjector for the project. For sure, the start-up time for LCLS-II would have increased significantly without this joint effort, extending the overall project by several years.

Superconducting accelerator — **Long view** LCLS-II will add a superconducting accelerator occupying one-third of SLAC’s original 2 mile-long linear accelerator tunnel. At the beam switchyard, electron beams from each linac will be directed to one of two new undulators to produce hard or soft X-ray pulses that are subsequently routed to the experimental halls. Credit: SLAC

Each partner brought something unique to the LCLS-II collaboration. While SLAC was still a relative newcomer to SRF technologies, the lab had a management team that was familiar with building large-scale accelerators (following successful delivery of the LCLS). The priority for SLAC was therefore to scale up its small nucleus of SRF experts by recruiting experienced SRF technologists and engineers to the staff team. In contrast, the JLab team brought an established track-record in the production of SRF cryomodules, having built its own machine, the Continuous Electron Beam Accelerator Facility (CEBAF), as well as cryomodules for the Spallation Neutron Source (SNS) linac at Oak Ridge National Laboratory in Tennessee. Cornell, too, came with a rich history in SRF R&D – capabilities that, in turn, helped to solidify the SRF cavity preparation process for LCLS-II.

Finally, Fermilab had, at the time, recently built two cutting-edge cryomodules of the same style as that chosen for LCLS-II. To fabricate these modules, Fermilab worked closely with the team at DESY to set up the same type of production infrastructure used on the European XFEL. From that perspective, the required tooling and fixtures were all ready to go for the LCLS-II project. While Fermilab was the “designer of record” for the SRF cryomodule, with primary responsibility for delivering a working design to meet LCLS-II requirements, the realisation of an optimised technology platform was a team effort involving SRF experts from across the collaboration.

Collective problems, collective solutions

While the European XFEL provided the template for the LCLS-II SRF cryomodule design, several key elements of the LCLS-II approach subsequently evolved to align with the continuous-wavelength (CW) operation requirements and the specifics of the SLAC tunnel. Success in tackling these technical challenges – across design, assembly, testing and transportation of the cryomodules – is testament to the strength of the LCLS-II collaboration and the collective efforts of the participating teams in the US and Europe.

Challenges are inevitable when developing new facilities at the limits of known technology

For one, the thermal performance specification of the SRF cavities exceeded the state-of-the-art and required development and industrialisation of the concept of nitrogen doping (a process in which SRF cavities are heat-treated in a nitrogen atmosphere to increase their cryogenic efficiency and, in turn, lower the overall operating costs of the linac). The nitrogen-doping technique was invented at Fermilab in 2012 but, prior to LCLS-II construction, had been used only in an R&D setting.

The priority was clear: to transfer the nitrogen-doping capability to LCLS-II’s industry partners, so that the cavity manufacturers could perform the necessary materials-processing before final helium-vessel jacketing. During this knowledge transfer, it was found that nitrogen-doped cavities are particularly sensitive to the base niobium sheet material – something the collaboration only realised once the cavity vendors were into full production. This resulted in a number of process changes for the heat treatment temperature, depending on which material supplier was used and the specific properties of the niobium sheet deployed in different production runs. JLab, for its part, held the contract for the cavities and pulled out all stops to ensure success.

**Production line** The use of two facilities to produce the SRF cryomodules – Fermilab and JLab (left) – ensured a compressed delivery schedule and increased flexibility within the LCLS-II programme. Credit: JLab

At the same time, the conversion from pulsed to CW operation necessitated a faster cooldown cycle for the SRF cavities, requiring several changes to the internal piping, a larger exhaust chimney on the helium vessel, as well as the addition of two new cryogenic valves per cryomodule. Also significant is the 0.5% slope in the longitudinal floor of the existing SLAC tunnel, which dictated careful attention to liquid-helium management in the cryomodules (with a separate two-phase line and liquid-level probes at both ends of every module).

However, the biggest setback during LCLS-II construction involved the loss of beamline vacuum during cryomodule transport. Specifically, two cryomodules had their beamlines vented and required complete disassembly and rebuilding – resulting in a five-month moratorium on shipping of completed cryomodules in the second half of 2019. It turns out that a small (what was thought to be inconsequential) change in a coupler flange resulted in the cold coupler assembly being susceptible to resonances excited by transport. The result was a bellows tear that vented the beamline. Unfortunately, initial “road-tests” with a similar, though not exactly identical, prototype cryomodule had not revealed this behaviour.

Such challenges are inevitable when developing new facilities at the limits of known technology. In the end, the problem was successfully addressed using the diverse talents of the collaboration to brainstorm solutions, with the available access ports allowing an elastomer wedge to be inserted to secure the vulnerable section. A key take-away here is the need for future projects to perform thorough transport analysis, verify the transport loads using mock-ups or dummy devices, and install adequate instrumentation to ensure granular data analysis before long-distance transport of mission-critical components.

**Heavy lifting** The last cryomodule from Fermilab was unloaded at the LCLS-II site at SLAC on 19 May 2021. Credit: J Orrell/SLAC

Upon completion of the assembly phase, all LCLS-II cryomodules were subsequently tested at either Fermilab or JLab, with one module tested at both locations to ensure reproducibility and consistency of results. For high Q₀ performance in nitrogen-doped cavities, cooldown flow rates of at least 30 g/s of liquid helium were found to give the best results, helping to expel magnetic flux that could otherwise be trapped in the cavity. Overall, cryomodule performance on the test stands exceeded specifications, with a total accelerating voltage per cryomodule of 158 MV (versus specification of 128 MV) and average Q₀ of 3 × 10¹⁰(versus specification of 2.7 × 10¹⁰). Looking ahead, attention is already shifting to the real-world cryomodule performance in the SLAC tunnel – something that was measured for the first time in 2022.

Transferable lessons

For all members of the collaboration working on the LCLS-II cryomodules, this challenging project holds many lessons. Most important is to build a strong team and use that strength to address problems in real-time as they arise. The mantra “we are all in this together” should be front-and-centre for any multi-institutional scientific endeavour – as it was in this case. Solutions need to be thought of in a more global sense, as the best answer might mean another collaborator taking more onto their plate. Collaboration implies true partnership and a working model very different to a transactional customer–vendor relationship.

From a planning perspective, it’s vital to ensure that the initial project cost and schedule are consistent with the technical challenges and preparedness of the infrastructure. Prototypes and pre-series production runs reduce risk and cost in the long term and should be part of the plan, but there must be sufficient time for data analysis and changes to be made after a prototype run in order for it to be useful. Time spent on detailed technical reviews is also time well spent. New designs of complex components need a comprehensive oversight and review, and should be controlled by a team, rather than a single individual, so that sign-off on any detailed design changes are made by an informed collective.

LCLS-II science: capturing atoms and molecules in motion like never before

The strobe-like pulses of the LCLS, which produced its first light in April 2009, are just a few millionths of a billionth of a second long, and a billion times brighter than previous X-ray sources. This enables users from a wide range of fields to take crisp pictures of atomic motions, watch chemical reactions unfold, probe the properties of materials and explore fundamental processes in living things. LCLS-II will provide a major jump in capability – moving from 120 pulses per second to 1 million, enabling experiments that were previously impossible. The scientific community has identified six areas where the unique capabilities of LCLS-II will be essential for further scientific progress:

Nanoscale materials dynamics, heterogeneity and fluctuations

Programmable trains of soft X-ray pulses at high rep rate will characterise spontaneous fluctuations and heterogeneities at the nanoscale across many decades, while coherent hard X-ray scattering will provide unprecedented spatial resolution of material structure, its evolution and relationship to functionality under operating conditions.

Fundamental energy and charge dynamics

High-repetition-rate soft X-rays will enable new techniques that will directly map charge distributions and reaction dynamics at the scale of molecules, while new nonlinear X-ray spectroscopies offer the potential to map quantum coherences in an element-specific way for the first time.

Catalysis and photocatalysis

Time-resolved, high-sensitivity, element- specific spectroscopy will provide the first direct view of charge dynamics and chemical processes at interfaces, characterise subtle conformational changes associated with charge accumulation, and capture rare chemical events in operating catalytic systems across multiple time and length scales – all of which are essential for designing new, more efficient systems for chemical transformation and solar-energy conversion.

Emergent phenomena in quantum materials

Fully coherent X-rays will enable new high- resolution spectroscopy techniques to map the collective excitations that define these new materials in unprecedented detail. Ultrashort X-ray pulses and optical fields will facilitate new methods for manipulating charge, spin and phonon modes to both advance fundamental understanding and point the way to new approaches for materials control.

Revealing biological function in real time

The high repetition rate of LCLS-II will provide a unique capability to follow the dynamics of macromolecules and interacting complexes in real time and in native environments. Advanced solution-scattering and coherent imaging techniques will characterise the conformational dynamics of heterogeneous ensembles of macromolecules, while the ability to generate “two-colour” hard X-ray pulses will resolve atomic-scale structural dynamics of biochemical processes that are often the first step leading to larger-scale protein motions.

Matter in extreme environments

The capability of LCLS-II to generate soft and hard X-ray pulses simultaneously will enable the creation and observation of extreme conditions that are far beyond our present reach, with the latter allowing the characterisation of unknown structural phases. Unprecedented spatial and temporal resolution will enable direct comparison with theoretical models relevant for inertial-confinement fusion and planetary science.

Work planning and control is another essential element for success and safety. This idea needs to be built into the “manufacturing system”, including into the cost and schedule, and to be part of each individual’s daily checklist. No one disagrees with this concept, but good intentions on their own will not suffice. As such, required safety documentation should be clear and unambiguous, and be reviewed by people with relevant expertise. Production data and documentation need to be collected, made easily available to the entire project team, and analysed regularly for trends, both positive and negative.

Supply chain, of course, is critical in any production environment – and LCLS-II is no exception. When possible, it is best to have parts procured, inspected, accepted and on-the-shelf before production begins, thereby eliminating possible workflow delays. Pre-stocking also allows adequate time to recycle and replace parts that do not meet project specifications. Also worth noting is that it’s often the smaller components – such as bellows, feedthroughs and copper-plated elements – that drive workflow slowdowns. A key insight from LCLS-II is to place purchase orders early, stay on top of vendor deliveries, and perform parts inspections as soon as possible post-delivery. Projects also benefit from having clearly articulated pass/fail criteria and established procedures for handling non-conformance – all of which alleviates the need to make critical go/no-go acceptance decisions in the face of schedule pressures.

As with many accelerator projects, LCLS-II is not an end-point in itself, more an evolutionary transition within a longer term roadmap

Finally, it’s worth highlighting the broader impact – both personal and professional – on individual team members participating in a big-science collaboration like LCLS-II. At the end of the build, what remained after designs were completed, problems solved, production rates met, and cryomodules delivered and installed, were the friendships that had been nurtured over several years. The collaboration amongst partners, both formal and informal, who truly cared about the project’s success, and had each other’s backs when there were issues arising: these are the things that solidified the mutual respect, the camaraderie and, in the end, made LCLS-II such a rewarding project.

First light

In April 2022 the new LCLS-II linac was successfully cooled to its 2 K operating temperature. The next step was to pump the SRF cavities with more than a megawatt of microwave power to accelerate the electron beam from the new source. Following further commissioning of the machine, first X-rays are expected to be produced in early 2023.

As with many accelerator projects, LCLS-II is not an end-point in itself, more an evolutionary transition within a longer term roadmap. In fact, work is already under way on LCLS-II HE – a project that will increase the energy of the CW SRF linac from 4 to 8 GeV, enabling the photon energy range to be extended to at least 13 keV, and potentially up to 20 keV at 1 MHz repetition rates. To ensure continuity of production for LCLS-II HE, 25 next-generation cryomodules are in the works, with even higher performance specifications versus their LCLS-II counterparts, while upgrades to the source and beam transport are also being finalised.

While the fascinating science opportunities for LCLS-II-HE continue to be refined and expanded, of one thing we can be certain: strong collaboration and the collective efforts of the participating teams are crucial.

This is an updated version of an article published in 2021 in a CERN Courier “In Focus” issue about US accelerator projects.

Science for peace? More than ever!

What happened? A tragedy fell upon Ukraine and found many in despair or in a dilemma. After 70 mainly peaceful years for much of Europe, we were surprised by war, because we had forgotten that it takes an effort to maintain peace.

Having witnessed the horrors of war first hand, several years as a soldier and then as a displaced person, I could not imagine that humanity would unleash another war on the continent. As one of its last witnesses, I wonder what advice should be passed on, especially to younger colleagues, about what to do in the short term, and perhaps more importantly, what to do afterwards.

Scientists have a special responsibility. Fortunately, there is no doubt today that science is independent of political doctrines. There is no “German physics” any more. We have established human relationships with our colleagues based on our enthusiasm for our profession, which has led to mutual trust and tolerance.

This has been practised at CERN for 70 years and continued at SESAME, where delegates from Israel, Palestine, Iran, Cyprus, Turkey and other governments sit peacefully around a table. Another offshoot of CERN, the South East European International Institute for Sustainable Technologies (SEEIIST), is in the making in the Balkans. Apart from fostering science, the aim is to transfer ethical achievements from science to politics: science diplomacy, as it has come to be known. In practice, this is done, for example, in the CERN Council where each government sends a representative and an additional scientist who work effectively together on a daily basis.

**Herwig Schopper** was CERN Director-General from 1981 to 1988, and is a founder of SESAME and the future SEEIIST facility. Credit: CERN-PHOTO-201906-169-6

In the case of imminent political conflicts, “Science for Peace” cannot of course help immediately, but occasionally opportunities arise even for this. In 1985, when disarmament negotiations between Gorbachev and Reagan in Geneva reached an impasse, one of the negotiators asked me to invite the key experts to CERN on neutral territory, and at a confidential dinner the knot was untied. This showed how trust built up in scientific cooperation can impact politics.

Hot crises put us in particularly difficult dilemmas. It is therefore understandable that the CERN Council has to follow, to a large extent, the guidelines of the individual governments and sometimes introduce harsh sanctions. This leads to considerable damage for many excellent projects, which should be mitigated as much as possible. But it seems equally important to prevent or at least alleviate human suffering among scientific colleagues and their families, and in doing so we should allow them tolerance and full freedom of expression. I am sure the CERN management will try to achieve this, as in the past.

Day after

But what I consider most important is to prepare for the situation after the war. Somehow and sometime there will be a solution to the Russian invasion. On that “day after”, it will be necessary to talk to each other again and build a new world out of the ruins. This was facilitated after World War II because, despite the Nazi reign of terror, some far-sighted scientists maintained human relations as well as scientific ones. I remember with pleasure how I was invited to spend a sabbatical year in 1948 in Sweden with Lise Meitner. I was also one of the first German citizens to be invited to a scientific conference in Israel in 1957, where I was received without resentment.

CERN was the first scientific organisation whose mission was not only to conduct excellent science, but also to help improve relations between nations. CERN did this initially in Europe with great success. Later, during the most intense period of the Cold War, it was CERN that signed an agreement with the Russian laboratory in Serpukhov in the 1960s. Together with contacts with JINR in Dubna, this offered one of the few opportunities for scientific West–East cooperation. CERN followed these principles during the occupation of the Czechoslovak Socialist Republic in 1968 and during the Afghanistan crisis in 1979.

The aim is to transfer ethical achievements from science to politics

CERN has become a symbol of what can be achieved when working on a common project without discrimination, for the benefit of science and humanity. In recent decades, when peace has reigned in Europe, this second goal of CERN has somewhat receded into the background. The present crisis reminds us to make greater efforts in this direction again, even more so when many powers disregard ethical principles or formal treaties by pretending that their fundamental interests are violated. Science for Peace tries to help create a minimum of human trust between governments. Without this, we run the risk that future political treaties will be based only on deterrence. That would be a gloomy world.

A vision for the day after requires courage and more Science for Peace than ever before.

Counting down to LISA

**Looking forward** Stefano Vitale of the University of Trento is co-lead of the LISA consortium and principal investigator for LISA Pathfinder. Credit: S Vitale

What is LISA?

LISA (Laser Interferometer Space Antenna) is a giant Michelson interferometer comprising three spacecraft that form an equilateral triangle with sides of about 2.5 million km. You can think of one satellite as the central building of a terrestrial observatory like Virgo or LIGO, and the other two as the end stations of the two interferometer arms. Mirrors at the two ends of each arm are replaced by a pair of free-falling test masses, the relative distance between which is measured by a laser interferometer. When a gravitational wave (GW) passes, it alternately stretches one arm and squeezes the other, causing these distances to oscillate by an almost imperceptible amount (just a few nm). The nature and position of the GW sources is encoded in the time evolution of this distortion. Unlike terrestrial observatories, which keep their arms locked in a fixed position, LISA must keep track of the satellite positions by counting the millions of wavelengths by which their separation changes each second. All interferometer signals are combined on the ground and a sophisticated analysis is used to determine the differential distance changes between the test masses.

What will LISA tell us that ground-based observatories can’t?

Most GW sources, such as the merger of two black holes detected for the first time by LIGO and Virgo in 2015, consist of binary systems; as the two compact companions spiral into each other, they generate GWs. In these extreme binary mergers, the frequency of the GWs decrease both with the increasing mass of the objects and with increasing distance from their final merger. GWs with frequencies down to about a few Hz, corresponding to objects with masses up to a few thousand solar masses, are detectable from the ground. Below that, however, Earth’s gravity is too noisy. To access milli-Hertz and sub-milli-Hertz frequencies we need to go to space. This low-frequency regime is the realm of supermassive objects with millions of solar masses located in galactic centres, and also where tens of thousands of compact objects in our galaxy, including some of the Virgo/LIGO black holes, emit their signals for years and centuries as they peacefully rotate around each other before entering the final few seconds of their collapse. The LISA mission will therefore be highly complementary to existing and future ground-based observatories such as the Einstein Telescope. Theorists are excited about the physics that can be probed by multiband GW astronomy.

When and how did you get involved in LISA?

LISA was an idea by Pete Bender and colleagues in the 1980s. It was first proposed to the European Space Agency (ESA) in 1993 as a medium-sized mission, an envelope that it could not possibly fit. Nevertheless, ESA got excited by the idea and studies immediately began toward a larger mission. I became aware of the project around that time, immediately fell in love with it and, in 1995, joined the team of enthusiastic scientists, led by Karsten Danzmann. At the time it was not clear that a detection of GWs from ground was possible, whereas unless general relativity was deadly wrong, LISA would certainly detect binary systems in our galaxy. It soon became clear that such a daring project needed a technology precursor, to prove the feasibility of test-mass freefall. This built on my field of expertise, and I became principal investigator, with Karsten as a co-principal investigator, of LISA Pathfinder.

What were the key findings of LISA Pathfinder?

Pathfinder essentially squeezed one of LISA’s arms from millions of kilometres to half a metre and placed it into a single spacecraft: two test masses in a near-perfect gravitational freefall with their relative distance tracked by a laser interferometer. It launched in December 2015 and exceeded all expectations. We were able to control and measure the relative motion of the test masses with unprecedented accuracy using innovative technologies comprising capacitive sensors, optical metrology and a micro-Newton thruster system, among others. By reducing and eliminating all sources of disturbance, Pathfinder observed the most perfect freefall ever created: the test masses were almost motionless with respect to each other, with a relative acceleration less than a millionth of a billionth of Earth’s gravitational acceleration.

What is LISA’s status today?

LISA is in its final study phase (“B1”) and marching toward adoption, possibly late next year, after which ESA will release the large industrial contracts to build the mission. Following Pathfinder, many necessary technologies are in a high state of maturity: the test masses will be the same, with only minor adjustments, and we also demonstrated a pm-resolution interferometer to detect the motion of the test masses inside the spacecraft – something we need in LISA, too. What we could not test in Pathfinder is the million-kilometre-long pm-resolution interferometer, which is very challenging. Whereas LIGO’s 4 km-long arms allow you to send laser light back and forth between the mirrors and reach kW powers, LISA will have a 1 W laser: if you try to reflect it off a small test-mass 2.5 million km away, you get back just 20 photons per second! The instrument therefore needs a transponder scheme: one spacecraft sends light to another, which collects and measures the frequency to see if there is a shift due to a passing GW. You do this with all six test masses (two per spacecraft), combining the signals in one heck of an analysis to make a “synthetic” LIGO. Since this is mostly a case of optics, you don’t need zero-g space tests, and based on laboratory evidence we are confident it will work. Although LISA is no longer a technology-research project, it will take a few more years to iron out some of the small problems and build the actual flight hardware, so there is no shortage of papers or PhD theses to be written.

How is the LISA consortium organised?

ESA’s science missions are often a collaboration in which ESA builds, launches and operates the satellite and its member states – via their universities and industries – contribute all or part of the scientific instruments, such as a telescope or a camera. NASA is a major partner with responsibilities that include the lasers, the device to discharge the test masses as they get charged up by cosmic rays, and the telescope to exchange laser beams among the satellites. Germany, which holds the consortium’s leadership role, also shares responsibility for a large part of the interferometry with the UK. Italy leads the development of the test-mass system; France the science data centre and the sophisticated ground testing of LISA optics; and Spain the science-diagnostics development. Critical hardware components are also contributed by Switzerland, the Netherlands, Belgium, the Czech Republic, Denmark and Poland, while scientists worldwide contribute to various aspects of the preparation of mission operation, data analysis and science utilisation. The LISA consortium has around 1500 members.

What is the estimated cost of the mission, and what is industry’s role?

A very crude estimate of the sum of ESA, NASA and member-state contributions may add up to something below two billion dollars. One of the main drivers of ESA’s scientific programme is to maintain the technological level of European aerospace, so the involvement of industry, in close cooperation with scientific institutes, is crucial. After having passed the adoption phase, ESA will grant contracts to prime industrial contractors who take responsibility for the mission. To foster industrial competition during the study phase, ESA has awarded contracts to two independent contractors, in our case Airbus and Thales Alenia. In addition, international partners and member-state contributions often, if not always, involve industry.

What scientific and technological synergies exist with other fields?

LISA will look for deviations from general relativity, in particular the case where compact objects fall into a supermassive black hole. In terms of their importance, deviations in general relativity are a very close cousin of deviations from the Standard Model of particle physics. Which will come first we don’t know, but LISA is certainly an outstanding laboratory for fundamental gravitational physics. Then there are expectations for cosmology, such as tracing the history of black-hole formation or maybe detecting stochastic backgrounds of GWs, such as “cusps” predicted in string theory. Wherever you push the frontiers to investigate the universe at large, you push the frontiers of fundamental interactions – so it’s not surprising that one of our best cosmologists now works at CERN! Technologically speaking, we just started a collaboration with CERN’s vacuum group. In LISA we have a tiny vacuum volume in the region where the test masses are located, and it is full of components and cables. It was a big challenge for Pathfinder, but for LISA we definitely need to understand more. The CERN vacuum group is really interested in understanding this, so we are very happy with this new collaboration. As with LIGO, Advanced Virgo and the Einstein Telescope, LISA is a CERN-recognised experiment.

There is no other space mission with as many papers published about its science expectations before it even leaves the ground

What’s the secret to maintaining the momentum in a complex, long-term global project in fundamental physics?

The LISA mission is so fascinating that it is “self-selling”. Scientists liked it, engineers liked it, industry liked it, space agencies like it. Obviously Pathfinder helped a lot – it meant that even in the darkest moments we knew we were “real”. But in the meantime, our theory colleagues did so much work. As far as I know, there is no other space mission with as many papers published about its science expectations before it even leaves the ground. It’s not just that the science is inspiring, but the fact that you can calculate things. The instrumentation is also so fascinating that students want to do it. With Pathfinder, we faced many difficulties. We were naïve in thinking that we could take this thing that we built in the lab and turn it into an industrial project. Of course we needed to grow and learn, but because we loved the project so much, we never ever gave up. One needs this mind-set and resilience to make big scientific projects work.

When do you envision launch?

Currently it’s planned for the mid-2030s. This is a bit in the future at my age, but I am grateful to have seen the launch of LISA Pathfinder and I am happy to think that many of my young colleagues will see it, and share the same emotions we did with Pathfinder, as a new era in GW astronomy opens up.

Determining the lifetime of the B_s

LHCb figure 1 — **Fig. 1.** The exponentially falling decay-time distribution of the decay B_s → J/ψη. Source: arXiv:2206.03088

As the LHCb experiment prepares for data taking with an upgraded detector for LHC Run 3, the rich harvest of results using data collected in Run 1 and Run 2 of the LHC continues.

A fascinating area of study is the quantum-mechanical oscillation of neutral mesons between their particle and antiparticle states, implying a coupled system of two mesons with different lifetimes. The phenomenology of the B_s system is particularly interesting as it provides a sensitive probe to physics beyond the Standard Model. A B_s meson oscillates with a frequency of about 3 × 10¹² Hz, or on average about nine times during its lifetime, τ. In addition, a sizeable difference between the decay widths of the heavy (Γ_H) and light (Γ_L) mass eigenstates is expected. Measuring the lifetime of a CP-even B_s-decay mode determines τ_L = 1/Γ_L.

LHCb has recently released a new and precise measurement of this parameter, making use of B_s → J/ψη decays selected from 5.7 fb^–1 of Run 2 data. The study improves the previous Run 1 precision by a factor of two. Due to the combinatorial background, the reconstruction of the η meson via its two-photon decay mode is a particular challenge for this analysis. Despite this, and even with the modest energy resolution of the calorimeter leading to a relatively broad mass peak overlapping partially with the signal from the B⁰ → J/ψη decay, a competitive accuracy has been achieved. By exploiting the latest machine-learning techniques to reduce the background and the well understood LHCb detector, the B_s → J/ψη decay is observed (figure 1), and τ_L is extracted from a two-dimensional fit to the mass and decay time.

LHCb figure 2 — **Fig. 2.** Summary of measurements of τ_L by LHCb of the B_s → J/ψφ decay mode along with the HFLAV average determined using the measurements of Γ_s and ΔΓ_s. The Standard Model prediction is shown by the grey band. Source: arXiv:2206.03088

The analysis finds τ_L = 1.445 ± 0.016 (stat) ± 0.008 (syst) ps, which is the most precise measurement of this quantity. Combined with the LHCb Run 1 study of this and the B_s → D_s⁺ D_s^– decay mode, τ_L = 1.437 ± 0.014 ps, which agrees well both with the Standard Model expectation (τ_L = 1.422 ± 0.013 ps) and the value inferred from measurements of Γ_s and ΔΓ_s in B_s → J/ψφ decays. Further improvement in the knowledge of τ_L is expected both by considering other CP-even B_s decays to final states containing η or η′ mesons, the B_s → D_s⁺ D_s^– dataset collected during Run 2 and from the upcoming Run 3.

Capturing the intangible

Every Nobel Prize comes with a story, and Leonard A Cole’s Chasing the Ghost offers a new perspective on that of Fred Reines, best known for discovering the electron neutrino with Clyde Cowan in 1956. While Cowan passed away in 1974, Reines went on to win the Nobel Prize in Physics for their discovery in 1995. Cole, Reines’s cousin, describes the life of Fred Reines – focusing on both his scientific career and extracurricular interests – in a personal way, showing obvious admiration for his elder cousin.

After participating in the Manhattan Project and assisting in developing nuclear weapons in the 1940s, Reines pivoted to study neutrinos, the fundamental particles emitted in nearly every nuclear reaction, which he describes as “the tiniest quantity of reality ever imagined by a human being”. While being tiny quantities, neutrinos are abundant, yet mysterious, and Reines’s work opened the door to better understand these particles. His research spanned the next five decades, and positions at universities and laboratories across the US, and the techniques that he developed to study neutrinos are used to this day.

Rainbows and Things

Throughout Chasing the Ghost, Cole splits his time between describing Reines’s career and his extracurricular pursuits. Even among his colleagues, Reines was known to be a prolific singer, performing with groups including the Los Alamos Light Opera Association and the Cleveland Orchestra Chorus. Time spent pondering these activities allowed Reines to connect better with non-science-major students when lecturing at universities. Reines famously taught his course “Rainbows and Things” to much acclaim at the University of California, Irvine, where he encouraged students to think deeply about the connection between classroom physics and the natural world. Cole explains that the course name, and much of its philosophy, stems from the play Finian’s Rainbow, which Reines performed in 1955.

Throughout his later life, it became apparent that Reines thought his accomplishments deserved more praise than they had received. In fact, it was only after he gave up hope of winning the Nobel Prize that he won it in 1995. Reines had been passed up on many occasions, including in 1988 when the team that discovered the second type of neutrinos was awarded the prize before him. Cole shares a humorous anecdote (in hindsight): at a CERN conference with both Reines and 1988 laureate Leon Lederman in attendance, a speaker suggested an experiment to search for the third type of neutrino, the tau neutrino. However, as the speaker lamented, it seemed as if no one would perform this type of experiment, “because evidently they only give a Nobel Prize for the detection of every other neutrino.” While the room may have burst into laughter, Fred Reines didn’t budge.

Regardless, Reines’s dedication to understand neutrinos persisted until the end of his life. Shortly before passing, when he heard of the ground-breaking news from Super-Kamiokande that neutrinos oscillate, he astutely asked “What’s the mass?”, understanding the implications of this result.

The work spearheaded by Reines and his contemporaries has made a lasting impact on the field of particle physics, that continues today. As Cole explains, the subfield of neutrino physics has blossomed to include large, international experimental collaborations, which have found even more unexpected results. Those results have spurred investigators to plan ambitious projects, such as the IceCube experiment in Antarctica, the DUNE experiment in the US, and Hyper-Kamiokande in Japan.

Inspiration

Today’s neutrino detectors are getting bigger and bigger. However, their forerunners can still serve a purpose: inspiration. Several detectors from Reines’s era are now exhibited, such as the Gargamelle detector at CERN. After discovering the electron neutrino, the race was on to build experiments to better understand neutrino properties, and Gargamelle was one such detector. Today, it is on display at the CERN Microcosm, perhaps inspiring a new generation of neutrino physicists.

Overall, Leonard A Cole’s Chasing the Ghost will inspire readers, especially those new to thinking about neutrino physics. Fred Reines’s work, with its focus on a deep understanding of these mysterious, abundant particles, continues to bear fruit to this day. There is no telling what the next neutrino experiments will uncover, but it’s a guarantee that sharp thinkers like Reines will be necessary in this field in the generations to come.

SESAME revives the ancient Near East

The IR microscope at SESAME — **Colouring the past** The painting technology of a Petra wall fragment explored using the IR microscope at SESAME. Credit: SESAME

The Synchrotron-light for Experimental Science and Applications in the Middle East (SESAME) is a 2.5 GeV third-generation synchrotron radiation (SR) source developed under the auspices of UNESCO and modelled after CERN. Located in Allan, Jordan, it aims to foster scientific and technological excellence as well as international cooperation amongst its members, which are currently Cyprus, Egypt, Iran, Israel, Jordan, Pakistan, Palestine and Turkey. As a user facility, SESAME hosts visiting scientists from a wide range of disciplines, allowing them to access advanced SR techniques that link the functions and properties of samples and materials to their micro, nano and atomic structure.

The location of SESAME is known for its richness in archaeological and cultural heritage. Many important museums, collections, research institutions and universities host departments dedicated to the study of materials and tools that are inextricably linked to prehistory and human history, demanding interdisciplinary research agendas and teams. As materials science and condensed-matter physics play an increasing role in understanding and reconstructing the properties of artefacts, SESAME offers a highly versatile tool for the researchers, conservators and cultural-heritage specialists in the region.

The high photon flux, small source size and low divergence available at SR sources allow for advanced spectroscopy and imaging techniques that are well suited for studying ancient and historical materials, and which often present very complex and heterogeneous structures. SR techniques are non-destructive, and the existence of several beamlines at SR facilities means that samples can easily be transferred and reanalysed using complementary techniques.

SESAME offers a versatile tool for researchers, conservators and cultural-heritage specialists in the region

At SESAME, an infrared microspectroscopy beamline, an X-ray fluorescence and absorption spectroscopy beamline, and a powder diffraction beamline are available, while a soft X-ray beamline called “HESEB” has been designed and constructed by five Helmholtz research centres and is now being commissioned. Next year, the BEAmline for Tomography at SESAME (BEATS) will also be completed, with the construction and commissioning of a beamline for hard X-ray full-field tomography. BEATS involves the INFN, The Cyprus Institute and the European SR facilities ALBA-CELLS (Spain), DESY (Germany), ESRF (France), Elettra (Italy), PSI (Switzerland) and SOLARIS (Poland).

To explore the potential of these beamlines, the First SESAME Cultural Heritage Day took place online on 16 February with more than 240 registrants in 39 countries. After a welcome by SESAME director Khaled Toukan and president of council Rolf Heuer, Mohamed ElMorsi (Conservation Centre, National Museum of Egyptian Civilization), Marine Cotte (ESRF) and Andrea Lausi (SESAME) presented overviews of ancient Egyptian cultural heritage, heritage studies at the ESRF, and the experimental capabilities of SESAME, respectively. This was followed by several research insights obtained by studies at SESAME and other SR facilities: Maram Na’es (TU Berlin) showed the reconstruction of colour in Petra paintings; Heinz-Eberhard Mahnke and Verena Lepper (Egyptian Museum and Papyrus Collection, FU/HU Berlin and HZB) explained how to analyse ancient Elephantine papyri using X-rays and tomography; Amir Rozatian (University of Isfahan) and Fatma Marii (University of Jordan) determined the material of pottery, glass, metal and textiles from Iran and ancient glass from the Petra church; and Gonca Dardeniz Arıkan (Istanbul University) provided an overview of current research into the metallurgy of Iran and Anatolia, the origins of glassmaking, and the future of cultural heritage studies in Turkey. Palaeontology with computed tomography and bioarchaeological samples were highlighted in talks by Kudakwashe Jakata (ESRF) and Kirsi Lorentz (The Cyprus Institute).

During the following discussions, it was clear that institutions devoted to the research, preservation and restoration of materials would benefit from developing research programmes in close cooperation with SESAME. Because of the multiple applications in archaeology, palaeontology, palaeo-environmental science and cultural heritage, it will be necessary to establish a multi-disciplinary working group, which should also share its expertise on practical issues such as handling, packaging, customs paperwork, shipping and insurance.

Accelerating a better world

The IAEA serves as a hub for the diverse use of accelerator technologies. Credit: IAEA

Tens of thousands of accelerators around the world help create radiopharmaceuticals, treat cancer, preserve food, monitor the environment, strengthen materials, understand fundamental physics, study the past, and even disclose crimes.

A first of its kind international conference, Accelerators for Research and Sustainable Development: From Good Practices Towards Socioeconomic Impact was organised by the International Atomic Energy Agency (IAEA) at its headquarters in Vienna from 23 to 27 May. It was held as a hybrid event attended by around 500 scientists from 72 IAEA member states. While focusing mainly on applications of accelerator science and technology, the conference was geared towards accelerator technologists, operators, users, entrepreneurs, and other stakeholders involved in applications of accelerator technologies as well as policy makers and regulators.

The far-reaching capabilities of accelerator technology help countries progress towards sustainable development
Rafael Mariano Grossi

“The far-reaching capabilities of accelerator technology help countries progress towards sustainable development,” said IAEA director general Rafael Mariano Grossi in his opening address. “IAEA’s work with accelerators helps to fulfil a core part of its ‘Atoms for Peace and Development’ mandate.” He also highlighted how accelerator technology plays a critical role in two IAEA initiatives launched over the past year: Rays of Hope, aimed at improving access to radiotherapy and cancer care in low- and middle-income countries, and NUTEC plastics, supporting countries in addressing plastic waste issues in the ocean and on land. Finally, he described IAEA plans to establish an accelerator of its own: a state-of-the-art ion-beam facility in Seibersdorf, Austria that will support research and help educate and train scientists.

The conference included sessions dedicated to case studies demonstrating socioeconomic impact as well as best practices in effective management, safe operation, and the sustainability of present and future accelerator facilities. It showcased the rich diversity in types of accelerators – from large-scale synchrotrons and spallation neutron sources, or medical cyclotrons and e-beam irradiators used for industrial applications, to smallscale electrostatic accelerators and compact-accelerator based neutron sources – and included updates in emerging accelerator technologies, such as laser-driven neutron and X-ray sources and their future applications. Six plenary sessions featuring 16 keynote talks captured the state of the art in various application domains, accompanied by 16 parallel and two poster sessions by young researchers.

During the summary and highlights session, important developments and future trends were presented:

• Large-scale accelerator facilities under development across the world – notably FAIR in Germany, SPIRAL-2 in France, FRIB in the US, RIBF in Japan, HIAF in China, RAON in Korea, DERICA in Russia and MYRRHA in Belgium – boost the development of advanced accelerator technologies, which are expected to deliver high-impact socioeconomical applications. Substantial interdisciplinary research programmes are foreseen from their beginning, and the IAEA could play an important role by strengthening the links and cooperation between all parties.

• Recent technology developments in Compact-Accelerator Neutron Sources (CANS) or High-Power CANS (HiCANS) are very promising. Among many projects, ERANS at RIKEN in Japan aims to realise a low-cost CANS capable of providing 1012 n/s for applications in materials research and ERANS-III a transportable CANS for testing the structure of bridges. On the HiCANS front, the French SONATE project aims to reach neutron flux levels comparable to the ageing fleet of low and medium power research reactors at least for some applications.

• CANS technology is promising for tools to fight cancer, for example via the Boron Neutron Capture Therapy (BNCT) method. Japan leads the way by operating or constructing 10 such in-hospital based facilities, with only a few other countries, e.g. Finland, considering similar technologies. Recent developments suggest that accelerator based BNCT treatments become soon more acceptable. IAEA could play an important coordinating role and as a technology bridge to developing countries to enable more widespread adoption.

• The role of accelerators in preserving cultural heritage objects and in detecting forgeries is becoming more vital, especially in countries that do not have the required capabilities. Ion-beam analysis and accelerator massspectrometry techniques are of particular relevance, and, again, the IAEA can assist by coordinating actions to disseminate knowledge, educating the relevant communities and possibly centralising the demands for expertise.

• The IAEA could simplify the supply of accelerator technologies between the different member states, enabling the installation and operation of facilities in low- and middle income countries, for example by structuring the scientific and technical accelerators communities, and educating young researchers and technicians via dedicated training schools.

• One of IAEA’s projects is to establish a stateof-the-art ion beam facility in Austria. This will enable applied research and provision of analytical services, as well as help educate and train scientists on the diverse applications of ion beams (including the production of secondary particles such as neutrons) and will enhance collaborations with both developed and developing countries.

• Ion-beam analysis (IBA) together with accelerator-mass spectroscopy (AMS) techniques are unique, reliable and costeffective for Environmental Monitoring and Climate Change Related Studies, for example in characterising environmental samples and investigating isotope ratio studies for chronology and environmental remediation AMS facilities with smaller footprints have increased their distribution worldwide, resulting in accessible and affordable measurements for interdisciplinary research, while other IBA techniques offer efficient analytical methods to characterise the chemical composition of particles from air pollution.

• Materials science and accelerators are now moving ahead hand in hand, from characterisation to modification of technologically important materials including semiconductors, nano-materials, materials for emerging quantum technologies and materials relevant to energy production. Testing materials with accelerator-based light and heavy-ion beams remains a unique possibility in the case of fusion materials and offers much faster radiation-damage studies than irradiation facilities at research reactors. Equally important is the accelerator-assisted creation of gaseous products such as hydrogen and helium that allows testing the radiation resilience in unmoderated neutron systems such as fast fission and fusion reactors.

• New developments in electron-beam accelerators for industrial applications were also mentioned, in particular their application to pollution control. E-beam system technologies are also widely employed in food safety. Reducing spoilage by extending the shelf-life of foods and reducing the potential for pathogens in and on foods will become major drivers for the adoption of these technologies, for which a deeper understanding of the related effects and resistance against radiation is mandatory.

Accelerator technologies evolve very fast, presenting a challenge for regulatory bodies to authorise and inspect accelerator facilities and activities. This conference demonstrated that thanks to recent technological breakthroughs in accelerator technology and associated instrumentation, accelerators are becoming an equally attractive alternative to other sources of ionising radiation such as gamma irradiators or research reactors, among other conventional techniques. Based on the success of this conference, it is expected that the IAEA will start a new series of accelerator community gatherings periodically from now on every two to three years.

Karl von Meyenn 1937–2022

Physicist and historian of science Karl von Meyenn passed away on 18 June 2022 in his hometown of Neuburg an der Donau. Karl was a CERN associate for many years, often to be found in Salle Pauli amongst the archive and library of Wolfgang Pauli, on whom he was one of the world’s leading experts.

Karl was born in Potsdam in 1937 and began studying physics in Chile, where his parents emigrated. He completed his doctorate in 1971 with Siegfried Flügge in Freiburg im Breisgau, then returned with his wife to Chile and taught at the Pontificia Universidad Católica until the military coup of 1973. Back in Germany, he worked first as a senior assistant to Helmut Reik at the faculty of physics in Freiburg, before specialising in the history of science with Armin Hermann at the Historical Institute of the University of Stuttgart in 1975. From 1985 to 1990 he was a professor of history of science at the Universitat Autònoma in Barcelona, after which he joined Hans-Peter Dürr at the Max Planck Institute for Physics in Munich. He also carried out research at the Institute of Theoretical Physics (with Frank Steiner) at the University of Ulm, and at CERN, where he devoted himself to Pauli’s scientific legacy.

Franca Pauli donated her husband’s scientific writings, library and other items to CERN during the 1960s and 1970s, and CERN took responsibility for safeguarding and making this valuable collection available. The Pauli Committee turned to Karl von Meyenn, who tracked down copies of other letters in public or private ownership, then collated this wealth of material into publishable form. Besides the monumental eight volumes of Pauli correspondence, Karl published a biographical anthology on the great physicists (Die großen Physiker) in 1997–1999, a two-volume selection of Erwin Schrödinger’s correspondence in 2011, and numerous essays, lectures and collaborative books on individual scientists and their interactions in developing new concepts in physics. In 2000 he was awarded the Marc-Auguste Pictet Medal of the Société de Physique et d’Histoire Naturelle de Genève for his work on the history of modern physics. Karl was a member of the Pauli Committee from 1994 and an honorary councillor at ETH Zurich since 2006. The library he leaves in Neuburg is a testimony to his great love of classical culture and broad cultural life. Combining great learning and rigorous scholarship with an engaging curiosity and enthusiasm, Karl was a stimulating and extremely likeable colleague. He will be sadly missed.

Run 3 physics gets under way

The start of Run 3 physics — **In tune** Cheering the start of Run 3 physics in the CERN Control Centre on 5 July. Credit: CERN

At 4.47 p.m. on Tuesday 5 July, applause broke out in the CERN Control Centre as LHC operators declared Stable Beams. After more than three years of upgrade and maintenance work across the machine and experiments, ALICE, ATLAS, CMS and LHCb started recording their first proton–proton collisions at an unprecedented energy of 13.6 TeV.

LHC Run 3 is set to last until December 2025. In addition to a slightly higher centre-of-mass energy than Run 2, the machine will operate at an increased average luminosity thanks to larger proton intensities and smaller transverse beam sizes. New or upgraded detectors and improved data readout and selection promise the experiments their greatest physics harvests yet. ATLAS and CMS each expect to record more collisions during Run 3 than in the two previous runs combined, while LHCb and ALICE hope for three and 50 times more data, respectively. Two new forward experiments, FASER and SND@LHC (CERN Courier July/August 2021 p7), also join the LHC-experiment family.

While pilot beams circulated in the LHC for a brief period in October 2021, the countdown to LHC Run 3 began in earnest on 22 April, when two beams of protons circulated in opposite directions at their injection energy of 450 GeV. Since then, operators have worked around the clock to ensure the smooth beginning of the LHC’s third run, which was livestreamed to the media on the afternoon of 5 July. True to form, the machine added drama to proceedings: a training quench that morning generated enough heat to warm up several magnets well above their operating temperature. The cryogenics team sprang into action, managing to recuperate operational conditions just in time for the live event, watched by more than 1.5 million people.

First 13.6 TeV collisions — **Run 3** ATLAS, CMS, ALICE and LHCb recorded their first proton–proton collisions at 13.6 TeV on 5 July. Credit: ATLAS, CMS, ALICE, LHCb

Since then, the intensity of the beams has been increased in carefully monitored steps. As the Courier went to press, 900 bunches each containing around 120 billion protons were circulating, with 2748 bunches expected by September. “Run 3 is going to be a game-changer for us,” says operations group leader Rende Steerenberg. “In Run 2, we exploited the LHC in its ‘normal’ hardware configuration as constructed. Now, after the injectors have been adapted, we can push the brightness and the intensity of the beams much more. Run 3 is also an important stepping-stone to the High-Luminosity LHC upgrade.”

Schedule change

In March, the CERN management announced a change to the LHC schedule. Long Shutdown 3 will now start in December 2025, one year later than in the previous baseline, and last for three instead of 2.5 years. Production schedules across the LHC’s lifetime will remain unaffected, while the change will allow work for the HL-LHC to be completed with appropriate schedule margins. The extended year-end technical stop (EYETS) is now scheduled to take place in 2024/2025 and to last for 17 weeks, while the two preceding EYETSs will be of the standard length of 13 weeks beam-to-beam.

The preferred scenarios and duration of ion runs during Run 3 remain to be confirmed, but are likely to take place in four week-long periods towards the end of each year. While the majority of the LHC’s heavy-ion runs employ lead ions, a novel addition to the Run 3 programme will be a short period of collisions between oxygen ions in 2024. As with the first xenon runs in 2017, colliding ions with masses that are intermediate between protons and lead allows the experiments to scan important physics regimes relevant to the study of high-energy QCD.

“Every time you make a step in energy, even if it’s not that large, and a step in the amount of data, you open up new physics opportunities,” said CERN Director-General Fabiola Gianotti. “And every time we start a new run, it’s always a new adventure. You have to recalibrate the detectors and the accelerator, so it’s always uncharted territory and always a big emotion.”

For full coverage of the physics targets at LHC Run 3, please see the May/June 2022 issue of CERN Courier.

Accelerating knowledge transfer with physics

Countries in Africa participating in ACP2021 — **Science for society** Map showing the countries in Africa with home institutes participating in ACP2021 (green). Credit: K Assamagan

Science and technology are key instruments for a society’s economic growth and development. Yet Africa’s science, innovation and education have been chronically under-funded. Transferring knowledge, building research capacity and developing competencies through training and education are major priorities for Africa in the 21st century. Physics combines these priorities by extending the frontiers of knowledge and inspiring young people. It is therefore essential to make basic knowledge of emerging technologies available and accessible to all African citizens to build a steady supply of trained and competent researchers.

In this spirit, the African School of Fundamental Physics and Applications was initiated in 2010 as a three-week biennial event. To increase networking opportunities among participants, the African Conference on Fundamental and Applied Physics (ACP) was included as a one-week extension of the school. The first edition was held in Namibia in 2018 and the second, co-organised jointly by Mohammed V University and Cadi Ayyad University in Morocco, was rebranded ACP2021, originally scheduled to take place in December but postponed due to COVID-19. The virtual event held from 7 to 11 March attracted more than 600 registrants, an order of magnitude higher than its first edition.

The ACP2021 scientific programme covered the three major physics areas of interest in Africa defined by the African Physical Society: particles and related applications; light sources and their applications; and cross-cutting fields covering accelerator physics, computing, instrumentation and detectors. The programme also included topics in quantum computing and quantum information, as well as machine learning and artificial intelligence. Furthermore, ACP2021 focused on topics related to physics education, community engagement, women in physics and early-career physicists. The agenda was stretched to accommodate different time zones and 15 parallel sessions took place.

Welcome speeches by Hassan Hbid (Cadi Ayyad University) and by Mohammed Rhachi (Mohammed V University) were followed by a plenary talk by former CERN Director-General Rolf Heuer, “Science bridging Cultures and Nations” and an overview of the African Strategy for Fundamental and Applied Physics (ASFAP). Launched in 2021, the ASFAP aims to increase African education and research capabilities, build the foundations and frameworks to attract the participation of African physicists, and establish a culture of awareness of grassroots physics activities contrary to the top-down strategies initiated by governments. Shamila Nair-Bedouelle (UNESCO) conveyed a deep appreciation of and support for the ASFAP initiative, which is aligned with the agenda of the United Nations Sustainable Development Goals. A rich panel discussion followed, raising different views on physics education and research roadmaps in Africa.

A central element of the ACP2021 physics programme is the ASFAP community planning meeting, where physics and community-engagement groups discussed progress in soliciting the community input that is critical for the ASFAP report. The report will outline the direction for the next decade to encourage and strengthen higher education, capacity building and scientific research in Africa.

The motivation and enthusiasm of the ACP2021 participants was notable, and the efforts in support of research and education across Africa were encouraged. The next ACP in 2023 will be hosted by South Africa.

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