Sometimes, it seems, mere proximity is the engine-room of opportunity. That’s certainly the case for two of Japan’s flagship large-scale research centres – the SPring-8 (Super Photon Ring 8 GeV) synchrotron facility and SACLA (the SPring-8 Angstrom Compact free-electron LAser) – which are co-located adjacent to each other on the main campus of Harima Science Park City in Hyogo Prefecture. 

It’s now two years since a joint research group from SPring-8 Center, which is managed by Japan’s premier research agency RIKEN, and the Japan Synchrotron Radiation Research Institute (JASRI), responsible for promoting the use of SPring-8, succeeded in utilising the linear accelerator of the SACLA X-ray free-electron laser (XFEL) facility as an injector for the SPring-8 storage ring. 

The upsides were immediate, impressive and – crucially – sustained. For starters, the 25-year-old injector of SPring-8, which hitherto was used exclusively for beam injection, has been superseded by the SACLA linear accelerator – reducing the electricity consumption needed to support SPring-8 operation by roughly 20%. Equally significant, the quality of the electron beam injected into the storage ring has been enhanced markedly – a breakthrough that will underpin the upgrade plan of SPring-8 to a fourth-generation light source (SPring-8-II) capable of generating X-ray beams hundreds of times brighter than the current facility. 

Legacy problems, creative solutions

By way of context, the previous, dedicated injector for the SPring-8 storage ring consisted of a 1 GeV linear accelerator and an 8 GeV synchrotron booster. The emittance of the resulting injection beam was about 200 nm-rad, which is far larger than the specification of the future SPring-8-II (at 10 nm-rad). There were legacy technology issues as well: both injector accelerators were more than 25 years old, while the associated (and somewhat decrepit) high-voltage power substation was itself in need of a root-and-branch overhaul. 

Cue some creative thinking and the game-changing idea of deploying SACLA’s low-emittance electron beam for SPring-8 beam injection (and in parallel to its core function as a source of XFEL photon beams for front-line research). Herein an engineering win-win began to take shape: on the one hand, the opportunity to decommission (rather than renovate) the high-voltage substation; on the other, to simultaneously reduce SPring-8’s electricity consumption significantly by shutting down the old injector accelerators (which were always maintained in operational mode despite the only intermittent requirement for beam injection). Underpinning it all is the fact that the SACLA linear accelerator is always online for the XFEL research users, so no additional cost or energy consumption accrues when sending a small number of electron-beam pulses from SACLA to the SPring-8 storage ring.

In the early design phase of SACLA (around 2008), and with engineers anticipating the future possibility of beam injection to SPring-8, the nominal beam energy of SACLA was set at 8 GeV to match the SPring-8 storage ring. The direction of the SACLA electron beam was also fixed towards SPring-8 to facilitate the envisaged beam-injection
scenario. To join things up, the two accelerators are linked by a beam transport line – the XFEL to Storage ring Beam Transport (XSBT) – which was constructed at the same time as the SACLA facility (see images “Better together” and “Joined up”). 

When it comes to the technical specifics, the SACLA beam repetition rate is set at 60 Hz, with the electron-beam pulses distributed on a pulse-by-pulse basis in three directions with the help of a “kicker” magnet: along two XFEL beamlines (BL2 and BL3) and down the XSBT beamline. What’s more, the electron-beam energies for XFEL user experiments are often adjusted in a range between 5 and 8 GeV depending on laser wavelengths, while the energy needs to be fixed at 8 GeV for the beam injection. It is therefore essential to control the electron-beam parameters pulse-by-pulse to allow XFEL user experiments and the SPring-8 beam injection to proceed in parallel. 

Synchronisation is nothing without control

Operationally, the preparation of the SACLA beam-injection scheme took about two years in design, planning and commissioning. The first task for the project team was to synchronise the two accelerators, given that they run on different reference clock frequencies (238 MHz for SACLA; 508 MHz for SPring-8). Since the two frequencies do not have a harmonic relation, the injection timing naturally goes off with respect to the target RF bucket of SPring-8 by a maximum ±2.1 ns. A novel timing system was subsequently developed to provide the necessary synchronisation. 

In top-up injection mode, which keeps a constant stored current within the storage ring, SPring-8 sends a beam request to SACLA when the stored current decreases under a certain threshold. Once SACLA receives the request, the timing system first searches out a point where the timing difference is at a minimum. By delaying the beam injection up to 197 µs, there should be a point where the timing difference becomes smaller than 105 ps. In a second step, a slight frequency modulation is applied to the SACLA 238 MHz reference clock, such that SACLA and SPring-8 are finally synchronised to within 3.8 ps (RMS).

Another significant piece of work involved retrofitting the accelerator control system. To change the accelerator parameters pulse-by-pulse, it is necessary for accelerator components to “know” the destination of the next beam pulse. That granular data (at the level of an individual pulse) is therefore sent through a “reflective memory network” to key components and subsystems – for example, RF sources and magnet power supplies – such that these devices can then operate with prestored parameters corresponding to the relevant beam destination. 

Meanwhile, all diagnostic data are saved in a database with a pulse tag number so that the measured data, such as beam trajectories and charges, can be distinguished among the three beam destinations (BL2, BL3 and XSBT). It is like there are three accelerators independently running with different beam energies and parameters. In day-to-day operation, three operators adjust and tune the accelerators by looking at the monitor and control panels for the three beams versus the respective destinations. Using the pulse tag number, XFEL users are also able to see which pulses were used for the beam injection, so that these pulses can be eliminated from their experimental data. 

A final core deliverable for the project team was to ensure the bulletproof reliability of SACLA – since the storage ring cannot operate without beam injection. In short, if the linear accelerator of SACLA fails to provide electron beams, close to 60 experiments on the SPring-8 beamlines are at risk of grinding to a halt. Redundancy is the key here: to promptly recover from unexpected troubles, the team installed back-up components for crucial accelerator devices, such as the electron gun, kicker magnet and RF sources.  

Going live

During the early-stage evaluation of the new beam-injection scheme, two other issues came into play: electron bunch purity and magnetic hysteresis of the kicker magnet. The electron bunch purity is a ratio of electron charges contained in an electron-injected RF bucket and an empty bucket on the storage ring. 

It is an important figure of merit – not least for ensuring low background noise in time-resolved experiments. A bunch purity of between 10^–8 and 10^–10 is typically required at SPring-8, while the electron bunch charge of SACLA is around 200 pC – i.e. even a single electron outside the beam pulse could degrade the bunch purity. 

It turned out, however, that a small number of electrons were detected 18 ns after the main beam pulse during the initial experimental runs. After detailed investigation, the project team found that these electrons were slipping out from the main pulse and making a round trip between two RF cavities at the injector section of SACLA. Consequently, they were delayed by 18 ns and being injected into unexpected RF buckets on the storage ring. The solution: remove the delayed electrons using an electric sweeper and an RF knockout system – a breakthrough that, in turn, yielded the required bunch purity of 10^–10. 

At the far-end of SACLA’s linear accelerator, electron-beam pulses are deflected horizontally in three directions by a kicker magnet. The polarity of the kicker current is negative for BL2, zero for BL3 and positive for XSBT. As a consequence, the beam orbits of the pulses just after the beam injection (i.e. two to three times a minute) deviate from the optimum trajectory inside the XFEL undulators – seriously degrading pointing stability and laser power within the SACLA beamlines. To overcome this issue, the excitation pattern of the kicker magnet is modified slightly after the beam injection, such that the hysteresis effect is now suppressed within an angular deviation of 1 µrad (i.e. less than 10 % of the laser spot size). 

With those “issues arising” addressed successfully by the project team, it’s instructive to look at the high-level operational performance of the new beam-injection arrangement. To fill up the storage ring with a nominal stored current of 100 mA, the electron beam is injected at 10 Hz from SACLA. The process takes about 10 minutes – roughly twice as fast as the old injector. Once the storage ring is filled, top-up injection gets underway to keep the stored current at a constant level. In top-up mode, the electron beam is injected typically 2–3 times every minute. Measuring the transverse beam sizes at the injection point of SPring-8 shows that the size of the electron beam from SACLA is significantly narrower versus that from the old injector, with the beam quality represented by emittance improving from 200 nm-rad to 1 nm-rad – more than satisfying the criteria for the future SPring-8-II.

Green machines

Alongside those sustained performance improvements, there are other notable wins to report for the SACLA beam-injection arrangement. After a probation period of six months, the old SPring-8 injector and its power station were shut down in April 2021, yielding a 5 MW saving in electricity consumption and an impressive 20% reduction in the SPring-8 power bill (i.e. versus current SPring-8 plus the old injector).

Further operational efficiencies are in the works for SPring-8-II given the global surge in energy prices and the shared commitment (with SACLA) towards carbon neutrality by 2050. By using cutting-edge, short-period, in-vacuum undulator technologies in SPring-8-II, for example, the electron-beam energy will be reduced from 8 GeV to 6 GeV without changing the X-ray radiation energy range. Replacing accelerator electromagnets with permanent magnets will enable additional reductions in power consumption. The ultimate goal of SPring-8-II, and with user operation pencilled in to begin no later than 2030, is a 50% reduction in power consumption versus the current SPring-8.

The ultimate goal is a 50% reduction in power consumption for SPring-8-II versus the current SPring-8

Similarly ambitious plans are taking shape for the SACLA-II upgrade, which will take place after the completion of SPring-8-II. The end-game: a 1 kHz repetition rate using conventional (rather than superconducting RF) accelerator technologies. With traditional RF acceleration, of course, more than 99.99 % of the input power is dissipated as heat – rather than accelerating the electron beam – so the challenge for SACLA-II is to boost this extremely low conversion efficiency, thereby increasing the repetition rate without increasing the power consumption. 

While none of this will be straightforward, it’s evident that the path to a “greener” and more sustainable accelerator complex is rapidly coming into view.

The estimated annual global incidence of new cancer cases was upwards of 19 million in 2020, with more than 70% of people suffering from the disease resident in low- and middle-income countries (JCO Global Oncology 2022 8 e2100358). What’s more, according to forecasts from the International Atomic Energy Agency published on World Cancer Day in February 2022, the total number of cancer deaths worldwide is forecast to rise by 60% over the next two decades – to 16 million people a year – with those same low- and middle-income countries suffering the brunt of the escalation. India finds itself in the eye of this healthcare storm, with the domestic burden of cancer cases projected at between 1.9 and 2 million in 2022 – a burden, moreover, that’s also projected to increase over time.

Fundamentally, this is a question of supply (high-quality cancer treatment) versus demand (rising cancer incidence) for India – not least when it comes to the challenges associated with rolling out accessible and affordable radiation therapy facilities at the national level. Right now, there are around 545 clinical radiotherapy units across India (180 ⁶⁰Co-based teletherapy systems and 365 electron linacs). Most of the e-linacs are supplied by commercial manufacturers, with 50% of these systems located in private hospitals – and therefore beyond the reach of the majority of Indian citizens. 

To drive down the cost of radiotherapy treatment, while simultaneously opening up access to more cancer patients, the Society for Applied Microwave Electronics Engineering and Research (SAMEER) in Mumbai has been prioritising technology innovation in e-linacs for several decades (with financial support from the central government’s Ministry of Electronics and Information Technology, also known as MeitY). 

Accessibility, affordability, availability 

A case study in this regard is the medical electronics division of SAMEER, which initiated an R&D programme for a 4 MeV e-linac for cancer therapy in the late 1980s. The initial outcome: an S-band, side-coupled linac (operating at π/2 mode at 2.998 GHz) developed for electron acceleration. The SAMEER development team later integrated the linac with other core subsystems in collaboration with the Central Scientific Instruments Organisation, Chandigarh, and the Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh, with the completed linac commissioned at PGIMER in 1991. 

This original machine was called Jeevan Jyoti-I. SAMEER engineers went on to build three more e-linac variations on the Jeevan Jyoti-I theme, with all units duly commissioned and operating in hospitals. Subsequently, under the Indian government’s Jai Vigyan initiative, SAMEER built six more radiotherapy units (with an increased energy of 6 MV) and installed these systems in hospitals. One more machine is being commissioned in 2022 – initially using commercial microwave sources from SAMEER (though these will eventually be replaced with a domestically developed 2.6 MW magnetron).

Multipurpose proton accelerators

India’s Department of Atomic Energy (DAE) plans to exploit the country’s rich natural sources of thorium to bolster the domestic nuclear energy programme, simultaneously exploring new methods for dealing with high-level nuclear waste as well as the at-scale production of medical radioisotopes for the diagnosis and treatment of cancer.

Consider the so-called accelerator-driven subcritical reactor (ADSR), a next-generation nuclear reactor design formed by coupling a substantially subcritical nuclear reactor core (using thorium as fuel) with a high-intensity, high-energy proton accelerator. The latter generates a copious beam of spallation neutrons to sustain the fission process – activating the thorium without needing to make the reactor critical (i.e. turning off the proton beam results in immediate and safe shut-down of the reactor). Another benefit of the ADSR scheme is the relatively short half-lives of the waste products.

Within this context, DAE’s R&D laboratories have started work on a high-current 1 GeV proton accelerator (see “Collective endeavour” figure). In the first phase of construction of a 20 MeV normal-conducting linac at Bhabha Atomic Research Centre (BARC), scientists accelerated a 2 mA proton beam from an ion source using a four-vane RF quadrupole (generating a 3 MeV proton beam with 65% transmission). Earlier this year, the BARC team boosted the proton energy to 6.8 MeV through the first drift-tube linac (with a peak beam current of 2.5 mA and an average beam current of 1 μA with 93% transmission). At Raja Ramanna Centre for Advanced Technology (RRCAT), meanwhile, several warm-front-end ion sources and associated subsystems are under construction (including low-energy beam transport, RF quadrupoles, medium-energy beam transport and a drift-tube linac). 

Operationally, collaboration is a defining theme of India’s R&D effort on proton accelerators – not least through its scientists’ direct participation in the Proton Improvement Plan II (PIP-II), an essential upgrade and ambitious reimagining of the Fermilab accelerator complex in the US. Several Indian institutions are front-and-centre in the PIP-II initiative, designing and developing room-temperature and superconducting magnets, superconducting RF cavities, cryomodules and RF amplifiers for the PIP-II project team. 

BARC and the Inter-University Accelerator Centre (IUAC) in New Delhi, for example, initially supplied two single-spoke-resonator cavities for testing at Fermilab, while end-to-end infrastructure for niobium-cavity fabrication and testing has been established at RRCAT. Several niobium superconducting cavities – required in both the PIP-II project and the Indian proton accelerator programme – have since been fabricatedand tested successfully.

Innovation pathways

One thing is clear: India’s e-linac R&D effort continues to gather momentum. The next step is to enhance the technology for dual photon energies (6 and 15 MeV) from the same linac, along with multiple electron energies (from 6 to 18 MeV) for treatment. A prototype of a novel dual-energy linac has already been put through its paces, delivering beam-on-target at SAMEER. The energy is varied by introducing a plunger in the coupling cavity in the acceleration section. Industry partners are being sought as the system undergoes final quality assurance and control checks.  

Parallel technology programmes – covering both e-linacs and proton cyclotrons – are also underway to support domestic production of medical radioisotopes used in the diagnosis and treatment of cancer. For example, a 30 MeV, 5–10 kW linac project (incorporating two 15 MeV sections) is being lined up for the production of ^99mTc from ⁹⁹Mo (the former being required in a nuclear imaging procedure called single-photon-emission computerised tomography, commonly known as SPECT). The ⁹⁹Mo will be produced from ¹⁰⁰Mo using Bremsstrahlung photons, with the latter emitted after accelerated electrons are incident on a target. Tests of the first accelerating structure (15 MeV) are in progress and the full energy of 30 MeV is expected to come online next year. 

India and CERN: a win–win partnership

Following India’s associate membership at CERN from 2017, the country’s scientists and engineers continue to build on a rich and diverse legacy of contributions spanning core accelerator technologies and participation in front-line high-energy physics experiments. This is a legacy that extends across more than 50 years of collaboration. In the 1990s, for example, the RRCAT contributed to LEP, while the Indian High-Energy Heavy Ion Physics Team contributed to the WA93 experiment at the CERN-SPS. An international cooperation agreement between India’s Department of Atomic Energy (DAE) and CERN was signed in 1992 to deepen ties and extend the scientific and technical cooperation between India and CERN. Those developments, in turn, paved the way for the decision (in 1996) of India’s Atomic Energy Commission to take part in the construction of the LHC – specifically, to contribute to the development of the CMS and ALICE detectors. India became a CERN Observer State in 2002, and the success of the DAE–CERN partnership on the LHC led to a new cooperation on novel accelerator technologies, shaping DAE’s participation in CERN’s Linac4, SPL and CTF3 projects. 

Elsewhere, the Variable Energy Cyclotron Centre (VECC) in Kolkata is leading a project to build an 18 MeV medical cyclotron – a machine that will reduce the cost of production for positron-emitting radioisotopes. In terms of operational specifics: the system will accelerate negative hydrogen ions (H^–) from an external, multicusp volume ion source, while a carbon stripper foil will alter the charge state of the ions from negative to positive ahead of extraction. Progress to date is encouraging: engineering design of the main magnet is complete and a 1 mA current has been extracted from the H^– ion source. 

Further technology innovation is evident in the field of hadron therapy, which uses proton or ion beams to deliver precision tumour targeting with zero exit dose – a capability that clinicians estimate could improve therapeutic outcomes in 15–20% of cancer patients who receive radiotherapy. Recognising the potential here, Indian clinics have recently purchased and installed two 230 MeV proton cyclotrons, supplied by Belgian equipment maker IBA, in a pivot towards next-generation cancer treatments. 

Further progress has been reported by a collaboration between SAMEER and KEK, Japan’s High-Energy Accelerator Research Organisation. Jointly, the two partners have completed conceptual design studies for a multi-ion therapy machine based on a novel digital accelerator concept. The system is basically a fast-cycling induction synchrotron with a specialised beam-handling capability. (For context, the accelerating devices of a conventional synchrotron, such as RF cavities, are replaced with induction devices in an induction synchrotron.) It is possible, for example, to inject particles at nearly 200 kV DC directly into the main ring and, as such, the induction synchrotron does not need a separate injector. 

In a related initiative, the Tata Memorial Centre Mumbai, and Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, have come up with a preliminary design for a 2 MeV injector and a 70–250 MeV proton synchrotron that may also be suitable for variable-energy beam delivery and other ion-beam therapies.

The Pakistan Atomic Energy Commission (PAEC) provides a focal point for the country’s diverse scientific, technological and engineering collaborations with CERN and other leading international accelerator facilities. Zoom in a little further and one of the engine-rooms for that collaborative endeavour is very much front-and-centre: the Pakistan Institute of Nuclear Science & Technology (PINSTECH).

Headquartered in Islamabad, PINSTECH is a premier R&D institute within PAEC and, by extension, one of Pakistan’s leading research centres. The institute’s wide-ranging research programme spans, among other things, isotope production and applications, materials science, radiation protection and health physics, as well as neutron science. That R&D effort is augmented by two operational nuclear research reactors: Pakistan Research Reactor-1 is a 10 MW pool-type reactor which is used to produce radioisotopes for medical applications; Pakistan Research Reactor-2 is a smaller reactor that’s used, chiefly, for neutron activation and teaching/training activities. 

Operationally, PINSTECH has facilities for the production of a range of reactor-based radioisotopes – including ⁹⁹Mo, ^99mTc generators, ¹³¹I, and ³²P – all of which are supplied to nuclear medicine centres across the country on a rolling basis. A key element of this programme is PINSTECH’s production of freeze-dried, radiopharmaceutical in-vivo diagnostic kits for nationwide distribution. 

During radioisotope production, target materials are placed into the research reactor for neutron irradiation, after which the irradiated targets are transferred into the “hot cells” of the facility for chemical separation, purification, quality control and dispatch. The Pakistan Nuclear Regulatory Authority and Drug Regulatory Authority of Pakistan regulate the end-to-end production process. 

Elsewhere within PINSTECH, and with support from CERN, researchers are developing a 5 MeV electron linac for radiotherapy applications – part of a national effort to scale technical capacity and capability in medical accelerator technologies.  

Radioisotope collaboration 

Collaboration is hard-wired into PAEC’s operational model and, as such, underpins Pakistan’s radioisotope production programme. PAEC and the International Atomic Energy Agency (IAEA), for example, have been working together in this area for many years and have completed a number of successful technical cooperation projects (with a joint project in the healthcare sector currently under way at the national level). 

CERN is another flagship R&D partner, with PINSTECH and the CERN–MEDICIS facility engaged in a cooperative effort focused on the production and study of innovative radioisotopes for medical diagnostics and treatment. Specifically, the two organisations are carrying out R&D activities on the production of novel medical (reactor- and accelerator-produced) radioisotopes such as ^195mPt, ¹⁶⁵Er, ²²⁵Ac, ¹⁵⁵Tb, ¹⁶¹Tb, ¹⁷⁷Lu – an endeavour, the two partners hope, that will ultimately extend to clinical trials. The bottom line is that Pakistan, which has high cancer mortality and morbidity within its population of 220 million, is keen to take advantage of cutting-edge radioisotope science for the at-scale diagnosis, treatment and management of cancer patients. 

Knowledge transfer 

With this in mind, PINSTECH scientists and engineers have been seconded to the CERN–MEDICIS team for the development of radiochemical activities – including a major contribution to the MEDICIS radiochemistry set-up for the purification of medical radioisotopes with both therapeutic and diagnostic properties (so-called “theranostic” combinations). PINSTECH engineers have also been working on the development of salt/aluminium foils for radio-isotope collection; the management of liquid radioactive waste at MEDICIS; a very promising R&D project relating to the production of ^195mPt for theranostic applications; and the development of a large-scale radiochemistry unit for handling higher levels of radioactivity. 

Fundamental to the PINSTECH-MEDICIS collaboration is the regular exchange of radioisotopes between the two partners. As an example, research quantities of ²²⁵Ac were recently shipped from CERN to Pakistan and, after chemical processing at PINSTECH, transferred to the Lahore-based INMOL cancer clinic (another PAEC facility). Here the ²²⁵Ac was used in the radiolabelling of a targeted “theranostic module” (with the chemical reaction to produce the radiopharmaceutical taking about 30 mins with a yield greater than 95%). 

For Pakistan, opportunity knocks, with the R&D collaboration between PINSTECH and MEDICIS key to the country’s long-term goal of strengthening the technical capability, commercial capacity and production infrastructure to secure a scalable domestic pipeline of novel and high-impact theranostic radioisotopes.

The Institute of High Energy Physics (IHEP), part of the Chinese Academy of Sciences, is in the vanguard of China’s expansive – and rapidly growing – scientific endeavours in elementary particle physics. One of the engine-rooms of that research effort is the IHEP accelerator division, the largest accelerator R&D programme in China, which has a similarly expansive remit covering the planning, design, construction and delivery of large-scale accelerator projects – on time and within budget – as well as the training and development of the next generation of accelerator scientists and engineers.

The origin story of the IHEP accelerator division can be traced back more than half a century. In 1964, Jialin Xie, the pioneer of Chinese accelerator science and technology, built the country’s first linear accelerator (a 30 MeV electron linac), laying the ground for the development of future large-scale accelerator facilities in China. 

Fast forward to the 1980s and IHEP’s construction of the Beijing Electron–Positron Collider (BEPC) – the foundation of the institute’s own accelerator science programme. Since then, a talented and multidisciplinary accelerator team has grown and developed alongside IHEP’s ambitious research programme in high-energy physics. Today, the IHEP accelerator team is made up of around 370 scientists and engineers, 110 postgraduate students, 26 postdocs and numerous guest scientists working across IHEP’s two campuses in Beijing and Dongguan (an industrial city close to Hong Kong). 

Project management: it’s all about delivery

The IHEP accelerator team works on advanced electron and proton accelerators, with deep domain knowledge and capabilities spanning a broad base of enabling technologies, including (but not limited to) precision mechanics; magnets and power supplies; ultrahigh vacuum components and nonevaporable-getter-(NEG)-coated vacuum chambers; cryogenic systems; superconducting magnets and RF cavities; RF power equipment and accelerating structures; as well as autocontrol systems and cutting-edge instrumentation for beam diagnostics and steering. 

Furthermore, that collective IHEP know-how – tying together fundamental accelerator science, technology innovation and systems engineering – has scaled  dramatically over the past 20 years as the team has overseen the construction and enhancement of several large-scale accelerator initiatives, including: the upgrade of the Beijing Electron–Positron Collider (BEPCII); a high-current proton injector for the Accelerator Driven Sub-critical System (ADS); the China Spallation Neutron Source (CSNS); the High Energy Photon Source (HEPS); as well as formative design work on the proposed 100 km Circular Electron–Positron Collider (CEPC). This is big science at scale – a diverse programme of accelerator R&D that required all manner of technology/engineering innovation along the way:  

1. BEPCII (2004–2009)

BEPCII was a five-year effort to upgrade the original BEPC with a new injector as well as replacing its single-ring collider with a double-ring architecture. Upon completion of the upgrade, BEPCII entered operation in 2009 with an adjustable beam energy in the range 1.0–2.3 GeV and a beam current of 910 mA. Significant progress was registered in terms of the core enabling technologies for a high-beam-current, high-luminosity electron–positron collider, as well as in the design optimisation, system integration and project management. The luminosity of BEPCII reached the design goal of 1 × 10³³ cm^–2.s^–1 at a beam energy of 1.89 GeV (100 times higher than that of BEPC). These successful outcomes informed and shaped subsequent IHEP accelerator projects, including the proposal for the future 100 km CEPC (see below). 

2. ADS (2010–2017)

ADS is a high-power, high-intensity proton machine that has potential applications in nuclear-waste transmutation as well as thorium-based energy production. Under the auspices of the Chinese Academy of Sciences, IHEP developed the world’s first high-frequency, high-power and continuous-wave (CW) proton injector for ADS – the building blocks consisting of a CW radiofrequency quadrupole (RFQ), a superconducting linac, a beam dump and transport lines. The RFQ operates in 325 MHz CW mode, providing a 3.2 MeV acceleration capability, while the superconducting spoke cavities installed in the 2 K cryomodules have successfully accelerated the 10.6 mA proton beam to an energy of 10.67 MeV. As such, ADS is the first proton linac operating at such a high beam current (and integrated by 14 superconducting spoke cavities with a record-breaking beta value of 0.12). 

3. CSNS (2011–2018) 

The CSNS is located on IHEP’s Dongguan campus and consists of an accelerator, a target station and several neutron instruments. The accelerator complex itself is made up of an H^– ion source (with 20 mA current), a four-vane RFQ linac (at 3 MeV), four drift-tube linac tanks (80 MeV), a rapid circling synchrotron (1.6 GeV/25 Hz) and various beam transfer lines. 

CSNS construction was completed in 2018, with the beam power reaching the design value of 100 kW in February 2020. The uncontrolled beam loss rate is less than 1 W/m thanks to IHEP’s custom-designed collimation system and successful mitigation of space-charge effects. Beam availability during 2021/22 operations reached 97.1% (with 5262 hours of effective beam-time on-target). 

The CSNS-II upgrade has since been approved, with construction scheduled to start in 2023. CSNS-II is designed to have a beam power of 500 kW – a capability that will be achieved by adding 20 superconducting double-spoke cavities and 24 six-cell ellipsoidal superconducting cavities to increase the linac beam energy to 300 MeV. Peak current intensity will, in turn, be scaled from 15 mA to 50 mA.

4. HEPS (2019–2025)

HEPS is a fourth-generation synchrotron radiation facility under construction in Huairou, a district in northern Beijing. HEPS consists of a 6 GeV storage ring with a circumference of about 1.3 km, a full energy booster, a 500 MeV linac, three transfer lines, multi-beamlines and corresponding experimental stations. In terms of performance, HEPS aims to have a beam current of 200 mA and a record-breaking ultralow emittance (better than 0.06 nm-rad), promising spectral brightness up to 1 × 10²² phs∙s^-1∙mm^-2∙mrad^-2∙(0.1% BW)^-1 in the typical hard X-ray regime. 

The storage ring consists of 48 hybrid seven-bend acrhomats, with alternating high- and low-beta straight sections to accommodate various types of insertion devices. An innovative and high-energy, accumulation-aided on-axis swap-out scheme is adopted for injection from the booster to the storage ring. In addition, the ultralow-emittance design imposes very-high-precision requirements on all equipment as well as beam diagnostics and controls (with temperature fluctuation in the tunnel also kept within 0.1 °C). 

Worth adding that HEPS will function as a “green accelerator”, with a 10 MW solar power generator (the largest solar power station in Beijing) on the roof of the storage ring serving as a test-case for future machines. 

The HEPS ground-breaking ceremony took place in 2019, with the linac and booster installation now nearing completion. Installation of the storage ring will be completed in 2023, with the first synchrotron X-rays expected in 2024. 

The CEPC blueprint: theory meets technology 

A legacy of successful project delivery and technology innovation on these accelerator initiatives means IHEP physicists are looking ahead to a bright future. Soon after the discovery of the Higgs boson at CERN a decade ago, IHEP scientists unveiled a grand plan to build the CEPC – which will function as a “Higgs factory” – followed by construction of a Super Proton–Proton Collider (SppC) to be housed in the same tunnel. The scope and ambition of these combined facilities would, were they to be realised, unquestionably position IHEP at the cutting-edge of particle physics and accelerator science for decades to come. 

At the same time, fleshing out the design, technology and engineering requirements for a next-generation accelerator complex like the CEPC is, by necessity, a collective endeavour, involving a network of collaborations with scientists and engineers at large-scale research facilities around the world. In terms of high-level design specifications, the circumference of the collider is optimised to be 100 km (based on the construction cost, operational performance and upgrade considerations for the SppC). Meanwhile, the lattice of the CEPC collider ring, as well as the interaction region, are specified so as to achieve high luminosities switchable among various energies corresponding to the Z, W and the Higgs bosons. A number of thorny challenges have already been overcome during the design phase, including beam–beam effects, strong collective instabilities, and radiation background and dose shielding. 

As with all big science initiatives, innovation and cost reduction are ever-present priorities. With this in mind, a number of new technologies are under study – for example, an electron-beam-driven plasma acceleration scheme for the linac injector, as well as the iron-based superconducting coils for the SppC. IHEP is also devoting its efforts to designing the CEPC as a dual-use machine – i.e. a Higgs factory on the one hand, as well as a high-flux, high-energy gamma-ray (up to 100 MeV) synchrotron light source with a multidisciplinary research programme of its own.

CEPC is, by necessity, a collective endeavour, involving a network of collaborations with scientists and engineers at large-scale research facilities around the world

Over the past decade, IHEP and its collaborators have been working on an extensive programme of technology R&D projects as part of the validation and iteration for the CEPC and SppC design studies. Significant progress can be seen along a number of coordinates, including: electropolishing and mid-temperature processing to yield state-of-the-art performance in the 1.3 GHz nine-cell and 650 MHz single-cell superconducting RF cavities; all design specifications met in prototypes of unprecedented low-field dipole and dual-aperture magnets; and prototype energy-efficient klystrons demonstrated an efficiency of 70.5% (closing in on the ultimate target value of 80%). Equally important in this regard is the fact that around 40% of the CEPC hardware requirement will exploit existing platform technologies that are already established at facilities like HEPS, CSNS and BEPCII.

Collaborate and accelerate

One thing is clear: cross-disciplinary and cross-border collaboration are going to be key to translating the technical vision underpinning the CEPC (and the SppC) into scientific reality. In this regard, IHEP is well placed, having a successful track-record of domestic collaboration with accelerator facilities across the country. IHEP scientists and engineers, for example, helped to build the Shanghai Light Source, and collaborated with the Institute of Modern Physics in Lanzhou to build ADS. Cooperation is underway now with the Shanghai Free Electron Laser Facility (to develop and produce superconducting RF cavities) as well as the Dalian Light Source (to build a complete superconducting accelerator module, including cavities and other components). 

It goes without saying that international collaboration is also hard-wired into IHEP’s operational model, with long-term R&D partnerships established in the US (e.g. Brookhaven National Laboratory and SLAC), Europe (CERN and DESY) and Japan (KEK) – and with many of these partners very much to the fore during the construction of BEPC/BEPCII, CSNS, ADS and HEPS. It works both ways, of course, with IHEP recently contributing to CERN’s High Luminosity-LHC upgrade with the provision of 13 units of superconducting corrector magnets. 

As for the future CEPC, the technology R&D effort is being led by IHEP with extensive support from domestic research institutes – including Peking University, Tsinghua University and Shanghai Jiao Tong University – and with additional inputs provided by an Institution Board of 32 universities and research centres. Meanwhile, the CEPC international network comprises an International Advisory Committee (IAC), International Accelerator Review Committee (IARC) and the International Detector R&D Review Committee (IDRDC). The global nature of the collaboration is evident in the CEPC Conceptual Design Report – which has some 1143 authors from 221 research institutes (including 144 overseas institutions across 24 countries) – while the CEPC study group has also signed more than 20 memoranda of understanding with research facilities and universities around the world. 

Wide-scale engagement is everything just now. As such, the CEPC accelerator team has been a participant in the commissioning of KEK’s SuperKEKB electron–positron
collider in Tsukuba, Japan, and has worked with the Future Circular Collider (FCC) team at CERN on fundamental studies of beam–beam interactions and related aspects of beam physics. There’s also been extensive outreach to industry via the CEPC Industry Promotion Consortium (CIPC). Established in 2017, the CIPC now has more than 70 industrial companies participating within China. 

For IHEP’s accelerator division, and its domestic and international partners, a new world of scientific opportunity is rapidly taking shape.