CERN’s transfer lines are undergoing significant renovations to make them more energy efficient while delivering high-quality beams reliably to experiments, write Konstantinos Papastergiou and Gilles Le Godec.
Just over 60 years ago, physicists and engineers at CERN were hard at work trying to tune the world’s first proton synchrotron, the PS. It was the first synchrotron of its kind, employing the strong-focusing principle to produce higher-energy beams within a smaller aperture and with a lower construction cost compared to, for example, the CERN synchrocyclotron. Little could physicists in 1959 imagine the maze of technical galleries and tunnels stemming out of the PS ring not many years later.
The first significant expansion to CERN’s accelerator complex was prompted by the 1962 discovery of the muon neutrino at the competing Alternating Gradient Synchrotron at Brookhaven National Laboratory in the US. Soon afterwards, CERN embarked on an ambitious programme starting with a new east experimental area, the PS booster and the first hadron collider – the Intersecting Storage Rings (ISR). A major challenge during this expansion was transferring the beam to targets, experiments and the ISR, which required that CERN build transfer lines that could handle different particles, different extraction energy levels and various duty cycles (see “In service” figure).
Transfer lines transport particle beams from one machine to another using powerful magnets. Once fully accelerated, a beam is given an ultra-fast “kick” off its trajectory by a kicker magnet and then guided away from the ring by one or more septum magnets. A series of focusing and defocusing quadrupole magnets contain the beams in the vacuum pipe while bending magnets direct them to their new destination (a target or a subsequent accelerator ring).
Making the connection
The first transfer lines linking two different CERN accelerators were TT1 and TT2, which were originally built for the ISR. The need to handle different particle energies and even different particle charges required continuous adjustment of the magnetic field at every extraction, typically once per second in the PS. One of the early challenges faced was a memory effect in the steel yokes of the magnets: alternating among different field values leaves a remnant field that changes the field density depending on the order of cycles played out before. Initially, complex solutions with secondary field-resetting coils were used. Later, magnetic reset was achieved by applying a predefined field excitation that brings the magnet to a reproducible state prior to the next physics cycle.
Solving the magnetic hysteresis problem was not the only hurdle that engineers faced. Handling rapid injections and extractions through the magnets was also a major challenge for the electronics of the time. The very first powering concept used machine/generator setups with adjustable speeds to modulate the electric current and consequently the field density in the transfer-line magnets. Each transfer line would have its own noisy generation plant that required a control room with specialised personnel (see “Early days” images). Modifying the mission-profile of a magnet to test new physics operations was a heavy and tedious operation.
Towards the end of 1960s, electrical motors in the west PS hall were replaced by the first semiconductor-operated thyristor rectifiers, which transformed the 50 Hz alternating grid voltage to a precisely regulated (to nearly 100 parts per million) current in the beamline magnets. They also occupied a fraction of the space, had lower power losses and were able to operate unsupervised. All of a sudden, transporting different particles with variable energies became possible at the touch of a knob. The timing could not have been better, as CERN prepared itself for the Super Proton Synchrotron (SPS) era, which would see yet more transfer lines added to its accelerator complex.
By the early 1980s the ISR had completed its mission, and the TT1 transfer line was decommissioned together with the storage rings. However, the phenomenal versatility of TT2 has allowed it to continue to extract particles for experiments. Today, virtually all user beams, except those for the East Area and ISOLDE, pass through the 300 m-long line. It delivers low-energy 3 GeV beams to “Dump 2” for machine development, 14 GeV beams to the SPS for various experiments in the North Area, 20 GeV beams towards the n_ToF facility, 26 GeV beams to the Antiproton Decelerator, and to the SPS – where protons are accelerated to 450 GeV before being injected into the LHC. While beams traverse TT2 in just over a microsecond, other beamlines, such as those in the East Area, spill particles out of the PS continuously for 450 ms towards the CLOUD experiment and other facilities – a process known as slow extraction.
Transfer lines are heavy users of electrical power, since typically their magnets are powered for long periods compared to the time it takes a beam to pass. During their last year of operation in 2017, for example, the East Area transfer lines accounted for 12% of all energy consumption by CERN’s PS/PSB injector complex. The reason for this inefficiency was the non-stop powering of the few dozen magnets used in each transfer line for the necessary focusing, steering and trajectory-correction functions. This old powering system, combined with a solid-yoke magnet structure, did not permit extraction of the magnetic field energy between beam operations.
CERN is looking at testing and implementing new systems that lower its environmental impact today and into the far future
For reference, a typical bending magnet absorbs the same energy as a high-performance car accelerating from 0 to 100 km/h, and must do so in a period of 0.5 s every 1.2 s for beams from the PS. To supply and recover all this energy between successive beam operations, powerful converters are required along with laminated steel magnet yokes, all of which became possible with the recent East Area renovation project.
Energy economy was the primary motivation for CERN to adopt the “Sirius” family of regenerative power converters for TT2 and, subsequently, the East Area and Booster transfer lines. While transfer lines typically absorb and return all the magnetic field energy from and to the power grid, the new Sirius power converter allows a more energy-efficient approach by recovering the magnetic field energy locally into electrolytic capacitors for re-use in the next physics cycle. Electrolytic capacitors are the only energy-storage technology that can withstand the approximately 200 million beam transports that a Sirius converter is expected to deliver during its lifetime, and the system employs between 15 and 420 such wine-bottle-sized units according to the magnet size and beam energy to be supplied (see “Transformational” image).
Sirius is also equipped with a front-end unit that can control the energy flow from the grid to match what is required to compensate the thermal losses in the system. By estimating in real time how much of the total energy can be recycled, Sirius has enabled the newly renovated East Area to be powered using only two large-distribution transformers rather than the seven transformers used in the past for the old 1960s thyristor rectifiers. To control the energy flow in the magnets, Sirius uses powerful silicon-based semiconductors that switch on and off 13,000 times per second. By adjusting the “on” time of the switches the average current in and out of the energy-storing units can be controlled with precision, while the high switching frequency allows rapid corrections of the generated voltage and current across the magnet.
The Sirius converters entered operation gradually from September 2020, and at present a total of 500 million magnetic cycles have been completed. Recent measurements made on the first circuits commissioned in the East Area demonstrated an energy consumption 95% lower than compared to the original 1960s figures. But above all, the primary role of Sirius is to provide current and hence magnetic field in transfer-line magnets to a precision of 10 parts per million, which enables excellent reproducibility for the beams coming down the lines. The most recent measurements demonstrated a stability better than 10 ppm during a 24-hour interval.
Unusual engineering model
CERN employs a rather unusual engineering model compared to those in industry. For Sirius, a team of experts and technicians from the electrical power converters group designed, prototyped and validated the power-converter design before issuing international tenders to procure the subsystems, assembly and testing. Engineers therefore have the opportunity to work with their counterparts in member-state industries, often helping them develop new manufacturing methods and skills. Sirius, for example, helped a magnetics-component manufacturer in Germany achieve a record precision in their manufacturing process and to improve their certification procedures for medium-power reactors. Another key partner acquired new knowledge in the manufacturing and testing of inoxidised water-cooling circuits, enabling the firm to expand its project portfolio.
Thanks to the CERN procurement process, Sirius components are built by a multitude of suppliers across Europe. For some, it was their first time working with CERN. For example, the converter-assembly contract was the first major (CHF 12 million) contract won by Romanian industry after the country’s accession to CERN five years ago. Other significant contributions were made by German, Dutch, French, UK, Danish and Swedish industries. Recent work by the CERN knowledge transfer group resulted in a contract with a Spanish firm that licensed the Sirius design for production for other laboratories, with the profits invested in R&D for future converter families.
Energy recycling tends to yield more impressive energy savings in fast-cycling accelerators and transfer lines, such as those in the PS. However, CERN is planning to deploy similar technologies in other experimental facilities such as the North Area that will undergo a major makeover in the following years. The codename for this new converter project is Polaris – a scalable converter family that can coast through the long extraction plateaus used in the SPS (see “Physics cycles” figure). The primary goal of the renovation, beyond better energy efficiency, is to restore the reliability and provide a 10-fold improvement in the precision of the magnetic field regulation.
Development efforts in the power-converters group do not stop here. The electrification of transportation and the net-zero carbon emission targets of many governments are also driving innovation in power electronics, which CERN might take advantage of. For example, wide bandgap semiconductors exhibit higher reverse-blocking capabilities and faster transitions that could allow switching at a rate of more than 40,000 Hz and therefore help to reduce size, losses and eliminate the audible noise emitted by power conversion altogether.
Another massive opportunity concerns energy storage, with CERN looking closely at the technologies driven by the battery mega-factories that are being built around the world. As part of our mission to provide the next generation of sustainable scientific facilities, as outlined in CERN’s recently released second environment report, we are looking at testing and implementing new systems to lower our environmental impact today and into the far future.