Stephen Sekula steve@hub.polari.us

Until today, a kilogram was defined as the mass of the International Prototype Kilogram (IPK), a platinum–iridium cylinder located in Paris, France. While all the other base units of the International System of Units (SI) had been redefined over the years based on fundamental constants of nature or atomic properties, the kilogram had remained since the late 19th century the only one to rely on a human-made artefact.

This changes today, and metrologists – those who study measurement – are excited. On the occasion of World Metrology Day, which commemorates the signing of the Metre Convention back in 1875, the kilogram has been given a new definition. From now on, it will be defined based on the most precise measurement ever made of the Planck constant, which can be expressed in terms of the SI units kilogram, metre and second. Since the latter two units are already defined by constants of nature, the value of a kilogram can be obtained without relying on comparing it with a physical reference block.

But measuring the Planck constant to a suitably high precision of ten parts per billion required decades of work by international teams across continents, and CERN played a small part in the endeavour.

In 1975, British physicist Bryan Kibble proposed a device, then known as a watt balance and now called the Kibble balance in his honour, which would allow the Planck constant to be measured precisely based on the IPK. Once the precision was achieved, the Planck constant’s value could be fixed and the definitions inverted, removing the kilogram’s dependence on the IPK. Several Kibble balances around the world were constructed to compare measurements, including one in Switzerland. METAS, the Swiss Federal Institute of Metrology, has been working on their Kibble balance project for almost two decades, the activity being led by Ali Eichenberger and Henri Baumann. Knowing CERN’s expertise in magnet systems, Eichenberger and Baumann reached out to the Laboratory to help prepare the required magnets.

“I am extremely proud to have participated in this adventure,” says Davide Tommasini from CERN’s Magnets, Superconductors and Cryostats group, who was directly involved in the project. “I do not know if the redefinition of the kilogram has a direct impact on the experiments at CERN, but the past teaches us that there are many new advancements which, at their initial moment, may not appear in their whole potential.”

In 2018, the Kibble balance in Canada measured the Planck constant with necessary ultrahigh precision, allowing a combination of measurements from around the world to help fix its value. But does it affect the value of the kilogram itself? Not really. “The Plank constant has been fixed at 6.626070150 × 10⁻³⁴ kg⋅m²/s using the IPK as standard,” explains Eichenberger. “So from today, one kilogram will stay the same. If the IPK drifts further with time then its value will change, but any mass calibration will have an uncertainty of the order of 20 parts per billion.”

So while it is a momentous occasion worthy of celebration, you won’t have to recalibrate your bathroom scales just yet.

The Proton Synchrotron (PS), which was CERN’s first synchrotron and which turns 60 this year, once held the record for the particle accelerator with the highest energy. Today, it forms a key link in CERN’s accelerator complex, mainly accelerating protons to 26 GeV before sending them to the Super Proton Synchrotron (SPS), but also delivering particles to several experimental areas such as the Antiproton Decelerator (AD). Over the course of Long Shutdown 2 (LS2), the PS will undergo a major overhaul to prepare it for the higher injection and beam intensities of the LHC’s Run 3 as well as for the High-Luminosity LHC.

One major component of the PS that will be consolidated is the magnet system. The synchrotron has a total of 100 main magnets within it (plus one reference magnet unit outside the ring), which bend and focus the particle beams as they whizz around it gaining energy. “During the last long shutdown (LS1) and at the beginning of LS2, the TE-MSC team performed various tests to identify weak points in the magnets,” explains Fernando Pedrosa, who is coordinating the LS2 work on the PS. The team identified 50 magnets needing refurbishment, of which seven were repaired during LS1 itself. “The remaining 43 magnets that need attention will be refurbished this year.”

Specifically, one of the elements, known as the pole-face windings, which is located between the beam pipe and the magnet yoke, needs replacing. In order to reach into the magnet innards to replace these elements, the magnet units have to be transferred to a workshop in building 151. Once disconnected, each magnet is placed onto a small locomotive system that drives them to the workshops. The locomotives themselves are over 50 years old, and their movement must be delicately managed. It takes ten hours to extract one magnet. So far, six magnets have been taken to the workshop and this work will last until 18 October 2019.

The workshop where the magnets are being treated is divided into two sections. In the first room, the vacuum chamber of the magnets is cut so as to access the pole-face windings. The magnet units are then taken to the second room, where prefabricated replacements are installed.

As mentioned in the previous LS2 Report, the PS Booster will see an increase in the energy it imparts to accelerating protons, from 1.4 GeV to 2 GeV. A new set of quadrupole magnets will be installed along the Booster-to-PS injection line, to increase the focusing strength required for the higher-energy beams. Higher-energy beams require higher-energy injection elements; therefore some elements will be replaced in the PS injection region as part of the LHC Injectors Upgrade (LIU) project, namely septum 42, kicker 45 and five bumper magnets.

Other improvements as part of the LIU project include the new cooling systems being installed to increase the cooling capacity of the PS. A new cooling station is being built at building 355, while one cooling tower in building 255 is being upgraded. The TT2 line, which is involved in the transfer from the PS to the SPS, will have its cooling system decoupled from the Booster’s, to allow the PS to operate independent of the Booster schedule. “The internal dumps of the PS, which are used in case the beam needs to be stopped, are also being changed, as are some other intercepting devices,” explains Pedrosa.

The LS2 operations are on a tight schedule,” notes Pedrosa, pointing out that works being performed on several interconnected systems create constraints for what can be done concurrently. As LS2 proceeds, we will bring you more news about the PS, including the installation of new instrumentation in wire scanners that help with beam-size measurement, an upgraded transverse-feedback system to stabilise the beam and more.

More pictures of the PS magnets are available on CDS: