Since April 22, protons have been circulating again in the 27-kilometer ring of the LHC, CERN's giant accelerator near Geneva. After a three-year maintenance shutdown, this is the beginning of run 3, which physicists expect will allow them to lift part of the veil that covers the mysteries of elementary matter, revealed during ultra energetic collisions between particles.
More precisely, in operation, in the LHC ring, nearly 3,000 bunches of protons, each containing more than 100 billion particles, circulate at almost the speed of light in one direction, and as many in the opposite direction. Thus, every 25 nanoseconds, two bunches of protons cross each other in the detectors, each crossing provoking a few dozen interactions between protons, from which new particles emerge carrying with them a message from the infinitely small.
The maintenance of this incessantly fast-moving broom is the job of CERN's accelerator specialists, whose task is to shape, accelerate and maintain the particle beams according to stringent specifications. During the maintenance phase, these physicists, engineers and technicians revised and improved the entire proton injection chain in the collider. At the end of a pharaonic project, this infrastructure, which includes a total of six separate acceleration systems for protons and ions, stretching over several kilometers, is now ready to push its own limits. During run 3, the quantity of particles contained in each proton bunch will be gradually doubled. Similarly, the proton density of the bunches will be continuously increased, with the result that the collision rate will be multiplied.
In concrete terms, the entire first stage of the LHC beam acceleration has been replaced. Whereas the Linac2, which dates from the 1970s, delivered particles with an energy of 50 MeV, its successor, the Linac4, increases the energy to 160 MeV.
This tripling of energy is directly related to the ultimate goal of increasing the rate of collisions in the LHC. "Charged particles repel each other, but as their kinetic energy increases, the magnetic field they generate intensifies, opposing this repulsion even more. It is then possible to increase the density of the bunches, which ultimately determines the number of interactions between protons at each crossing," explains Malika Meddahi, deputy director of accelerators and technology at CERN.
To accomplish its task, Linac4 is composed of four separate accelerating structures. The first, a high-intensity radio frequency quadrupole (RFQ), gives negative hydrogen ions (H-, a hydrogen atom and an extra electron) their first acceleration while already giving the beam its bunch structure. "This is crucial because the entire particle acceleration is the result of successive radio-frequency cavities that deliver an alternating electric field. To be accelerated, the beam must therefore be cut into pieces as soon as it is created so that the particles enter each structure at the precise moment when the field is in the right direction. Conversely, a continuous beam would see its different sections alternately accelerated and decelerated," explains Sébastien Bousson, director of the accelerator physics division at IJCLab (Orsay).
Within the RFQ pipe, this first cut is the result of an electromagnetic field with a diabolical geometry, itself dictated by the precise shape of a solid copper part several meters long. As Pierre Védrine, head of the accelerator, cryogenics and magnetism department at CEA Paris Saclay, whose teams, along with those of the CNRS, participated in the design of this RFQ, explains, "This is a difficult part to design and delicate to machine as it is necessary to guarantee this shape to within a few tens of microns over a very long distance."
After leaving the RFQ, the H- ions then pass through three other more conventional radio frequency structures, but "state of the art", admires Sébastien Bousson, who adds as a connoisseur: “the choice of these structures is the result of fine optimization in terms of principle, design and cost, linked to the fact that in this first phase of acceleration, the speed of the ions, still far from the speed of light, varies enormously. In the end, the Linac4 is an impressive machine in terms of energy and intensity. Moreover, it has been designed, manufactured and started up with the required level of reliability and performance in a timely manner, which is a real achievement."
After passing through the Linac4, the H- ions are then sent to an amplifier, the PSB (Proton Synchrotron Booster), via a highly innovative injection chain, where they are stripped of their two electrons, so that only protons remain. Completely overhauled during the shutdown, this set of four superimposed synchrotrons with a circumference of 157 meters accelerates protons up to 2 GeV, compared to 1.4 GeV in its previous version. “In particular, we have changed all the injection and extraction systems, the accelerator cavity system, the power supply system for the magnets to match the increased energy of the beams, and the cooling system," says Malika Meddahi. “All of the magnets have also been changed or renovated, as well as the beam instrumentation system and the unwanted beams absorber systems.”
PS? This is the next acceleration stage, a 628-meter circumference proton synchrotron where protons are boosted to 25 GeV before being injected into the 7-kilometer circumference SPS, or super proton synchrotron. The energy of the particles then reaches 450 GeV before injection into the LHC ring.
More precisely, the proton bunches are injected in pairs or quads in the PS, which then become four trains of 72 bunches in the SPS and finally 10 trains of 288 bunches in the giant accelerator, where their energy will finally be increased from 450 GeV to 6.8 TeV. "The nominal situation is reached at the end of a sequence of about one hour during which, all along the injection chain, the protons are accelerated little by little while complex radiofrequency manipulations give the beam its shape and tempo in a succession of mergers and splits of the bunches", describes Malika Meddahi.
Like the first stages of the injection chain, the PS and SPS have undergone a real facelift. For example, all the corrective magnets of the PS, which allow the brightness of the proton bunches in the ring to be maintained, have been changed. The old ones dated from 1959. In addition, the radio frequency accelerator system and the beam instrumentation system have been improved and/or changed.
Also, all the radiofrequency installations of the SPS have been changed in order to adapt them to a brighter beam. As for the inner surface of the accelerator's vacuum chambers in the critical areas, it has been completely covered with a layer of amorphous carbon to minimize beam-wall interaction and, in so doing, limit losses. "We looked at element by element how to win to improve the beam characteristics," notes the physicist.
Once injected and accelerated in the LHC's 27-kilometer ring, the beams circle for about eight hours at a rate of 11,245 revolutions per second. Their quality degraded by successive crossings, they are finally released from their circular trajectory to hit a huge-meter long graphite block placed in a 12-millimeter thick stainless steel tube, all encased in an iron shield of 7 tons filled with nitrogen gas.
And for good reason: during the first two data taking campaigns of the collider, its beams contained an energy of 320 megajoules, that is to say, the energy of a high-speed train launched at 150 km/h. So after 10 years of absorbing shocks, the absorbers finally showed some signs of fatigue, forcing CERN staff to review the support system. Especially since during run 3, the energy of the beams will reach 555 megajoules, causing the temperature of the absorbers to rise to 1500 °C in 100 microseconds, compared to 1000 °C before.
Similarly, the SPS beam dump has been changed in order to "hold" the proton concentrate produced by the new injection chain. It is simple: the old one could not handle the concentrated intensities in an even smaller volume that will become the norm at the end of run 3. As for the new one, composed of an assembly of graphite, molybdenum and tungsten surrounded by concrete and a cast-iron and marble shielding, 9 meters long, it required the modification of 150 meters of the accelerator tunnel for the installation of fast deflection magnets capable of deflecting and diluting the beam, even in case of emergency.
Since the beginning of the year, each of the accelerator's systems and its injection chain have been restarted without a hitch, and there is nothing to stop protons from colliding again soon at an unprecedented energy and frequency. This will give physicists the opportunity to make fantastic discoveries.
Translated from an article written by Mathieu Grousson (les Chemineurs), published here
Améliorations apportées à la chaîne des accélérateurs du CERN pendant le 2e arrêt technique du LHC. ©2022 CERN