The accelerator at CERN that led to the discovery of the Higgs boson in 2012—the Large Hadron Collider—has been out of operation since early this summer. At the end of June, the last proton beams circulated, and the detectors recorded what are likely to be their final proton collisions for the time being. After that, the keys to the accelerator were handed over to the High Luminosity LHC team.
That HL-LHC team, along with a whole army of engineers, technicians, and builders, is tasked with delivering a significantly improved version of the LHC accelerator by 2030. The goal of the high-luminosity project is to make it ten times more effective. Roughly one and a half kilometers of the 27-kilometer-long underground particle collider will be completely replaced with new technology.
Segments
In the IT String Facility, one of the countless workshop halls on the CERN campus just outside Geneva, a glimpse of the future CERN accelerator has been visible for about two years now. In a nearly 100-meter-long trough, three of the typical tubular beam magnets used to build particle accelerators are linked together. These are for the HiLumi.
The light blue, bright yellow, and orange-red segments are so-called inner triplet beam-focusing magnets, which will later be installed near the large ATLAS and CMD detectors. Essentially, they are superconducting coils that ensure a beam of charged particles, such as protons, travels through the accelerator in as compact a line as possible—roughly ten times closer together than the magnets of the old LHC could achieve.
HiLumi LHC magnet test string at CERN.
When two such concentrated beams collide head-on, this results in up to ten times more proton-proton collisions, allowing physicists to peer deep into the world of particles. It is estimated that the HiLumi-LHC will produce 200 simultaneous particle collisions per beam collision.
An Abundance of Collisions
In total, the machine is expected to produce some 360 million Higgs particles—ten times as many as the LHC has produced in eighteen years.
This abundance of events will enable much more refined analyses of the Higgs mechanism. In addition, rare particles and particle processes that were previously hidden in the noise may become visible in the data.
The key to these enormous improvements is a much stronger magnetic field that compresses the beam and keeps the proton flow in a firm grip despite their mutual repulsion. The strong field is made possible by the use of a new superconducting material, niobium-tin, which, when deeply cooled, can handle much stronger electric currents than the currently standard niobium-titanium. More current through a coil produces a stronger magnetic field.
The IT String facility at CERN, a test setup for beam magnets for the HiLumi-LHC accelerator.
In early July, CERN reported a successful test of the complete setup under realistic conditions. Cooled to 1.9 Kelvin (271 degrees below zero), the intended maximum current of 16,230 amperes was achieved in the coils. This produces a magnetic field of over 11 Tesla, about 35 percent stronger than the old magnets.
Magnet Training
The record-breaking power of the magnets was by no means the only goal of the realistic test. The experiment also examined the behavior of the magnets. Accelerator magnets, for example, must be “trained” to reach their maximum performance, partly because the coils deform, move, and settle due to the enormous magnetic forces. During the tests at the IT String Facility, the procedures for this were extensively tested.
The new inner triplet beam magnets are the foundation of the future high-luminosity accelerator at CERN, but at the same time, it is more challenging to precisely collide such tightly focused beams with one another.
Work on the crab cavities for the HiLumi LHC.
For this reason, just before the collisions, the beams are slightly lifted by a special magnet so that they will strike each other over a larger surface area, resulting in more particle collisions. This summer, the first of these so-called crab cavities also arrived at CERN. In the coming years, they will be installed in the new accelerator.
Additional Tunnels
Work on the HiLumi program has been underway at CERN since 2011, first on paper and through extensive tests and experiments, and later with the construction of magnets and other hardware. At two locations along the accelerator ring—one in Switzerland and one in France—additional tunnels and new above-ground facilities for cooling and high-voltage power have been built. Starting now, parts of the LHC accelerator are being removed from the tunnels and replaced with the new systems.
A number of older LHC magnets with malfunctions will also be replaced. Recently, all magnets in the tunnel were tested for stability and their shutdown behavior, known as quenching. In addition, other parts of the expanded accelerator complex in Geneva are also being addressed during this Long Shutdown 3. According to the schedule, the operation is set to be completed in 2030, after which the physicists will regain control of the accelerator.
Next episode: What will the HiLumi LHC mean for physics?
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