CERN Accelerating science

HL-LHC superconducting quadrupole successfully tested

The quadrupole magnet being prepared for a test at Brookhaven National Laboratory. (Image: Brookhaven National Laboratory)

 

A quadrupole magnet for the high-luminosity LHC (HL-LHC) has been tested successfully in the US, attaining a conductor peak field of 11.4 T – a record for a focusing magnet ready for installation in an accelerator. The 4.2 m-long, 150-mm-single-aperture device is based on the superconductor niobium tin (Nb3Sn) and is one of several quadrupoles being built by US labs and CERN for the HL-LHC, where they will squeeze the proton beams more tightly within the ATLAS and CMS experiments to produce a higher luminosity. The result follows successful tests carried out last year at CERN of the first accelerator-ready Nb3Sn dipole magnet, and both of these milestones are soon to be followed by tests of other 7.2 m and 4.2 m quadrupole magnets at CERN and the US.

“This copious harvest comes after significant recent R&D on niobium-tin superconducting magnet technology and is the best answer to the question if HL-LHC is on time: it is,” says HL-LHC project leader Lucio Rossi of CERN. “We should also underline that this full-length, accelerator-ready magnet performance record is a real textbook case for international collaboration in the accelerator domain: since the very beginning the three US labs and CERN teamed up and managed to have a common and very synergic R&D, particularly for the quadrupole magnet that is the cornerstone of the upgrade. This has resulted in substantial savings and improved output.”

The current LHC magnets, which have been tested to a bore field of 8.3 T and are currently operated at 7.7 T at 1.9 K for 6.5 TeV operation, are made from the superconductor niobium-titanium (Nb-Ti). As the transport properties of Nb-Ti are limited for fields beyond 10-11 T at 1.9 K, HL-LHC magnets call for a move to Nb3Sn, which remain superconducting for much higher fields. Although Nb3Sn has been studied for decades and is already in widespread use in solenoids for NMR — not to mention underpinning the large coils, presently being manufactured, that will be used to contain and control the plasma in the ITER fusion experiment – it is more challenging than Nb-Ti to work with: once formed, the Nb3Sn compound becomes brittle and strain sensitive and therefore much harder than niobium-titanium alloy to process into cables to be wound with the accuracy required to achieve the performance and field quality of state-of-the-art accelerator magnets.

Researchers at Fermilab, Brookhaven National Laboratory and Lawrence Berkeley National Laboratory are to provide a total of 16 quadrupole magnets for the interactions regions of the HL-LHC, which is due to operate from 2027. The purpose of a quadrupole magnet is to produce a field gradient in the radial direction with respect to the beam, allowing charged-particle beams to be focused. A test was carried out at Brookhaven in January, when the team operated the 8-tonne quadrupole magnet continuously at a nominal field gradient of around 130 T/m and a temperature of 1.9 K for five hours. Eight longer quadrupole magnets (each providing an equivalent “cold mass” as two US quadrupole magnets) are being produced by CERN.

“We’ve demonstrated that this first quadrupole magnet behaves successfully and according to design, based on the multiyear development effort made possible by DOE investments in this new technology,” said Fermilab’s Giorgio Apollinari, head of the US Accelerator Upgrade Project in a Fermilab press release. “It’s a very cutting-edge magnet,” added Kathleen Amm, who is Brookhaven’s representative for the project.

Dipole tests at CERN

In addition to stronger focusing magnets, the HL-LHC requires new dipole magnets positioned on either side of a collimator to correct off-momentum protons in the high-intensity beam. To gain the required space in the magnetic lattice, Nb3Sn dipole magnets of shorter length and higher field than the current LHC dipole magnets are needed. In July 2019 the CERN magnet group successfully tested a full-length, 5.3-m, 60-mm-twin-aperture dipole magnet – the longest Nb3Sn magnet tested so far – and achieved a nominal bore field of 11.2 T at 1.9 K (corresponding to a conductor peak field of 11.8 T).

“This multi-year effort on Nb3Sn, which we are running together with the US, and our partner laboratories in Europe, is leading to a major breakthrough in accelerator magnet technology, from which CERN, and the whole particle physics community, will profit for the years to come,” says Luca Bottura, head of the CERN magnet group.

The dipole- and quadrupole-magnet milestones also send a positive signal about the viability of future hadron colliders beyond the LHC, which are expected to rely on Nb3Sn magnets with fields of up to 16 T. To this end, CERN and the US labs are achieving impressive results in the performance of Nb3Sn conductor in various demonstrator magnets. In February, the CERN magnet group produced a record field of 16.36 T at 1.9 K (16.5 T conductor peak field) in the centre of a short “enhanced racetrack model coil” demonstrator, with no useful aperture, which was developed in the framework of the Future Circular Collider study. In June 2019, as part of the US Magnet Development Programme, a short “cos-theta” dipole magnet with an aperture of 60 mm reached a bore field of 14.1 T at 4.5 K at Fermilab. Beyond magnets, says Rossi, the HL-LHC is also breaking new ground in superconducting-RF crab cavities, advanced material collimators and 120 kA links based on novel MgB2 superconductors.

Next steps

Before they can constitute fully operational accelerator magnets which could be installed in the HL-LHC, both these quadrupole magnets and the dipole magnets must be connected in pairs (the longer CERN quadrupole magnets are single units). Each magnet in a pair has the same winding, and differs only in its mechanical interfaces and details of its electrical circuitry. Tests of the remaining halves of the quadrupole- and dipole-magnet pairs were scheduled to take place in the US and at CERN during the coming months, with the dipole magnet pairs to be installed in the LHC tunnel this year. Given the current global situation, this plan will have to be reviewed, which is now the high-priority discussion within the HL-LHC project.

This news appeared first at the CERN Courier

This news was also featured at Fermilab

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A novel composite for HL-LHC collimators

Collimator installation in 2018. (Image: CERN)

 

During the long-shutdown 2 (LS2), the Large Hadron Collider (LHC) collimation system will be upgraded, by the production and installation of four new primary and eight secondary collimators for beam halo cleaning, as well as four collimators in the dispersion suppression region. Additional 20% units will be produced and kept as spare. Collimators are a crucial part of an accelerator, allowing the controlled deposition of beam loses in specific locations, thus minimising its impact of radiation in the collider and detectors.

For the halo cleaning, the main goal of the upgrade is to decrease the impedance of the collimation system. This must be done by replacing the existing Carbon-Fibre-Carbon composite (CFC), used in in the collimator jaws, which are the elements actively intercepting the primary beam particles and the particle shower, with a higher-electrically-conductive material.

The primary and secondary collimators under production will thus be equipped with a novel composite, named Molybdenum-Graphite (MoGr), co-developed in the past years by CERN and Brevetti Bizz, an Italian company (Figure 1). The material possesses extraordinary thermo-physical properties, including a thermal conductivity more than doubles that of copper and a low density (2.5 g/cm3).  Even more important for collimators, the new composite’s electrical conductivity is higher than that of CFC by a factor of 5. In the case of secondary collimators, the material will also be coated with a thin layer (6 µm) of metallic molybdenum, further boosting the surface electrical conductivity by an additional order of magnitude.

Figure 1: Molybdenum-Graphite block (left) and tapered extremity (right). (Image: CERN)

 

Finally, MoGr will also be adopted for the jaw tapered extremities, which host the Beam Position Monitor (BPM). Past experiments in the HiRadMat facility [1], in fact, have shown that the copper alloy previously used for these elements was destroyed in case of accidental beam impact on the jaw, while MoGr survives under the accidental design scenario, without any damage (Figure 2).

Figure 2: Impact of 288 proton bunches (SPS beam) on a copper-alloy standard tapered extremity (left) and on a MoGr tapering (right).  (Image: CERN)

 

The production of MoGr blocks for the HL-LHC collimators is a challenging task [2].

In particular, the material must be sintered at extremely high temperature, above the melting point of the molybdenum carbide, which is formed during the process (> 2600 °C). This must be done applying an intense pressure to the mould, in a process known as spark-plasma sintering. After sintering, the material is submitted to another high-temperature cycle, this time pressure-less, for stress relieving, before being finally machined to very tight dimensional tolerances. The production contract for the MoGr was assigned to Nanoker, a company specialized in ceramic materials, sited in Oviedo (Spain). Production is currently ongoing, with a 60% completion, and an estimated end of production by the end of 2019.

The blocks are typically shipped to CERN in batches, with an average lead-time of one batch (i.e. material for one collimator plus spare) per month. After reception, the components are submitted to a thorough acceptance campaign, including dimensional and thermo-physical measurements, before being prepared for the molybdenum coating, which is performed by the Danish Technical Institute (DTI), sited in Aarhus (Denmark). The coating itself is an additional technological challenge: it must have a good adherence with the substrate, no impurities, and regular grain size.

Excellent results are being obtained with the high-power impulse magnetron sputtering technique (HiPIMS), where a pulsed Krypton plasma is generated between a Molybdenum cathode and an anode. Mo atoms (partially ionized) are sputtered away from the anode and deposited on the MoGr substrate. This technique guarantees an electrical surface conductivity equal to the maximum value theoretically possible, i.e. that of pure metallic bulk molybdenum. After the coated blocks are shipped back to CERN, they are submitted to a final UHV test, to ensure compatibility with the specifications for component installation in the LHC, before the final delivery to the company in charge of the full collimator production.

Technical challenges were experienced between the passage from an R&D phase to the industrial production, but they were solved thanks to good team-work within the High Luminosity-LHC project. The ongoing production of MoGr is, beyond a key ingredient for the HL-LHC upgrade, an important step towards industrialization of this material, which has potential interest in high-end fields such as aerospace, nuclear energy, fusion, heat dissipation, and any other domain where low density and high thermal and electrical properties are necessary. For this reason, the Knowledge Transfer group at CERN is significantly involved in the industrialization of the material, whereas further material optimization is in the scope of H2020 European projects, such as ARIES [3].

Further reading:

  1. G. Gobbi et al., “Novel LHC collimator materials: High-energy Hadron beam impact tests and non‑destructive post‑irradiation examination”.  Mechanics of Advanced Materials and Structures, 2019, DOI: 10.1080/15376494.2018.1518501.
  2. J. Guardia-Valenzuela et al., “Development and properties of high thermal conductivity molybdenum carbide - graphite composites”. Carbon, 135, pp. 72, 2018.
  3. More information on H2020 ARIES project: https://aries.web.cern.ch/
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