CERN Accelerating science

by Panos Charitos (CERN)

A new furnace for the heat-treatment of superconducting coils is currently commissioned at CERN’s Large Magnet Facility. It is the last large equipment that will allow the production of superconducting coils needed for the HL-LHC upgrade and future colliders explored under the FCC study. 

HL-LHC targets to increase the integrated luminosity by a factor of 10, resulting in an integrated luminosity of 3000 fb−1. The higher luminosities will allow higher-precision measurements and enable scientists to collect data at a much faster rate. HL-LHC calls for the development of new 11 Tesla dipole magnets with a total length of 11 meters to replace some of the existing 8 Tesla LHC dipole magnets. The development of these magnets was launched at the end of 2010 in the framework of a collaboration between the magnet groups of CERN and the US laboratory FNAL. Another major improvement for HL-LHC is the reduction of the beam size near the collision points. This requires the development of 150 mm single aperture quadrupoles to be installed in the interaction regions around the ATLAS and CMS experiments. The new quadrupoles, to be operated at a peak field of nearly 12 Tesla, are developed in a joint collaboration between CERN and the US-LHC Accelerator Research Program (LARP).

The FCC study has launched an R&D programme for the development of 16 Tesla magnets. In order to keep the protons on a circular track at the record-breaking energies of 100 TeV foreseen for a future hadron circular collider, scientists have to design and demonstrate very powerful magnets accelerator. The FCC relies on the use of magnets with a nominal field of 16 Tesla, almost double compared to the nominal 8.3 Tesla of the superconducting magnets used in the LHC.

To attain the goals posed by the HL-LHC and the FCC study, the use of new superconducting materials is needed. The niobium-titanium alloy that is currently used for the LHC superconducting magnets doesn’t allow to go higher than 9 T at 1.9 Kelvin in accelerator magnets. A new superconductor, based on the metallic compound niobium-three-tin (Nb3Sn) is at present the only practical option to reach such a high magnetic fields. Nb3Sn coils can sustain the required current densities to create magnetic fields of up to 16 T. Therefore it could fulfil the requirements of the HL-LHC upgrade as well as allow to realize a future circular hadron collider like the one explored by the FCC study.

In order to use Nb3Sn for new magnets one should understand in depth its properties and have the manufacturing processes at hand at a reasonable cost. Nb3Sn is much more complicated to work with compared to niobium titanium. A high temperature heat treatment (HTHT) is needed to form Nb3Sn superconductor via a solid state reaction of the primary components. This is a long process that lasts about two weeks. During the HTHT, the coils reach different temperature plateaus up to 665 oC.

After this process the material becomes brittle as ceramic. This poses a challenge for the manufacturing processes of the Nb3Sn superconducting coils. “Nb3Sn has been chosen for the next generation of superconducting magnets. The field achieved with this material can reach up to 16T. The production of such coils is complex as we must first wind the coils and then perform the heat treatment - a technique called "wind and react" - that will allow the formation of Nb3Sn” explains Friedrich Lackner, the project engineer who supervises the long quadropule coil production for HL-LHC. He continues: "Submitting an entire coil of many meters length, with its content of insulators and residuals of organics, to a high temperature treatment is a complicated process. Therefore, the oven to accommodate the heat treatment of Nb3Sn coils requires latest technology to achieve temperature uniformity and process stability."

Friedrich Lackner, project engineer who supervises the long quadropule coil production for HL-LHC, explains the special features of the new furnace.

In 2010 CERN launched the procurement of industrial furnaces to perform the in-house thermal treatment of the new coils. Before the construction of the long coils for future high-field dipole and quadrupole magnets, experts study and develop the process based on a short-model programme based on 2.5 meter magnets. This is a necessary step before starting the construction of the 5.5 m coils for the dipole magnets (that will be installed in the LHC tunnel during the Long Shutdown 2 in 2018) and the 8 meter long coils needed for new quadrupoles.

CERN’s Large Magnet Test facility has been equipped with different furnaces for treating the new superconducting materials forming a full production chain for the new coils. The first piece of this chain is the so-called HB160 high-temperature furnace arrived at CERN in 2011 soon followed by the installation of GLO750.

In 2014, the installation of GLO2000 in the Large Magnet Test hall allowed the treatment of 6.5 m long coils. The first 5.5 m long coils were wound in the CERN Large Magnet to build the HL-LHC 11 Tesla dipole magnets successfully tested this year. The results fulfilled the requirements that were specified for the furnace proving an excellent collaboration between CERN, the US laboratories in developing Nb3Sn coils that can meet the requirements of the HL-LHC project.

On May, the last missing piece of this chain arrived at CERN. The new furnace, called GL010000, will allow the heat treatment of coils with length of up to 11 meters while it can reach temperatures up to 900 oC providing a sufficient margin for future challenges. The new furnace will allow CERN to lead a rich R&D effort in the production of superconducting coils and the development of high-field magnets. Moreover, what makes the new oven unique is the high temperature homogeneity that can be achieved with a tolerance of +/- 3 oC for up to two weeks in an Argon atmosphere. The whole gas guiding system is optimized to achieve the nominal temperature as fast as possible and to achieve the best possible uniformity during the heat treatment.

GL010000 will be mainly used for treating Nb3Sn quadrupole coils magnets for the HL-LHC while it can also be used for prototyping the 16 Tesla magnets for FCC. The first copper practice coil for the quadrupole magnets (MQXF) will be wounded in early 2016 and a full RHT based on this coil will be performed in summer 2016.

“The completion of this project that started as an initiative for HL-LHC but will be also useful for the new magnets needed for FCC, puts CERN and the global HEP community in a unique position in the development of new powerful accelerator magnets” says Lucio Rossi, leader of the HL-LHC project.

The installation of the new furnace at CERN’s Large Magnet Facility (LMF) will help scientists researching and developing the new materials needed for future colliders to understand the superconductor development based on this Nb3Sn alloy, and allow CERN to lead the production of superconducting coils and the development of high-field magnets.

The successful test of 11 Tesla magnets for the HL-LHC upgrade and the developments for the 16 Tesla magnets programme of the FCC study proves that the future is bright for developing high-field accelerator magnets. The realization of more powerful magnets will allow extending the luminosity and energy frontiers and pave the way to answer some of the fundamental questions that lie open after the Higgs discovery.