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LARP leads way for niobium-tin magnets
 

A major milestone has been achieved. December 2009 saw the successful US-based tests of a new, improved magnet to withstand planned increases in luminosity of the Large Hadron Collider (LHC).

Our story begins in 1932, when Ernest Lawrence of the University of California, Berkley manufactured the first cyclotron - a circular particle accelerator. Since then, accelerator-based experiments have been the primary source of new discoveries in particle physics.

Fast forward almost 80 years to 2010. The LHC at CERN has just achieved record-breaking collision energies, and is expected to progress toward its full design performance in the years to come.

LHC upgrades will increase luminosity to expand the collider’s physics reach. Increased luminosity and increased collisions means more data in less time. However the "inner triplet" LHC magnets will be subjected to much higher radiation and heat than they are presently designed to withstand. These inner triplets are superconducting quadrupole magnets (see right-hand image) that focus the beams just before collision. The magnets currently in place already operate at the limits of well-established niobium-titanium (Nb-Ti) technology. The time is right for a technology based on a new material.

From Niobium-Titanium to Niobium-Tin

  


Diagram of a current inner triplet magnet in the Large Hadron Collider (LHC) at CERN. These quadrupole magnets are responsible for focussing the particle beams just before the collision points. The magnets sit either side of each of the four LHC detectors (ATLAS, CMS, ALICE & LHCb). When the LHC is upgraded, these magnets will be replaced by new, improved designs, which could be based on the promising results coming from the US-based LARP (LHC Accelerator Research Program).
Image courtesy of Fermilab. Thumbnail image main page courtesy of Billy Alexander, stock.xchng
 
 
 
 

Niobium-tin (Nb3Sn) is a superconducting material that supports higher magnetic fields and larger temperature margins than niobium-titanium.
 



Loading of a superconducting coil of niobium-tin into the high temperature reaction oven at Brookhaven National Laboratory. Niobium-tin is a brittle material that needs to be shaped via heat treatment of 650 to 700 degrees Celsius. Image courtesy: Brookhaven National Laboratory (BNL)

  

But unlike niobium-titanium, niobium-tin is brittle and cannot be drawn into thin filaments. Instead it has to be shaped using high-temperature heat treatment of 650 to 700 degrees Celsius. In the fully reacted state, the filaments are extremely sensitive to strain. Therefore, attempting to wind reacted cable into coils can cause breakages and unacceptable critical current degradation at the ends.

These properties lead to considerable engineering challenges, and in order to use this material effectively new approaches to magnet design and fabrication have to be developed. Deformations of the wire internal structure during cabling need to be minimized. The coils are wound using un-reacted cable so that components are still ductile. Heat treatment is performed after coil winding, requiring the use of special insulation and coil components. In order to avoid degradation of the conductor performance, new support structures and assembly methods have to be developed to prevent excessive strain at all stages.
 

Worldwide attraction to high field magnet research

R&D programmes are underway worldwide to address the technology challenges of very high field magnets using brittle superconductors. In keeping with a long-standing tradition of the particle physics community, a strong collaborative network has formed among the different programmes. This network includes large efforts supported by formal organisations as well as direct links among investigators, to address issues of mutual interest.

In the United States, the Department of Energy’s Office of High Energy Physics has supported niobium-tin magnet research at several national laboratories and universities through its Advanced Accelerator Technology Program. Starting in 2004, the LHC Accelerator Research Program (LARP), a collaboration of Brookhaven National Laboratory (BNL), Fermilab (FNAL), Lawrence Berkley Natiional Laboratory (LBNL) and Stanford Linear Accelerator Center (SLAC), has been coordinating the US effort to develop prototype magnets for the LHC upgrades.

In the European Union, the development of advanced magnet technology is currently coordinated through the High Field Magnet (HFM) work package of EuCARD.

In Japan, the National Institute for Materials Science (NIMS) and KEK laboratories have been focussing on the development of magnets based on niobium-aluminum. This material has structure and properties similar to niobium-tin, and promises a greater tolerance to strain but is in a less advanced stage of development.

Results of the LHC Accelerator Research Program (LARP)

The LARP effort initially centred on a series of short quadrupole models at Fermilab and Berkeley Lab and, in parallel, a four-metre-long magnet based on racetrack coils, built at Brookhaven and Berkeley Labs. The next step involved the combined resources of all three laboratories: the fabrication of a long, large-aperture quadrupole magnet.

 

 

Long Quadrupole magnet assembly at Lawrence Berkeley National Laboratory. Image courtesy: Lawrence Berkeley National Laboratory (LBNL).
 


In 2005, the US Department of Energy, CERN, and LARP agreed to set a goal of reaching, before the end of 2009, a gradient, or rate of increase in field strength, of 200 Tesla per metre (200 T/m) in a four-metre-long superconducting quadrupole magnet with a 90-millimetre bore for housing the beam pipe.

This goal was met last December by the first "Long Quadrupole Shell" model. The magnet’s superconducting coils performed well, as did its mechanical structure, based on a thick aluminum cylinder (shell) that supports the superconducting coils against the large forces generated by high magnetic fields and electrical currents. The magnet's ability to withstand quenches – sudden transitions from superconductivity to normal conductivity with resulting heating – was also excellent.

Although the successful test of the long quadrupole was a major milestone, it is only one of several steps needed to fully qualify the new technology for use in the LHC. One goal is to further increase the field gradient in the long quadrupole, both to explore the limits of the technology and to reproduce the performance levels demonstrated in short models. A second goal is to address other critical accelerator requirements, such as field quality and alignment, through a new series of models with an even larger aperture (120 millimetres). The first model of the new design is currently undergoing final assembly and is expected to be tested in the coming months. A technical summary of the programme results and next steps was given at CERN in January 2010.

EuCARD working with LARP

In Europe, Nb3Sn magnet models were successfully built at the beginning of the 90’s; with the construction of the LHC, R&D activity stopped with the exception of the Saclay CEA project of an LHC-like quadrupole with Nb3Sn coils. Then, thanks to the EC cofunded FP6 project, CARE, R&D restarted with an initiative for European industries to develop a Nb3Sn conductor (CARE-NED). The EuCARD project continues this High Field Magnets (HFM) research, aiming to construct a large aperture 13 Tesla dipole to be used as a cable test station.

EuCARD profits from synergies with the LARP programme on many topics, from the conductor development to avoid instabilities, to the design and instrumentation of the mechanical structure needed to keep the large electromagnetic forces.

During the past two years, CERN has also tested some Nb3Sn models built by the LARP collaboration, acquiring the technology relative to mechanical structures based on aluminum shells that allow a precise control of the stress in the coils. These tests have allowed detailed exploration into issues such as the maximal stress tolerable in a Nb3Sn magnet, a hot topic in current magnet R&D.

Eric Prebys, the LARP programme director, will present "Accelerator R&D in US/LARP" on Thursday 15 April 2010 at the first EuCARD Annual Meeting.

- Kate Kahle, CERN, EuCARD-DCO (WP2); GianLuca Sabbi, Lawrence Berkeley National Laboratory; Ezio Todesco, CERN, EuCARD-HFM (WP7)

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