CERN and U.S. Labs Test Quadrupole Magnets for HL-LHC Project

U.S. government labs Brookhaven, Lawrence Berkeley National Lab, and the Fermi National Accelerator Lab (Fermilab), in conjunction with CERN, have successfully tested a superconducting quadrupole magnet with a peak field of 11.4 T. This is the highest peak field ever demonstrated by a focusing magnet ready for installation in an accelerator. The magnet will be installed in the High Luminosity Large Hadron Collider (HL-LHC),

“HL-LHC will be a tool to explore the unknown at the edges of human knowledge,” said Giorgio Apollinari, the former head of Fermilab’s Technical Division and now in charge of the US contribution to HL-LHC. “It will offer the possibility to fully characterize the newly discovered Higgs boson as well as shed light on dark matter, dark energy and beyond-the-Standard Model physics for several years until the next tools become available.”

The function of a quadrupole magnet is to generate a field gradient in the radial direction of the beam, which allows the charged-particle beam to be more focused. The 150mm-single-aperture device is 4.2m long and fabricated from niobium tin (Nb3Sn). It is one of several quadrupole magnets in construction for the HL-LHC in an effort to pack proton beams more densely within ATLAS and CMS to achieve higher luminosity.

“All superconducting synchrotron accelerators at the forefront of physics research (Tevatron, HERA, RHIC, LHC) have been based so far on NbTi magnets and limited to operational magnetic fields around 8 T,” Apollinari pointed out. “This application is breaking that barrier with the demonstrated operational field around 12 T and the possibility, in future applications, to reach higher levels around 15-16 T.”

Lab Cooperation Accelerated Magnet Development

The three U.S. labs will produce a total of 16 quadrupole magnets for the HL-LHC, which is set to begin operations in 2027. CERN will produce an additional eight.
This most recent quadrupole magnet also represents a landmark of international cooperation for building an accelerator. The three U.S. labs and CERN have been working together closely on the development and design of the new magnet. This tight-knit collaboration has resulted in higher efficiency and lower costs of production.

“A quadrupole is made by four coils, so sharing the construction of the coils with an identical design among different labs has provided the opportunity to proceed faster in the development and with lower, affordable investments at the various labs,” Apollinari explained. An added benefit of collaboration is “the intellectual contributions coming from different research groups which brings critical diversity to the development of new technologies.”

Nb3Sn Enables Higher Magnetic Fields

Existing superconducting LHC magnets, fabricated from niobium titanium (NbTi) have been tested to a bore field of 8.3 T and operate at 7.7 T at 1.9 K for 6.5 TeV operation. They have transport property limitations beyond 10-11 T at 1.9 K, thus prompting the shift to Nb3Sn, which can remain superconducting for much higher fields. However, Nb3Sn can be more difficult to work with, since once formed, the compound is brittle and strain sensitive, which present a number of design obstacles for fabricating cables with the required performance and field quality specs for accelerator magnets.

“The Nb3Sn technology is unforgiving due to the brittleness of the material and especially when used in complicated coil shapes like the ones needed for accelerator magnets,” emphasized Apollinari. “Clearly specified handling procedures and tight QC steps are vital ingredients needed to insure fabrication success.”

The HL-LHC will additionally require a set of dipole magnets on either side of a collimator to correct for off-momentum protons in the beam. These magnets will be Nb3Sn fabrications of shorter length and higher field than the existing LHC dipole magnets.

Labs Breaking Ground with Novel Magnets

These landmark magnets suggest a promising future for hadron colliders beyond the LHC, which will employ Nb3Sn magnets with fields of 16 T. CERN and the U.S. labs have demonstrated a succession of impressive magnet tests during the past year, and will soon roll out testing for additional 7.2 m and 4.2 m quadrupole magnets. .

In June last year, Fermilab tested a short “cos-theta” dipole magnet with a 60 mm aperture with a bore field of 14.1 T at 4.5 K. Last July, CERN successfully tested a full-length 5.3 m, 60 mm twin aperature dipole magnet, a record for Nb3Sn magnets, which achieved a bore field of 11.2 T at 1.9 K.

In January, Brookhaven undertook the continuous operation of an 8 tonne quadrupole magnet at a nominal field gradient of 130 T/m at 1.9 K for a 5-hour duration. In February, CERN demonstrated a short “enhanced racetrack model coil” with a record field of 16.36 T at 1.9 K with a 16.5 T conductor peak field. The HL-LHC has also set records with its superconducting-RF crab cavities, advanced material collimators, and 120 kA links based on MgB2 superconductors.

“Various efforts across the world are presently taking place to develop a path for the exploration of the physical world at the limits of human knowledge,” indicated Apollinari. “The exploration can only take place if the tools at our disposals are adequate. Developing magnets at high fields and superconducting links for critical application are providing humankind with the proper set of tools to map the path.”

Quadupole and Dipole Installation May Be Delayed by COVID-19

The next tests will be based around coupling the quadrupole and dipole magnets in pairs. Each individual magnet has the same winding, but differs in its mechanical interfaces and electrical circuitry. The remaining halves of quadrupole and dipole pairs has been scheduled to be installed in the coming months, leading up to the installation of the dipole magnets in the LHC tunnel, but this schedule is being reviewed with the COVID-19 pandemic in mind.
Apollinari concluded: “The next big step is to build strings of several magnets at CERN with contributions from multiple HL-LHC collaborators for a complete system test before installation in the HL-LHC.“

Funding for the U.S. labs was provided by the U.S. Department of Energy, Office of Science, High Energy Physics.

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