“Only perfect practice makes perfect:” Breaking the 11-Tesla barrier

Giorgio Apollinari

Giorgio Apollinari, head of the Technical Division, wrote this column.

We all know the saying that practice makes perfect. But it is worth remembering that the famous coach of a football team located north of Chicago Bears territory pointed out that “only perfect practice makes perfect”—the mere repetition of activities is not a sure way to achieve perfection. Vince Lombardi understood that in practice itself there is a constant need to focus, improve and advance.

Lombardi’s insight applies to the development of new technologies for future accelerators. It is especially true when it comes to the development of magnet prototypes.

Less than one year ago, in July 2012, I reported that a 2-meter-long collared dipole magnet prototype—with accelerator-quality features, made of Nb3Sn and built at Fermilab—achieved a magnetic field of 10.4 Tesla, close to our goal of producing an 11-Tesla magnet. That was a major achievement. But we soon discovered that the magnet’s performance was limited due to the damage its superconducting cable must have suffered during the reaction process that created the magnet’s superconducting material. (Nb3Sn is a brittle conductor that is very susceptible to mechanical damage during the reaction process.)

In less than six months, we built a new 1-meter-long prototype and began testing it at the beginning of 2013. Building a new prototype magnet was more than good practice. Following Lombardi’s advice, we used this opportunity to implement innovations introduced by our magnet scientists. The new prototype features a cored superconducting cable to reduce parasitic eddy currents that can limit a magnet’s performance. We also used the new prototype magnet to test an advanced Nb3Sn strand design, which is a promising design for the future production of quadrupole magnets for the LHC luminosity upgrade.

On March 7, our prototype magnet reached a current of 12.54 kiloamps with a calculated field of 11.5 Tesla, thus surpassing the 11-Tesla goal we had set. (As I explained in my previous column, the development of these magnets is part of our general R&D efforts to develop stronger magnets with realistic accelerator qualities for future machines such as a Muon Collider or a Very Large Hadron Collider. The use of such magnets in the foreseen LHC luminosity upgrades is an intermediate step.)

I would like to stress the short time it took us to make and test a new prototype and achieve very good results. It shows the maturity of this magnet technology. An easy extrapolation of our latest results tells us that this magnet technology would allow us to build a Muon Collider with a diameter of approximately 5 kilometers that would fit completely on Fermilab site and achieve center-of-mass collision energies of approximately 10 TeV.

The worldwide high-energy physics community is now involved in a process that I, only half-jokingly, call “generational facilities planning,” in the sense that we are planning for facilities that will be exploited mainly by future generations. I am hopeful that further progress in superconducting materials and magnets development will allow us to plan hadron colliders that could exceed 100 TeV center-of-mass energies, about eight times more than the design energy of the Large Hadron Collider at CERN.