Monday, January 24, 2022

New particle accelerator magnet achieves record ramping speed

Cost- and energy-efficient rapid cycling magnets play a critical role in particle physics research. Their performance determines how frequently a circular particle accelerator can receive a bunch of particles, propel them to higher energy, send them to an experiment or target station, and then repeat all over again.

A team of engineers at the U.S. Department of Energy’s Fermi National Particle Accelerator Laboratory has demonstrated the world’s fastest magnetic ramping rates for particle accelerator magnets. They achieved this record by using magnets made with energy-efficient, high-temperature superconducting material.

Today, the world’s most powerful accelerator, the Large Hadron Collider (LHC), also uses very strong magnetic fields to keep particles on track as they fly inside the chamber at near the speed of light. Magnetic fields within the LHC can ramp up to almost 8 Tesla in approximately 20 minutes, while the accelerator propels particles to 6.5 TeV. This corresponds to a ramping rate of about 0.006 Tesla per second (T/s) and is much slower than the ramping rate of conventional accelerator magnets operating at room temperature.

Despite the many attractive features of superconducting wire, some particle accelerators still use magnets with copper conductors operating at room temperature. These include the 3 GeV proton ring at JPARC in Japan, which features a magnetic field that changes at a rate of 70 T/s, and the 8 GeV Booster ring at Fermilab, which achieves a ramping rate of 30 T/s.

Henryk Piekarz of Fermilab’s Accelerator Division controls the flow of cryogens in the high-temperature superconductor magnet prototype.
Henryk Piekarz of Fermilab’s Accelerator Division controls the flow of cryogens in the high-temperature superconductor magnet prototype. Credit: Ryan Postel, Fermilab

One problem with using superconducting magnetism for these purposes is the heating of the superconductor during ramping due to eddy currents that can create large heat depositions in the superconductor. This heating rapidly increases with the increase of field amplitude and the ramping rate. Another problem is the very small margin for temperature variation in the traditional low-temperature superconductors. Even a small increase in temperature can transition these superconducting magnets into their normal conducting, resistive state.

The Fermilab researchers have found a solution to these problems in a material known as yttrium barium copper oxide (YBCO), which boasts unique properties of “high-temperature” superconducting. Using this material, the team has designed a magnet and operated it at temperatures between 6 and 20 Kelvin and up to 1,000 amps of electrical current.

During the tests, they found that the new superconducting accelerator magnet could ramp up at a rate of up to 290 T/s while achieving a peak magnetic field strength of about 0.5 Tesla. This is obviously a far cry from 8 Tesla of LHC, but the researcher hopes to achieve even higher magnetic field strength by increasing the electrical current running through the magnet while maintaining the superior ramping rate.

They further plan to expand the power supply capabilities in the future, possibly achieving even higher ramping rates, as they will carry out further studies on the ultimate capabilities of this advanced magnet technology. According to the researchers, the development of these fast-cycling magnets is critical for future neutrino research, featuring rapid-cycling proton synchrotrons, particle injectors for the proposed Future Circular Collider.

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