A niobium-tin magnet could be the key to unlocking the heavy ion accelerator’s potential

Researchers from Berkeley Lab’s Accelerator Technology and Applied Physics (ATAP) Division teamed up with colleagues at Michigan State University’s Rare Isotope Beam Facility (FRIB), the accelerator the most powerful heavy ion magnet in the world, to develop a new superconducting magnet based on niobium-tin (Nb₃Sn technology).

This magnet, the first of its kind, could significantly improve the performance and capabilities of the FRIB, paving the way for new applications in medicine, industry and research. The article is published in the journal IEEE Transactions on Applied Superconductivity.

At FRIB, beams of ionized atoms (ions) of elements spanning the periodic table, including heavy elements like uranium, are accelerated to half the speed of light. When these beams collide with a target, they break up to create short-lived isotopes.

By studying these rare isotopes, some of which have never been observed, scientists can better understand the structure of matter and the formation of our universe.

“A key component of the FRIB is an electron cyclotron resonance ion source, or ECRIS, which creates high current, high charge state ions for injection into the accelerator beamline,” explains Tengming Shen , researcher in the ATAP Superconducting Magnets Program (SMP). ) who is leading the development of the new magnet.

“This ECRIS uses a sextupolar magnet and solenoid to confine electrons and ions in a plasma. The electrons are then heated with high-frequency microwaves (28 GHz), creating high-energy electrons that strip the electrons of neutral atoms from the plasma to produce high-charge state ions. (This configuration, Shen notes, is based on the Versatile ECRIS for Nuclear Science (VENUS) design used in Berkeley Lab’s cyclotron accelerator.)

Made at Berkeley Lab, this sextupolar magnet is wound with superconducting niobium-titanium (Nb-Ti) coils. However, 28 GHz Nb-Ti magnets have a maximum field strength of 6.7 Tesla (T) at the liquid helium temperatures (4.2 Kelvin, -452.1°Fahrenheit) at which ECIRS operates.

Shen says that to improve the facility’s performance and expand its range of applications, ECRIS must be built with magnets capable of producing higher magnetic fields to enable operation at higher microwave frequencies.

“Our goal is to increase the microwave frequency to above 45 GHz. At this frequency, the maximum magnetic field increases up to 10.8 T; however, the current carrying capacity of the Nb-Ti material decreases considerably.”

To this end, the researchers chose a magnet design based on superconducting Nb coils.₃Sn. Coils in Nb₃Sn can carry a high current density of over 100 amps per square millimeter at much higher magnetic fields, potentially up to 22 T, than those produced by Nb-Ti at 4.2 K.

However, while the superconducting properties of Nb₃Sn exceeds those of Nb-Ti, according to Shen, the conductive characteristics of Nb₃Sn are very different from those of Nb-Ti.

“For example, unlike Nb-Ti, Nb₃Sn is fragile and sensitive to stress. In addition, the Nb coils₃Sn undergoes dimensional changes during manufacturing, which requires better management of the manufacturing process.

“Additionally, the magnet is constructed using small conductors rather than the large Rutherford wires used in current magnet designs and requires approximately three hundred turns for each coil.”

He says these factors add to the complexity of producing the coils and assembling a magnet.

“Consequently,” he continues, “making Nb₃Sn coils are more difficult, especially for this one-of-a-kind magnet for which no model currently exists. Therefore, creating such a magnet requires extensive experience in the design and manufacturing of superconducting magnets. »

Fortunately, Berkeley Lab has considerable experience working with Nb₃Sn-based magnets. Last year, for example, the lab successfully fabricated and assembled the first set of Nb quadrupole magnets.₃Sn superconducting cables.

This work is part of the US Accelerator Upgrade Project’s ongoing contribution to the High-Luminosity Large Hadron Collider Upgrade Project, which aims to improve the capabilities of the Large Hadron Collider, promising new discoveries in high energy and particle physics.

The development of this ECRIS magnet “is a great example of how R&D on high-field accelerator magnets for future colliders can benefit other scientific applications,” says Soren Prestemon, deputy director of technology at the ATAP and head of the SMP.

“In addition, this provides an excellent opportunity for our talented team of scientists, engineers and technical staff to contribute directly to new and operational facilities like FRIB and to the advancement of high energy physics research.”

Shen says the team has already done a lot of magnetic and mechanical design calculations to handle Nb.₃Sensitive nature to Sn stains.

“We also evaluated the conductor manufacturing process, conducted winding and manufacturing trials, and developed a new design that addresses the challenges of coil manufacturing. We are about to complete the practical coil, tool design, manufacturing procedures and processes.

He adds that work has already begun on winding a full-size prototype coil and they intend to test a full-length version soon to verify its superconducting performance. If the test proves successful, he says they plan to develop, build and test a 28 GHz system with “keeping an eye on future upgrades.”

The new magnet design based on Nb₃Sn” technology will lead to a higher magnetic field than the current Nb-Ti source, providing superior performance while providing a greater safety margin. More importantly, it enables new ECR source design to operate at higher frequencies higher (up to 45 GHz) and increased plasma power.

Once completed, he says the magnet “will ensure that FRIB remains on the frontier of basic scientific research.”