Culminating decades of research, scientists at three DOE national laboratories have deployed a cutting-edge, fully functional magnetic device known as an undulator that uses superconducting wire made of niobium and tin.
BY MICHAEL MATZ | MAY 16, 2023
Scientists at the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, have achieved an important milestone in the development of next-generation superconducting magnets for light source facilities. After several years of research, they have built and installed at the APS a functional, full-size version of a magnetic device that would improve the performance of existing synchrotron and free-electron laser (FEL) facilities. Equipped with this device, these facilities could broaden their capabilities and provide an enhanced source of X-rays to researchers.
The device is known as a superconducting undulator (SCU). It consists of two strings of magnets made of a special alloy of niobium and tin (Nb3Sn). The new device is the product of a long-running collaboration between Argonne, DOE’s Lawrence Berkeley National Laboratory, and DOE’s Fermi National Accelerator Laboratory. The collaborative project was funded by the Accelerator and Detector Program at the DOE Office of Basic Energy Sciences. It marks the first time ever that this technology has been installed on an operational particle accelerator.
“There have been attempts at several institutions to build a niobium-tin undulator, but never before has anyone succeeded in building an operational device that researchers can use in experiments,” said Efim Gluskin, a distinguished fellow at Argonne and principal investigator of the project. “Development of such novel undulators would expand the capabilities of existing light-source facilities and might provide an opportunity for more compact, lower-cost light source facilities in the future.”
A Higher Magnetic Field
The centerpiece of the APS is a particle accelerator known as a storage ring that circulates electrons. Electrons wiggle or undulate as they pass through the periodic magnetic field of the undulator, causing the electrons to emit X-ray beams, which researchers use in the APS’s experimental stations. The most advanced undulators are those with a small magnetic period and a high magnetic field. (The period is the distance between either two south poles or between two north poles along the string of magnets.)
In order to make such devices work and outperform all existing undulators, the superconducting technology delivers record-high magnetic fields generated by extremely high current running through small-diameter superconducting wire. The higher an undulator’s magnetic field strength—and the smaller its magnetic period—the more intense high-energy X-ray beams it can create.
Up until now, the APS has used superconducting undulators made of a niobium-titanium alloy (NbTi). This superconducting material has been used for decades, and undulators based on that technology are close to their performance limit. The Nb3Sn undulator recently installed on the APS’s storage ring exceeds that limit and paves the way toward building SCUs with smaller periods.
“This is very important because electrons in this SCU will produce brighter high energy X-ray beams compared to other undulators of the same length,” said Gluskin. “SCUs with small periods deliver X-rays with shorter wavelengths that allow researchers to view deeper inside materials.”
Overcoming Many Technical Hurdles
In 2020, Argonne, Berkeley, and Fermilab successfully completed the construction and testing of a half-meter-long prototype of the Nb3Sn undulator magnet. Since then, the researchers have focused on developing a full-size, one-meter-long version. This endeavor has presented numerous challenges, which the researchers have addressed with innovative features.
“The device must comply with rigorous technical specifications to ensure reliable performance and fulfill the practical and experimental needs of users,” said Argonne scientist Ibrahim Kesgin, the project manager.
Kesgin and the engineers from the APS SCU team designed and constructed the SCU. It is comprised of two strings of magnets positioned in a specific configuration to generate a magnetic field with an alternating sign along the strings. These magnets are composed of soft iron formers with racetrack winding grooves that house the bundle of niobium-3 tin wires and provide mechanical support.
Additionally, a special high-temperature-resistant insulation is utilized between the niobium-3 tin wires and magnet formers to withstand high voltages that may occur during a quench. A quench is a backlash that happens when the magnets lose their ability to conduct current without resistance. It can damage the magnets.
Fermilab has developed a process that entails placing magnets in a specialized oven heated to 1,200° Fahrenheit. This high temperature heat treatment triggers reactions that lead to the formation of the niobium-3 tin alloy in the wire.
The assembled SCU magnetic structure was instrumented with diagnostic elements and enclosed within a 2-meter-long vacuum vessel. To maintain superconductivity, the magnets are indirectly cooled by liquid helium and kept at a temperature of 4.3 Kelvin (-452° Fahrenheit).
The maximum operating current of Nb3Snwire is approximately twice that of niobium-titanium wire currently used in undulators. As a result of this high current, the device stores significantly more energy. Although the device is stable under normal conditions, any unexpected perturbation could result in a loss of its superconductivity, leading to a quench and requiring the safe removal of the stored energy.
To overcome this challenge, Berkeley Lab researchers built a system that protects the device during a quench. Such a system has never been needed before since other undulators don’t store nearly as much energy.
The Outlook for Advanced Undulators
The team’s successful integration of the Nb3SnSCU into the APS storage ring demonstrated the device’s ability to generate higher magnetic fields, marking a significant milestone in undulator technology.
This accomplishment sets the stage for further development of even more advanced superconducting materials. For example, high-temperature superconductors have significantly higher operating temperatures than Nb3Snand can potentially be cooled with easier-to-handle materials for lower cost application.
An important next step would be to further develop SCUs made with high-temperature superconductors or other state-of-the-art materials, with an aim to produce significantly higher magnetic fields than today’s niobium titanium SCUs. Alternatively, technology development could focus on reducing the magnetic periods of these SCUs without compromising their magnetic field strength.
Continued progress can enhance existing light-source facilities while potentially helping to enable more compact, affordable light-source facilities.