A team of researchers have designed, built, and tested two devices, called superconducting undulators, which could make X-ray free-electron lasers (FELs) more powerful, versatile, compact, and durable.

This Berkeley Lab-developed device, a niobium-tin superconducting undulator prototype, set a record in magnetic field strength for a device of its kind. This type of undulator could be used to wiggle electron beams to emit light for a next generation of X-ray lasers. (Credit: Marilyn Chung/Berkeley Lab

X-ray FELs are powerful tools for studying the microscopic structure and other properties of samples, such as proteins that are key to drug design, exotic materials relevant to electronics and energy applications, and chemistry that is central to industrial processes like fuel production.

The development effort was motivated by SLAC National Accelerator Laboratory's upgrade of its Linac Coherent Light Source (LCLS), the nation's only X-ray FEL. This upgrade, now underway, is known as LCLS-II. All existing X-ray FELS, including both LCLS and LCLS-II, use permanent magnet undulators to generate intense pulses of X-rays. These devices produce X-ray light by passing high-energy bunches of electrons through alternating magnetic fields produced by a series of permanent magnets.

Superconducting undulators (SCUs) offer another technical solution and are considered among the most promising technologies to improve the performance of the next generation FELs, and of other types of light sources, such as Berkeley Lab's Advanced Light Source (ALS) and Argonne's Advanced Photon Source (APS).

SCUs replace the permanent magnets in the undulator with superconducting coils. The prototype SCUs have successfully produced stronger magnetic fields than conventional undulators of the same size. Higher fields, in turn, can produce higher-energy free-electron laser light to open up a broader range of experiments.

Berkeley Lab's 1.5-meter-long prototype undulator, which uses a superconducting material known as niobium-tin (Nb3Sn), set a record in magnetic field strength for a device of its design during testing at the Lab in September 2016.

Argonne's test of another superconducting material, niobium-titanium (NbTi), successfully reached its performance goal, and additionally passed a bevy of quality tests. Niobium-titanium has a lower maximum magnetic field strength than niobium-tin, but is further along in its development.

Niobium-tin is a brittle material that cannot be drawn into a wire. For practical use, a pliable wire, which contains the components that will form niobium-tin when heat-treated, is used for winding the undulator coils. The full undulator coil is then heat-treated in a furnace at 1200°F. The niobium-tin wire is wound around a steel frame to form tightly wrapped coils in an alternating arrangement. The precision of the winding is critical for the performance of the device. One of the challenges was to maintain precision in its winding despite large temperature variations.

After the heat treatment, the coils are placed in a mold and impregnated with epoxy to hold them in place. To achieve a superconducting state and demonstrate its performance, the device was immersed in a bath of liquid helium to cool it down to about minus 450°F.

Another challenge was in developing a fast shutoff to prevent catastrophic failure during an event known as quenching. During a quench, there is a sudden loss of superconductivity that can be caused by a small amount of heat generation. Uncontrolled quenching could lead to rapid heating that might damage the niobium-tin and surrounding copper and ruin the device. This is a critical issue for the niobium-tin undulators due to the extraordinary current densities they can support. A quench-protection system that can detect the occurrence of quenching within a couple of milliseconds and shut down its effects within 10 milliseconds was developed to address the issue.

For more information, contact Glenn Roberts Jr. 510-486-5582.