Magnetics Business & Technology - Summer 2017 - 12

RESEARCH & DEVELOPMENT
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 freeelectron 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.
"This is a much-anticipated innovation," said Wim Leemans, Director, Accelerator Technology and Applied Physics (ATAP). "Higher
performance in a smaller footprint is something that benefits everyone: the laboratories that host the facilities, the funding agencies,
and above all, the user community."
Argonne's test of another superconducting material, niobiumtitanium, 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.
"The superconducting technology in general, and especially with
the niobium tin, lived up to its promise of being the highest performer," said Ross Schlueter, Head of the Magnetics Department in
Berkeley Lab's Engineering Division. "We're very excited about this
world record. This device allows you to get a much higher photon
energy" from a given electron beam energy.
"We have expertise here both in free-electron laser undulators,
as demonstrated in our role in leading the construction of LCLS-II's
undulators, and in synchrotron undulator development at the ALS,"
said Soren Prestemon, Director of the Berkeley Center for Magnet
Technology (BCMT), which brings together the Accelerator Technology and Applied Physics Division (ATAP) and Engineering Division,
to design and build a range of magnetic devices for scientific, medical and other applications.
Diego Arbelaez, the lead engineer in the development of Berkeley
Lab's device, said earlier work at the Lab in building superconducting
undulator prototypes for a different project were useful in informing
the latest design, though there were still plenty of challenges.
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

A close-up view of the superconducting undulator prototype developed
at Berkeley Lab. To construct the undulator, researchers wound a pliable wire in alternating coils around a steel frame. The pliable wire is
baked to form a niobium-tin compound that is very brittle but is capable of achieving high magnetic fields when chilled to superconducting temperatures. (Credit: Marilyn Chung/Berkeley Lab)

Magnetics Business & Technology * Summer 2017

the undulator coils. The full undulator coil is then heat-treated in a
furnace at 1,200°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. Arbelaez said, "One of the questions was whether you can maintain
precision in its winding even though you are going through these
large temperature variations."
After the heat treatment, the coils are placed in a mold and impregnated with epoxy to hold the superconducting coils in place. To
achieve a superconducting state and demonstrate its record-setting
performance, the device was immersed in a bath of liquid helium to
cool it down to about -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. Berkeley Lab's
Marcos Turqueti led the effort to engineer a quench-protection system that can detect the occurrence of quenching within a couple
thousandths of a second and shut down its effects within 10 thousandths of a second.
Arbelaez also helped devise a system to correct for magneticfield errors while the undulator is in its superconducting state.
SLAC's Paul Emma, the accelerator physics lead for LCLS-II, coordinated the superconducting undulator development effort. Emma
said that the niobium-tin superconducting undulator developed at
Berkeley Lab shows potential but may require more extensive continuing R&D than Argonne's niobium-titanium prototype. Argonne
earlier developed superconducting undulators that are in use at its
APS, and Berkeley Lab also hopes to add superconducting undulators at its ALS.
"With superconducting undulators," Emma said, "you don't necessarily lower the cost but you get better performance for the same
stretch of undulator."
A superconducting undulator of an equivalent length to a permanent magnetic undulator could produce light that is at least two to
three times, perhaps up to 10 times, more powerful, and could also
access a wider range in X-ray wavelengths, Emma said, producing a
more efficient FEL.
Superconducting undulators also have no macroscopic moving
parts, so they could conceivably be tuned more quickly with high
precision. Superconductors also are far less prone to damage by
high-intensity radiation than permanent-magnet materials, a significant issue in high-power accelerators such as those that will be
installed for LCLS-II.
There appears to be a clear path forward to developing superconducting undulators for upgrades of existing and new X-ray freeelectron lasers, Emma said, and for other types of light sources.
"Superconducting undulators will be the technology we go to
eventually, whether it's in the next 10 or 20 years," he said. "They
are powerful enough to produce the light we are going to need - I
think it's going to happen. People know it's a big enough step, and
we've got to get there."
The Advanced Light Source, Advanced Photon Source, and Linac
Coherent Light Source are DOE Office of Science User Facilities.
The development of the superconducting undulator prototypes was
supported by the DOE's Office of Science."

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