Development of niobium gilding for use in accelerator components could reduce costs and improve performance. Image: Jefferson Laboratory |
For
thousands of years, craftsmen have applied gilding, a thin layer of
gold, to objects to enhance their value. Now, researchers at Dept. of
Energy’s Jefferson Lab are using this same idea to enhance materials for
accelerator science.
As
the number of uses for accelerators grows, scientists are faced with
the challenge of building low-cost accelerators that offer high
performance. And to meet that challenge, scientists at Jefferson Lab are
combining a bit of the old with the new.
Jefferson
Lab SRF Institute Research Scientist Anne-Marie Valente and her
colleagues are exploring the application of a thin film of niobium to
other materials, such as copper, to make better accelerator components.
“We
want to replace, in certain projects, the bulk niobium with niobium on
copper. The niobium on copper has potential advantages over bulk
niobium,” she explains.
Jefferson
Lab’s Continuous Electron Beam Accelerator Facility accelerator depends
on niobium, a rare metal, for more than 300 critical components. But
during recent research and development efforts, JLab scientists reached
the theoretical limit of niobium’s performance, and it’s still far short
of what’s needed for many proposed accelerator-based applications. In
addition, niobium components are expensive, with individual units in
CEBAF costing upwards of $30,000 each.
Anne-Marie
Valente, an SRF Institute research scientist, and her colleagues are
using a technique called energetic condensation by electron cyclotron
resonance to produce and apply thin films of niobium to copper, aluminum
and other test metals in the ECR Plasma Deposition Lab.
Beyond bulk niobium
Early
linear accelerators used accelerating components made of copper. The
metal was a good conductor of the energy, but copper accelerators have
their limits. They are capable of handling continuously high fields to
accelerate particles but can only be used for short periods without
overheating.
To
get beyond these limitations, accelerator scientists turned to niobium.
When chilled to near absolute zero, niobium becomes superconducting,
losing its resistance to the flow of energy through it. This allows
niobium accelerators to continuously accelerate particles at greater
energies without overheating.
However,
niobium also has its limits. It is susceptible to developing “hot
spots” on its surface when pushed to higher efficiencies. These hot
spots are the result of impurities embedded in the niobium or defects on
the component surface.
“Because
of the thermal properties of the niobium, you are not going to
dissipate that energy, and you end up getting thermal breakdown,”
Valente explains. “With copper, you have a better chance of dissipating
that energy, and that local heating is not going to be as critical or
it’s going to allow you to go further before you get to breakdown.”
Sputter-on films
Other laboratories have explored thin-film niobium on copper and other materials as an alternative to pure niobium components.
“In some projects already, the niobium on copper has already been a good competitor to bulk niobium,” Valente says.
In
the 1990s, for instance, accelerator scientists used a technique called
sputtering to apply a thin film of niobium to copper accelerator
components for the Large Electron-Positron Collider at CERN in Europe.
The
components exhibited many of the positive characteristics of bulk
niobium components and were used at single-digit temperatures. But the
thin-film-niobium-copper components in LEP did not reach the greater
efficiencies needed for future accelerators.
“The
sputtering technique used for LEP generated a lot of defects in the
film, and these can affect performance when you try to reach higher
fields,” Valente explains.
LEP
did demonstrate that thin-film-niobium-copper components can offer many
of the benefits that bulk niobium components have over those made of
copper. Valente aims to build on that work to optimize
thin-film-niobium-copper component technology.
Reaching resonance
Valente
and her colleagues are using a technique called energetic condensation
by electron cyclotron resonance to produce and apply thin films of
niobium to copper, aluminum and other test metals in JLab’s ECR Plasma
Deposition Lab. In this technique, bulk niobium pellets are vaporized,
and a cloud of electrons whipped into cycloidal motion by the effect of
powerful magnets associated with an electrical field is used to ionize
the niobium vapor. The niobium ions are than deposited on a copper
sample. Altering the plasma or the energy of the niobium ions arriving
on the sample affects the thickness and other properties of the thin
film.
Once
the copper has been coated with niobium, the sample is analyzed.
Analyses conducted on the sample, including Electron Backscatter
Diffraction, X-ray diffraction, Transmission Electron Microscopy and
Auger Electron Spectroscopy, are used to characterize the structure and
surface of the film. Then, the samples are subjected to tests that
predict their ability to accelerate particles. This is important should
the samples be reproduced as accelerator components called cavities.
“That’s
one of the things that we’re trying to work on: producing films that we
can apply to the surface of a cavity,” Valente explains. “The objective
is not just to create good films, but to optimize the SRF performance
of the films.”
Once
she and her colleagues have several good thin-film recipes for
cavities, they aim to move beyond sample production to producing full
cavities to test their performance in Jefferson Lab’s cavity production
facilities.
More money-saving layers
Valente
and her colleagues are also testing the feasibility of multiple layers
of different films to enhance performance. By layering, they hope to get
better performance out of accelerator components, while operating them
at higher temperatures than the current limit of single-digit
temperatures above absolute zero.
“You
have a superconductor over an insulator over a superconductor. You’re
basically getting the qualities of the higher operating temperature
material to enhance the performance of the niobium,” Valente says.
She
says that while using less niobium in cavity production offers savings
during construction of new accelerators, reducing the amount of
refrigeration needed to run those accelerators can drastically reduce
the ongoing operation costs over the lifetime of a machine.
“If you operate at liquid helium temperatures, you wouldn’t need such an expensive refrigerating system.” Valente confirms.
In
the meantime, Valente and her colleagues are busy in the lab, pushing
the boundaries of the science behind these ideas for producing ever more
efficient, cheaper accelerators for a wide range of beneficial
applications.
SOURCE: Jefferson Laboratory
