Tuesday, February 9, 2010
In a 1954 speech to the American Physical Society, the
University of Chicago's Enrico
Fermi fancifully envisioned a particle accelerator that encircled
the globe. Such would be the ultimate theoretical outcome, Fermi
surmised, of the quest for the ever-more powerful accelerators
needed to discover new laws of physics.
|
| Superconducting radio frequency cavities are a
key technology for next-generation accelerators and the future of
particle physics. |
"How much energy you can put into a particle per meter
corresponds directly to how big the machine is," says Steven
Sibener, the Carl William Eisendrath Professor in Chemistry and the
James Franck Institute at UChicago. This means that future
accelerators must either grow to inconceivable sizes, at great
costs, or they must somehow pump far more energy into each particle
per meter of acceleration than modern technology will allow.
Sibener and Lance Cooley, AB'86, of the Fermi National
Accelerator Laboratory, are working on the latter option with $1.5
million in funding from the U.S. Department of Energy. They aim to
improve the efficiency of superconducting radio frequency (SRF)
cavities made of niobium to accelerate beams of subatomic particles
in the next generation of high-energy physics experiments.
The result could be accelerators powerful enough to open new
frontiers in physics without the need for a massive increase in
size.
A key to such efforts is niobium, a metallic element that
becomes superconducting at very low temperatures. In fact,
niobium's superconducting characteristics are the best among the
elements, providing the capacity to carry thousands of times more
electric current than normal conductivity through copper. When
highly pure, niobium also efficiently sheds any heat generated at
flaws and defects to its cryogenic coolant. Niobium SRF cavities
thus will comprise the heart of future particle accelerators,
including the proposed International Linear Collider.
Enabling collider technology
"The niobium superconducting cavity is enabling technology for
anything that is high-power, high-energy, or high-intensity for
linear colliders," says Cooley, the SRF Materials Group Leader at
Fermilab. Cooley works with niobium cooled to 2 Kelvin (minus-455.8
degrees Fahrenheit) to maximize its superconducting
characteristics. "We use superconductors because it's friction-free
electricity, which saves on the operating wall-plug power," he
says.
As an undergraduate at UChicago in the 1980s, Cooley conducted
research for his senior project in the laboratory of Thomas
Rosenbaum, Provost and the John T. Wilson Distinguished Service
Professor in Physics. It was then that Cooley became interested in
superconductivity. His interest in Fermilab and its accelerators
was motivated by another UChicago faculty member, Professor
Emeritus and Nobel Laureate James Cronin. Cooley arrived at
Fermilab in 2007, and soon after, met Sibener to discuss niobium
surface chemistry at the recommendation of mutual colleagues.
Pushing particle beams
Niobium has assumed greater importance in plans for the next
round of linear colliders. The current generation of ring
colliders, including Fermilab's Tevatron and Europe's newly
operating Large Hadron Collider, use thousands of niobium-titanium
superconducting magnets to steer and focus their beams of charged
particles, which travel in great loops before being steered into
collisions that can reveal fundamental properties of matter.
Cavities are a small part of these machines, providing a momentary
push to the particles each time they orbit the ring.
But linear colliders, including Stanford's current linear
accelerator, Fermilab's proposed Project X, and the proposed ILC,
string together thousands of cavities into one long line. The
resulting linear accelerator creates an immense electric field to
push the particle beams toward their collision in a single pass,
without any need for steering and recirculating them.
The emergence of niobium SRF cavity technology over the past 20
years makes it possible for each resonating cavity to utilize
superconductivity to produce high-power output through low-power
input, with an estimated gain in quality factor of 100,000 over
Stanford's copper cavities. But many aspects of the system are not
yet optimal.
Niobium is processed according to laboratory recipes that could
benefit from a firm grounding in materials science, Cooley says.
"Just how precisely a given recipe is followed depends on
laboratory culture, attention to detail by individual operators,
arrangement of tasks based on what is perceived to be important,
and so on," Cooley says. "The true impact of different processing
steps is just beginning to emerge as the university scientists like
Steve step in and produce basic understanding."
The microscopes in Sibener's laboratory enable researchers to
observe the behavior of individual atoms. With the earlier seed
grant, Sibener's team found that niobium's reaction with oxygen
produced a variety of surface oxides and defects that suggested to
Cooley and others explanations of observed changes in real-world
SRF cavities.
"This is some of the purest niobium you can find in the world,
actually," says Sibener, displaying a mirror-like wafer of the
material in his office at the Gordon Center for Integrative
Science. His research group will closely examine the material to
determine exactly which oxides and defects at the surface of
niobium crystals lead to loss of superconductivity under extreme
conditions.
"If the Fermilab-UChicago collaboration is successful," says
Cooley, "it will allow new types of accelerators to be built at
great cost savings."
SOURCE