Argonne physicists create landmark accelerator gradient
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ARGONNE, Ill. (Sept. 14, 2007) — The Argonne
Wakefield Accelerator (AWA)
Group at the U.S. Department of Energy's Argonne National Laboratory works
on particle accelerators in much the same way that horsepower junkies work
on muscle cars. Although their research doesn't involve turbochargers, stall
torque converters or cat back exhaust systems, the AWA group obsesses over
the power of their machine.
This past spring, Manoel Conde and his colleagues in the AWA group, led by
physicist Wei Gai, crossed a major frontier in accelerator science: They generated
an accelerating gradient five times stronger than that used in traditional
linear accelerators. The gradient – a region of increasing electrical potential – works
like an electrical "motor," transferring enormous amounts of energy
to charged particles as they shoot through an accelerator. By substituting
a dielectric for the copper medium used to support these accelerating electric
fields, Conde and his colleagues generated a gradient of 100 MV/m; traditional
linear accelerators generally cannot support a gradient higher than 20 MV/m.
Higher gradients could allow for more compact accelerators with the same or
greater energy as today's high-energy accelerators, which often stretch for
miles. Aside from extending the energy frontier, high-gradient acceleration
will allow for the miniaturization of the medium- and low-energy accelerators
used in hospitals for radiation therapy, improving their accuracy while reducing
their cost, Gai said.
High-energy physicists realize that they cannot continue their decades-long
trend of building ever-longer accelerators, said John Power, a physicist in
the AWA group. He sees projects like CERN's Large
Hadron Collider, with a circumference
of about 17 miles, or the proposed 20-to-25-mile-long International
Linear Collider (ILC) as nearing the ceiling for an accelerator's physical size.
"Right now, high-energy physicists are building accelerators that are
approaching practical limits in term of their length and cost," Power
said. "We're pushing technology to the limit and accelerators are getting
to be extremely long – to get to the next level we need to raise the gradient."
Physicists have started to look for ways to make accelerators more powerful.
The limiting factor lies in finding a suitable alternative to the traditional
copper accelerators that Power called "the workhorses of high-energy physics
over the last 50 years."
Traditionally, copper-based accelerators have operated at gradients below
approximately 20 MV/m. Recently, accelerator scientists developed an advanced
copper accelerator and were able to achieve a gradient of 65 MV/m, above which
point the accelerator suffered irreversible damage, according to Power. It
is believed that above this level the strong electric fields produce a phenomenon
known as field emission, in which electrons circulating in the highly conductive
metal get pulled out of the copper surface, generating discharges that can
damage the accelerator.
In the quest to achieve a higher gradient, the AWA group turned to dielectrics – or
insulating materials – as potential alternatives. "If dielectrics can
support high gradients, we've got a great shot at a working accelerator," Power
said. "The power sources and the size of the supporting structures are
the same as that of traditional accelerators, which is a great strength of
the dielectric scheme."
While Gai cautioned that other challenges still impede the realization of
a high-gradient dielectric accelerator, he agreed with Power's observation. "This
is one of several promising advanced technologies on the horizon, along with
laser and plasma acceleration and various types of metallic-based accelerators," he
said.
Unlike traditional accelerators, which employ only one particle beam, wakefield
accelerators use two beams: a “drive beam” to create the high gradient and
a “main beam” to be accelerated. Within the drive beam, one or several highly
charged "bunches" of electrons are shot through the dielectric structure.
Like a speedboat rushing over a lake, each drive bunch leaves behind an electromagnetic
wake. The electric gradients produced by these wakes are then used to accelerate
the main beam. By synchronizing the emission of each successive drive bunch
with the wake's waves, Yusof explained, scientists can create stronger wakes
that produce higher gradients.
Although the 100 MV/m figure constitutes a true landmark for accelerator physicists
— "it's a milestone we've pursued for 10 years," Power said — dielectrics
or similar materials could produce even higher gradients. The absolute upper
bound on solid-based accelerators is believed to be on the order of 10 GV/m,
above which point solids are expected to ionize and destabilize. That
figure represents a 100-fold gain over the gradients AWA scientists
have generated, leaving considerable room for later improvements.
Power believes that the successful construction of higher-gradient accelerators
will precipitate a revolution, as high-energy physicists — the AWA's "primary
customers" — will gain access to a whole new regime of energies. "Right
now," he said, "the 20-mile ILC is expected to give you about half a tera-electronvolt
(TeV) of energy. If you take the same 20 miles and apply high-gradient technology,
you can get a couple of TeV and perhaps even more in the future. That opens
up many different possibilities."
Argonne's wakefield accelerator research is funded by the High
Energy Physics Program in DOE's Office of Science.
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By Jared Sagoff
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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