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When it Comes to Accelerators,
What is Cold?
By
Heather Rock Woods
Superconductivity arises in special materials at super cold
temperatures. At these temperatures—a few degrees above absolute
zero—the materials’ electrical resistance virtually vanishes.
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A Fermilab technician works
on an array of superconducting niobium cavities at Fermilab.
(Photo courtesy of Fermilab) |
Superconducting technology will be used to accelerate electrons and
positrons into extremely energetic collisions in the proposed
International Linear Collider (ILC).
This
summer, the International Technical Recommendation Panel (ITRP) decided
that the international physics community should design the ILC linear
accelerators (linacs) with cold technology, rather than the warm
technology espoused by SLAC and other institutions.
The
panel stressed that both technologies were mature and viable. SLAC has
strongly promoted the project, independent of the technology choice, and
is now refocusing its efforts to optimally design and achieve a machine
with cold technology (see TIP, September
3, 2004).
SLAC is
thoroughly familiar with warm technology. A lower-energy version runs
the linac here. The particles travel through the center of copper
cavities kept at 113 degrees Fahrenheit.
So, How
Does Cold Work?
Greg
Loew (DO), who spent two years steeped in warm and cold details as chair
of the ILC Technical Review Committee, explained the cold technology.
Cavities (roughly seven inches in diameter with a hole through the
middle) are cooled to 1.8 Kelvin (271 degrees Celcius below the freezing
point of water). The super-cooled cavities are made of a metal called
niobium that looks like stainless steel. At that temperature, niobium is
superconducting. The electrons in the niobium material (not the
electrons being accelerated through the niobium cavity holes) flow with
virtually no resistance, like pairs of skaters on perfectly smooth ice.
The ILC
calls for two linacs (one for electrons, one for positrons) pointed at
each other. Whether a linac is warm or cold, particles get accelerated
by microwave power that is injected into the array of cavities. The
microwave power generates longitudinal electric fields and cylindrical
magnetic fields. The electric field attracts (or pulls) the particles
traveling through the cavity, giving them an energy boost.
Because
they have almost no resistance, the superconducting cavities can hold
the microwave power longer. In warm technology, some of the power ends
up heating the copper cavity walls—which have some resistance.
Power
Cost vs. Energy Reach
“Superconducting cavities allow you to store microwave energy very
efficiently for a long time,” said David Burke (ILC).
Linacs
accelerate particles in ‘trains’ with multiple cars—bunches of
particles—containing a cargo of 20 billion particles per bunch for the
cold design, or 7.5 billion particles per bunch for the warm design.
Because cold cavities store microwave energy longer, each microwave
pulse can accelerate longer bunch trains. Cold trains carry 15 times
more bunches than warm trains, but arrive less frequently—five times
each second compared to 120 times each second for warm. In the end, the
two designs generate similar luminosities, or event rates, for
experiments.
Compared to warm technology, cold technology uses less electricity from
the power company while accelerating longer energy-efficient trains.
That is like adding loaded railroad cars to a steam train without
needing more coal to power the train.
However, some of what superconducting technology gains by saving power,
it loses in particle energy.
Niobium
surrenders its superconductivity when exposed to too strong a magnetic
field (see sidebar). Accordingly, the cold linacs will use lower
magnetic and electric fields which means particles will get a smaller
tug, and gain less energy, for every meter traveled. Thus, a cold
machine needs to be longer than a warm machine to reach the final beam
energy of 250 Giga electron volts (GeV) for each beam. The machine
length will be determined by the design, but will be at least 20 miles
long from the end of one linac to the end of the other.
Back to
the Future
Helping
design a cold machine won’t be completely new to SLAC, Loew pointed out.
In the late 1960s, people at SLAC and Stanford explored how to equip the
linac with superconducting cavities, but it proved unfeasible at the
time. Now the time appears right for super-energetic cold linacs.
For
more information on the linear collider, see:
http://www-project.slac.stanford.edu/ilc/
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