Accelerator on the move,
but scientists compensate for tidal effects
BY ANDREI SERYI AND P.A.
MOORE
For the past 30 years, a
fixed landmark on the Peninsula for small planes and
commuters has been the two-mile-long linear accelerator
(linac) at Stanford. In reality, this fixture moves
daily, although commuters surely do not notice a movement
less than the width of a human hair. The movement results
from forces exerted by the sun, moon and tides. Though
scientists can make instrument corrections to ensure that
movements do not endanger experiments, the phenomenon
could influence decisions about where future accelerators
are built.
"With our instruments
we can see the push and pull of the ocean tides, and the
effects of atmospheric pressure on the tunnel where the
accelerator is housed," says Andrei Seryi, a
physicist at Stanford Linear Accelerator Center (SLAC).
"This kind of information will help with research
for a future machine."
That future machine is
called the Next Linear Collider, or NLC. Seryi is part of
the team working on R&D for the NLC. Its design calls
for a 20-mile length. Given its size, the NLC will not be
built at Stanford. Its location is a political decision,
since it depends on the countries involved and the
funding sources. Using electrons as a probe of matter,
the NLC will operate at an initial energy 10 times that
of SLAC, 500 billion electron volts (GeV) compared to
SLAC's 50 GeV. Higher energies will allow physicists to
study forces of nature beyond the so-called
"standard model" of current physics. The
billion-dollar project is still in the R&D phase. If
approved, construction could begin in 2004.
Subatomic particle
collision requires extreme precision. Movement could
cause particle beams to miss each other at the desired
collision point, so tunnel stability is important. The
world record of focusing electron beams was achieved at
the Final Focus Test Facility at SLAC. Beams were focused
to a 70-nanometer spot -- one-tenth the wavelength of
visible light and about 20 times smaller than the typical
beam size of the Stanford Linear Collider, an apparatus
used in previous SLAC experiments. The NLC would reduce
beam size by another factor of 20.
A major construction
consideration is what kind of tunnel to build. Options
include cutting a tunnel in the dirt and covering it
after it is filled with the accelerator pipe (the
technique used for the SLAC linac) or boring a hole into
bedrock (the method chosen for the now defunct
Superconducting Supercollider in Texas). A cut-and-cover
tunnel is cheaper and easier to build, but the stability
of such a tunnel must be carefully investigated. The SLAC
linac tunnel is an ideal test site for such studies.
Scientists have studied
movement of the SLAC tunnel in the past. "Our linac
tunnel has a laser alignment system, so it's a unique
location for studying long-term relative transverse
motion over long distances," says physicist Chris
Adolphsen. Physicist Gordon Bowden performed tunnel
stability measurements for periods from several minutes
to a day in November 1995. Repeating these measurements
over much longer periods of time filled in a missing gap
in the data.
"By cross-correlating
the measured data with other parameters like atmospheric
pressure, we can determine which factors are partly
responsible for tunnel motion," says Seryi, who
conducted research during the holiday break in December
when the accelerator was shut down. The data acquisition
system recorded transverse deformation of the tunnel
center with respect to its ends every second for a month.
The measurements revealed
several unexpected facts. One is that the observed motion
has very clear daily and half-daily periods. Detailed
analysis confirmed that this motion is indeed tidal --
that is, produced by gravitational attraction of the moon
and sun on the Earth.
The amplitude of the
observed tidal motion was surprisingly large -- about 10
microns, or a hundred times larger than expected. This
anomaly is explained by SLAC's location near the Pacific
Coast. When the ocean tides change the water level at the
shore, this water produces additional pressure that
increases the deformation of the nearby Earth. Called
"ocean loading," this phenomenon has been known
to geophysicists for more than 30 years.
This is only the second
observation of the impact of tidal motions on an
accelerator -- the first being at CERN physics laboratory
in Geneva. CERN scientists noticed tiny changes in the
energy of the beam of particles in a machine called LEP
(for Large Electron-Positron Collider), and, with the
help of SLAC's Gerry Fischer (now deceased), were able to
correlate these changes with the phases of the moon.
As the Earth stretches
periodically from tidal forces, the LEP machine stretches
a few millimeters from its circumference of about 27
kilometers. The transverse tidal deformation observed at
SLAC is much smaller and would be nearly undetectable if
not enhanced by ocean loading. The 10 microns of SLAC
movement are equivalent to about one-half of
one-thousandth of an inch. This type of precision and
more is necessary to collide subatomic particles.
While knowing about tidal
deformations aids in building the future linear collider,
such deformations are of little real concern to
experimentalists. "Tidal motion is slow, very
predictable and has quite a long wavelength, all of which
make it quite harmless to our current machine or to a
future machine," says Seryi.
Another unexpected
observation that could have more impact on tunnel
construction and a future collider site was the influence
of atmospheric pressure variations on tunnel deformation.
Variation of ground materials and the contour of
landscape along the SLAC tunnel appear responsible for
this effect. Landscape and ground properties can vary on
much shorter length scales than do tidal motions.
"Our accelerator
tunnel can easily cope with misalignments which have a
long wavelength," says Seryi. "The short
wavelengths could be more of a problem since they spoil
the beam quality. Now we know better ways to decrease
this effect."
SLAC's two-mile
accelerator has been working well for over 30 years. But
for the next generation machine, builders will certainly
take tidal motion and the landscape into consideration. A
flat and homogeneous site would be ideal. California's
Central Valley might be a good spot, according to SLAC
scientists, but they add that their colleagues at
FermiLab near Chicago might prefer a mid-west prairie.
"Our goal is that
this machine be built, and built in the near
future," says David Burke, the NLC project leader.
"This is big science. To achieve our goal requires
broad national and international commitment and
cooperation. It's a fascinating blend of scientific
passion, cultural awareness, and political acumen. A
little luck will help too." SR
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