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