LIGO’s optical components quietly reside in a colossal vacuum chamber encompassing 10,000 cubic meters (353,000 cubic feet), with an air pressure of 10-9 torr, or one-trillionth of an atmosphere.
Creating and maintaining this vacuum is absolutely essential to LIGO’s operation. LIGO's vacuum environment prevents sound waves from causing vibrations on the mirrors since sound cannot travel through a vacuum. In addition, if LIGO did not operate in a vacuum, temperature variations from air inside the beam tubes would alter the shape LIGO’s optics enough to destroy the quality of the laser beam. Air currents within the tubes would also cause the laser to refract making gravitational-wave detection impossible. Both of these noise sources can be eliminated by operating in a vacuum.
It took 1100 hours (40 days) of constant pumpdown to evacuate the chambers to their optimal operating pressure. In that time, turbo-pump vacuums removed the bulk of the air in the tubes while the tubes themselves were heated to 150-170 degrees C for 30 days to drive out residual gases.
Maintaining this vacuum requires sophisticated monitors and controls as well as the constant operation of ion pumps that extract molecules outgassing from the tubes and other structures inside the vacuum systems. Stray water molecules are also removed by continuously operating liquid nitrogen cryopumps.
LIGO’s vacuum tubes were constructed of spiral-welded 304L stainless steel a mere 3 mm thick. With its relatively low carbon content, 304L steel is resistant to corrosion, especially at the critical welded seams. Rust did grow on the interior of the vacuum tubes during their manufacture in the 1990’s, so when the tubes were installed at LIGO, the interiors of the tubes were meticulously polished and cleaned to remove rust, significantly reducing the likelihood that oxide flakes will fall through the laser beam or migrate onto optical surfaces, the latter being potentially disastrous to LIGO’s mission.
Beam Tube Installation
LIGO's arms are long enough that the curvature of the Earth itself was a complicating factor when installing the vacuum tubes. It wasn’t enough for LIGO’s civil engineers to smooth a level path and assemble each arm’s tubes in a straight line. To ensure a perfectly level beam path, the Earth’s curvature (more than a vertical meter over the length of each arm) was countered by GPS-assisted earth-moving and high-precision concrete work. Reinforced concrete floors 75 cm thick under the interferometers also minimize leak-through of seismic vibrations. These floors are separated from slabs that support labs and offices and from footings that hold the vertical framing. Each of these levels of engineering was designed to support the path and stability of LIGO’s laser beam as it passes through the interferometer and to keep the entire instrument as noise-free as possible.
The 1.2 m diameter beam tubes were created in 19 to 20 m-long segments, rolled into a tube with a continuous spiral weld (far left photo). While a mathematically perfect cylinder will not collapse under pressure, any small imperfection in a real tube would allow it to buckle (a crushed vacuum tube would be catastrophic). To prevent collapse, LIGO's tubes are supported with stiffener rings that provide a significant layer of resistance to buckling under the extreme pressure of the atmosphere. The tubes must withstand these stresses for at least 20 years.