Get out of here!
We wish it was that easy to get the air out of our chambers and tube! The LIGO vacuum system is one of the largest in the world. Why do we need a vacuum? And what does it mean?
All the key components in the LIGO interferometer are inside vacuum vessels: big tanks and pipes with basically all the air pumped out of them. The main 4-km-long LIGO arms are about one meter in diameter and are at a pressure of one billionth of a torr (1e-9 torr). For comparison, the normal average atmospheric pressure is 760 torr, meaning that inside the LIGO arms there is one thousandth of a billionth of the amount of air in the atmosphere, that is 1e-12! If we were to collect all the residual air in the arms and get it out, it would fill a volume that’s less than a drop of water, at normal pressure.
Why do we need to operate in a vacuum? There are two main reasons: first, to avoid contamination from dust and other substances; second, to isolate the laser beam from air pressure fluctuation (sound) that would create a signal much larger than gravitational waves.
How do we reach and maintain this level of vacuum? First of all, to get all the air out we need several kinds of pumps. First, roughing pumps: basically simple pistons that suck the air out of the vacuum vessels: those are good for going from atmospheric pressure down to a fraction of a torr, after that they’re not efficient anymore. At that point we use turbo pumps: something like airplane turbines that spin a thousand times per second. Each time a molecule hits the fans, it’s kicked out of the way. Those pumps are good to get down to about one millionth of a torr. Below that we need a combination of ion pumps and cryogenic pumps. The first uses electrical potential to attract and capture molecules, while the second relies on cold surfaces to freeze molecules. All those together allow the LIGO vacuum vessel to reach 1e-9 torr.
But that’s not enough. All parts that are in “contact” with the vacuum need to be cleaned and baked, to remove all contaminants from the surface that would slowly evaporate. All metals adsorb gasses on the surface: we need to heat them up in an auxiliary vacuum chamber and keep pumping out the gas that evaporates. That’s the only way to ensure all new parts installed in vacuum won’t contaminate the system.
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