I spent the bulk of yesterday afternoon doing vacuum system work, specifically working on the system to feed gas into the atomic beam source. My feelings about this can be inferred from the Facebook status message I set at the time: “Chad Orzel abhors a vacuum.”
The apparatus I’m building uses laser cooling to decelerate an atomic beam of krypton atoms in a particular metastable state. This works brilliantly to slow metastable krypton atoms down, but the only atoms affected by the laser are krypton atoms– everything else continues along unimpeded. As a result, the entire experiment needs to be carried out in an ultra-high vacuum chamber, so I have a large and shiny apparatus with multiple vacuum pumps to ensure that all the air is removed from the path of the krypton atoms:
Of course, in order to work with metastable krypton, I need to get krypton into the system and excited to the proper state. And that’s where the “moron” part comes in.
The process of getting krypton into the chamber is pretty involved. The krypton gas starts in a lecture bottle where is is stored at a pressure of several atmospheres. There’s a valve on the top of the bottle, then a gas regulator that limits the pressure at which the gas exits into the rest of the system. Then there are two more valves, with about four inches of copper tubing between them, then about five feet of copper tubing, before a needle valve with a micrometer readout so the opening can be precisely controlled. After that, there’s a gauge, then a length of glass tubing in which we excite a plasma discharge to prepare a small fraction of the atoms into the proper state for laser cooling.
The big series of valves is because the gas in the bottle is at a pressure of several atmospheres, while the pressure inside the vacuum chamber is about one billionth of an atmosphere (10-7 – 10-6 torr). We need to be able to carefully control the pressure of the gas in the discharge region, and cut it down to a level that doesn’t overwhelm the pumps.
Of course, we also need to be able to pump all the air out of all that tubing. The glass tubing is a half-inch in (outer) diameter, and the copper tubing is a quarter-inch OD. There’s about ten feet of tubing in total, and getting the air out of it is a little like sucking water through a ten-foot drinking straw– it takes forever, and the only thing you can do to speed it up is to open all those valves as far as possible.
Of course, it’s important that you close all the valves back down after pumping the air out. It’s also critically important to close the regulator down, as well, before you open the valve on the gas bottle. Otherwise, you wind up filling six feet of narrow tubing with krypton gas at about three atmospheres, and you get to spend an entire afternoon pumping it back out again.
Which is why I abhor a vacuum…