Tuesday, June 06, 2006

Experimental Thoughts

Nothing worked today. Everything went to hell. The experiment we tried didn't work.

In order to do our big experiment we need do a small one first. We have a plasma, which is essentially a charged gas, and we try to figure out how dense it is. To be exact, we need 10^19 electrons per cubic centimeter of space in order to be in the right way... but we don't know whether we can get it yet.

How do we create the plasma? Simple. Like the neon lights, we put a gas in a chamber and then apply a voltage across it. The electric field of the voltage is strong enough to cause electrons that were moving around the nucleus of an atom to be released and travel free. If this happens 10^19 times, we have the plasma we need. (In our case we have Hydrogen gas instead of neon or xenon, and we apply 20,000 Volts instead of 220, because we need so many electrons free!)

The big question that we have been dealing with the last few months is: how do we measure how many electrons we have? How do we measure the plasma density?

The latest thing we tried was an interferometer. We take a laser and split it in two beams. The first beam travels through the air, the second beam travels equal distance through the plasma. Then we add the beams at the end. If the times they travelled are exactly equal, a certain pattern emerges when you sum the beams; if they are not, another certain pattern emerges.

Light slows down when it travels through a medium. The same thing happens with plasma: when the laser beam goes through, it slows down by an amount that depends on the density of the plasma. The more electrons in the way of the beam, the more it slows down, and hence it produces a different pattern. So by looking at the pattern we can trackback and calculate how many electrons the laser found on its way, and hence find the density.

Well, nothing worked. For one thing, the laser pattern that emerges is very sensitive to the distance travelled. A few nm of difference (less than a human hair!) will create its own patterns, so we cannot distinguish if teh pattern we see is due to the plasma or due to someone walking by, closing a door, or flushing the toilet! All kinds of vibrations are around and they affect the measurement.

Second, when you apply 20,000 volts, well, it is very strong. If there is air, it may cause a spark (that's why we have the whole thing inside vacuum). But there are always particles of air around, and instead of the voltage reaching the plasma it creates currents on its way there and is gone before it ever reaches it!

Tomorrow we will try it again, more carefully and more cleanly. We will see...