Wednesday, July 20, 2011

Current labwork: Vacuums

My mother is begging me to write about my current summer research.  I have already talked about my general research topic, but let's talk a little more about the experimental details.  Let's talk about this thing:

My reaction (and possibly yours too) is, "What the hell is that?"  I've toured a bunch of physics labs, and many of them are inhabited by these strange Physics Devices.  They just look like jumbles of spheres, windows, tubes, wires, and aluminum foil.  Who knows what those mad physicists are up to?

But after working on these Physics Devices for weeks, I've unlocked some of their secrets.  In short, they are vacuum chambers.

Actually, they are all sorts of different devices for different experiments, but vacuum chambers are a very common component.  A vacuum has obvious utility for lots of experiments.  In my own experiment, air will interact with the surfaces of the superconductors, and thus ruin the experimental results (which are very surface-sensitive).  Therefore, the superconductor has to be kept under vacuum.

Why do vacuum chambers look the way they do?  First, you need those spheres to prevent air from coming into the vacuum.  And then you need the windows so you can actually see what's happening inside.  You need additional spheres as loading chambers (so you don't let the air in everywhere whenever you load a material).  And then you may need additional chambers between the main one and the loading chamber in order to slowly step up the vacuum power.  There are also multiple vacuum pumps and pressure gauges.

And then, you need some way to transfer materials from the loading chamber to the main chamber.  This is tricky.  I'm going to zoom in on the solution.

That long cylinder is a manipulator arm.  On the very right end is something you can slide back and forth to move the arm.  Typically, you'd have a sample (eg a superconductor) screwed onto a "stage" in the loading chamber, and you'd use the manipulator arm to unscrew the sample, move it into the main chamber, and then screw it onto another stage.

What's with all the aluminum foil?  Water molecules tend to stick to the inner surfaces of the vacuum chamber, and they slowly come loose, to the detriment of the vacuum.  We need a really good vacuum, down to 10-11 atmospheres or so!  Therefore, before starting any experiments, we need to heat up the chamber, and boil off all the water so it can be pumped out.  This is called a "bake-out".  The aluminum foil is there to keep the heat in during the bake-out.  Grad students are usually too lazy to take the foil off afterwards, especially since they'll just have to put it on again for the next bake-out.

The wires are there to supply power and record data.  All the data ends up on a computer, which is actually where I do most of my work.  But that's boring to describe, so back to the vacuum.

Depending on the experiment, there could be lots of other attachments to the vacuum chamber.  In my experiment, we need to take off a layer of the superconductor while it's in the vacuum in order to expose a new surface that has never touched air before.  This is also quite tricky.  The solution involves gluing a little peg to the sample beforehand, and then using a "wobble stick" in order to jab the peg off.  I am not making this up.

My experiment also requires the addition of a hemispherical analyzer (shaped like a hemisphere), and a laser (which comes with a whole new jumble of lenses, mirrors, cameras, and other optics).  There's probably even more stuff that I don't understand.  Who am I kidding, I don't even fully understand the things I've described!


Deralterchemiker said...

Describe the pump, which I think is the most interesting (and vulnerable) part of the system.

miller said...

There are multiple pumps: rough pumps, cryo pumps, ion pumps, none of which I really understand.