ok let's start zoomed out and oversimplified:
Atoms:
Atoms, totally a thing! they're small. really small. a million of them fit across a single human hair. tiny, but they still take up space.
they often like to group up with other atoms to form molecules.
Gasses:
Hey so gasses exist. –you're breathing some right now.
what are gasses made of? molecules.
what are those molecules doing? mostly just bouncing around. –off each other, off the walls
how fast are they moving? fast. really fast. usually over a thousand miles per hour (you are being pelted by molecules right now).
the speed of an individual molecule depends on its temperature and its mass: cold things fly slower, heavy things fly slower.
but that speed is just for individual molecules; the gas as a whole has no great speed or direction because the molecules bounce off each other randomly in every possible direction.
how many molecules are in any given space? a lot! the room you're sitting in has about a gazillion gas molecules in it.
BUT: the number of molecules in a room depends on the pressure of the room: lower pressure = fewer molecules.
Gas Pressure:
When a gas molecule bounces off of something it pushes that something. If that something is another gas molecule that other molecule goes flying off. And if that something is much bigger, for example a metal box, the molecule will push on the box just the same, but the box will hardly budge. What happens when I add more molecules to the box? all of those little bounces add up. That's pressure: the push from all the molecules. Add enough molecules and the pressure will rise so high that the box might pop!
What happens if we remove, instead of add, molecules?
Vacuum:
When the pressure is lower than the air air around us, we call it a relative vacuum.
When there are absolutely no molecules whatsoever in a space, we call it a perfect vacuum.
A perfect vacuum never happens. All vacuum chambers leak. And even in the furthest reaches of space, a stray molecule here and there flies past.
and now getting into nitty gritty details and definitions:
mean free path: the average distance that a gas molecule can travel before colliding with another gas molecule.
as pressure drops, mean free path increases, because there are less molecules to run into. this effects the behavior of gasses.
Behaviors of gasses:
Viscous flow:
Under normal atmospheric conditions-- gas molecules are constantly running into each other (short mean free path) and bouncing off.
In fact, when you blow out a candle it's not the molecules from your lungs that put out the flame (there are too many air molecules in the way); your lung-air molecules merely knock forward the air molecules in front of you face, which knock forward the air molecules above the cake, which in turn knock the air molecules in the flame away from the wick, taking their heat with them, and extinguishing the flame. This domino-chain like behavior is called viscous flow, and it's what we're used to.
But what happens when the pressure gets so low that molecules hardly run into each other?
Molecular Flow:
When a molecule is more likely to hit a chamber wall than it is to hit another molecule the gas starts behaving in strange ways: wind does not blow and pumps cannot suck. We call this behavior molecular flow, see figure above.
It's like the world's largest air hockey table, with a few pucks zipping about. Even if I manage to knock one puck, it is unlikely to hit any other pucks on it's way to the goal. Wind does not blow and pumps cannot suck because there are not enough molecules around for domino-chain effects (like blowing out a candle) to occur.
But if a pump can't pull air after the gas enters molecular flow, how can pressure be dropped any lower? They Trap, like the goal of an air hockey table.
Pumping:
Here at LCLS, we keep using pumps after the gas has started behaving as a molecular flow, if they can't suck, how do they pump? The pump is like the air hockey goal. when
So let's relate this to our vacuum chambers: it starts at atmospheric pressure, which is in viscous flow. We turn on a pump, and after a few minutes, the pressure has dropped enough to be in molecular flow. And under molecular flow, wind doesn't blow and pumps don't suck, the atoms are just too far apart!
So how does a pump pump if it can't suck? It traps. It's just a trap waiting for a molecule to fly in, and once it does, it can't get back out. Pumping achieved!
conductance
all vacuum chambers leak
contamination
molecules sticking to walls intermittently
main culprits:
water - from moisture in the air
hydrocarbons (oils, plastics)
how to defeat contamination
prevent it!
use gloves, change them frequently
clean anything going inside the vacuum
opening the chamber? pump nitrogen into it - prevents water from getting in
nitrogen gas has no water vapor, unlike air.
bake it
making the chamber hot will get help those sticky molecules (water and oil) move along faster