Intro

This is a beginners beginner guide to vacuum theory for beginners, and assumes the reader knows nothing about vacuums. It introduces concepts and defines some of the language used to discuss vacuum technology, and is intended to lay the groundwork for deeper understanding. 

This Guide starts off way zoomed out and oversimplified, building off itself as it goes, introducing new terms along the way, ending still zoomed out and oversimplified --but a bit less so. There's so much more depth behind these subjects than document acknowledges. More technical discussions and descriptions can be found within the other pages of LCLS Vacuum Support Training. But 

Side note: many of the pictures are linked to external material, including videos, lessons, and product pages; click to explore, but they aren't necessary, and may go way beyond the scope of this introductory guide.

Happy reading!

Starting at the bottom, Setting the scene

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. Usually as a mix of different molecule types. Air is a mix of over ten types (mostly nitrogen and oxygen). People work very hard to collect gasses that aren't mixed. 

what are those gas 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 individual molecules depend on their temperature and mass: cold things fly slower, heavy things fly slower.

but that speed is only 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 gas in the room: fewer molecules = lower pressure.

Gas Pressure

When a gas molecule bounces off of something it pushes on that something. If that something is another gas molecule that other molecule goes flying off. 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, it's way too big.

Ah but 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! 

Now what happens if we remove, instead of add, molecules? Vacuum. 

Vacuum

When the pressure in the box is lower than the 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.

but 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; though it probably won't hit another molecule for many lifetimes. 

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 is called the mean free path.

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 your mouth, 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.

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). 

Imagine 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. That is molecular flow. Wind does not blow and pumps cannot suck because there are not enough molecules around for domino-chain effects to occur (like blowing out a candle). 

Now, onto the machines that care so much about gasses and their behaviors: Vacuum chambers

Vacuum Chambers

The Chamber itself

 just a box for holding nothing. 

perhaps with holes to attach stuff, like pumps.

Pumping

A pump can't pull air after gasses enter molecular flow. But if that pump Can't suck, what does it do? It traps. Like the goal on an air hockey table traps the puck when it flies in.

So how likely is a molecule to fly into a pump that traps? That depends on how big the tube is leading to the pump.

Conductance

A long/narrow tube restricts gas flow more than a wide/short tube. The rate of gas flow through a tube is called the tube's conductance; the long/narrow tube has the lower conductance of the two.

Sadly adding a bigger pump to suck faster though the tiny tube only works in viscous flow. In molecular flow, the pump will only ever remove molecules at the rate they naturally fly through the smallest/longest tube in leading to the pump.  The lowest conductance point sets the pace.  =(

different pumps for different pressures:

Pumps come in many different shapes, sizes, and types. 

When we want to get to really low pressures (which is most of the time here at LCLS) we use layered tiers of pumping: where we turn on one type, wait until the pressure stops dropping, then turn on the next, and again when the pressure stops dropping again we turn on a third type. 

The first layer is good for pumping room pressure down through viscous flow and a bit into molecular flow. The second layer works best in molecular flow, and gets  (Fun Fact: for similar reasons we have to use layered tiers of pressure gauges as well)

for more details on how they actually work, see the confluence page on vacuum pumps. and for that matter, also see the page on gauges, we layer those too.

Pressure, revisited: this time it's chambers.

Partial pressure

So gasses mix. How does mixing gasses change pressure? The pressures add up. For example:

In the mixed chamber the oxygen and nitrogen are still contributing the same pressures they had when they were separate. Those contributing pressures are called partial pressures. Oxygen's partial pressure in the mixed chamber is 2 units of pressure.

partial pressure

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

Units of pressure, log scales

Helpful slides: Vacuum Science and Technology for Accelerator Vacuum Systems