TRANSCRIPT OF
"QUICK CLASSROOM: TALKING ABOUT SCIENCE"
WHAT IS PLASMA?

Narration by Alec Gallimore, Arthur F. Thurnau Professor of Aerospace Engineering, Associate Dean for Academic Program and Initiatives, University of Michigan.

The field of research that we do is electropropulsion. Sometimes we call it plasma propulsion. And a logical 
question is, what is plasma?

 Well, it's not what's running through your veins. That's 
 a different kind of plasma. 
 
 The type of plasma that we're talking about is actually 
 a state of matter. Some people call it the "Fourth 
State of Matter." 

If you start with the three states of matter that we 
understand (mostly)--solids, liquids, and gasses--
that's a natural segue toward understanding what a 
plasma is. 

If you start with, let's say, a solid piece of water ice. That's solid phase. 

If you put energy into it, what's going to happen?
If you put enough energy into it, it melts. It goes 
into a liquid phase. 

If you put more energy into it--you raise the 
temperature--it'll eventually boil and go into a 
vapor phase or a gas phase. 

Now if you put a LOT more energy into it, what'll 
happen is that the atoms in the molecules will break 
apart. And then even MORE energy what'll actually 
happen is that the atoms, individual atoms, will 
start breaking apart. 

An atom is nothing more than a nucleus made up of 
protons and neutrons, with you could think of it as
a cloud of electrons floating around it. And if you 
put enough energy into that, what'll happen is that 
the electrons gain so much energy that they're actually 
able to break away and come free from the atoms. 

So what you're left with then is a free electron, 
which is negative, and then an ion, which has been 
depleted of one or more electrons, which has a 
positive charge. 

First order, what you can think of a plasma is a 
cloud full of positive and negative charged particles. 

[shows rocket engine]

This is a "Hall Effect Thruster," and this type of 
engine would be used for anything from deep-space 
probes to very large earth-orbiting spacecraft like 
communications satellites.

The way a Hall Effect thruster works is that we'd  
have a tank [shows back side of thruster] which we're 
not showing right here, full of propellant. And the 
propellant would be something like Xenon or Krypton, 
a very easy gas to store. We have tube back here 
[points to back of thruster where tank would be 
attached] that injects the Xenon gas in the back of
this discharge channel we call it. 

[points to bracket at top of thruster] Then what we 
do is we'd have normally a tube here that we call a 
cathode, that emits negatively charged particles 
called electrons. You could think of this tube as 
essentially emitting a series of lightning bolts, if you will, steady-state lightning bolts. 

What the lightning bolts do is essentially heat up 
the Xenon particles to such a degree that it 
actually becomes a plasma. It's heated so high it 
becomes a plasma. 

And once it becomes a plasma now, we're able to apply the 
applied electromagnetic field to accelerate the 
plasma. And we're able to accelerate the plasma 
anywhere from about 40,000 miles per hour up to 
about 100,000 miles per hour.