Plasma Physics

If you want to understand fusion, there is one thing you cannot avoid: the very form the fuel itself takes, namely plasma. The core of the Sun and the inside of a fusion reactor are both made of the plasma we are trying to confine. This section is a learning roadmap that guides you through the order in which to study plasma physics so that your understanding builds up without strain.

What Is Plasma (High-School-Level Overview)

Heat a solid and it becomes a liquid; heat it further and it becomes a gas. Heat a gas even more and more, and electrons are stripped from the atoms, leaving negative electrons and positive ions flying around separately. This is plasma, and it is called the fourth state of matter, following solid, liquid, and gas.

Plasma has three interesting properties. The first is quasi-neutrality: electrons and ions are mixed in nearly equal numbers, so as a whole the positive and negative charges balance out. The second is collective behavior: because electric forces reach far, the motion of a single particle influences many of its neighbors all at once. Thanks to this, plasma ripples and swirls, showing lively behavior quite different from a gas. The third is a strong response to electromagnetic fields: you can control the shape and motion of plasma with magnets and electric fields. It is thanks to this property that a fusion reactor can confine plasma with magnetic fields.

The temperature needed for fusion reaches about 100 million degrees. At such high temperatures, matter is always in the plasma state, so the physics of fusion is, in its entirety, also the physics of plasma.

What You Will Learn in This Section

We build up plasma physics from micro to macro, in order: starting from the motion of a single particle, then the behavior of the collective, then a description that views the whole as a fluid, and finally turbulence and transport.

Prerequisite Knowledge

Before you read on, it is reassuring to first look through the Basics section. Once you have a grasp of the big picture—what fusion is, why high temperatures are needed, and what kind of devices are used to study it—the discussion here becomes much easier to follow. It is also a good idea to recall the elementary mechanics and electromagnetism from high school physics.

Recommended Order of Study

1. Motion of Charged Particles: First, learn how electrons and ions each move, one particle at a time, in magnetic and electric fields. Cyclotron motion, in which particles wind around magnetic field lines, and drift, in which they gradually slide sideways, form the foundation of the vocabulary of plasma physics.
2. Debye Shielding: Next, learn the collective properties that appear when many particles gather together. This is the mechanism by which plasma cancels out electric fields and maintains quasi-neutrality, and it is the starting point of what makes plasma plasma.
3. Magnetohydrodynamics: This is a description that treats the whole plasma as an electrically charged fluid rather than particle by particle. It becomes the basis for thinking about plasma equilibrium and large-scale stability.
4. Plasma Instabilities: Learn when confined plasma breaks down. This is the lesson where you learn the true nature of the turbulence that hinders fusion.
5. Transport: Learn how heat and particles escape out of the plasma. This is a theme that determines confinement performance and is directly tied to the success or failure of fusion.
6. Principles of Heating: Finally, learn how to heat plasma up to the high temperatures needed for fusion. Here, everything you have learned about particles, waves, and transport comes together.

Reading in order is best, but you are welcome to start from whichever page interests you. If something feels difficult, try going back to the previous page.

Sections to Explore Next

Once you have studied plasma physics all the way through, go and see how it is actually put to use in real devices. When you move on to the engineering of confinement—methods that confine with magnetic fields, methods that confine by inertia, and so on—you will begin to see how the physics you learned here connects to device design.