Confinement Methods

To make fusion happen, you have to prepare a plasma (a gas in which electrons and ions fly around separately) hotter than 100 million degrees so that atomic nuclei can be slammed together. But a gas that hot cools down the instant it touches a wall, no matter what container you put it in. So the starting point of fusion research is always the same question: how do you keep hot plasma from touching the walls? This trick of “confinement” is the theme of this entire section.

The Three Ideas of Confinement (High School)

The ways to hold hot plasma fall broadly into three groups. Distinguishing them by “which force is used” makes the big picture easy to grasp.

The Sun confines plasma by gravity. The Sun packs plasma toward its center with its own tremendous weight. It is a very stable method, but Earth-sized gravity is nowhere near enough, so it cannot be used in ground-based devices. How stars shine is covered in Fusion Basics.

Confinement by magnetic fields is the mainstream of ground-based research. Because plasma is a collection of electrically charged particles, its direction of travel can be bent by the force of a magnet (a magnetic field). Particles travel while spiraling around the magnetic field lines, so if you close the field lines neatly into a loop shape, the plasma keeps running around and around without touching the walls. Picture the plasma running along invisible rails made of magnets.

Confinement by inertia is laser fusion. A tiny pellet of fuel is crushed in an instant by lasers from all directions, and the reaction is made to happen before it has time to blow apart. The idea is “finish it before it escapes,” and the confinement time is only about a trillionth of a second.

What You Will Learn in This Section

We recommend reading the following pages in order from the top. The first half builds the foundation of magnetic confinement, and the second half expands into representative devices and alternative methods.

• Basics of Magnetic Configuration: The common foundation for how magnetic fields hold plasma. You will learn why the shape is a loop (torus) and how the field lines are twisted. Please read this first.
• Tokamak: The most researched magnetic confinement method to date. Its defining feature is running a current through the plasma itself to twist the magnetic field.
• Stellarator: A method that twists the magnetic field using only external coils. Because it does not rely on a current, it is well suited to continuous operation.
• Magnetic Mirror: A method that strengthens both ends of a straight (not looped) magnetic field to bounce the plasma back like a plug.
• Inertial Confinement Fusion: A different family of method that uses no magnetic field, compressing fuel in an instant with lasers.

What to Know Before Reading

This section becomes much easier to understand if you read the following two sections first.

• Fusion Basics: Covers what fusion is in the first place and why high temperature and high density are needed.
• Plasma Physics: Covers how charged particles move in a magnetic field (Larmor motion and so on). This is the foundation for the discussion of magnetic confinement.

Even if you have not read them yet, each page explains terms when they first appear, so feel free to go back and check whenever something catches your attention. Let’s start with Basics of Magnetic Configuration.