[NF.9] Inertial Confinement Fusion - Basic Principles

As I promised in the previous article, I will now be a bit more detailed about two mainstream fusion confinement principles. I will give an irrational advantage to ICF.

Reminder:

Before I start, I will remind you what I have already said about inertial confinement:

This principle uses fast compression of the plasma usually with lasers. The plasma is produced by compression of a small tablet (size of a pill) that is centered in the large box/container (size of a house). In such way, plasma cools down until it reaches the wall. The compression makes plasma so dense, that it is about a hundred times larger than that of the lead! However, the confinement time is quite short - a billionth of the second, due to the pulsed nature of the process.

Main terms

"Inertial" in the name comes from the historical reason. The physics is identical to the hydrogen bomb, where fission bomb momentum (inertia) is used as compression source.

The "tablet" I mention in the above is usually called microcapsule or microballoon, as it is not larger than a millimeter (so you can measure its diameter or radius in micrometers). The microcapsule is solid ice, made out of fusion-relevant atom(s) - either Deuterium or Deuterium-Tritium.

Driver is what gives us momentum - in a hydrogen bomb, it is fission bomb explosion, while for peaceful energy purpose it is usually a laser, while sometimes it can be an ion beam or Z-pinch (sometime later I can talk about this guys).

Inertial fusion can be direct or indirect. In former laser directly (ha!) hits the microcapsule. In latter microcapsule is set in the hohlraum (German for "cavity") whose interior reflects X-rays towards the microcapsule in a more unique (symmetric) fashion.

Whatever kind of inertial fusion one wants to do, the microcapsule will go through the following stages after the laser impact:

Source: Wiki Commons

1. Driver impact

The driver (whatever it is) heats up the microcapsule and ablates the surface material, creating plasma. Due to driver momentum, some plasma particles move TOWARDS center like a rocket!

2. Compression

The plasma is melting the rest of microcapsule and compressing it TOWARDS the center. At the same time, other plasma particles fly away FROM the center.

3. Implosion

Interaction of outgoing plasma and driver creates shock-wave TOWARDS the center and compresses the microcapsule even more!

4. Fusion

Finally, fusion reactions are starting to happen.

Trouble

The main issue with inertial confinement is laser efficiency, microcapsule production (more words about this in the upcoming weeks), and asymmetrical implosion.

The last point is the main part of physics research in inertial confinement. And it is the main reason why hohlraum is introduced - to increase the symmetry. Just imagine dozens of 10-meter laser beams concentrating on the 1-millimeter ball and you will realize the difficulty ;)

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Note that the names of 4 stages are my own simplifications. Also, expect the corresponding article on Friday about magnetic confinement fusion.

Please feel free to ask anything in the comments. I will either try to answer you in the comment or even make the new post.