A log fire or a combustion engine?

When I accused the Sun of cheating, I referred to the two main ways of attempting nuclear fusion: either using magnetic fields to stop the ions from escaping and giving them plenty of opportunities to collide, or by quickly crushing the ions to get a short, intense burst of fusion.

Why magnetic fields? Well, it turns out charged particles spiral around field lines. Most people are familiar with field lines from experiments with iron filings and bar magnets at school, but field lines are also a more abstract concept that makes it easier to think about magnetism. If you placed an ion or an electron near a field line, it will spiral around it in a helix, moving along the field line but never straying far from it.


Central image from Wikipedia, used under CC BY-SA 3.0

For fusion using magnets, the trick is to make the field lines themselves loop around and connect up again so the particles can never escape and have to collide and fuse. It’s a bit like jousting – two knights charging in opposite directions might well miss. However, if instead of a straight lines, the knights rode around in circles, then sooner or later someone’s going to get a lance through the chest.


Alternatively, how can a plasma be crushed? We need something to put pressure on the outside of the plasma and squeeze it tight. If you’ve ever tried to squeeze a water balloon with your hands, you’ll know they tend to just explode, leaving you soaked. This is because the pressure you apply isn’t uniform all over the balloon. The same phenomenon is a big problem with crushing plasmas (more technically known as ‘inertial confinement’) and a lot of effort goes into getting the driving pressure very uniform. Often lasers, x-rays or jets of heavy ions are used.

I like to think of these two techniques as like a log fire or a combustion engine. Magnetic confinement is like a log fire – you chuck a load of hydrogen on the fire, and it slowly fuses. As the fusion process releases energy, that energy then heats up the next load of hydrogen you pump into your reactor, just like a log fire slowly heats up and then burns the next log you chuck on the fire. You can keep the process going indefinitely if you just keep adding fuel.*

Inertial confinement is more like a combustion engine. First of all, a big piston applies pressure to the fuel, making it dense and hot. When it gets dense and hot enough, the fuel ignites, giving out a huge amount of energy in a short time. This energy can be captured, and some of it needs to be reused to compress the next batch of fuel – in an engine, the piston captures the energy, but in a reactor the captured energy would be used to recharge the lasers or whatever you’re using to compress the fuel. 

It is possible to combine magnetic and inertial confinement, and that’s the basis of magnetised liner inertial fusion that I’ll try and discuss soon.

*Though if you turn off the magnets, the ions will fly off and the reaction will stop. This is one reason fusion is so much safer than fission – it’s very easy to stop!

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