ScienceWise - Autumn 2011

Lighting the fire

How mathematical modelling may be the key to successful fusion power

The air molecules in a room move constantly, colliding with each other, the walls and anything else that happens to be in there. The velocity of these molecules is what gives the air its temperature, the faster the motion the hotter the air. However not all the molecules are moving at the same speed. They follow what’s known as a Maxwell- Boltzmann distribution, a bell curve of velocities that mathematically describes most fluids in equilibrium. The physics of fluids with such Maxwellian temperature distributions is well established and models many ordinary every-day fluids extremely well. However when it comes to more complicated situations such as the flow of super high temperature plasma in a fusion reactor, it begins to fall down. Dr Mathew Hole and Dr Michael Fitzgerald are two scientists aiming to put this right.
“Most of our existing theoretical models of plasma fusion reactors rely to a greater or lesser extent on simple Maxwellian distributions,” Dr Hole explains, “But in a fusion reactor we know that there are times when the distributions are far from Maxwellian.“ This is because of the extreme conditions within a reactor and the fact that unlike many fluids, plasma is electrically charged.

Fusion can only take place when the plasma reaches a temperature of many millions of degrees and the only way to hold something that hot is within a magnetic bottle. The problem is that you can’t get it that hot without using powerful microwave fields and injected beams of super energetic particles. Both make the plasma non-Maxwellian, and the introduced hot particles generate their own magnetic fields as they twist and weave along an externally applied magnetic containment field.  The beam particles also inject momentum, spinning up the plasma. Now add to that the additional magnetic and electric fields generated by charged energetic alpha particles produced by fusion reaction, and you have mathematician’s nightmare.

Unfortunately, there’s no way round creating a solid theoretical model of any potential reactor. Such undertakings are multi billion dollar affairs so you can’t make one on a “suck it and see” basis. If after ten billion dollars and ten years construction you realise it should have been a different shape or used a different metal in the walls, your credibility and finances are blown. As a result, governments and scientists alike, are very interested in improving the theoretical models on which the designs are based.

“What our project is looking to do is put theoretical modelling of plasmas on a more solid footing by building a mathematical model that isn’t simply Maxwellian. We hoping to do this either by incorporating multiple  fluids representing different energetic populations, and/or allowing those fluids to be anisotropic.” Dr Hole explains.
An anisotropic fluid is one in which  its properties such as pressure or temperature are different in one direction from another. “Think about a swimming pool,” Dr Fitzgerald says. “When your head gets to the bottom your ears begin to hurt because of the pressure. It doesn’t matter which way your head turns, the pressure’s the same. That’s an isotropic fluid.”

“Now imagine you’re underwater in a pool and someone’s squirting a hose into. You’d feel pressure in your ear one way but when you turn your head sideways, it’s gone – at least from your ear. That’s an anisotropic fluid, and that’s very much more like the situation in a fusion reactor when the energetic heating beam is injected.”
Although a heating beam is a great way to get a fusion plasma up to temperature, it’s not desirable to have to keep it running all the time because it’s highly expensive. Dr Fitzgerald draws analogy with a campfire. “Imagine making a campfire out of something like tea bags. If you hold your lighter under them they will burn, and you might do better keeping warm than with just the lighter alone, but pull the lighter away and it will just smoulder out. You couldn’t afford to do that, you’d waste hundreds of lighters.”

Ideally, you want a situation where once up to temperature, the energetic alpha particles produced by fusion reactions provide the heating for the new fuel – a  so called burning plasma. “A burning plasma is more like a campfire made of sticks,” Dr Fitzgerald says, “Once it’s well lit you can take the lighter away and it just keeps on going by itself.”
“What we are hoping to do is improve the physics basis  for reactors that once lit will simply pour out vast amounts of clean, green energy.”

The project is being run in collaboration with the MAST experimental fusion facility in the UK and H1-NF facility here in Australia.  “We’ll be testing our models against known real world data sets and hopefully tuning them to the conditions inside MAST” Dr Hole says.

 

 

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