Saturday, September 22, 2007

Part 2 - the D+He3 fusion reactor

A Deuterium-Helium3 fusion reaction will be our next step. It may well be our last step, since the D-He3 fusion reaction produces the most energy per unit mass of *any* possible reaction. [except for matter-antimatter - but antimatter does not occur naturally around here.]
The fusion reaction we'll use is:

D + He3 -> He4 + H

This is a *great* equation - the reaction produces no neutrons! In the D-T reactor discussed previously we had these pesky neutrons flying out of our containment trap since (being neutrally charged) we were unable to contain them. We made a virtue out of this fact by utilizing the speedy neutrons as best we could... but still it would be better if they weren't there at all.
If everything is charged it (hopefully!) stays in the trap. Better yet, we can extract energy from a fast He4 and H by electromagnetically braking them, converting their speed into energy with ~95% efficiency. Back in the D-T days, we turned speed into energy with efficiencies more like 35%.

And H and He4 aren't dangerous gases, they can be vented to the environment without issue.

It's all good!

We have the key concepts in place to grok any kind of fusion... we have a Bunsen Burner to heat the reaction chamber to 100 million degrees and we have a magnetic trap good enough to hold a small reaction chamber under high pressure so fusion can take place. We're good to go with D-He3... except for three little nagging details:

* All magnetic reaction chambers leak. If they leak too much, they can't sustain the pressure/temperature we need. Currently, the best magnetic traps in the world are just barely able to contain the D-T reaction and they are not good enough to contain D-He3. They need to be roughly 100 times better. This sounds like a big problem... but over the last 50 years we have improved our magnetic reaction chambers by a factor of over 10000. There is nothing fundamental about making them better still, we "just" need to do what we're doing, only better. So we should have it figured out by 2020.

* A magnetic trap good enough to contain a D-He3 fusion reaction will also permit D-D fusion. Without going into details, a bit of D-D fusion will be occuring in the magnetic trap, right alongside D-He3. And unfortunately, D-D makes fast neutrons. We will probably solve this problem by ignoring it.

* He3 doesn't exist on the Earth and it is energy-inefficient to manufacture it. But the Sun very kindly planted a small amount of it on the Moon. We could go fetch it. Cherei had a good blog post discussing the details here:

The Moon has enough He3 to power our civilization at current rates for about 1000 years. By then we'll be able to get it from the Mother Lode: The gas giant planets!

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