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Comment by stavros | Hacker News Reader

So does eg Q=100 mean that it'll still go out on its own at some point? Ie is Q a measure of duration/total energy output for a given energy expenditure?

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Yes, it means you will get out 100 times more energy than you put in, but at that point it does still go out if you don't keep supplying that external heat.

At Q=infinity, you get out as much energy as you want compared to what you put in initially, but it keeps burning (as long as you add fuel and remove ashes, that is). At that point you pay an initial start-up cost and can fuse as much as you want.

I see, thanks! So this doesn't take into account the hydrogen, just the external energy/heat you put in? I guess it must, otherwise Q=inf would be free energy.

Correct. The energy balance comes from the fuel mass that is converted to energy with E=mc^2.

Right, thanks for the explanation!

You still have to run the cryogenic system for the superconductors and the pittance of power to keep the confinement coil power supplies. Q=inf means ignition: an entirely self-heated plasma. The collisions of the helium ash on the un-burnt fuel are enough to bring the fuel above the flash point. In this way it is like a campfire.

To keep operating you need to extract heat, make power, and drive it from that power, or it goes out or blows radioactive burning molten lithium all over.

In practice, no such plant will ever exist, because there is no plausible prospect of the power being competitively priced.

In the same way there was no plausible prospect of launching cargo to orbit for <$1000/kg at the turn of the century?

No, in the sense that the costs for the competition, renewables + storage, are still falling at a reliably exponential rate, leaving everything else far, far behind. There is no reasonable prospect of catching up even ignoring capital cost and just comparing operating expense. The cost of maintaining steam turbines will not be coming down at all, never mind going to zero.

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