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This cannot work: the power balance doesn't close for a 25 MWe tokamak. Otherwise people would have made plans to build one already. Tokamaks want to be big for physics reasons, way before economics reasons. This is part of why DEMO is so big. If ARC could be any smaller it would be.

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Is there a limit to how big before the effect falls off, or will we see twenty gigawatt net power output mega-tokamaks with rings five stories tall?

Fission seems to like to be big too but for different mostly logistical reasons. It makes all the nasty and hard aspects a lot easier if you do a lot of it on one site rather than a little on many sites, and it avoids having to ship nasty fissile material around too much.

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