This. We already have a fusion reactor... it achieved ignition some 5 billion years ago with enough fuel to sustain itself for about 10 billion years (5 down,

This. We already have a fusion reactor... it achieved ignition some 5 billion years ago with enough fuel to sustain itself for about 10 billion years (5 down, 5 remaining). It's conveniently located at a safe distance suspended in empty space at the center of our solar system. All we have to do is harvest the energy it's producing, and we have the tech to do that, including a road-map for growing civilization by many orders of magnitude using that energy (up to a full Dyson sphere). As for the notion that somehow you can harvest useful energy from a fusion reaction using anything less than the mass of a star... well, at the very best it isn't likely to have a very good EROEI (energy return on energy invested) because no matter how much progress we make it will always be expensive to ignite and then contain a mini-star. And it's quite possible, even likely, that the EROEI will always remain negative.