The inner surface of the Wendelstein plasma vessel is water-cooled (it has to be, or it would eventually melt), so in the future you could drive a generator from the cooling system.

There is a separate engineering problem about how to keep the plasma vessel from warming the magnets (which need be near absolute zero). In the Wendelstein this works by putting most of the device inside a vacuum, to provide better thermal isolation. So the full system is a vacuum chamber which contains the magnets which wrap around the plasma vessel which wraps around the plasma.

There is an article about the cooling system here: https://www.ipp.mpg.de/ippcms/eng/presse/pi/02_10_pi

> But how much of that G > 1 energy has ever been harvested?

None, because we haven't achieved G > 1 yet. :)

Per https://physics.stackexchange.com/questions/70209/how-is-ene..., hydrogen fusion produces helium and neutrons; neutrons are unaffected by magnetic fields, so they escape confinement and impact the reactor walls, heating them. The heat is simply vented in current test rigs, but in a working model it can be used to produce steam to power generators. That's specifically for tokamaks, but I would guess the process is similar for stellarators.

It's great how even our most advanced energy generation technologies are still just steam boilers at heart. That hasn't changed in 305 years.

I've read about some theoretical designs that would open the field to allow all negative charged particles out and use these electrons as power directly.

But first, designs would have to be able to keep the containment pressure/temperature much higher than it currently is.

https://en.wikipedia.org/wiki/Aneutronic_fusion#Energy_captu...

The Focus Fusion http://lppfusion.com/ reactor does direct conversion to electricity.

A Shame these guys are the least funded contender in this race and could do their research on a rounding error from ITAR.

Steam initially, but effect ways of converting all that heat to electricity are pretty limited at the moment.

One way to convert the heat from radioactive decay into electricity is through thermocouples (typically found in RTGs used on deep space probes and Mars exploration craft). They're incredibly inefficient though, less efficient than simply turning steam turbines.

So if the neutrons are untouched by magnetic fields, don't they also heat the magnets? I can see them passing through walls to boil water, but any heat created in the magnets would be very difficult to harvest given the temperatures involved. How much energy would it take to keep the magnets cool in the presence of a sustained (ie days) fusion reaction nearby? Generating such a flux in a bottle surrounded by things that need to be kept so cool sounds a bit of a fool's errand... but I really want practical fusion to be a thing.

Water is a good neutron absorber. Maybe the water is in front of the magnets.