[pfdietz]

I was withholding judgment on General Fusion. They had a concept that, at least in principle, would allow engineering limits on the power density at the first wall to be evaded (since the first wall would be thick liquid metal, not a solid that could sustain damage.)

But they gave up on their original "acoustic" compression scheme, and now will compress the plasma via subsonic motion of the liquid metal. This scheme involves a solid conductor going down the middle of the chamber. It will be exposed to orders of magnitude higher radiation flux than the first wall of mainstream fusion concepts, as well as pulsed loads from magnetic fields up to 100T (which correspond to pressures far higher than the chamber of a gun, and higher than the deepest point in the oceans.) Getting this conductor to survive even one shot would require heroic engineering; keeping it cooled and together as its material properties rapidly degraded would require superheroic engineering.

[lambdatronics]

I heard that the shock wave was spalling off the front few cm of the liquid metal wall as a spray of jets/droplets -- the shock reflects from the vacuum interface & where the wave interfered destructively with itself, the pressure dropped to zero, and cavitation occurred. This happens in a random & uneven fashion.

[pfdietz]

That sounds very plausible. Do you have a reference I could cite?

[lambdatronics]

Here it is: https://generalfusion.com/wp-content/uploads/2016/08/Suponit...

Edit: Here's a second one that's similar (but behind a paywall). http://www.sciencedirect.com/science/article/pii/S0045793013...

The preprint is here: https://arxiv.org/abs/1310.6010v2

Edit 2: Here's a better one with video frames from the experiment showing the jets. https://generalfusion.com/wp-content/uploads/2016/08/Richard...

[pfdietz]

Cool, thanks!

[nickparker]

Can you point me to where they say the central cones are a change in their hypothetical production design?

I was under the impression it's just there for diagnostics in the sub-scale prototype.

[pfdietz]

The abandonment of the acoustic compression scheme is explained in this poster.

https://generalfusion.com/wp-content/uploads/2018/07/ICPP_20...

The move to a spherical tokamak is explained in page 9 of these slides.

https://arpa-e.energy.gov/sites/default/files/11_LABERGE.pdf

The slower compression of the non-acoustic scheme requires better confinement than the spheromak provided.

[nickparker]

Hmm yeah that does sound like a permanent change in the second link doesn't it? Too bad, I was really rooting for their plan.

[pfdietz]

The Turchi mentioned there was associated with Los Alamos, where the LINUS concept was investigated. General Fusion seems to be moving in that direction, so this slide from Turchi at Compact Fusion Systems could be of interest. It has a focus on practical engineering and cost issues that I find encouraging.

https://nucleus.iaea.org/sites/fusionportal/Shared%20Documen...