For what it's worth, people have been wondering about fusion with sonoluminescence for about 20 years (that I know of) but probably longer:

https://en.wikipedia.org/wiki/Sonoluminescence

Since fluids are approximately incompressible, they can concentrate a tremendous amount of pressure and heat in a small space when rammed together with no route for escape:

https://en.wikipedia.org/wiki/Water_hammer

Since we know that the hydrogen in water will fuse on its own under enough pressure, we can run the calculation using the water hammer formula;

maximum pressure = 0.0135 * velocity * length / time

The pressure at the center of the sun is 26.5 petapascals (2.65e16 pascals):

https://en.wikipedia.org/wiki/Solar_core

If we use a length of 1 meter, and velocity of 1 meter per second, we can see what compression time is required to achieve the pressure at the core of the sun using a water hammer:

2.65e16 = 0.0135 * 1 * 1 / time

So:

time = 0.0135 * 1 * 1 / 26.5e15 = 5.09e-19 seconds

This is about 1/2 an attosecond (10^-18 seconds). I was hoping for something more in the femtosecond range because we’re used to dealing with that timescale for capacitance, etc. Lead has 11.4 times the density of water, so for liquid lead the formula would be:

time = 0.1539 * 1 * 1 / 26.5e15 = 5.81e-18 seconds

And with 10 times the length and 10 times the velocity:

time = 0.1539 * 10 * 10 / 26.5e15 = 5.81e-16 seconds

Now we are to the femtosecond range. They don’t give the dimensions of the sphere or the speed of the pistons in the video. But a 10 meter radius with lead moving at 10 meters per second seems feasible.

So this is the maximum time that the liquid must remain compressed at the center without finding a way out (via quantum tunneling or something, who knows at these extremes) in order to reach the desired pressure. I don’t know if the shape of the bubble needs to be spherical or if any geometry works. I also don’t know how temperature affects it (the temperature will likely be higher than for sonoluminescence since the bubble is bigger). But we do know that the general idea works, because fusion bombs are ignited by a fission bomb compressing hydrogen to the necessary heat and pressure.

The main problem will be with the hydrogen plasma diffusing into the metal. But since pressure falls off gradually from the center, the cross section should be large enough that it may not pose much of an issue. I also wonder if the presence of the metal atoms might create smaller channels through which the hydrogen must pass. That would take advantage of the electromagnetic force holding atoms apart, similarly to how magnetic confinement works in a Tokamak or Farnsworth Fusor, and create pinch points where atoms are more likely to fuse.

The final reactor design should probably use liquid hydrogen because a bigger cross section could fuse, because the P=pLv/t formula is linear. F=ma means that lighter atoms accelerate linearly faster under the same force, but also decrease final pressure linearly. So if my math is right, final pressure should be within the same order of magnitude for all atoms (hopefully someone can check this). For example, liquid hydrogen has a density of 0.07 grams per cubic centimeter, so is 14.29 times less dense than water, so the final V for liquid hydrogen should be about 14.29 times higher than water and 14.28 * 11.4 = 162.9 times higher than lead. But final pressure would be 14.29 and 162.9 times lower respectively so result in nearly the same maximum pressure.

Also they should tune the geometry so that the 1 second firing rate matches the bounce frequency of the liquid.

Thermodynamic efficiency probably won’t be great at first, with the Carnot limit of:

https://en.wikipedia.org/wiki/Carnot%27s_theorem_(thermodyna...

efficiency = 1 - (cold temperature)/(hot temperature)

Which for typical heat engines operating at 400-600 kelvin is 25-50%. So they’ll need to overengineer it by at least a factor of 2-4 to achieve unity.

Maybe the first reactor design gets 10% of unity, maybe the second one gets 20%. It will be interesting to see the final formula for reactor size vs gain factor (Q), because then we’ll know if it can simply be scaled up by a few orders of magnitude or if something in the physics prevents it from working if it’s smaller than a star. Of course this principle is the same for any fusion reactor. Wouldn’t it be wonderful if they find that all that matters is the strength of the pistons and that the reactor can be scaled down to fit in a shipping container, just running at a higher frequency!

My gut feeling is that the General Fusion reactor design is going to work. It will almost certainly achieve fusion (detect neutrons or excess heat) just like with sonoluminescence. Will it be over unity? Well.. I would wager more money on this design than any others that I’ve seen.