[boxed]

From what I understood, laser fusion needs laser efficiencies not just 40x better than what NIF uses, but like 3 or 4 orders of magnitude more efficient than the state of the art. Seems like a non-starter.

[DennisP]

NIF's lasers are 0.5% efficient. Equivalent modern lasers are 20% efficient. Both of these sources have both numbers:

https://physicsworld.com/a/national-ignition-facilitys-ignit...

https://pubs.aip.org/physicstoday/Online/31501/The-commercia...

[boxed]

80% energy loss on the way in would make the entire thing unworkable immediately. That's my point.

[DennisP]

Not at all. If you have that, plus 30% efficiency in a turbine, but you've got a 35X gain from fusion, then overall you have .2 * .3 * 50 = 2.1X overall gain.

NIF's latest shot is 2 MJ in, 7 MJ out[1], for a 3.5X fusion gain. So they've got an order of magnitude to go before getting to modestly practical levels. NIF seems to scale much better than linear with respect to the laser power, so an order of magnitude better gain is probably not a big change to the device.

(This does neglect energy loss in the hohlraum, so it assumes that direct drive laser fusion will get similar results. There are several projects working on that. Based on another comment here[2], the main reason for the hohlraum was that it made the experiment more relevant to weapons.)

[1] https://physicsworld.com/a/fusion-industry-meets-in-london-t...

[2] https://news.ycombinator.com/item?id=43935891

[bbatha]

Everything fusion reactor design needs similar gains in some part of the stack outside of the fusion parts to make it a viable power source: tokamaks need magnets to be orders of magnitude better, the lining for the reactors needs to last for much longer, the whole steam conversion mess, etc.

[DennisP]

Commercial REBCO tape is an entirely sufficient superconductor for tokamaks. At this point the limiting factor for the magnetic field is the structural strength of the reactor. Tokamak output scales with the square of size and the fourth power of magnetic field strength, and using REBCO, the CFS ARC design should get practical power output from a reactor much smaller than ITER.

[pfdietz]

Much smaller than ITER, but ITER is so huge a reactor could be much smaller and still be too big to be practical.

[DennisP]

About the size of JET. It's definitely practical in the sense that we can build it and it's likely to produce overall net power. Whether it will be competitive is another issue, and for that I agree with you that other designs, like Helion, have a better shot.

[pfdietz]

That's not a definition of "practical" that I would use. "Possible", perhaps, but practical implies effectiveness and suitability, and without competitiveness that isn't there.