Laser Fusion Put On a Slow Burn By US Government

Hello, I'm sorry to say this, but aneutronic fusion is probably never going to be a practical energy source.

There's a reason D-T fusion is the focus. One problem is that all the aneutronic fusion reactions involve higher-Z (higher atomic number) nuclei. Higher Z nuclei have worse energy loss via Bremsstrahlung radiation than the D-T or D-D reactions. In a plasma hot enough to sustain fusion reactions, the electrons and ions are banging against each other, and every hit potentially makes X-rays or gamma rays, converting thermal energy into light. In a reasonable-sized thermal plasma, these photons pretty much just leave without interacting again, thus cooling the plasma.

People have calculated that the energy loss rate from Bremsstrahlung in a thermal plasma composed of atoms capable of doing aneutronic fusion would exceed the rate that the fusion reactions would heat it. Thus, the plasma would cool right off, the flame would in effect "go out" because it would lose heat faster than it created heat via fusion.

In a star, this works out, because a star is so very, very big that the photons from Bremsstrahlung are re-captured within the star: i.e., the heat can't escape because of sheer mass in the way. We're never going to pull that size and density off in a lab or an engineering installation.

Now, if you can somehow arrange for the plasma to NOT be thermal, you may be able to beat this issue. However, keeping a plasma from thermalizing requires a large energy input, and is very hard to arrange for and preserve long enough to get energy from fusions. Inertial confinement might work (laser or Z-pinch or the like), there you potentially have very high densities for maybe "long enough" for Bremsstrahlung not to eat your lunch: I don't know. However, both laser and Z-type installations seem very hard engineering problems.

The wikipedia on "aneutronic fusion" discusses these issues some as well.

Anyway, that's one reason most are happily ignoring aneutronic fusion entirely. Another is that much higher temperatures are required for the aneutronic fusion reactions, and we haven't even got D-T going yet and that is the lowest temperature fusion reaction. D-T is where I would put my money, too, given the results of the physics calculations.

--PM

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/1diZdNYIUMc/story01.htm

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