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This is a basic question, but it something I have been wondering ever since I learned about inertially confined fusion. In the ICF reactor configurations I am aware of, the drivers that are used to compress and heat the pellet are electrically pumped lasers. They coherently build up a laser pulse in the 10-40 TW range, then dump it quickly onto the target. They have been unable to reach net gain energy because the energy confinement isn't good enough, and haven't been able to drive multiple shots because that would overheat the lasers.

So this is my question: If you were to build a massive solar reflector farm and concentrate the same 10-40 TW (which is this case means an array of 1-4 square kilometers) would you be able to ignite the pellet and get net gain fusion? The reflectors would not have the same overheating issues as the lasers, they would get their energy for free, and best of all be able to pump in energy continuously.

Would this work? And if not, what would be the reasons it wouldn't?

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  • $\begingroup$ It isn't possible, because you can't heat an object higher than 6000K by focusing sunlight, due to the second law. (6000K is the surface temperature of the Sun.) Hopefully someone can write a good answer explaining this in more detail. $\endgroup$
    – N. Virgo
    Commented Feb 24, 2018 at 2:38

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Using reflectors, lenses etc for sunlight, one cannot create temperatures exceeding the temperature of the surface of the Sun, ~6000 C ~ 0.6 eV, according to the second law of thermodynamics. This temperature is way below ignition-relevant black body radiation in ICF hohlraums, ~300 eV, cited in ICF literature, e.g., Phys. Plasmas 18, 032706 2011. So, for indirect drive with solar radiation it looks pretty hopeless. For direct drive, i.e, directly focusing sunlight on the surface of ICF capsule, one obvious problem is that sunlight is not coherent so it cannot be focused well.

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  • $\begingroup$ Wait if that is true, how does the current icf reactor design get up to fusion temperature using infrared radiation? The photon energy is way below the 10-50 keV that is needed for fusion. $\endgroup$
    – user11377
    Commented Feb 24, 2018 at 15:47
  • $\begingroup$ The black body radiation in the hohlraum has to be at ~300 eV temperature to drive an ICF pellet. With optics (mirrors, lenses etc) you cannot create 300 eV black body radiation if the source is 0.6 ev black body radiation. That's a thermodynamic constraint. Do we agree on this? But with black body radiation of needed parameters, the pellet is accelerated (imploded) and when it stagnates all this mechanical energy becomes thermal energy, so this is how T~kev temperature is achieved. Imagine an air gun firing a bullet, the bullet hits a wall and becomes hot, way above the air in the gun. $\endgroup$
    – Maxim Umansky
    Commented Feb 24, 2018 at 17:20
  • $\begingroup$ He's asking how you can get 300 eV black body radiation in the hohlraum with a laser with per-photon energy much below this. But the laser does not have a black body distribution of energy across wavelengths. $\endgroup$
    – EL_DON
    Commented Jul 4, 2018 at 14:46
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I think that one question here is whether one can concentrate all the 1-4 square km sunlight into a beam diameter comparable to the pellet, which is in the mm range. A further problem is that you need the very sudden application of the light beam power to achieve inertial confinement fusion. This means that you would have to be able to switch the concentrated solar light beam with extremely high speed. Whether this can be accomplished seems to be a technical problem. .

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