Could focused sunlight ignite a pellet of lithium deuteride? 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? 
 A: 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.
A: 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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