I’m actually wondering why gaseous substances are used as they have a bad thermal conductivity (or actually heat capacity) per volume. I see that Hydrogen and Helium are advantageous because of their low Z (protons) so they don’t absorb the x-rays as the cooling cycles have to run through the photovoltaic layers. But maybe the cooling of the outer layers should be done by a liquid beryllium-hydrogen or boron-hydrogen compound to get more heat transferred. Or are they too corrosive? Maybe resort then to liquid carbon-hydrogen.
