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The question:

At temperatures above ~5000K are not stable any solid or liquid materials or even more complex molecules (such as fullerenes and Polycyclic aromatic hydrocarbons) which emit/absorb wide-spectral black-body radiation effectively.

Simple molecules and atoms which survive such temperatures have very narrow absorption/emission lines and small absorption/emission cross-section. => They are transparent for thermal radiation.

Is there anything which would emit/absorb broadband thermal radiation at temperatures >10,000 K ?

What about plasma? I guess plasma must be both highly ionized (~hot) and at the same time dense in order to have high opacity and absorption for thermal light (Am I right?). This is a bit contradictory. Density is inversely proportional to temperature if we are limited by pressure let's say 100MPa, while ionization is increasing with temperature. Is it possible to increase opacity/absorption of gas/plasma at ~10,000-100,000 K by seeding with alkali metal which release electron very easily ?

background:

Gas core nuclear reactor, and derived gas core nuclear rocket is a fission nuclear reactor theoretically able to achieve temperatures above 5000K (some proposals talk about 40,000-100,000 Kelvin ) which is important for high energy efficiency of electricity production (using MHD generators ) and high specific impulse of nuclear rocket.

There is a concept of space propulsion called nuclear lightbulb rocket which should achieve high specific impulse exhausting moleculer/atomic hydrogen propellant at velocity up to 20-40 km/s which means temperature ~25,000-90,000 Kelvin.

For effective function of such engine it is necessary that all the heat from gaseous nuclear core is transferred to propellant by thermal radiation. However, hydrogen propellant itself is almost transparent for thermal radiation. This would cause that the heat is transfered to the walls of rocket nozzle instead, which would melt the walls and destroy the reactor. In order to make propellant opaque it was proposed that tiny dust particles of tungsten or hafnium-tantalum carbide particles would be dispersed in propellant gas. These particles would however evaporate above ~5000K making propellant gas transparent again.

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