Because of the extreme differences between electron mass and nuclear mass, kinetic energy exchange between electrons and nuclei is very inefficient. As a result, a plasma with different electron temperature and nuclei temperature can exist for a long time. For example, the electron gas in fluorescent lamps can reach 20,000K while ions and molecules are only slightly above the room temperature. So I am curious whether it is possible to make a plasma with cold electron gas and hot nuclei. For example, in tokamak devices, only the thermal movement of deuterium and tritium nuclei is implicated in the nuclear fusion. The thermal movement of electrons OTOH drains energy away via bremsstrahlung radiation. If we can heat up the nuclei directly instead of heating up the electron gas, will the electron gas eventually cool down due to the radiation loss?
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$\begingroup$ "if we can heat up the nuclei directly" I cannot think of a way to do that. It can be done in accelerators , but I think it is the mass difference that cannot be overcome in plasma. $\endgroup$– anna vCommented Oct 1, 2021 at 8:15
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$\begingroup$ Particle accelerator is one way. The energy of fusion reaction also releases as the kinetic energy of nuclei. $\endgroup$– 哲煜黄Commented Oct 1, 2021 at 9:03
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$\begingroup$ Yes, but the electrons are still there in the plasma, and even become runaway pure.mpg.de/rest/items/item_3073013_5/component/file_3173424/… , $\endgroup$– anna vCommented Oct 1, 2021 at 9:20
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$\begingroup$ You can use things like electrostatic ion cyclotron waves and other, ion-dependent instabilities that predominantly heat the ions only. $\endgroup$– honeste_vivereCommented Oct 1, 2021 at 13:42
1 Answer
To some degree, yes, you can heat up ions directly without heating the electrons. This can be done through ion cyclotron resonance heating. Due to the Lorentz force, ions in a magnetized plasma (like that inside a tokamak) will gyrate around field lines. That is, they move in circular orbits around magnetic field lines with a very specific frequency. You can inject electromagnetic radiation into the plasma with that specific frequency (and the right polarization), such that these waves resonate with the ion gyration motion and a transfer of energy to the ions takes place. This gives the ions more kinetic energy, i.e. raises their temperature. This process will give virtually no energy to the electrons. However, even though energy transfer between electrons and ions is inefficient, transfer of energy from ions to electrons is much more efficient (something heavy "bumping into" something light will easily send it flying, but the opposite is not true). So, the electron temperature will much more rapidly equilibrate to a higher ion temperature than the other way around. So yes, it's possible, but creating large differences is very difficult and even more difficult to sustain for long periods.
