[hamdani yusuf (OP)]

It's often mentioned that strong magnetic field prevents plasma particles from hitting tokamak's inner wall, thus protecting it from thermal conductance and convection. But how is it protected from thermal radiation?

[Bored chemist]

Mainly distance.
Tokamaks are big and the confined plasma is in the middle of the toroid so it's quite some distance from the wall.

[alancalverd]

Although the plasma temperature is very high, the power dissipated in it is only a few megawatts, which is easily dissipated by cooling the torus jacket. Compared with the 240 MW of water cooling for the JET field coils, this is not a big deal.

[evan_au]

How can tokamak's inner wall withstand thermal radiation of confined plasma?

Since it is a plasma, there is no line spectrum in the visible/UV spectrum.

The thermal radiation spectrum is determined by the black body temperature of millions of degrees.
- There is relatively little visible, infra-red or UV radiation; this can be taken care of by making the inner surface very reflective at these wavelengths
- There is a considerable amount of thermal X-Ray radiation, which penetrates the wall, depositing energy deep in the metal, where channels carry coolant to carry away the heat.
- Nuclear energy is also released in the form of gamma rays
- A lot of energy is released in the form of neutrons, which are not contained by the magnetic field, and not stopped very well by the metal walls. These are stopped by neutron absorbers, which are required to generate Tritium to fuel the reactor.
- Another large part of energy generation is in the form of neutrinos, which aren't stopped by the reactor walls (or the Earth), so they don't contribute to heating of the reactor.

Most fusion reactors spend most of their lifetime running on Hydrogen and Deuterium, and producing virtually no neutrons, gamma rays (or neutrinos).
- Near the end of their lifetime, they will try a Deuterium-Tritium mixture to see how close they can get to "break-even".
       - It is only this phase of testing that generates neutrons and tests the Tritium generation part of the fuel cycle.
       - It also makes the structure radioactive, which makes it harder to modify the design
See: https://en.wikipedia.org/wiki/Plasma-facing_material

[Bored chemist]

These work because they are full of a deuterium plasma
https://en.wikipedia.org/wiki/Deuterium_arc_lamp

They do a fine job of producing UV (and visible light) from the recombination reaction where electrons re-attach to deuterium nuclei producing a continuum emission spectrum


It's not clear to me why the tokamak plasma would be different. The density is a bit lower but, the whole point of a tokamak is to fuse nuclei so the pressure can't be that low; there must be some molecules present.

[evan_au]

It's not clear to me why the tokamak plasma would be different... there must be some molecules present.

It is true that "neutral beam injection" is often used to inject fresh fuel into a fusion reactor through the strong magnetic field lines.
- But these are not neutral molecules - they are first put through a particle accelerator to boost them to an energy of around 4MeV before being neutralised into neutral atoms and fired into the reactor.

With an operating temperature of around 150 million degrees, neutral atoms don't survive the first collision with an ion in the plasma. There really isn't a population of molecules present in the plasma.

See: https://en.wikipedia.org/wiki/Neutral_beam_injection

[Bored chemist]

There is a considerable amount of thermal X-Ray radiation

What does the spectrum of that look like?

[Bored chemist]

There really isn't a population of molecules present in the plasma.

https://www.researchgate.net/publication/292072580_Molecular_emission_in_the_edge_plasma_of_T-10_tokamak