[femto]

Measuring the temperature is the same as measuring the average energy of particles in the plasma, which is the same as measuring the average speed of the particles. Basically you do that by measuring radiation, doing it in several different ways, so you can test the accuracy of your measurement. This article gives an overview:

https://www.scienceinschool.org/article/2013/fusion-4/

Incidentally, the plasma is of such low density (ie. few particles) that it has little stored heat energy. As soon as the plasma touches anything it cools down and fusion stops. You're unlikely to get a mushroom cloud out of today's fusion reactors, as there is just not enough stored energy in them. (Might that change if they are scaled to the point where large amounts of energy can be extracted, a bit like the bang out of a large charged capacitor?)

[wrp]

> ...the plasma is of such low density...

I believe the chambers are operated at a pressure of 1/10,000th of an atmosphere.

[ncmncm]

When the thousands of tons of lithium hydroxide catch fire, you will notice.

[pfdietz]

Having lithium hydroxide catch fire would be quite a trick. Are you mixing it with chlorine trifluoride?

[ncmncm]

Lithium hydride. The oxide is what you get after it explodes.

[pfdietz]

I don't think LiH makes sense in a DT fusion reactor blanket. Neutron economy there is very important, and H will absorb some neutrons.

[ncmncm]

If the H in your LiH is deuterium, absorbing neutrons would be OK, because that makes tritium. Not quite as much as lithium absorbing them, but better than anything else.

Hundreds of tons of deuterium in your thousands of tons of terrifyingly inflammable LiD would be expensive. But if we balk at expense, we won't get fusion power.

Most of us do, in fact, balk at expense, for reasons. But the topic here is what would be needed for fusion to be made to work at all. $2B worth of deuterium to help breed tritium is not much for a $100B fusion plant.

Extracting your few grams of tritium every day from a thousand tons of LiD may be called somebody else's problem.

[femto]

What's lithium hydroxide used for in fusion reactors?

Edit: A bit more searching turned up an answer. It's a neutron absorber. So it's used as a shield to stop the neutrons leaving the reactor?

[ncmncm]

The lithium + neutrons would breed the tritium you need for fuel to burn tomorrow.

Extracting parts-per-billion of tritium distributed throughout thousands of tons of hot, brittle, radioactive, super-flammable LiH would be no picnic. Melting it probably would not make that easier.

Fortunately, no one will need to.

[ncmncm]

Oops, autocorrect strikes again.

The blanket is lithium hydride, not hydroxide.

Some people talk about adding beryllium.