If you start to think of 'temperature' of individual particles as 'speed with which they move' that is a useful rough approximation of trying to figure out what it means that something has a particular temperature. Containing the plasma is hard not just because of the temperature it is at but simply because it tends to destroy anything that contains it and that doesn't really change all that much for 15 million degrees Celsius, 30, 100 or 150. What it does change is that at 150 million degrees Celsius you have some hope of extracting useful work from a very small quantity of plasma. If you don't get it up to those temperatures - again, as far as I understand it - then you will always be putting in more energy than you are gaining because of some fundamental physics limitations.

So the smaller you make your reactor the hotter you'll have to make it to make it net positive. This leads to the counter intuitive result that making a much larger reactor is actually quite possibly easier than making a really small one. The rate of heat loss is much smaller for a larger reactor and so it becomes easier to sustain the reaction and to extract useful energy from it.

It is very well possible that none of the reactors currently on the drawing board and under construction are going to be working well enough to give us a sustained reaction resulting in net yield. But we're getting closer and closer to that and there is some (small) chance that I will still see this in my lifetime.

The catch is that as long as you can't get a small reactor to work getting funding for a much larger one (which you actually may be able to get to work) is going to be extremely difficult. We like to see proof before we scale up. In this case it may well be that such small scale proof can't be done or can't be done in a way that it it will convince backers that a larger scale device will work.

> What it does change is that at 150 million degrees Celsius you have some hope of extracting useful work from a very small quantity of plasma

It's also nice when your reaction quits flinging antimatter at your containment vessel :)

Yes, true. Apropos antimatter, recently I read here on HN somewhere that lightning generates antimatter as well, and it made me wonder if earthquakes do too but I haven't been able to get a clear answer on that. Fascinating stuff.

https://news.ycombinator.com/item?id=36749663

Larger reactors may simplify the plasma physics, but it complicates the materials engineering significantly. A huge problem already is creating a structure that can bear the weight of the reaction vessels and the gigantic magnetic fields used for containing the plasma, and also continue to do so for a decent amount of time after being exposed to the constant neutron bombardment of D-T fusion.

I very much doubt currently known materials and structures could be used to construct a reactor 10 times the size of ITER.

This is a high value comment - it's got a hook that I sort of mostly get, but follows that up with jargon laden things I don't understand well, and your references gave me a half hour of rabbit-hole learning. I appreciate the casual knowledge you've passed along. Thanks!