Yes, that's the Lawson Criterion. The product of density, temperature, and confinement time has to be above a threshold to reach ignition.

The second thing I don't know, thats outside my expertise.

I couldn't make it out this year, I'm glad it was better.

I'm glad you were able to attend at all, a lot of the US national labs people were not there due to COVID travel rules.

Ok, thanks for the clarification. Maybe I misunderstood what his post was getting at. I have heard that proton-Boron is basically impossible because the brems losses at the temperature where reactivity is sufficient will always be higher, or almost always higher. I think this is what you are saying, you can't just focus on the temperature. Sam Wurzel had a great talk on this at APS 2021, clarifying Qeng vs Qsci and how that changes depending on your recirculating power fraction and other factors.

The DT reaction that most groups believe is the economical choice for fusion given our current technology level. It is not the easiest fusion reaction to do, I believe that would be muon catalyzed fusion. It is not the cleanest fusion reaction, that would probably be proton-boron which just makes three alpha particles and rarely makes a neutron. It's not the fusion reaction with the most available fuel, that would be pure hydrogen fusion like the sun does. DT seems to be the sweet spot when you combine all the relevant factors together.

The reactivity curve of DT is favorable compared to other similar options like DD and DHe3. It occurs at lower temperatures, which means it is easier to build a fusion reactor to reach those conditions. For most power plant schemes you want to reach ignition of the fuel, meaning the energy coming out of the fuel also heats the fuel more than the external heat source, like lighting a match. It is a lot easier to build a power plant with a fuel that is more like gasoline, where a little spark sets it ablaze, rather than a block of rubber, which requires significant heating to get it to burn.

The other big advantage of DT is that half the fuel is very abundant. Deuterium is relatively easy to find, you can buy it at Praxair in T-cylinders. You need to breed tritium in the fusion reactor itself, but the consumable is lithium, which is also pretty easy to find. Because it's a nuclear reactor each power plant is only burning maybe hundreds or thousands of kilos of lithium a year, which is nothing on the world scale. If we can design a plant that has a viable tritium breeding cycle, and there are many proposals that seem promising, this should be solvable. This is likely a lot easier than making a fusion reactor that achieves 10x higher temperatures, which is what you need to do a straight DD reaction.

Source: I work at one of those private fusion companies. Engineer, not a physicist.

Isn't that exactly what Robus said? The DT reaction is favorable because it reaches high reactivity at lower temperatures. https://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/Fusion_rxnrate.svg/330px-Fusion_rxnrate.svg.png