A system of particles in thermal equilibrium will contain a small fraction whose kinetic energy exceeds the average by a factor of 10. Also, the energy available in the center-of-mass frame of two colliding particles is higher if they happen to be moving in opposite directions. That could give you another factor of up to 2. In any event, I don't think you want your fuel fusing all at once!

Expanding on this, this 10KeV temperature is the average of all particles, and there's a distribution around this average. Some will be higher, and thus more capable of colliding with high energy.

In addition to that, whether two nuclei fuse is also dependent on how squarely they collide. A glancing blow intuitively allows both nuclei to push each other away a lot easier than if they experience a head-on collision.

A graph of the fusion rate such as [1] shows that even at much lower average temperatures, fusion will occasionally happen when two higher-than-average nuclei collide head-on. As the temperature gets higher, this rate increases as more particles have enough energy and less require those head-on collisions. The peak rate for D-T according to this graph is about 70kEv.

I can't speak to why the grandparent's link referenced 100kEv, as it's been a decade since I last studied this and I'm very rusty.

[1]: https://en.wikipedia.org/wiki/File:Fusion_rxnrate.svg

How does one control the chain reaction of the fusion process? I understand fission reactors using control rods to absorb some of the neutrons to prevent those neutrons from hitting other fissile particles, but this seems like a harder problem. Those particles fusing at a "low" temperature in the distribution of equilibrium cause additional fusion reactions at higher temperature thresholds in other particles because of the energy released, if my mental model is correct? And assuming the fuel is gaseous, the idea of a control/absorptive retarder seems like a much harder problem. Edit: Oh! Maybe they reduce the strength of the magnetic containment field? Which reduces pressure inside the reaction chamber and thus reducing temperature?

Basically, yeah. It’s very hard to sustain a reaction like this, so to stop it all you need to do is stop trying your hardest to keep it stable.

In addition to this there is also a quantum effect called quantum tunneling [1] which allows for a very tiny probability for particles with insufficient energy to fuse upon collision anyway.

[1] https://en.wikipedia.org/wiki/Quantum_tunnelling#Nuclear_fus...

Sometimes you want to fuse all of the fuel at once, but the technology for doing that has been available since the 1950s.

The Wiki entry also mentioned "two effects that lower the actual temperature needed", one being average kinetic energy and the other quantum tunneling "if [nuclei] have nearly enough energy." The term "nearly" isn't precisely defined though.

There isn’t a hard limit. But as you get farther away from the amount of energy that would be “enough” without tunneling, the probability of tunneling falls off exponentially.

Quantum tunneling effects are generally (maybe always?) exponentially unlikely, in the sense that the probability of tunneling will decay exponentially with the discrepancy between energy-available and energy-required.