Nuclear Fusion

What to observe

1. Start on Cold gas. The nuclei drift and bounce and simply refuse to touch. Two positive charges repel, and at ordinary energies they turn back long before they meet. Nothing fuses, the reactivity is essentially zero, and no amount of waiting changes that. This is why fusion is hard: the fuel does not want to burn.
2. Raise the temperature. Now the nuclei are moving fast enough that a few of them tunnel through the barrier they cannot climb, and fusion flashes begin. Watch the reactivity curve on the right climb almost vertically: fusion power is savagely sensitive to temperature, which is why you need a hundred million degrees before anything useful happens.
3. Switch the fuel at a fixed temperature. D-T lights up first and brightest: it has the lowest barrier and the fattest cross section, which is why every first-generation reactor burns it. D-D and D-³He need it far hotter, and plain hydrogen, the star-maker, barely reacts at all.
4. Load Tokamak. Temperature, density and confinement together push the triple product n·T·τ across the Lawson line, and the alpha particles the fusion throws off now heat the plasma faster than it leaks. The panel flips to ignited: the fire feeds itself. That single crossing is the goal of every fusion machine ever built.
5. Drop the confinement time back down. The heat escapes faster than fusion can replace it and ignition dies, even though nothing about the fuel changed. Confinement is the third leg, and the hardest: holding a hundred-million-degree gas still, for long enough, without touching it.
6. Load The Sun. Here is the twist: the Sun is not hot by fusion standards, only fifteen million degrees, and plain hydrogen at that temperature reacts so feebly that a given proton waits billions of years to find a partner. Read the wait time. The Sun works not because it burns hard but because it is unfathomably large and gravity holds it forever. Its core makes less power per cubic metre than a compost heap.
7. Load H-bomb: D-T squeezed to stellar density and heated by a fission trigger. Now the reactivity, the density squared and the released energy all pile up at once, and the whole charge burns in the time light crosses it. Same equation as the Sun and the tokamak, driven off the top of every scale.
8. Fusion, a star and a bomb are one process at three settings. What separates them is never the physics, only how hot, how dense, and for how long.