Demystifying the Coulomb barrier

It's time to reframe the Coulomb barrier as a hindrance factor in a quantum state transition. This perspective reveals how established techniques for accelerating state transitions apply to fusion.

Much is written about nuclear fusion these days and often enough, the technical challenge underlying fusion is described as “overcoming the Coulomb barrier.“ What is meant by “Coulomb barrier“ is the repulsive potential that results from two nuclei, with their positively charged protons, coming close to each other.

Much like pressing two magnets together with their North poles facing, the closer the two get, the stronger the repulsion. The implication is simple: one needs to apply a great deal of force to get them close — close enough for the attractive nuclear strong force to kick in and hold them together. What results is a framing of nuclear fusion as a sort of billiard game, where balls (nuclei) are imbued with kinetic energy to forcefully collide and fuse.

This billiard table analogy is useful but misleading. It frames fusion as a brute-force collision game, where particles must smash into one another at high speed to “attack” the Coulomb barrier. Yet, this view risks narrowing our perspective to solutions that rely solely on high-energy collisions.

There exist, however, Coulomb barrier equivalents in all kinds of different contexts — many of which are routinely “overcome” by more elegant means than particle collisions. Think about what the notion of a Coulomb barrier really means: it represents the probabilistic inertia that hinders a system to immediately go from one configuration to another, energetically more favorable one. This is akin to how in an atomic state transition, e.g. from the 2p state to the 1s state in the hydrogen atom, the transition is not instantaneous, because the electron cloud involved cannot shift configuration without probabilistic “resistance.” This is why state transitions are associated with a half-life, and excited states with a lifetime.

So we can think of the cause for time constants in atomic state transitions as Coulomb barrier equivalents of sorts. These hindrances are weaker than in the nuclear case, as is reflected in their shorter lifetimes.

Returning to nuclei, let us consider a fusion reaction not as a simple collision event that “attacks” and “overcomes” the Coulomb barrier, but rather as a state transition. This state transition, from an energetically higher state to an energetically lower state, is, however, hindered by the probabilistic resistance that the reconfiguration of nucleons represents. With this alternate framing, the full toolset of atomic physics to accelerate state transitions comes into the view for the purpose of accelerating nuclear fusion transitions.

And that is ultimately what we aim to do in the field of nucleonics.