The Coulomb Barrier & Cross-Sections

Helium-3's double charge raises the Coulomb barrier for D–He-3, pushing the useful reaction temperature into the tens of keV.

Why the barrier is higher

Two nuclei must approach within the range of the strong force to fuse, against their mutual electrostatic repulsion. That repulsion scales with the product of the charges, Z₁Z₂. For D–T it is 1×1; for D–³He it is 1×2. The larger barrier means fewer ions have enough energy to tunnel through it at a given temperature.

Quantum tunnelling lets fusion occur below the classical barrier height, and the reaction rate is dominated by ions in the high-energy tail of the Maxwellian distribution — the Gamow peak. Because the D–³He barrier is higher, its Gamow peak sits at higher energy, and the plasma must be hotter to populate it.

The consequence

This single fact — helium-3's +2 charge — is why the burner runs near 90 keV rather than 15 keV, why its triple product target is higher, and why its confinement must be correspondingly strong. It is a fixed feature of the fuel, not a design choice that can be relaxed.

There is no way to lower the barrier — it is set by the nuclear charges — so the only levers are temperature, density, and confinement time, which is why D–³He forces a higher triple product than D–T. This is the root cause that propagates into every downstream gate: the field the plug needs, the regime it must reach, and the recirculating power the engineering gain must overcome all trace back to helium-3's +2 charge.

• Barrier ∝ Z₁Z₂; D–³He = 1×2
• Higher barrier → higher Gamow peak energy
• Reaction driven by the Maxwellian tail
• Fixes the ~90 keV operating requirement