D-3He Reaction Products & Their Energies

The burner's fuel yields a fast proton and a helium-4 nucleus; their energies and spectra set every requirement downstream in the DEC train.

The products, exactly

The reaction is D + 3He → 4He + p, releasing 18.3 MeV of kinetic energy. By conservation of momentum the lighter proton takes the larger share: about 14.7 MeV to the proton and about 3.6 MeV to the helium-4 nucleus. Both are positively charged, so both can be guided out along the magnetic field and decelerated in a converter.

Why the spectrum, not just the peak, matters

If every proton left at exactly 14.7 MeV, one perfectly tuned retarding voltage would recover nearly all of it. In practice the energy is spread: fusion happens over a range of plasma conditions, particles scatter before they leave, and the two species differ by a factor of four in energy. A converter must therefore accept a band of energies, which is why a single electrostatic grid is not enough and the train is staged.

Downstream consequences

• Two species, two energy scales — the train must handle both the fast proton and the slower helium-4.
• A broad spectrum — favoring traveling-wave conversion, which sorts particles by velocity in time.
• High per-particle energy — demanding high-voltage standoff at the collectors.
• Charged-dominant yield — enabling the ~95% direct share that keeps water use low.

A useful way to picture the momentum split: the proton and the helium-4 leave back to back, and because momentum is shared equally while energy scales inversely with mass, the lighter proton carries roughly four times the energy of the helium-4. That single fact — one very fast light particle and one slower heavy one — is why no single retarding voltage suits both, and why the burner's converter is a train rather than one grid.

Every later page in this section — TWDEC bunching, MHD conversion, thermionic topping, collector design — is ultimately a response to these two numbers and their spread. Hold the pair 14.7 MeV and 3.6 MeV in mind and the rest of the DEC train reads as a series of answers to the question they pose: how do you cleanly recover the energy of two charged species born with a four-to-one energy ratio and a thermal spread around each?