The Deuterium–Helium-3 Mix Ratio

The ratio of deuterium to helium-3 sets the balance between fusion power and neutron production, and is chosen to keep the neutron fraction low.

A dial between power and cleanliness

The fuel mix is not simply one-to-one. The deuterium-to-helium-3 ratio is a design choice with a real trade-off. More deuterium raises the rate of D–D and D–T side reactions, which increases neutron output; more helium-3 favors the clean primary channel but demands more of the scarce isotope and higher temperatures to burn. The Aegis design point places this ratio where the neutron fraction settles at 5.44% of fusion power.

Why not maximize helium-3?

In principle pushing the mix toward pure helium-3 would minimize neutrons. In practice this raises the required temperature and the helium-3 demand, and a mirror plasma still contains deuterium that reacts with itself regardless. The chosen ratio is a balance: low enough neutron fraction to keep the machine clean and long-lived, while remaining reachable in the tandem-mirror configuration and economical in its use of bred helium-3.

• Ratio trades fusion power against neutron production
• Excess deuterium raises D–D and D–T neutron channels
• Excess helium-3 is cleaner but harder to burn and scarcer
• Design point: neutron fraction 5.44% at the chosen ratio

Because the ratio is set by injection, it is maintained actively during the burn. Diagnostics track the actual in-plasma composition and the control system trims injection to hold the target — the mix ratio is enforced, not assumed.