Bremsstrahlung Radiation Losses
Electrons decelerating in the field of ions radiate X-rays; this bremsstrahlung is a major, unavoidable loss channel for a hot D–He-3 plasma.
The dominant radiative loss
When electrons pass near ions they decelerate and emit X-ray photons — bremsstrahlung ("braking radiation"). The radiated power scales as ne2 Zeff √Te. Helium-3 has charge 2, so a D–3He plasma radiates more bremsstrahlung than a pure hydrogenic one at the same density and temperature.
Bremsstrahlung is fundamental: it cannot be shielded or reflected back into the plasma the way some other losses can, because the plasma is transparent to hard X-rays. For D–3He, the ratio of bremsstrahlung loss to fusion power is far less favorable than for D–T, and it sets a floor on the temperature and purity the plasma must maintain to stay net-positive.
Why it constrains the design
Keeping Zeff low means keeping impurities out; running at high ion temperature relative to electron temperature helps, because fusion power tracks Ti while bremsstrahlung tracks Te. The burner design assumes a hot-ion mode where Ti exceeds Te to keep the radiation balance workable.
Because the loss escapes the plasma entirely, it sets a hard lower bound on the fusion power density needed for net gain — the plasma must out-produce its own X-ray glow. For D–3He that bound is far closer to the achievable fusion power than it is for D–T, which is precisely why impurity control and the hot-ion mode are treated as requirements rather than refinements.
- Pbr ∝ ne2 Zeff √Te
- He-3's Z=2 raises radiated power
- Plasma is transparent to the X-rays — they leave
- Hot-ion operation (Ti > Te) helps the balance