Impurity Radiation

Heavy impurities from the walls radiate strongly and raise Z_eff; keeping the plasma clean is essential to the D–He-3 power balance.

Why cleanliness is physics, not housekeeping

Impurities — wall-eroded metals, oxygen, carbon — are far more effective radiators than the fuel ions because radiated power scales with Z² and, for line radiation, even more steeply. A small concentration of a high-Z impurity can dominate the plasma's radiative losses and raise the effective charge Z_eff, worsening bremsstrahlung across the board.

For D–³He this is especially punishing because the power balance is already tight against bremsstrahlung. Even a modest impurity influx can tip the balance from net-positive to net-negative. So the first wall, plug, and expander surfaces must be chosen and conditioned to minimise erosion, and impurities that do enter must be exhausted with the ash.

The requirement

Keeping Z_eff near its clean value is a hard operating requirement, not a nicety. It shapes material choices, wall conditioning, and the exhaust design, and it is one reason the burner's surfaces and their maintenance receive close attention in the design.

Because the tolerance for impurities is so tight, the burner treats wall material selection, conditioning, and impurity exhaust as coupled requirements set by the power balance, not as independent engineering choices. A machine that could tolerate a dirtier plasma would have more design freedom; D–³He's radiation-limited balance removes that freedom and makes cleanliness a physics constraint.

• Radiated power ∝ Z²; high-Z dominates
• Small impurity fractions can dominate losses
• D–³He balance is already bremsstrahlung-tight
• Drives material choice and wall conditioning