bremsstrahlung is the X-ray radiation emitted when free electrons decelerate in the electric fields of ions — an unavoidable energy-loss channel in every hot plasma.
The word is German, compounding bremsen, "to brake," with Strahlung, "radiation": braking radiation. The mechanism follows from classical electromagnetism — any accelerated charge radiates — so whenever an electron swings past an ion and its path bends, it emits a photon; at fusion temperatures those photons are X-rays that stream out of the 📝plasma carrying energy with them. Because no confinement scheme can stop it, bremsstrahlung sets a floor under a fusion plasma's power balance: fusion heating must outrun radiative loss before net energy is possible.
The loss scales with the square of the ions' charge, which drives two consequences. In deuterium-tritium machines it makes purity an operational imperative — a small fraction of high-charge impurity ions sputtered from the walls multiplies the radiated power, which is tracked through the plasma's effective charge, Zeff. For 📝aneutronic fusion it is close to disqualifying: fuels like proton–boron-11 carry higher nuclear charge and demand temperatures roughly ten times higher, and whether such a plasma can fuse faster than it radiates remains an open question. Bremsstrahlung is distinct from line radiation, emitted at discrete wavelengths by incompletely stripped impurity ions, and from synchrotron radiation, emitted by electrons gyrating in the magnetic field.