⁴This is the well known free-free radiation process which predominates for photon energies k much larger than the fundamental Rydberg state Z²I _H of the heaviest impurity in the plasma, where I_H ≃ 13.6eV is the fundamental Rydberg state of hydrogen. For the carbon, Z = 6 and the condition is k ≫ 0.5keV, while for iron with Z = 26, it is k ≫ 9keV. At lower energies, one must therefore consider the free-bound radiation process.